Phytate-calcium ratio dietary

Dietary intake data for calcium, phytate and phytate/calcium ratio are summarized in Table II. Mean calcium intake for HS-I was about 300 mg greater than the recommended dietary allowance (RDA) of 800 mg established for adults by the National Research Council (8) and for HS-II just slightly less than the RDA. The range of calcium intakes, because of different caloric needs was from 927 to 1490... 

Two metabolic balance studies were conducted using healthy adult men to study the effect of phytate on bioavailability of dietary calcium. Dietary treatments were each 15 days in duration. In the first study, a mean daily calcium balance of 208+58 (SD) mg was observed when 2.0 g of phytate from 36 g of whole wheat bran was consumed daily with 1100 mg of calcium, phytate/calcium molar ratio 0.11. Calcium balance was 184+87 mg when 36 g of dephytinized bran was consumed with the same intake of calcium, phytate/calcium molar ratio 0.01. In the second study, calcium intake was 740 mg/day. 

Apparent absorption (intake minus fecal excretion) of calcium decreased when the diet contained muffins with added sodium phytate to increase the molar ratio of phytate/calcium from 0.04 to 0.14 and 0.24. One-half of the men excreted more calcium in feces than was consumed when the high phytate diet was consumed. People consuming diets with molar ratios of phytate/calcium exceeding 0.2 may be at risk of calcium deficiency because of low bioavailability of dietary calcium unless physiological adjustments can be accomplished that maintain homeostasis. 

HS-II. The mean (+ SD) daily apparent absorption values were 153+77, 94+65 and 23+66 mg for 0.5, 1.7 and 2.9 g phytate intakes, respectively. There was a significant difference between all means P <0.05, by ANOVA. A plot of apparent absorption of calcium vs the dietary phytate/calcium molar ratio is shown in Figure 1. 

Because phytate intakes were essentially constant, but calcium intake varied with caloric needs, a range of phytate/calcium molar ratios resulted with each phytate level. The correlation coefficient for apparent absorption and dietary phytate/zinc molar ratio was 0.62, P<0.01. The equation of the regression line was y =... 

Figure 1. Dietary phytate/calcium molar ratio and apparent absorption of calcium. Each triangle is the mean for one individual for 3 consecutive 5-day menu cyles. See text for study details, HS-II.

We have not completed the analyses of self-chosen diets consumed by the subjects in HS-II and HS-III, but preliminary indications are that there was a wide range in intakes of calcium and of phytate/ calcium molar ratios. Stools and urine were collected while the self-chosen diets were being collected. Comparison of calcium metabolic balance parameters when consuming the self-chosen diets and the controlled diets of HS-II may provide an insight on dietary calcium requirement by these individuals. Some advocate the present RDA of 800 mg for adult men is too low (17), however, unpublished (R.D. Reynolds et al.) data from dietary intakes in a developing country indicate much lower usual intakes of calcium with relatively high phytate/calcium molar ratio. 

Kinetic synergism of calcium and zinc with phytate causes a complexatlon less soluble than either separately. Saliva and pancreatic fluid secrete large quantities of zinc equivalent to as much as three times the dietary intake which is also vulnerable to phytate complexatlon. The mechanism of phytate action in the gastrointestinal tract is related to complexatlon and subsequent prevention of absorption and reabsorption of zinc. The complexatlon can be equated to a phytate zlnc molar ratio and the relative hazard may be subsequently estimated from such data. 

Oberleas (.9) first suggested that the molar ratio of phytate to zinc might be useful for prediction of the zinc bioavailability from phytate-rich foods. Molar ratios of greater than 20 1 seemed to be indicative of poorly available zinc. O Dell (10), Morris and Ellis (11) and Davies and Olpin (12) have all pointed out the importance of the calcium content of the diet to the phytate to zinc molar ratio. Higher dietary calcium clearly depresses zinc bioavailability at phytate to zinc molar ratios of less than 20 1 in diets fed to rats. 

As pointed out earlier in this review, increasing the level of dietary calcium decreases the zinc bioavailability from phytate-containing foods. Presumably the mechanism is through the formation of chemical complexes containing zinc, phytate and calcium which are insoluble at intestinal pH and nonabsorbable (24). Recently, our laboratories used slope ratio techniques to compare the bioavailability of zinc contained in calcium sulfate-and in magnesium chloride-precipitated soybean curd (Tofu) to that of zinc added as the carbonate to egg white diets by slope ratio techniques (25). Total dietary calcium level in all diets was adjusted to 0.7% with calcium carbonate. The results (not shown) indicated that the relative availability of zinc from both tofu preparations was 51% as measured by weight gain and 36-39% for bone zinc. These results are similar to those reported for full fat soy flour (16) in Table I. 

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