Wheat bran bioavailability

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. 

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... 

Fermentation of plant foods generally increases mineral bioavailability. Studies by Ranhotra and coworkers (121, 122 for example) have shown increased available zinc from breads and cookies that have undergone yeast fermentation. Also, iron was shown by these workers to be more highly available from unfortified breads than from breads fortified with wheat bran, soy flour or other whole grain vegetable flours. 

Phytate has been studied extensively with regard to mineral (mostly Zn and Ca) status of animals, and it has been shown to reduce whole-body Mn retention in rats (12). Phytate, however, is not present in the neutral detergent fiber or in the ash component of feedstuffs. Therefore, phytate does not appear to be responsible for the reduction of Mn uptake in chicks fed corn, soybean meal, wheat bran or fish meal (9). That phytate negatively impacts Mn nutriture also disagrees with the research of Reinhold et aK (13), who reported that fiber, and not phytate, was the pTTmary factor determining bioavailability of divalent mineral elements in breads. 

Wheat bran contains several different forms of dietary fiber including hemicellulose. Some but not all purified dietary fiber sources have been found to have an adverse effect on manganese bioavailability (10). 

We conducted a human metabolic balance study to test the concept of dietary phytate/zinc molar ratio as a predictor of zinc bioavailability to humans. Using unaltered or enzymatically dephytinlzed wheat bran with ordinary foods we attained phytate zinc molar ratios of about 1 and 12 with relatively high intakes of dietary fiber in the menus and found no difference in zinc balance. Retrospectively, the result may be qualified on the basis of the magnitude of the zinc Intake and possible adaptive or homestatic responses over the period of the study. A second study was then conducted and a wider range of phytate/zinc molar ratio was provided than in the first study. We will briefly outline the first study, give a progress report on the second study and, with some information on phytate intakes obtained by our laboratory, discuss the nutritional implication. 

Carter EG and Carpenter KJ (1982) The bioavailability for humans of bound niacin from wheat bran. American Journal of Clinical Nutrition 36, 855-61. 

Glycosides of this vitamin (sometimes denoted as vitamin B3) were not described, however, there exists an evidence that bound niacin from wheat bran (termed niac)4in) has a single nicotinic acid moiety at least partially linked to an aromatic amine with glucose, xylose, and arabinose in a 6 3 1 molar ratio per molecule, with approximately three cinnamic acid esters [152,177,178]. It seems that these glycosidic complexes limit the bioavailability of the nicotinamide and for its liberation they must be treated, e. g., by soaking com in a lime solution, traditionally performed in Central America during production of tortillas [179]. 

Phytate, Wheat Bran, and Bioavailability of Dietary Iron... 

Monoferric phytate is the major fraction of iron in wheat bran, and is a highly bioavailable form of dietary iron in contrast to insoluble di- or tetra-ferric phytate. Monoferric phytate equilibrates with the miscible nonheme iron pool of a meal in extrinsic label iron absorption tests. Whole wheat bran depressed absorption by humans of nonheme iron in a meal. Dephytinized wheat bran also inhibited nonheme iron absorption by humans and the inhibition could not be clearly attributed to either the insoluble or soluble fractions of the dephytinized bran. Adult men who consumed 36 g of wheat bran per day had positive iron balances. Iron balance was not increased when dephytinized bran was consumed. The form of ferric phytate must be known to properly explain the effect of phytic acid on iron absorption. The overall meal composition must be considered to evaluate the effect of wheat bran on iron nutrition of humans. 

About a decade ago we began studies to identify the chemical nature of iron in wheat as possibly a first step in devising a means to improve the assimilation of the iron in wheat-based foods. In this communication we will discuss the evidence for our belief that monoferric phytate is the endogenous form of iron in bran, some physical-chemical characteristics of monoferric phytate, and studies in animals and humans of the bioavailability of iron in wheat bran and monoferric phytate. 

Animal studies. In the initial studies (.6), a partially purified monoferric phytate prepared from extracts of wheat bran and two synthetic preparations were bioassayed using rats. The hemoglobin depletion-repletion method was used and the relative biological value (RBV) was computed by slope ratio. Compared to the response to ferrous ammonium sulfate as 100, the RBV for the monoferric phytate prepared from wheat bran was 99, and for the two synthetic preparations 101 and 97. The 95 confidence interval for the preparation from wheat bran was 87-111. The iron of monoferric phytate is highly bioavailable to rats. 

Human studies. Simpson et al. (14) measured absorption by humans of free monoferric phytate from meals of both high and low bioavailability. Absorption was measured by the extrinsic tag method (15). Iron (2 mg) was added to the meals in each pair of absorption tests as either monoferric phytate or FeClj, each compound was labeled with either 55 Fe or 59Fe. The absorption ratio of monoferric phytate to FeClj was not significantly different from unity (Table III). The free biological form of iron in wheat bran, monoferric phytate, was no less well absorbed than the dietary nonheme iron in the meals. Absorption of both compounds was several fold greater from the standard (STD) than from the semisynthetic (SS) meal reflecting the relative bioavailability of nonheme iron from the two types of meals. 

Table IV presents the RBV for the iron of three breakfast cereals not fortified with iron. The two ready-to-eat cereals were sources of highly bioavailable iron, but the instant cereal was significantly lower in bioavailability than the reference salt. We have not studied the chemical nature of iron in commercially available wheat based foods. The iron that remains in the residue of 1.2 M ammonium acetate extracted wheat bran was only 71 as bioavailable as the extracted monoferric phytate (6j, but the iron of enzymatically dephytinized wheat bran, which may be complexed with amino acids, is highly bioavailable (11).

The major fraction of iron in wheat bran is monoferric phytate, which is soluble and equilibrates with the miscible dietary nonheme iron pool of a meal. The di- and tetra-ferric phytates are much less soluble than monoferric phytate and probably do not equilibrate with the miscible nonheme iron pool of a meal. Bioavailability studies using either ferric phytate or sodium phytate must be evaluated in light of the form of ferric phytate or whether an insoluble ferric phytate may have been produced in the food. Wheat bran depresses absorption of dietary nonheme iron by humans. On the basis of available evidence, that effect of wheat bran cannot be unequivocally attributed to either the phytate or fiber component alone and might be influenced by interactions between the fiber, phytate and iron in the whole bran. Adult men maintained adequate positive iron balance when they consumed 36 g of whole bran each... 

In contrast with some other fiber sources such as wheat bran no significant decrease in mean transit times has been reported by most researchers (7,9,18,29,30) investigating the effect of pectin on bowel function or mineral bioavailability. Only one study ( 5) reported a significant decrease in transit time from a starting time averaging 63 hours. No significant difference in transit time between high and low methoxyl pectins was observed by Judd and Truswell (19). 

Liver, nuts and eggs are the most important food sources for humans. Bioavailability is very variable. Sources include molasses (150%), brewers yeast (100%), soybeans (100%), wheat bran (50%), walnuts (50%), peanuts (50%), oats (30%), eggs (15%) and cauliflower (15%) (Fig. 26.22). 

Folate is widely distributed in foodstuffs but is unstable to cooking and has reduced bioavailability in fiiiits and v e-tables. Sources include brewers yeast (800%), beef liver (100-500%), broccoli (100%), spinach (50%), Brussels sprouts (50%), peas (50%), wheat bran (40%), bananas (25%), oranges (15%) and tomatoes (15%) (Fig. 26.24). 

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