Oxalates, effect bioavailability

Many other dietary factors have been reported to affect calcium bioavailability. Phytate, fiber, cellulose, uronic acids, sodium alginate, oxalate, fat (only in the presence of steatorrhea), and alcohol have been reported to decrease calcium bioavailability (15). Lactose and medium chain triglyceride increase it (15). FTuoride also affects calcium retention primarily by stimulating bone formation thereby decreasing calcium excretion (33-38). The effects of fluoride on calcium utilization have been variable (34,38,39). 

Review of studies on the effect of oxalic acid on calcium bioavailability in rats and in humans indicates that most of the research was done between 1930 and 1950. Decreased availability of calcium in young rats was reported when spinach containing oxalic acid was fed with low calcium diets. The extent of the effect of oxalic acid on calcium availability was shown to be related to levels of calcium and oxalic acid, as well as the presence of vitamin D in the diet. In human studies there was generally no effect of oxalic acid on calcium balance however, in a few studies decreased calcium balances were reported. There is recent evidence that oxalic acid consumed along with a moderately high level of fiber intake may have adverse effects on calcium balance of human subjects. 

Several comprehensive reviews on oxalic acid have been published in which effects on calcium metabolism were discussed (7-11). A review of studies on the effect of oxalic acid on calcium bioavailability in rats and humans is presented in this paper. 

In the studies on humans there appeared to be decreased calcium balances when 200 g or more of spinach per day was included in the diet. In two of the studies in which women were fed spinach, calcium intakes were below the Recommended Dietary Allowance of 800 mg/day (37). Some studies were conducted for short period of a week or less, which may not be sufficient time to adjust to a change in diet. From measurement of calcium excretion in urine after a test meal, it was shown that the calcium in oxalate-containing vegetables was less well-absorbed than that of milk or of vegetables not containing oxalic acid. However, this would not necessarily affect calcium balance, since the total amount of calcium in the diet would have to be considered. The effect of a combination of oxalic acid and fiber on calcium bioavailability should be further investigated. 

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... 

Little agreement has been reached as to which dietary components or which food processes physiologially affect mineral availability. Many plant foods contain phytic acid, oxalic acid or other dietary fiber components that can be shown to chelate minerals. The effect of these dietary substances upon the final bioavailability of the mineral in question will depend upon the digestibility of the chelate (106). 

Little information is available on the effect of oxalic acid on zinc bioavailability. In one study, Welch et al. ( fed weanling rats zinc-deficient diets with and without 0.75% sodium oxalate. The rats were dosed orally with zinc-labeled spinach leaves or zinc-labeled zinc sulfate. Dietary oxalate enhanced the availability of radioactive zinc from zinc sulfate, but had no effect on zinc from spinach leaves. Absorption and retention of zinc was greater from spinach leaves than from zinc sulfate. 

Iron bioavailability is affected by valence state, form, solubility, particle size, and com-plexation which in turn may be affected by the food matrix. Complexation of iron has been found to have either a positive or negative effect on availability, with such compounds as ascorbic acid and fructose increasing availability and oxalates, phytates, phosphates and food fibers perhaps decreasing availability. Availability has also been shown to be directly correlated to acid solubility. We have found that acidity tends to increase ionization as well as favoring the ferrous state which has greater solubility at... 

All other oxalates are sparingly soluble in water which would dramatically effect the bioavailability of the metal ions involved. Table III illustrates both the solubility and stability constants for a few selected metals (35.). It may be seen that with calcium it forms a practically insoluble salt at neutral or alkaline pH, being soluble to the extent of 0.67 mg per 100 ml of water at pH 7.0 sind 13°C. Zinc also has limited solubility (0.79 mg/100 ml, 18°) while Fe+2 and Fe+3 show solubilities of 22.0 ml/100 ml and "very soluble" respectively. These chemical facts and their effedt on iron absorption have recently been substantiated in a biological sense by Van Campen and Welch (36) who investigated the availability to rats of iron from two varieties of spinach. Also they compared the absorption of iron between FeClj and Fe-oxalate as well as the effects of adding 0.75% oxalate to the diet. They found that absorption of iron from both varieties of spinach was comparable to that from FeClj and that the iron was equally avail-... 

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