Phytate dietary

This book is based on the symposium that was designed to assess the current perspective and future direction of research on the nutritional bioavailability of zinc. Inhibitors suspected of interfering with the absorption of zinc are some factors that infiuence bioavailability. Phytates, dietary fibers, proteins, nonenzymatic browning products, and certain micronutrients are among these substances. These inhibitors are covered in various chapters. 

Dietary fiber and phytate. Dietary fiber and phytate are known as potential inhibitors of the absorption of divalent cations however, the literature regarding the effect of dietary fiber and phytate on the bioavailability of minerals is contradictory. Data by Yannai and Sachs (1993) indicate that phytate does not affect methylmercury absorption. Yannai and Sachs (1993) compared the absorption by rats of mercury found intrinsically in experimental fish meal with and without added phytate and found no significant differences in the absorption of Hg (93 5%) between 2 experimental fish meal diets (containing 1.4 mol Hg/kg diet), with or without added sodium phytate. The authors speculated that phytate might be preferentially bound to zinc, iron, and copper, which were present at much higher concentrations in the diet. 

Vitamin D withdrawal is an obvious treatment for D toxicity (219). However, because of the 5—7 d half-life of plasma vitamin D and 20—30 d half-life of 25-hydroxy vitamin D, it may not be immediately successful. A prompt reduction in dietary calcium is also indicated to reduce hypercalcemia. Sodium phytate can aid in reducing intestinal calcium transport. Calcitonin glucagon and glucocorticoid therapy have also been reported to reduce semm calcium resulting from D intoxication (210). 

Defatted soy flour was suspended in water to 8% (v/w) protein, pH was adjusted to 6 by HCl before 640 mg/L Novozyme 415 (a-galactosidase), 10 mg/L Phytase L (Novo Nordisk) and 53 mg enzyme protein/L rhamnogalacturonase B were added. After 4 hours at 50 C the pH was adjusted to 8 by NaOH, and the slurry was centrifuged. The supernatant was pasteurized (85°C, 5 minutes) and freezedried. Protein was measured as 6.25 x Kjeldahl N. Phytate was measured as described in [16], and dietary fibres were analysed as described in [17]. 

The composition of the resulting soy product is shown on Table V, where it is seen that the protein content was unchanged, whereas raffinose, stachyose and phytate were almost removed, and the amount of dietary fibres was improved. This demonstrates that the availability of relatively pure pectinolytic enzymes, like the used RGase B, opens up for the new types of soy processes and products. 

Graf, E. and Eaton, J.W. (1985). Dietary suppression of colonic cancer. Fiber or phytate. Cancer 56, 717-718. 

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). 

The effect of phytic acid on Ca2+ bioavailability is still in dispute. Some early nutritionists reported rachitogenic properties of dietary phytate based on feeding studies using puppies (16-18) and epidemiological studies on British-born children of Asian immigrants... 

These conclusions were seriously questioned (21-24 and recent results indicate that the bioavailability of Ca2+ is the same from a casein diet as from a high-phytate soy concentrate (25). Oberleas contends that, in the presence of adequate amounts of Ca2+ and vitamin D, dietary phytate is not rachitogenic, even though it may bind substantial amounts of Ca2+ (J 5). This controversy, the relative paucity of available information, and the growing incidence of Ca2+ deficiency prompted us to investigate further the chemical interactions between Ca2+ and phytate and to assess its effect on the bioavailability of Ca2+ administered to mice by gavage. 

From our results on solubility properties of Ca2+-phytate complexes and their bioavailability to mice we conclude that, at least in our experimental system, phytate has no detrimental effect on Ca2+ bioavailability. Indeed, as we have argued elsewhere, adequate levels of dietary phytate may actually be beneficial due to the... 

The obliteration of these iron-dependent oxidative processes by phytate suggests that this ubiquitous and abundant plant component functions as a natural antioxidant, preventing oxidative damage during storage of seeds. Surface treatment with phytic acid of various fruits and vegetables preserved their color, texture and flavor (7). Thus, dietary phytate may be a superior substitute for presently employed food preservatives, many of which pose significant health hazards. Additional applications of phytic acid are summarized in a recent review (34). 

By virtue of rendering iron catalytically inactive, dietary phytate may also suppress the incidence of colonic cancer (J2j>). Intestinal aerobic bacteria and/or minor inflammatory events generate substantial amounts of O27 leading to OH formation and lipid peroxidation. These two processes are thought to be important elements in tissue injury which occurs during inflammation. This argument is compatible with the observation that colonic cancer is frequently preceded, or accompanied, by pigmentation of the colonic epithelium lipofuscin, a byproduct of lipid... 

In conclusion, phytic acid forms soluble complexes with Ca2+ at intestinal pH under a variety of conditions and fails to inhibit Ca2 bioavailability to mice in our experimental system. Despite the hazard in direct extrapolation of results obtained with animals kept on a well-defined dietary regimen to humans consuming a complex diet, many elements of which affect Ca2+ bioavailability, our data demonstrate the need for a reevaluation of the putative antinutritional properties of dietary phytate. Our further contention that adequate levels of dietary phytate may actually be beneficial due to its food preserving properties and its protection against colonic cancer will warrant a prospective epidemiological human study designed to assess the longterm effects of dietary phytate on mineral bioavailability and inflammatory bowel diseases. 

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. 

Two metabolic balance studies conducted in our laboratory have yielded information relative to the effect of phytate and dietary fiber on calcium bioavailabilty. In the first study, a relatively high intake of dietary fiber was consumed with a 10-fold difference in phytate intake from wheat bran. In the second study three levels of phytate were consumed with a low amount of dephytinized bran as the principal dietary fiber source. The two higher phytate levels in the latter study were attained using sodium phytate. 

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... 

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. 

Our studies do not resolve the question of phytate vs fiber for the effect of wheat bran on dietary calcium bioavailability. Phytate level clearly affected apparent absorption of calcium in HS-II in the presence of an amount of the water insoluble fraction of dephytinized bran equivalent to 12 g of untreated bran and the phytate supplied as sodium phytate. An additional trial using untreated bran and the same amount of fiber as the water insoluble fraction with sodium phytate could resolve the question of fiber vs phytate. In HS-I, the balances were positive when a relatively large amount of bran, 36 g/day, was consumed. Calcium intakes were possibly higher than most men consume, but under the dietary conditions imposed for 15 days, the phytate and fiber of 36 g of bran did not express an adverse effect on calcium balance. 

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... 

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