Abstract
Oxalic acid is among the most abundant organic acids found in different biospheres, including plants, as an end product of metabolism. It forms either soluble or insoluble salts with monovalent or divalent cations, respectively. Then, consumption of oxalic acid-rich foods in human diets, particularly leafy vegetables (e.g., spinach, tea, and rhubarb), affects minerals absorption such as calcium. Meanwhile, its high level in blood is associated with many diseases such as hyperoxaluria systemic oxalosis and is thus classified among potential anti-nutrients. Various factors have affected oxalic acid levels in foods, including agricultural traits and consumption practices. Hence, the current review aimed at rediscovering oxalic acid dietary sources, metabolism, and the various processes employed to reduce its content in foods, and consequently, health harmful effects. Among them are physical/cooking, chemical, fermentation, and biotechnological processing. Recent biotechnological approaches have been attempted to produce transgenic crops remodeling oxalate metabolism, particularly its degradation. The soluble form of oxalate seems to be better absorbed and more harmful than insoluble salts in foods aiding in kidney stones formation. Cooking (e.g., boiling, microwaving, and steaming) appears as a useful management strategy to reduce soluble oxalate and, therefore, lowering oxaluria. The present review provides new perspectives on different processing methods to lower oxalate in essential vegetables highlighting their advantages or any limitations to aid improve these foods nutritional value and consumption.
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Introduction
Oxalic acid (C2O42−), chemically known as ethanedioic acid, is among the most abundant low molecular weight dicarboxylic organic acids identified in the rhizosphere beside to lactic, malic, and citric acids (Xiang et al., 2020; Zhao et al., 2017). It can be found in soil, minerals, bacteria, fungi, and in Planta. It exhibits strong acidity, chelating, and reducing abilities. It forms either soluble (often as salts of sodium and potassium or as free acid) or insoluble salt (produced with divalent metals like calcium or copper) (Grąz, 2024). Oxalic acid production and roles have been widely investigated in fungi, animals, and bacteria compared to Planta (Jiao et al., 2022; Palmieri et al., 2019). For example, it is known that fungi excrete a significant amount of oxalic acid, as a byproduct of carbohydrates metabolism (Grąz, 2024). Considering that it is produced by a number of phytopathogenic fungi during plant colonization, oxalic acid is also regarded as a pathogenesis factor in plants resulting in a potential loss of crop production (Kumar et al., 2019). It facilitates the breakdown of plant cell walls by maximizing the impact of the enzymes released by these phytopathogens that function best at an acidic pH (Kumar et al., 2019). Nevertheless, it is also involved in various positive functions in plants, including regulation of plant growth and development, response to biotic and abiotic stresses such as heavy metal detoxification and plant defense, and food quality (Li et al., 2022b). A significant portion of the world’s oxalic acid production is left behind on soil surfaces as leaf litter, where microbes oxidize it, raising pH levels and changing carbonate equilibria. Carbon dioxide is stored in soils as insoluble calcite (CaCO3) as a result of calcium oxalate metabolism, a process known as the oxalate-carbonate pathway (Cowan et al., 2024).
Different plant species have variable natural levels of oxalate which is produced as an end product of metabolism accounting for 3–80% of the dry mass (Akhtar et al., 2011). Oxalic acid is typically deposited in the form of calcium oxalate crystals, oxalic acid exists as intracellular or extracellular deposits in angiosperms’ tissues. Examples include the vacuoles of specialized cells known as crystal idioblasts. Nonetheless, most crystals in gymnosperms are found in the cell wall playing a role in controlling plants’ levels of free calcium (Kumar et al., 2019; Li et al., 2022b). Furthermore, numerous factors affect oxalate content in the same plant, including agricultural practices (Ghanati et al., 2024) such as genotypes (e.g., black tea vs. green tea leaves (Ferraro et al., 2020) and taro leaves cultivar (Du Thanh et al., 2017), season of harvesting (e.g., tea leaves harvested in autumn vs. spring) (Hönow et al., 2010), and maturity stage (e.g., 58% total oxalates in mature leaves of silver beet (Beta vulgaris) versus up to 89% in young counterpart) (Simpson et al., 2009).
Both exogenous sources, consumed in dietary sources especially green leafy vegetables alongside endogenous sources as byproducts of amino acid metabolism, provide oxalic acid or oxalates inside the body. A balance between the body’s excretion, exogenous supply, and endogenous sources is required to maintain oxalate homeostasis (Baltazar et al., 2023; Ermer et al., 2023). Its presence in a variety of food sources may have detrimental effects on the body’s ability to absorb nutrients putting it under the classification of “anti-nutrient” alongside phytates, tannins, glycoalkaloids, proteinase inhibitors, nitrites, nitrates, and cyanogenic glycosides. Anti-nutrient effect of oxalic acid is mostly attributed for its capacity to chelate minerals such as sodium, potassium, calcium, iron, and magnesium, reducing their bioavailability (Dagostin, 2017; Gupta, 2018). High oxalate levels have consequently been linked to many diseases such as hyperoxaluria systemic oxalosis in extreme cases resulting in nephropathy and acute renal failure due to kidney stones formation which are composed mainly of calcium phosphate and calcium oxalate, cardiovascular disease, and osteoporosis (Dassanayake & Gnanathasan, 2012; Ermer et al., 2016; Petroski & Minich, 2020; Rendina et al., 2020; Salgado et al., 2023). It is important to highlight, though, that hyperoxaluria can be primary or secondary, with the former being attributed to genetic disorders in oxalate metabolism, and the latter by non-genetic factors related to diet, gut microbiota, renal, and metabolic diseases (Baltazar et al., 2023).
It is therefore imperative to monitor oxalate consumption and to consider food processing or agricultural practices that can limit its level in dietary sources below the hazardous levels (Ghanati et al., 2024; Huynh et al., 2022). Example of processing method impact on oxalate include rhubarb stalks cooking (e.g., boiling, steaming, and blanching) (García-Herrera et al., 2020; Ghosh Das & Savage, 2013). Employing cooking techniques that greatly reduce soluble oxalate presents a potential strategy for lowering oxaluria in those prone to the development of kidney stones since soluble forms of oxalate seem to be more bioavailable than insoluble sources (López-Moreno et al., 2022). Moreover, the addition of calcium salts (e.g., calcium carbonate and calcium chloride to the oxalates-rich food source (e.g., spinach leaves, Spinacia oleracea L.) upon cooking could also lead to oxalate reduction, particularly by converting soluble oxalates into insoluble form (Bong et al., 2017; Faudon & Savage, 2014; Nguyen & Savage, 2020). Since various genes have been reported to affect oxalate biosynthesis (e.g., SoGLO2, SoGLO3, SoOXACs, SoMLS, SoMDH1, SoMDH2, and SoMDH4) (Cai et al., 2018), other processing includes development of genetically engineered crops manipulating its metabolism, and hence, reduce production mostly derived via three main precursors namely, glyoxylate/glycolate, oxaloacetate, and ascorbate (Kumar et al., 2019). Furthermore, the human gut is home to symbiotic organisms such as Oxalobacter and Lactobacillus, which act synergistically to modify certain metabolic pathways and aid in the breakdown of oxalates, thereby preventing the formation of stones (Sadaf et al., 2017). Patients with enteric hyperoxaluria and those who have recurring calcium oxalate stones have been shown to have reduced intestine colonization of Oxalobacter formigenes (Wigner et al., 2022).
This thorough review aims to analyze the various aspects of oxalate content in foods, including its occurrence, biosynthesis, metabolism, and processing techniques to lower its level in dietary sources and subsequently harmful health effects, as well as its versatile applications as an agrochemical and food preservative. Physical, chemical, biological, and biotechnological processing techniques are explored in an effort to find the best methods for reducing oxalates without compromising food values.
Searching Criteria
The following databases were used to choose the articles: Web of Science, PubMed, Google Scholar, and Scopus. The search phrases “biosynthesis” OR “metabolism” OR “Dietary sources” OR “degradation” OR “cooking” OR “fermentation” OR “maturity” OR “harvesting season” OR “transgenic crops” AND “oxalate” OR “hyperoxaluria” were used to find the relevant articles. Only English-language and published articles in impacted journals with full accessible text were included in this review. The used articles were not limited to a specific kind, while the timing of publishing has focused on the last 15 years. These criteria have resulted in more than thousands of articles in Google Scholar. Excluding articles discussing bacterial and fungal sources and related pathophysiology to hyperoxaluria has reduced the number to about 3700. Also, extraction and analytical determinations were to finally about 130 articles that were suitable candidates for the objectives of the current review article.
Dietary Sources of Oxalates
The oxalate content of different foods is key to regulating the exogenous oxalate supply inside the body. Several studies have measured total oxalate levels in foods and consequently labeled them as high oxalate-containing foods such as spinach, taro, and rhubarb (Han et al., 2015; Ruan et al., 2013; Salgado et al., 2023). The average daily consumption of oxalate in English diets has been estimated to range from 70 to 150 mg, with tea appearing to account for the largest amount of oxalate (Siener et al., 2017). Raw legumes are common constituents in various dishes worldwide and also rich sources of oxalate, where the highest amount (370 mg/100 g DW) is found in soybean, followed by common bean (98–117 mg/100 g DW), chickpea (192 mg/100 g DW), lentil and peas (168–293 mg/100 g DW), and bean (Petroski & Minich, 2020). Generally, increased vegetable consumption by vegetarians can result in a higher intake of oxalates, which may decrease the availability of calcium, particularly women, who need higher dietary calcium intake especially at certain life stages (Craig, 2010; Galchenko et al., 2023). For vegetarians, avoiding foods high in oxalate is more challenging than for omnivores. For omnivores, the typical daily oxalate intake is between 70 and 930 mg; however, for vegetarians, the range is between 80 and 2000 mg (Vanhanen & Savage, 2015).
It should be noted that the total oxalate content of a food is a combination of the soluble and insoluble oxalate, with the excess soluble oxalate to play a much more significant role in diseases and chronic kidney disease (CKD) progression (Ermer et al., 2016; Salgado et al., 2023). A comparison of the soluble oxalate content in particular would give a more accurate representation of the risk degree. Table 1 lists differences in soluble oxalate content in different foods commonly consumed by households. For instance, oxalate content in spinach is distributed in a nearly normal range: 400–1700 mg/100 g on a fresh weight (FW) basis or 5–15% on a dry weight basis; of which, 20–80% were found to be insoluble (Bong et al., 2017). Very high-risk foods were designated as those that contain more than 100 mg/100 g FW, while high-risk foods were assigned to those that encompass more than 50 mg/100g FW. For all types of foods presented, the samples analyzed were raw, suggesting more needed work to indicate changes in oxalate levels with different food processing methods for results to be conclusive.
Biosynthesis and Degradation of Oxalic Acid in Plants
Three mechanisms, including the oxaloacetate, ascorbic acid, and glycolate/glyoxylate pathways, have been linked to the production of oxalate in plants, Fig. 1. According to previous studies, oxalate has three main precursors including ascorbate, oxaloacetate, and glyoxylate/glycolate. Oxalate builds up during photorespiration and the glyoxylate cycle in plants as a result of the oxidation of glyoxylate/glycolate (Goldsmith et al., 2022; Yu et al., 2010). Oxalate and acetate can also be produced via the oxidation of oxaloacetate. l-Ascorbic acid is another important precursor of oxalate found in many plant species (Kumar et al., 2019). Oxalate in spinach may be regulated by the glyoxylate cycle, ascorbate degradation, and photorespiratory pathway, according to the expression profiles of genes linked to various physiological processes. Such hypothesis is supported with the roles that acyl-activating enzyme 3 (AAE3), ascorbate metabolism-related genes, and isocitrate lyase (ICL) aiding oxalate homeostasis in spinach (Cai et al., 2018; Joshi et al., 2021). Moreover, transcriptome sequencing (RNA-Seq) of spinach leaves and root tissues represented by two genotypes led to the annotation of a total of 2308 leaf- and 1686 root-specific differentially expressed genes (DEGs) to be involved in oxalate metabolism (Joshi et al., 2021).
Biosynthesis (blue text) and catabolism/fate (red text) pathways of oxalic acid in plants from its biochemical precursors and breakdown products, respectively. Ascorbic acid is the principal substrate to produce oxalic acid in plants, while in most other organisms, glycolic, glyoxalic, isocitric, oxaloacetic, and ascorbic acid are also involved precursors following the findings of Chakraborty, et al. (Chakraborty et al., 2013) and Ul Hasan, et al. (Hasan et al., 2023)
However, with regards to oxalate catabolism in Planta, three distinct processes, i.e., oxidation, decarboxylation, and acetylation are known to break down excess oxalate, with acetylation pathway as the most well-studied pathway. Oxalate is initially converted by oxalyl-CoA synthetase into oxalyl-CoA, which is subsequently broken down via oxalyl-CoA decarboxylase into formyl-CoA and CO2. Next, formyl-CoA hydrolase catalyzes the breakdown of formyl-CoA into formate. Lastly, formate dehydrogenase converts formate into CO2 and H2O (Li et al., 2022a; Lou et al., 2016). Annotation of these genes was further utilized to create transgenic tomato (Solanum lycopersicum) that expressed an oxalate decarboxylase (OXDC) leading to a possible metabolic remodeling of oxalic acid in the transgenic fruit (Chakraborty et al., 2013).
Differences in Oxalate Levels: Agricultural and Consumption Practices
While the data presented in Table 1 is categorized per food source, it is important to consider a multitude of other factors that could impact oxalate levels even for the same food source. Other variables that have been shown to influence oxalate levels in foods include mostly agricultural variables (e.g., cultivar, region, and harvesting season). Consumption practices/habits were also found to affect oxalate levels to include the part of the plant that is consumed (stems/leaves/stalks, etc.), the serving size of the dietary source (given that different food items are, most commonly, consumed with differing frequencies and portions), and lastly any relevant processing of the food before consumption (to be discussed in more detail in the following sections). The combination of all the aforementioned variables is thus crucial when evaluating oxalate levels in food. Figure 2 shows the different agricultural and consumption practices that affect oxalate levels in dietary sources, with the next subsections to discuss these variables/practices in detail from different food sources.
Agricultural and consumption practices that affect oxalate levels in dietary sources
Agricultural Practices
Cultivar/Variety Type
It has been documented that oxalate content varies significantly in context to plant genotypes/cultivars (Freidig & Goldman, 2011; Hang et al., 2013; Kristl et al., 2021). A study performed by Cai et al. on 2 spinach varieties cultivated in China (SP14 and SP104) revealed SP14 showed higher soluble (567.0 mg/100 g FW) and total oxalate (1164.0 mg/100 g FW) content compared to SP104 at 2.3- and 2.6-fold increase in the case of mature leaf lamina, respectively (Cai et al., 2018). Also, the total oxalate contents in green (Actinidia deliciosa L.) and golden (Actinidia chinensis L.) kiwifruit varied from 12.7–84.3 and 7.8–45 mg/100 g FW, respectively (Nguyễn & Savage, 2013c).
Harvesting Season
Green tea leaves are another example that showed variable oxalate content with regard to harvesting season, with mature leaves collected in autumn to encompass more oxalate than small and young leaves harvested in spring. Same pattern was observed in case of Chinese green tea leaves collected in autumn found to encompass higher oxalate level of 66.0–89.1 versus 24.2–28.6 mg/L in spring (Brzezicha-Cirocka et al., 2016). Although comparison of the soluble oxalate content in different foods appears to be more logical approach than total oxalate content, it is still merely a generic comparison which does not account for other factors, e.g., extraction and analytical methods (Filho et al., 2023; Nguyễn & Savage, 2013a; Salgado et al., 2023).
Plant Maturity Stage
It has been reported that oxalate accumulation varies among numerous plant species at different phases of plant development. For instance, tea younger leaves are richer in oxalate than older ones. This effect could explain the higher oxalate levels in black and oolong teas produced from old mature leaves compared to the green counterpart (Brzezicha-Cirocka et al., 2016). Nevertheless, cocoa beans (Theobroma cacao L.) investigated regarding oxalate content at four different phases of maturity: immature, mature, half-ripe, and fully ripe showed comparable total oxalate levels ranging from 600 to 670 mg/100 g dry matter (DM) (Nguyễn et al., 2018) suggestive of plant type effect and not to be generalized for all dietary food. Similarly, the effect of maturity was also found to affect oxalate content in taro (Colocasia esculenta var. Schott) leaves grown in New Zealand. The study was performed on young (100–200 mm long) and older leaves (200–300 mm long). The young taro leaves had again a greater total oxalate concentration (589 mg/100 g FW) than the older leaves (433 mg/100 Fw), of which, soluble oxalates represented 74% (Du Thanh et al., 2017).
Nitrogen Fertilizers
The exposure of plants to nitrogen containing chemicals in the soil, specifically nitrate, has been found to stimulate oxalate accumulation. Many studies have shown that plants treated with nitrate fertilizers generally accumulated higher oxalate levels as compared to untreated, or even those treated with other nitrogen fertilizers such as ammonium, urea, and organic nitrogen fertilizers (Liu et al., 2015). Since it is well known that nitrate can efficiently lead to oxalate accumulation in vegetables (Al Daini et al., 2013), a study investigating the effect of nitrate fertilizer on spinach leaves reported that spinach leaves were significantly affected by nitrate treatments. The results showed that the total oxalate content in the leaves of 2 genotypes of spinach treated with 20.0 mmol L−1 nitrate increased by 21.3% and 23.6%, compared to those treated with 2 mmol L−1 nitrate (Liu et al., 2015). Similar findings were reported in rice leaves revealing the same positive correlation between nitrate treatment and oxalate biosynthesis exists (Miyagi et al., 2020). Although the exact molecular mechanism behind such interaction remains unclear and requires further examination at transcript and protein levels. It has been suggested that increase in nitrate ions within plants inhibits oxalate oxidase, resulting in higher oxalate levels (Ghaly et al., 2017), but other studies showed that nitrate reductase activity was shown to be positively linked with oxalate levels in rice leaves (Al Daini et al., 2013). The reported relationship presented in the aforementioned studies suggest that controlling nitrate levels in plantation could be an effective method for total oxalate reduction.
Consumption Practices
Aside from the individual habits (e.g., smoking, opium consumption, and alcohol drinking (Zainodini et al., 2023) which may affect oxalate bioavailability and consequent kidney stone formation, other factors such as the consumed part and serving size shall be mainly considered. A daily fluid intake of at least 2.5 to 3.0 L is advised to ensure an appropriate urine volume. The oxalate content of beverages should be taken into consideration, as a high dietary oxalate consumption can significantly contribute to urinary oxalate excretion, a key risk factor for the formation of calcium oxalate stones (Siener et al., 2017).
Consumed Part of the Plant
The edible plant part is also a factor affecting the oxalate level suggestive for variable metabolic mechanisms among organs (Kang et al., 2019). For example, the study of Cai et al. performed in 2 Chinese cultivars of spinach, the mature leaf lamina and mature leaf petiole of SP14 showed soluble oxalate levels at 56.7 and 27.3 mg/g FW, respectively, which were 1.7 and 2.3 times higher than those of SP104. In contrast, juvenile leaf lamina and leaf petiole of SP14 showed comparable soluble oxalate levels of 32–34 mg/g FW. Although there were minor variations between juvenile leaf laminas and petioles, soluble oxalate levels in the leaf laminas were still noticeably larger than those in the petioles (Cai et al., 2018). In addition, different parts of kiwifruit, i.e., pulp, skin, and seeds, showed a comparable oxalates profile. For instance, the golden cultivar showed soluble oxalate levels at 8.5, 11.9, and 12.6 mg/100 g FW in pulp, skin, and seeds, respectively. However, the total oxalate contents varied significantly with 15.7, 55.4, and 97.3 mg/100 g FW in pulp, skin, and seeds, respectively (Nguyễn & Savage, 2013c). This might indicate that insoluble oxalate represented higher percentages especially in seeds which need further genetic investigations to determine action mechanisms responsible for such variability.
Moreover, investigation of some edible Mediterranean non-cultivated vegetables such as Silybum marianum (L.) Gaertn., Beta maritima L., and Sonchus oleraceus L. showed high oxalic acid levels at 1030, 581, and 539 mg/100 g FW, respectively. These contents were determined in their basal leaves which are traditionally consumed in Spain. Hence, these species are recommended to be avoided by individuals who are prone to form kidney oxalate calculus (Morales et al., 2014).
Serving Size
With regards to serving size as a factor, while parsley, a very high-risk food in reference to its soluble oxalate content, it is not typically consumed at large amounts, making it far less of a risk food. This is opposed to sweet potatoes or white beans, which although have a lower soluble oxalate content than parsley, are typically consumed at much higher levels, potentially presenting a higher risk food than that of parsley (Crivelli et al., 2020). In a typical western diet, dietary oxalate ranges from 70 to 930 mg/day depending on food choice and serving sizes (Nguyen, 2012).
Oxalic Acid Roles and Industrial Applications
Roles in Fungi, Bacteria, Plants, and Animals
Oxalic acid is a chemically versatile substance that is widely present in ecosystems, suggesting that it may be important for interactions and ecosystem function (Hasan et al., 2023). Roles of oxalic acid in fungi, bacteria, plants, and animals are summarized by Palmieri et al. (2019) and shown in Fig. 3.
Proposed roles of oxalic acid in different organisms, including fungi, bacteria, plants, and animals following the findings of Palmieri et al. (2019)
Aside from roles in fungi, bacteria, and animals, oxalic acid plays pivotal functions for plants, Fig. 3. It is hypothesized that oxalic acid is important for calcium homeostasis, ionic equilibrium, heavy metal detoxification, and plant defense against herbivores (Li et al., 2022b) (Khan et al., 2024). Through biochemical control of cellular processes as well as signal transduction, calcium plays a significant role in plant metabolism. However, calcium is poisonous to cells at concentrations of 10−6–10−8 M (Palmieri et al., 2019). As a result, insoluble calcium oxalate complexes represent a sink for controlling calcium levels to maintain appropriate cell function. Furthermore, calcium oxalate crystals can be used as a calcium storage container. More crystals will form when the amount of calcium in cells is too high, but if there is not enough calcium, crystals will disintegrate and release the calcium and carbon needed for cellular activities (Gómez-Espinoza et al., 2020).
Moreover, plants lack a skeleton, with maintenance of hydrostatic pressure necessary for a plant to remain erect depends on ionic equilibrium. Since oxalic acid can balance the charge of the positive ions and amino acids in a cell, it may therefore be important for maintaining ion homeostasis. Furthermore, oxalic acid contributes to the detoxification of heavy metals. For instance, a significant issue for plants grown in acidic soil is aluminum poisoning. Different mechanisms have evolved by plants against aluminium toxicity under these circumstances. The formation of oxalate is linked to both the internal tolerance of aluminum and its exclusion from plant roots. Whether in the rhizosphere or inside the plant, aluminum will be contained as non-toxic aluminum-oxalate salts (Lou et al., 2016; Zhang et al., 2019). Oxalate may also offer protection against herbivores as crystals can form in leaves in reaction to herbivory or as a preventative measure to fortify themselves (Paiva, 2021).
Industrial and Agricultural Applications of Oxalic Acid
Due to acidity, metal chelating, and antioxidant-stimulating activities, oxalic acid exhibits diverse applications in both industry and agriculture. It can be used as a pre- or post-harvest treatments, deactivation of preservatives containing copper, detoxification of aluminum toxicity, and remediation of organic contaminants (Hasan et al., 2023; Serna-Escolano et al., 2021) as explained in more details in the next subsections.
Pre- and Post-harvest Treatments
The use of pre-harvest treatments with natural elicitor chemicals, which trigger a tree response and delay the fruit’s maturation process during cold storage, is one of the common approaches in agronomy being helpful to reduce losses and improve the functional qualities of fruits and vegetables during storage (Barberis et al., 2019; Cefola & Pace, 2015; Moreno-Escamilla et al., 2018; Viacava et al., 2018). Among these agrochemicals, oxalic acid is one elicitor which has been reported to be applied either as pre- or post-harvest treatments to inhibit physiological processes such as respiration, ethylene production, and water loss, delaying ripening and senescence. By reducing oxidative stress, oxalic acid treatment for banana fruit prevented unwanted storage effects such as chilling injury, enzymatic browning, and flesh softening. This treatment included dipping in 20 mM solutions for 10 min, then keeping at room temperature (23 ± 2 °C) and 75–90% relative humidity (Huang et al., 2013).
It was also reported that applying oxalic acid as pre-harvest treatment to fruits specifically aided to postpone aging and to maintain the quality of sweet cherries, peaches, and pomegranates (García-Pastor et al., 2020). Moreover, pre-harvest treatment of lemon (Citrus limon (L.) Burm. F) trees with 1 mM of oxalic acid resulted in fruits with maintained firmness, total soluble solids (TSS), and total acidity (TA) compared with the control while reducing weight loss (WL) after storage for 35 days at 10 °C. Moreover, the flavedo of citrus fruit showed increased activity of the antioxidant CAT, APX, and POD compared to the controls. Likewise, fruit from the oxalic acid-treated trees had a higher total phenolic content (TPC) in the flavedo and juice than in the control presenting an added value especially if fruits are targeted for increased phenolics to be used in nutraceuticals. Profiling of flavonoids in response to oxalic acid treatment should now be followed to determine changes in individual compounds. Hence, the incidence of deterioration after harvest was decreased by preharvest oxalic acid treatments that strengthened lemon fruits’ antioxidant system (Serna-Escolano et al., 2021), which has yet to be tested using in vivo assay.
While the exact action mechanisms of oxalic acid is unknown, post-harvest treatments in lemon and banana fruits have been linked to scavenging reactive oxygen species (ROS) buildup (Huang et al., 2013; Serna-Escolano et al., 2021), and maintaining high ascorbic acid and ATP levels in post-harvest oxalic acid-treated kiwifruit (Chunqiang et al., 2017). Additionally, post-harvest oxalic acid treatment of muskmelon (Cucumis melo L. cv. Yindi) fruit at 50 mM containing 0.05% Tween 80 for 10 min. against pink rot caused by Trichothecium roseum resulted in an average infection incidence and lesion diameter on days three through seven following T. roseum inoculation that were 25.1% and 16.9% lower than those of the control, respectively (Deng et al., 2015). Also, jujube (Ziziphus jujuba Mill. cv Dongzao) fruit treatment at 5 mM oxalic acid solution for 10 min, then dried in air at 20 °C for 2 h, was linked to a decrease in decay during storage (Wang et al., 2009). Such effect was associated with reduction of ethylene production in jujube fruit treated by oxalic acid by about threefold as opposed to control fruit These effects were explained by increased activity of antioxidant enzymes associated with disease resistance, such as catalase (CAT), ascorbate peroxidase (APX), superoxide dismutase (SOD), and peroxidase (POD) (Deng et al., 2015; Wang et al., 2009).
Food Preservation
Previous reports have been carried out on the application of organic acids and their salts for decontamination and extending shelf life of beef and poultry, including oxalic acid, based on its antimicrobial action as typical for acidulants (Gonzalez-Fandos et al., 2020; Kumar et al., 2018). Additionally, commercial pectin preparations containing oxalic acid were able to prevent the browning of fresh, raw Granny Smith apple juice due to enzymatic reaction. These preparations when added to fresh, unrefrigerated apple juice at a concentration of 0.5% (w/v), totally stopped browning after 24 h. This effect was confirmed, where the anti-browning action of the commercial pectin was eliminated or significantly diminished upon the removal of oxalic acid, which has yet to be tested in other fruits. Additionally, it was shown that pure oxalic acid at the same concentration of 0.5% prevented fresh juice from browning confirming the previous finding. However, it should be considered that oxalic acid has not been approved officially as a food additive yet by the US Food and Drug Administration (FDA).
Oxalate Absorption and Metabolism in Humans
Oxalate is introduced to the body in one of two forms: endogenous oxalate and exogenous oxalate as summarized in Fig. 4. Endogenous oxalate is the oxalate produced by the body as a metabolite from the breakdown of substances such as ascorbic acid, glyoxylate, hydroxyproline, glycine, and protein (Ermer et al., 2023; Franceschi & Loewus, 2020). The liver is the primary producer of endogenous soluble oxalate, and oxalate produced by the liver enters the bloodstream (Petroski & Minich, 2020). On the other hand, exogenous oxalate is obtained from high oxalate dietary sources, mostly being plant foods (specifically leafy greens), whole grains, and nuts (Han et al., 2015).
Sources of oxalate and pathways to excretion from the body. Red arrows indicate pathways ending in urinary excretion. Blue arrows indicate pathways ending in fecal excretion. Double asterisks (**) indicate associated health risk or disease
Transcellular anion transporters, particularly the solute-linked carrier (SLC)−26 family, and paracellular fluxes through tight junctions are responsible for oxalate absorption from the digestive tract (Crivelli et al., 2020). It seems that the quantity of oxalate that is consumed and absorbed by the intestine ranges from 2 to 20%, depending on the co-consumption of fiber, calcium, and magnesium (Crivelli et al., 2020), where the peak blood level happens 1–6 h after consumption of foods high in oxalate content (Savage & Mårtensson, 2010). Malabsorptive intestinal disorders in case of surgical removal of intestinal segments, such as after Roux-en-Y gastric bypass (RYGB) surgery operated for weight loss, have been linked to hyperabsorption of oxalate, i.e., enteric hyperoxaluria with urine oxalate excretion ≥ 45 mg/day that resulted in urinary calcium oxalate supersaturation (Kumar et al., 2011; Witting et al., 2021). So, dietary oxalate may not be recognized as a great problem since absorption is limited and based on several factors, including co-consumed foods (Crivelli et al., 2020).
Dietary oxalate contains both, soluble and insoluble oxalate, with insoluble oxalate to be excreted from the body as feces, whereas, soluble oxalate is absorbed in multiple organs of the gastrointestinal tract such as the stomach, small intestine, and large intestine (Salgado et al., 2023). Absorbed oxalate can chelate minerals such as calcium and magnesium, forming insoluble fecal oxalate salts that can be excreted from the body. However, oxalate remaining in the soluble form is absorbed in the intestines and to reach the blood stream (Ermer et al., 2016; Petroski & Minich, 2020). Therefore, it is recommended to increase the intake of calcium supplements or calcium-rich foods (e.g., dairy products as hard cheese (1% w/w) and nuts containing up to 0.6% w/w) (Cormick & Belizán, 2019) together with foods high in oxalic acid in order to eliminate calcium oxalate from the stomach and lower blood oxalate levels (Duraiswamy et al., 2023). It should be possible to counteract any potential inhibitory effects from dietary oxalates with a typical calcium diet of 800–1,000 mg/day (Petroski & Minich, 2020). However, it has been reported that it is recommended that oxalic acid/calcium ratio should be monitored and be not more than 2.5 (9:4 as the optimal limit) in foods to prevent the consequent harmful effects, i.e., formation of kidney stones and decrease of calcium bioavailability (Lee & Lin, 2011).
While bacteria, fungi, and plants all have a mechanism for the catabolism of oxalate, vertebrates, including humans, do not exhibit such machinery (Chakraborty et al., 2013). Oxalate is broken down by O. formigenes, which also maintains a vital symbiotic relationship with its hosts by controlling the intestinal absorption of oxalic acid, i.e., oxalic acid homeostasis (Lee et al., 2014). There is no known gastrointestinal route for the excretion of oxalate; thus, once it is consumed, it must be eliminated through the kidneys (Karamad et al., 2022; Miller et al., 2017). Nonetheless, the least lethal dose of oxalic acid for an adult is approximately 5 g, indicating its comparatively low toxicity (Morales et al., 2014). Serum oxalate, obtained from either the endogenous or exogenous pathway, is processed by the kidney and is either excreted in urine, or is chelated with calcium ions in the kidney leading to the formation of calcium oxalate crystals, and contributing to serious health risks such as kidney stone formation (nephropathy) (D’Alessandro et al., 2019).
Processing Methods to Reduce Total Oxalate Levels in Dietary Sources
Aside from factors discussed before affecting oxalate levels inside the body, processing methods appear to play a more critical role in reducing oxalate (particularly soluble oxalate). Such methods can be classified as physical, chemical, biological, and/or combinations. Alongside the preparation method, it is vital to consider all other relevant variables, as illustrated in Fig. 2. Nevertheless, for the sake of generic comparison, many studies were included in this review, while taking note of such variables. The next subsections shall provide application of each technique, highlighting its advantages and or any limitations for comparison among these approaches to present best methods to reduce oxalate in different dietary sources. In addition, Table 2 and Table 3 summarized and compared relevant literature that reported the use of processing methods for oxalate level reduction in foods.
Physical/Cooking Processing Methods
Oxalate levels are mostly reduced in cooked foods. Some physical procedures that have been associated with a decrease in oxalate include, but are not limited to, common household cooking methods to include boiling, steaming, roasting, dehydration, soaking, blanching, and frying attributed to oxalate water solubility (Mashitoa et al., 2021; Petroski & Minich, 2020; Salgado et al., 2023). While the aforementioned physical processing methods have been linked to a reduction in oxalate, the oxalate content of roasted peanuts, cashews, and almonds was not significantly affected (Petroski & Minich, 2020). Whether such an effect is common for all roasting procedures or identified only in nuts should be examined in other dietary sources such as coffee for results to be conclusive. Moreover, it was believed that the physical binding of oxalates to food fiber, and consequently, eliminated during juice processing and the amount of oxalates in the juice might be considerably lower. Vanhanen and Savage, however, demonstrated that large amounts of soluble oxalates were added regardless of the presence of pulp fiber, depending on the type and quantity of vegetables used to prepare green juices (Vanhanen & Savage, 2015).
One study in pumpkin leaves revealed that boiling showed the largest reduction at 82% of total oxalate of the sample, while microwaving, frying, and steaming, showed reductions of 76.5%, 62.1%, and 51.5% respectively, clearly presenting boiling as the most effective technique among different physical methods (Mashitoa et al., 2021). Another study on Galega kale leaves showed similar results using microwaving that showed a 35.8% reduction on total oxalate, while boiling and steaming, showed reduction at 34.2% and 12.7% respectively, marking that both microwaving (at 900 W for 20 or 30 min) and boiling (for 20 or 30 min) as almost equally effective (Armesto et al., 2019). In contrast, a study on sweet potato varieties showed that while boiling (until the root was easily accessible with a fork) was most effective for Whitesp and Yellowsp sweet potato, dehydration in an air oven at 70 °C for 12 h was a more efficient technique for Kabode sweet potato (Abong’ et al., 2021), and suggestive for matrix effect on processing method impact towards oxalate reduction. Such results warrant comparing each technique in different cultivars or varieties within each food type for results to be conclusive. Moreover, while frying reduced oxalate in pumpkin leaves by 62.1%, it increased oxalate levels in the bitter melon or Bitter Guord fruit (Momordica charantia) in the Indian and Vietnamese cultivar by 6.5% and 23.4%, respectively (Bong & Savage, 2018), suggestive for a food matrix effect. However, frying reduced oxalate by 15.6% in case of the fruits of Malaysian Bitter Guord (Bong & Savage, 2018).
Table 2 summarizes the effect of different physical processing methods (e.g., boiling, steaming, microwaving, frying, and dehydration) on oxalate levels (mg/100 g) of some dietary sources. A study showed that boiling spinach and carrots resulted in a total decrease in oxalate that ranged from 30 to 87%, while steaming resulted in a much less reduction level. Similarly, after boiling 12 of the 13 fresh Thai vegetables for 30 min, soluble oxalate was reduced by 30.4–65.0%. Among these, the soluble oxalates in taro corms were reduced by 34.2% (Juajun et al., 2012). Several studies have attributed the general effectiveness of boiling in particular to the water solubility of oxalate making it susceptible to degradation using boiling (Sotelo et al., 2010). In fact, previously mentioned increase in oxalate in case of Indian and Vietnamese Bitter Guord frying was attributed by the loss of moisture during frying, which prohibits oxalate from leaching onto the water (Bong & Savage, 2018). Conclusively, the effectiveness of any processing method involving high heat has been attributed to likewise the thermal instability of oxalate. Also, heat treatment has been found to cause changes in cell permeability which increases the chemical extraction of some organic compounds from plant tissue and decrease in foods, including anti-nutrients (e.g., nitrate, phytate, and oxalate) (Singh et al., 2015).
Soaking of legumes seed (e.g., soybean, lentils, fava beans, peas, and chickpeas) was found to be an effective strategy to reduce oxalate content. Seeds germination/sprouting is induced by soaking which decreased not only oxalate level, but also lectins. Soaking in distilled water, (1:5 seed:water ratio) for 4 h at room temperature, and then cooked, at 95 °C in water for 1 h, succeeded in decreasing the total oxalate and soluble oxalate by 17.4–51.9% and 26.7–56.3%, respectively in various legume seeds. Particularly, the raw soybean showed the highest total oxalate (370.5 mg/100 g DM) and soluble oxalate (200.7 mg/100 g) levels. However, the total oxalates decreased to 178.2 5 mg/100 g DM (51.9%) and 125.4 5 mg/100 g DM (66.2%) in soaked and cooked seeds, respectively. Also, soaked and cooked seeds showed soluble oxalate reduction to 87.7 mg/100 g DM (56.3%) and 52.3 5 mg/100 g DM (74%), respectively (Shi et al., 2018). In addition, legumes’ total and soluble oxalate content is potentially reduced by soaking overnight followed by boiling or autoclaving (Bhat & Karim, 2009; Sharma, 2021). Also, germination of cereals such as pearl millet (Pennisetum typhoideum) resulted in oxalate reduction, where the total oxalate content decreased significantly (p < 0.05) by as much as 48% for Maharashtra Rabi Bajra and as much as 24% for the Kalukombu cultivar. This decrease may have been caused by the soluble oxalate content leaching during steeping (Suma & Urooj, 2014).
Addition of Salts and Change of pH
One of the most examined chemical processing methods to reduce oxalate entails processing the food sourc 2e with a calcium source, most typically a calcium salt such as calcium chloride, calcium carbonate, and calcium sulfate (Bong et al., 2017; Huynh et al., 2020, 2022; Vanhanen, 2018; Wang et al., 2019). The addition of calcium salt to food upon cooking as a processing method can be classified as a physio-chemical one. One study investigated the effect of adding calcium carbonate (with a calcium content 29.3% as determined by IPC analysis), calcium chloride (25.6%), calcium citrate (24.7%), and calcium sulphate (33.7%) at 0, 50, 100, 200, 300, 400, and 500 mg/100 g of homogenized spinach leaves and measured the amount of soluble oxalate remaining after treatment. The study not only reported an inverse relationship between the mass of salt and remaining oxalate in spinach, but also revealed that among the different tested calcium salts, i.e., calcium chloride exerted the highest percentage reduction in soluble oxalate with only 1.75% remaining oxalate (Bong et al., 2017). Likewise, a different study on green juices, made from common fruit and vegetables available in New Zealand, reported calcium chloride to reduce soluble oxalate content by 98.3% (Vanhanen, 2018). The same study also investigated the effect of soaking spinach leaves in a calcium chloride solution and reported a 50% and 28% reduction in soluble and insoluble oxalate, respectively (Vanhanen, 2018).
A similar approach was reported by another study that measured reduction in oxalate in rhubarb stalks by pairing them with a calcium source during cooking but using milk rather than calcium salt. This study reported a soluble oxalate reduction of 65.9% and 74.5% upon cooking rhubarb with trim and standard milk, respectively (Nguyen & Savage, 2020), and posing milk as good alternative considering its common use as ingredient in many recipes. Another study also measured the oxalate content of taro leaves after baking them either alone or combined with cow milk or coconut milk prior to cooking. This study also reported a decrease in oxalate when food was combined with milk (Savage & Mårtensson, 2010). Comparison of different milk types and at different fat levels should follow to identify best milk composition to be added to reduce oxalate levels especially considering that it can be readily adopted in home prepared foods.
Moreover, another study investigating oxalate in cocoa beans studied the effects of six calcium sources (three of which were calcium salts and the remaining three were milk types) and likewise assessed the effectiveness of ultrasound assistance in reducing oxalate (Huynh et al., 2020). This physio-chemical study reported the top three effective methods to include ultrasonication (39.6% reduction), followed by the addition of calcium sulphate (31.8% reduction), and lastly pairing with high calcium milk (21.4% reduction) (Huynh et al., 2020). The rationale behind the effectiveness of adding a calcium source in oxalate reduction is attributed to oxalate binding to the free calcium ions from the added source (Bong et al., 2017; Huynh et al., 2022). This consequently converts soluble oxalate into insoluble oxalate to be excreted in feces as opposed to bloodstream, as shown in Fig. 4. An analogous approach is adding a magnesium salt enabling the oxalate to bind to magnesium ions, following the same pathway as previously discussed (Riley et al., 2013).
The pairing of calcium sources, although frequently reported as effective, is heavily influenced by pH (Huynh et al., 2022; Vanhanen, 2018). Therefore, in considering such processing method in preparation of oxalate rich foods, pH levels must be adjusted accordingly. A study exploring potential strategies for reduction of soluble oxalate content in green juices reported that the optimum conditions for extracting oxalate from green juice included a pH of 0.93 and showed that the least amount of oxalate extracted was at a pH of 4.6 (Vanhanen, 2018). Note however that the temperature values were not consistent across both sets of conditions. Nevertheless, this suggested that slightly acidic pH values are optimum for oxalate reduction. This can be explained by the findings of another study on the oxalate content of silver beet leaves (B. vulgaris), which showed that at pH values less than 6 in the presence of low fat milk, the proportion of fully deprotonated divalent oxalate ion (C2O42−) sharply decrease (Simpson et al., 2009). As a result, there is a reduced potential for the ion to bind with other mineral cations such as calcium to form insoluble oxalates. Therefore, upon pairing an oxalate-rich food with a source of calcium intended to convert the soluble oxalate into insoluble oxalate, it is vital to ensure neutral pH values as this will guarantee the highest amount of C2O42− that can readily bind to the calcium ion (Simpson et al., 2009).
Finally, soaking in brine has been applied with some oxalate-rich foods such as the silver beetroot leaves (B. vulgaris var. cicla) has been reported. The study soaked the leaves in a ratio of 1:2 in 10% salt water for 2 h at room temperature, then 9 h in the fridge. The salted silver beet leaves showed a decrease in the amount of total oxalate content from 4.3 ± 0.2 mg/100 g to 3.7 ± 0.2 mg/100 g FW (Wadamori et al., 2014). Table 3 summarizes the effect of salts addition processing method of some dietary sources.
Fermentation Processing Method
Microorganisms are helpful in breaking down plant polysaccharides and polyphenols being able to improve the nutrients’ bioavailability and digestibility (Difo et al., 2013). Fermentation is one of the most discussed biological processing procedures shown to reduce oxalate in food sources (Difo et al., 2015; Hajimohammadi et al., 2020; Olawoye & Gbadamosi, 2017), especially considering fermentation positive effects on improving food shelf life and sensory attributes presenting an added value.
One study on sesame seed showed that fermentation decreased oxalate level by 69% at a time period of 12 days (Hajimohammadi et al., 2020), and likewise observed in case of fermentation, in calabash pots lined uniformly with banana leaves, of green amaranth (Amaranthus viridis L.) seed as manifested by 20.8% reduction in total oxalate level (Olawoye & Gbadamosi, 2017). Further investigation of different fermentation setups, viz open versus controlled fermentation in Vigna racemosa seeds, revealed that controlled fermentation led to large reduction of 96.9% compared with 59.1% in case of open fermentation process (Difo et al., 2015). Screening of different microorganisms, especially those reported in food fermentation, i.e., Lactobacillus acidophilus, and Aspergillus niger should now be followed to identify the best strain type and exact underlying mechanism. Reduction of oxalate with fermentation in foods has been attributed to microorganisms arising in the fermentation process degrading oxalate-mineral complex (Sahoo et al., 2023). It is therefore reasonable that controlled fermentation to be more effective than open fermentation as it includes more than one microorganism. While fermentation alone stands as an effective processing method for oxalate reduction, pairing it with another processing method can enhance efficiency further due to a synergistic action. A study investigating anti-nutrient content of sandbox tree (Hura crepitans) seed studied effects of cooking and fermentation separately and in combination as what is often termed hurdle technology in food. The study revealed that cooked fermented seeds had less oxalate than separate fermented or cooked unfermented seeds (Osungbade et al., 2016). Further studies into individual and combined effects of the different processing methods would aid identify best measures to reduce oxalate, while maintaining other food health benefits especially in case of oxalate rich foods. Recently, the use of modified microbes has emerged as a novel approach to the management and prevention of kidney stones (Wan et al., 2024). Table 3 summarizes the effect of fermentation processing method of some dietary sources.
Enzymatic Treatment Method
The addition of exogenous enzymes for oxalate reduction has also been extensively examined as potential approach for oxalate reduction considering its specificity targeting oxalate without affecting other phytochemicals in food, as possible (Tharifkhan et al., 2021). One study exploring oxalate in taro flour found that addition of oxalate oxidase (produced by Fusarium oxysporum RBP3) resulted in a 98.3% removal of total oxalate (Kizhakedathil et al., 2022). Similarly, a different study investigating taro though of a different cultivar, reported a 97% reduction in total oxalate upon treatment with oxalate oxidase produced by Ochrobactrum intermedium CL6 (Kumar & Belur, 2018). The advantage of such processing method is primarily a preservation of properties of the food source and almost with zero changes in characteristics of food composition and nutritional value (Kizhakedathil et al., 2022; Kumar & Belur, 2018). Moreover, expression of the various oxalate-degrading enzymes has been employed in probiotic dairy and pharmaceutical preparations for oxalate degradation (Abratt & Reid, 2010; Youssef, 2024). In addition, decrease in the amount of oxalate excreted in urine using an animal model was achieved through the administration of liposome-encapsulated oxalate oxidase taken from Bougainvillea glabra (Dahiya & Pundir, 2013). Encapsulation can aid protect enzyme and extend its application for commercial purposes (Xu et al., 2024).
Furthermore, exogenous addition of phytase to oxalate-rich foods was applied as beans (e.g., red kidney bean and white bean) and bran (e.g., wheat, oat, and barley) to decrease soluble oxalate content. The application of phytase resulted in a rise in phosphate content; however, the effect on the concentration of soluble oxalate differed. When compared to bean samples, wheat and oat bran shown a substantial decrease (p < 0.05) in soluble oxalate. It has been found that the concentration of calcium ions affects the correlation between the calcium:phosphate molar ratio and phosphate release, which in turn affects the concentration of soluble oxalate (Israr et al., 2017). Table 3 summarizes the effect of enzymatic treatment of some dietary sources.
Biotechnology/Development of Transgenic Crops Methods
Aside from physicochemical and biological processing methods, genetic manipulation and development of transgenic crops has been investigated with advances in plant biotechnology especially with anti-nutrients rich crops, including oxalic acid (Duraiswamy et al., 2023). Metabolic remodeling of oxalic acid metabolism has been reported in transgenic crops found capable to breakdown oxalic acid (Chakraborty et al., 2013). Kumar et al. have recently reviewed the various genes and enzymes involved in oxalate breakdown in bacteria, fungi, and plants. The enzymes oxalate oxidase (EC 1.2.3.4), oxalate decarboxylase (EC 4.1.1.2), oxalyl-CoA synthetase (EC 6.2.1.8) or oxalate-CoA ligase, and oxalyl-CoA decarboxylase (EC 4.1.1.8)/formyl-CoA transferase (EC 2.8.3.16) are responsible for breaking down oxalate (Kumar et al., 2019). Therefore, different oxalate-metabolizing enzymes, and in particular oxalate-degrading enzymes, have been targeted by genetic engineering to control oxalate level. The genes encoding the oxalate-metabolizing enzymes, derived from microbes or plants, have found extensive application in various biotechnological fields, including crop enhancement, the creation of oxalate-free genetically engineered food, and human diagnostic and therapeutic uses (Kumar et al., 2016; Li et al., 2022a).
Examples include OXDC which catalyzes the substrate-specific decarboxylation breakdown of oxalic acid, resulting in the formation of carbon dioxide and formic acid. OXDC is a multimeric manganese-containing enzyme that belongs to the cupin protein superfamily (Mäkelä et al., 2010). Development of transgenic tomato (Solanum lycopersicum) plants (E8.2-OXDC tomato plants) that express an OXDC (FvOXDC) derived from the fungus Flammulina velutipes that selectively expressed in the fruit. The oxalate concentration of these E8.2-OXDC fruits was reduced by up to 90%, concomitant with increases in calcium, iron, and citrate (27–42%). The breakdown product formic acid content was confirmed to be increased to 30–67% suggestive for its catalytic efficiency. Also, oxalic acid precursor showed a 10–12% reduction. At this experiment, neither carbon dioxide assimilation rates nor any apparent morphological changes in transgenic tomato were identified by OXDC expression, which has yet to be examined in other oxalate-rich food such as spinach (Chakraborty et al., 2013; Pereira Menezes Reis et al., 2022).
The heterologous expression of a 43 KD-sized OXDC was successfully performed from YvrK gene of Bacillus subtilis by recombinant E. coli (pBy). The purified OXDC degraded more than 50% of oxalate when it was treated with MnCl2 and sodium oxalate at a pH of 5 and a temperature of 28 °C. Also, in vivo investigations carried out in the hyperoxaluric rat model showed that the recombinant enzyme was able to lower urine oxalate levels significantly following the oral administration of sodium oxalate (Lee et al., 2014). Besides, development of recombinant Lactobacillus plantarum expressing OXDC (gene taken from Bacillus subtilis) was able to break down oxalate in probiotic dairy and pharmaceutical products (Anbazhagan et al., 2013). Furthermore, treatment for calcium oxalate stone disease involves expressing heterologous OXDC in human embryo kidney cell line (HEK293 cells) to guard against oxidative damage caused by oxalate is possible which shielded cells from oxidative damage and may therefore be a useful therapeutic approach for preventing calcium oxalate stone disease (Albert et al., 2017). Table 3 summarizes some reported examples for the use of biotechnology/development of transgenic crops methods of some dietary sources, in addition to in vitro and in vivo application.
Challenges and Future Work
Management of anti-nutrients, including phytic acid, lectins, tannins, protease inhibitors, and oxalates by soaking, sprouting, heating, boiling, and fermentation has attracted increasing attention in previous literature considering that they compromise the food value. Despite few research to investigate the impact on other nutrient contents such as minerals, the effect on the other nutraceuticals, which may be heat-sensitive, have not been commonly assessed for these foods after reduction of oxalate levels. Hence, monitoring of not only minerals, but also vitamins, essential fatty and amino acids are highly recommended to assess how removal approaches can impact main macro- and micronutrients in these food sources. Similarly, low nitrogen levels, i.e., nitrates, in soil fertilizers is recommended to decrease the oxalate content, but the crops growth and quality may be worsened associated with nitrogen deficiency. Nitrogen-deficient plants produce smaller-than-normal fruit, leaves, and shoots. Also, they showed more crimson than usual leaves in Autumn and drop earlier than usual. Then, this challenge should be considered carefully and nitrogen level optimization in used fertilizers is a priority in oxalate-rich crops. Besides, younger leaves are usually richer in oxalate contents, despite their desirable and fine taste for most consumers. Hence, application of metabolomics approach based on various analytical platforms (e.g., LC–MS, NMR, and IR) for oxalate screening and metabolic profiling, in addition to simple quantification method, is suggestive for future work monitoring of different quality aspects concomitantly, and in all similar studies to remove oxalate from dietary sources. Evaluation of the gut microbiome using multi-omics has been recently applied in uncommon hyperoxaluric circumstances.
In addition, development of transgenic less oxalate-producing crops is still in early stages and, therefore, requires more studies to investigate if they are nutritional food candidate and can replace the natural counterparts, alongside their safety as genetically modified foods (GMF). Moreover, similar enzymes to OXDC derived from probiotics, such as oxalate oxidase, should be targeted in other research after thorough investigations of biosynthetic/degradation pathways. Oxalate oxidase belongs to the cupin superfamily of proteins, which exhibits significant similarities to OXDC at the amino acid level. Attempts to decrease oxalate content in foods may lead to higher calcium bioavailability resulting in hypercalcemia, and therefore, it is better to monitor the oxalic acid:calcium ratio to be not more than 2.5 in foods and should be assessed in all studies to identify best methods. Finally, development of drugs or nutraceuticals which have high affinity to oxalate and consequently are able to adsorb oxalates in human gastrointestinal tract or blood may be recognized as a potential solution to get rid of most exogenous dietary and endogenous sources, especially for people who are at risk for formation of kidney stones. Among biological methods for removal of oxalates, developments of drugs containing degrading enzymes produced by probiotics, i.e., microbial enzymes-derived, may be potential for future studies considering the rich metabolic machinery in that niche and several health benefits of probiotics to improve food value. Aided by advances in analytics and bacterial culturing, identification of active bacterial strains should be expedited. Recently, the use of modified microbes has emerged as a novel approach to the management and prevention of kidney stones. With all these challenges and future perspective, optimization of these methods is highly recommended for efficacy assurance alongside reduction of the anti-nutrient content.
Conclusion
The current review highlighted different aspects about oxalic acid in foods as a potential anti-nutrient. It can be found either in soluble or insoluble forms in various dietary sources, mostly in vegetables. Also, its inclusion in various applications in industry and agriculture has aided in increasing its food contents. Despite oxalic acid’s key functions for plants, the soluble form was found to be harmful due to its effect on minerals bioavailability as calcium. In addition, its intestinal absorption results in chelation of calcium in blood and formation of kidney stones due to lack of a degradation mechanism in vertebrates, including human, warranting for developing low oxalate foods or processing methods for its removal in food maximizing their nutritional attributes. Interestingly, previous literature showed that levels in foods vary in context to several factors, including agricultural practices (e.g., plant genotype, harvesting season, and region of cultivation) and consumption habits (e.g., edible parts and serving size). Despite a lack of consensus on the most effective management strategies, physical processing method, boiling and microwaving seem to be especially effective in reducing oxalate levels compared to steaming and baking. Additionally, calcium or magnesium salts or milk reduced oxalate levels through ion-pairing in relatively low pH posing administration of calcium as potential adjunct treatment for reducing absorption of dietary oxalate. Nitrate levels in soil are a determining factor for oxalate accumulation in a direct relationship. So, low soil nitrates are recommended to reduce oxalate in foods, but growth and quality attributes shall be affected. Moreover, the oxalate-metabolizing enzymes' genes such as oxalate decarboxylase, which are derived from microorganisms or plants, have been recently used in a variety of biotechnological disciplines, including crop improvement, the development of genetically altered foods free from oxalate, and human diagnostic and therapeutic applications. Further studies should now follow to generate drugs and probiotics containing and producing oxalate-metabolizing enzymes, considering its higher safety and less likely to affect food nutritional value. Also, more genetic research is required to investigate plausible molecular mechanisms underlying the varying levels of variability among plant organs.
Data Availability
No datasets were generated or analysed during the current study.
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M.F.: Conceptualization, writing-review and editing, and Supervision. A.Z. and G.A.: data collection, analysis, writing-original draft. A.Z.: preparing figures, writing-review, and editing. All authors have approved the final version that was submitted.
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Zayed, A., Adly, G.M. & Farag, M.A. Management Strategies for the Anti-nutrient Oxalic Acid in Foods: A Comprehensive Overview of Its Dietary Sources, Roles, Metabolism, and Processing. Food Bioprocess Technol 18, 4280–4300 (2025). https://doi.org/10.1007/s11947-024-03726-0
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DOI: https://doi.org/10.1007/s11947-024-03726-0