Fiber, dietary, effect bioavailability

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. 

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. 

The mechanisms by which various forms of dietary fiber influence calcium bioavailability apparently also differ. In some cases, apparent dietary fiber effects on calcium bioavailability may be secondary to effects on bile acid and salt secretion and reabsorption or to other dietary components. 

Johnson BF, Rodin SM, Hoch K, Shekar V. The effect of dietary fiber on the bioavailability of digoxin in capsules. J Clin Pharmacol (1987) 27, 487-90. 

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). 

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). 

Toma, R.B., and D.J. Curtis. 1986. Dietary fiber Effect on mineral bioavailability. Food Technol. 40, no. 2 111-116. 

Manganese is a nutritionally important trace element for chicks. Dietary energy and protein sources contain very little bioavailable Mn, and these feed ingredients reduce the biopotency of inorganic Mn supplements. This adverse effect is exerted primarily in the intestine as a result of reduced Mn absorption and is mediated by the fiber and/or ash components of the feedstuffs. Gut absorption efficiencies are higher when a phytate-and fiber-free casein-dextrose diet is fed than when a corn-soybean meal diet is fed. Dietary interrelationships exist between Mn and Co and between Mn and Fe. Cobalt increases Mn absorption and may precipitate Mn toxicosis. Excess dietary Mn reduces Fe utilization, but excess Fe does not affect Mn utilization. Eimeria acervulina infection increases Mn absorption. 

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). 

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. 

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... 

The effect of dietary fibers or fiber-rich foods on iron bioavailability in animals. 

Lutz M, Espinoza J, Arancibia A, ArayaM, Pacheco I, Brunser O. Effect of structured dietary fiber on bioavailability of amoxicillin. Clin Pharmacol Ther (1987) 42, 220-4. 

It is proposed that polyphenols and fiber are the main contributors to the observed effects, either individually or in a synergistic interaction. However, bioavailability and metabolism of the numerous apple polyphenols is not completely understood. Moreover, limited data are available for the characterization of apple fiber, in particular pectin, in the different apple varieties and the impact of the physicochemical properties and structure on the degree of effect. Suitably powered randomly controlled, human dietary intervention studies with long intervention periods are required to clarify the protective effects of apples and juice consumption. Further in vivo animal models are also necessary to explore the potential multiple mechanisms of actions. 

---