Oxalate food sources

Figure 5-1 Examples of Metal Chelates. Only the relevant portions of the molecules are shown. The chelate formers are (A) thiocarbamate, (B) phosphate, (C) thioacid, (D) diamine, (E) o-phenantrolin, (F) a-aminoacid, (G) o-diphenol, (H) oxalic acid. Source From K. Pfeilsticker, Food Components as Metal Chelates, Food Sci. Technol., Vol. 3, pp. 45-51, 1970.

Calcium oxalate monohydrate Sodium formate oxalic acid source Potassium oxalate oxidant, food L-Ascorbic acid oxidant, rocket fuels Perchloryl fluoride oxidant, selective... 

Wheat bran has been the fiber source most commonly used to study effects of dietary fiber on calcium absorption in controlled laboratory studies. However, wheat bran and other forms of fiber as they occur in food products present several disadvantages in terms of definition and by concurrently altering intakes of other substances or materials known or suspected of having an adverse effect on the bioavailability of calcium such as phytates and oxalates (5,13,17,22-28). Several studies have been conducted which have sought to separate or compare the effects of phytate and fiber... 

As noted above, disagreement has often been observed among different studies on the effects of fiber, phytic acid and protein source on mineral utilization. Some possible reasons include (a) estimates of absorption from single meals (with or without previous consumption of the same foods used in the test meal which may also affect results) may not always be equivalent to results from multi-day balance studies, (b) in balance studies, the failure to allow sufficient time (e.g., 1-2 weeks or more) for adaptation may alter the findings, (c) variations in the compositions of meals or diets, including mineral levels, between studies may influence the results obtained, and (d) the persons used as subjects vary and this may have an affect. In addition, in the fiber studies, the levels, types, and particle size of fiber fed have varied widely and levels of other possibly confounding components (e.g., caffeine, tanins, oxalates) may have differed. 

Various constituents in plant foods can impede Ca absorption. Plant-based diets can be high in oxalate and phytate, which are recognized as inhibitors of Ca absorption. In fact, Ca absorption is considered to be inversely proportional to oxalic acid content of the food (Weaver et al, 1999). Phytic acid poses Ca absorption problems for those species imable to endogenously synthesize phytase (e.g., humans, birds, and pigs). The Ca in CCM is chelated with the citrate and malate anions, which may make CCM less reactive than other sources of Ca toward food components known to interact with Ca " cations. For example, Lihono et al (1997a) reported data suggesting that the Ca in CCM may be less likely to complex with phytates than other Ca salts. Qn this basis, CCM may be more appropriate for the fortification of soy or other phytic acid-containing products. 

Xylitol is as sweet as sucrose and has been used as a food additive. Because it does not induce formation of dental plaque, it is used as a replacement for sucrose in chewing gum. It appeared to be an ideal sugar substitute for diabetics. However, despite the fact that it is already naturally present in the body, ingestion of large amounts of xylitol causes bladder tumors as well as oxalate stones in rats and mice. Its use has, therefore, been largely discontinued. A possible source of the problem may lie in the conversion by fructokinase of some of the xylitol to D-xylulose 1 -P, which can be cleaved by the xylulose 1-P aldolase to dihydroxy acetone P and glycolaldehyde. 

Iron usually occurs as the Fe(III) form in food. This is also the form released from iron pots when food is cooked in them. However, iron must be in the Fe(II) state to be absorbed. Reduction from Fe(III) to Fe(II) can be accomplished by ascorbate (vitamin C) or by succinate. Factors that affect absorption include the solubility of a given compound of iron, the presence of antacids in the digestive tract, and the source of the iron. To give some examples, iron may form insoluble complexes with phosphate or oxalate, and the presence of antacids in the digestive tract may decrease iron absorption. Iron from meats is more easily absorbed than iron from plant sources. 

Organic matter not only plays a direct role as a source of calcium and magnesium, but it is also a food for microorganisms that form carbon dioxide and many organic acids, such as acetic, butyric, lactic and oxalic. These acids react with limestones and other minerals and thereby continually release available nutrients. In the presence of an abundant supply of humus much of the released calcium and magnesium that is not immediately absorbed by plants may be adsorbed by the humus and thereby be prevented from loss by leaching. ... 

Oxalates—Nutritionists have long been concerned about whether oxalates, which are present in such foods as rhubarb and spinach, might bind with sufficient amounts of calcium and/or iron so as to produce deficiencies of these minerals. However, experiments have shown that such effects are not likely to occur in man unless (1) the mineral intake is marginal and (2) unusually large amounts of these foods are eaten, along with other sources of oxalates like almonds, beet greens, cashew nuts, cocoa, or tea. ... 

NOTE WELL The biological availability of zinc in different foods varies widely meats and seafoods are much better sources of available zinc than vegetables. Zinc availability is adversely affected by phytates (found in whole grains and beans), high calcium, oxalates (in rhubarb and spinach), high fiber, copper (from drinking water conveyed in copper piping), and EDTA (an additive used in certain canned foods). 

Premna microphylla Turcz is an important commercial crop historically used in China to prepare green tofu and currently cultivated on a large scale by Chinese farmers because its leaves are used to extract pectin [13]. Using ammonium oxalate for the extraction, a pectin yield of 20.61% was attained with a degree of esterification of 14.90%. Based on the former results, the use of Premna microphylla Turcz leaves, as a source of LDE pectin is recommended by the authors. They indicate that the obtained pectin has great potential as commercial gelling agent or a thickener in the food industry. 

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