Phytate-calcium ratio

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.

Figure 2. Apparent absorption of calcium for 9 consecutive 5-day menu cycles with phytate/calcium molar ratio <0.05, HS-III. Each individual is represented by a different symbol.

For HS-I, with calcium intakes of about 1100 mg/day, no difference was observed in either apparent absorption or balance of calcium over the last 10 of 15 days when the phytate intake was 0.2 or 2.0 g/day. The molar ratio of phytate/calcium was either 0.01 or 0.1 in HS-I. In HS-II the calcium intake was lower, about 740 mg/day, but the same across three levels of phytic acid, 0.5, 1.7 and 2.9 g/day. The phytate/calcium molar ratios were 0.04, 0.14 and 0.24. Apparent absorption of calcium for the 15-day diet treatment period became progressively less as the molar ratio of phytate/calcium increased, to the extent that 6 of 12 individuals excreted more calcium in the feces than they consumed when the mean ratio was 0.24. About 200 mg of calcium was excreted daily in the urine by... 

The brown or whole meal bread diets employed by previous investigators were often variable in calcium and phytate intakes, not only between individuals, but by the same individual subjected to different diet treatments. Nevertheless an estimate of the molar ratio of phytate/calcium in the brown or whole meal bread diets used by McCance and Widdowson (UO), Walker et al. (11) and Reinhold et al. (2, 12) is 0.25 or greater. These investigators observed either negative or less positive calcium balance and apparent absorption when the brown bread diets were consumed compared to white bread diets with phytate/calcium molar ratios less than 0.05. Our results support their findings. Reinhold et al. (2) and McCance and Widdowson (33) used sodium phytate in some studies as well as whole wheat bread and observed similar results. 

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. 

Figure 4 illustrates the quantity of precipitate formed when equimolar concentrations of phytate, zinc and/or calcium (1 1 or 2 1) are mixed in an open vessel and pH s adjusted first to less than 3, than carefully to the appropriate pH. The pH range between 3 and 9 was selected to encompass the physiologically important range. The results indicate that 1) calcium and phytate, in equimolar concentration, are quite soluble at all pH s under these conditions 2) zinc phytate is less soluble than calcium phytate and at pH 6 is less soluble than calcium at twice the molar concentration 3) zinc, calcium and phytate in all combinations tested was less soluble than either zinc or calcium phytate or the sum of these alone at pH 6. The pH 6 is very important physiologically because this is the approximate pH of the duodenum and upper jejunum, an area of the gastrointestinal tract in which zinc must be absorbed. At pH 6 and a 2 1 1 calcium zinc phytate molar ratio, 98% of the zinc was in the precipitate (, 39). 

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. 

Zinc Is a trace element and both calcium and phytate are present In many foods In macro quantities therefore, an In vitro model was developed to study the quantity of zinc available for absorption with these more physiological ratios. The results, Figure 5, Indicate that as the ratio of calclum phytate Increases there Is a decrease In the uncomplexed zinc In solution which would be available for absorption (28, 38, 40). EUTA added Into this same model Increased the soluble zinc (38) Indicating that soluble and absorbable chelating compounds may compete with phytate and make some zinc available for absorption or reabsorp-tlon. The Implications of the studies described above are that the Interaction of phytate with zinc and calcium Involves a chemical rather than a physiological reaction. 

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. 

Calcium Effects on Zinc Bioavailability for the Rat and the Human. It should be pointed out at this juncture that the nutrient requirement of calcium for the rat is much higher than for man. In fact, the molar ratio of calcium to zinc in excess of 660 1 is recommended for rat diets, while for man the ratio is between 80 1 and 160 1. To feed rats molar ratios of calcium and zinc similar to human requirements would necessitate either a very calcium deficient diet or one containing zinc at a level well in excess of the requirement. Neither choice is nutritionally suitable for demonstrating an effect of phytate on zinc availability. 

Soy Product Phytate to zinc molar ratio Calcium to zinc molar ratio Dialyzability (%)... 

Simple predictors, such as the phytate to zinc molar ratio, will not be accurate for human diets. The aggravating effect of calcium and perhaps of magnesium seen in rat and in vivo tests may or may not be of practical significance to man. We can, however, develop a list of factors that have been shown to affect zinc bioavailability from foods. These are listed in Table VII. 

Absorption of calcium is influenced by many factors. The maintenance of normal gastric acidity facilitates absorption, and vitamin D enhances it. Absorption is increased by protein, lactose, and citrates in the diet. Absorption is decreased by the presence of phytates (in cereals), oxalates, and fatty acids, since in each instance insoluble calcium salts are formed. The ratio of calcium to phosphorus in the diet influences absorption. Calcium enters the gastrointestinal tract not only in food but as a component of digestive secretions as much as 0.3 to 0.8 g. daily may be secreted into the gastrointestinal tract. Reabsorption of this calcium is influenced by the factors just enumerated. 

Phytic acid (myo-inositol-l,2,3,4,5,6-hexakisdihydrogen phosphate) occurs in a number of important crops, especially cereals, legumes and oilseeds. The main form is a mixed calcium and magnesium salt, which is called phytin. Phytate phosphorus has reduced biological utihsation and lower utihsation than other minerals (Ca, Mg and Zn and Fe in particular). The contents of phytic acid in some food materials and foods, and the ratio of phytate phosphorus to total phosphorus, are shown in Table 6.5. 

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