W HEAT B RAN

12.5.2 W HEAT B RAN

Wheat bran is a widely consumed fiber that has been proposed to negatively influence calcium absorption. 2,3 One of the earlier studies that triggered interest to determine

Carbohydrates and Mineral Metabolism

the effect of wheat bran on mineral absorption showed that feeding bread partly made from wheat whole meal resulted in a decreased calcium balance in two men. 4 It is important to recognize that the presence of phytate in this preparation of insoluble fiber may have been more of a factor for the reduced calcium absorption. This possibility was nicely shown in a study conducted in rats fed graded amounts of wheat bran for 2 weeks and measures of calcium bioavailability obtained using

orally administered tracer 45 Ca. The results indicated that the addition of wheat bran up to 15% of the diet had no effect on altering 45 Ca absorption. 76 A good explanation for the inconsistent findings in rats, compared to studies conducted in pigs where reduced calcium bioavailability was observed when they were fed wheat bran, is the presence of active phytase enzyme in the rodent intestine. Earlier studies conducted in human subjects reported that feeding 16 g of wheat bran/day to eight ileostomy

patients did not impair calcium absorption from the small intestine. 75 Other human studies conducted in six men, where the phytate content was controlled by adding sodium phytate to obtain an equal amount of phytate for each loaf of bread, con-

cluded that wheat bran had no significant effect on calcium absorption. 77 Intrinsically

45 Ca-labeled wheat during plant growth propagation was used as a measure of interactions with calcium and other dietary constituents. This distinct experimental

design produced results that had wheat products, but not wheat bran, showing no significant effect on calcium absorption in 26 healthy adult women. 78 It is noteworthy

that leavening bread was found to improve the calcium absorption, which can be explained on the basis that yeasts can produce phytases, and thus reduce the phytic acid content during bread making.

Other in vitro studies have supported the hypothesis that wheat bran by itself has the potential to decrease mineral bioavailability, since the insoluble fraction of wheat bran has a large potential to bind calcium even after removal of phytate. 79 This is seen when calcium ions are added to an insoluble fiber residue of dephytinized wheat bran, and the calcium binding capacity is determined using flame atomic absorption spectroscopy after acid washing the wheat bran. A similar technique was used to determine the binding capacity of wheat bran, rice bran, and oat fiber for

calcium and other minerals. 80 Wheat bran was found to bind significantly more calcium than rice bran and oat fiber, respectively, which suggests that wheat bran

has either more specific calcium binding sites or a similar number of sites, but a higher affinity toward calcium than other fibers. An in vitro method that involves simulating gastrointestinal digestion estimated the effect of wheat and barley fiber

on calcium bioavailability by measuring 45 Ca dialyzability in test meals. 81 Results indicated that 45 Ca dialyzability was reduced more by wheat fiber than barley fiber, relative to control. Again, it is important to note that wheat fiber contains a 3.2-fold higher content of phytate than the barley fiber. Cell culture studies utilizing Caco-

2 cells (i.e., the human colon adenocarcinoma) as a model test system to determine the effect of barley hull and wheat bran, as well as the dephytinized barley and wheat fiber on calcium absorption, showed that wheat bran significantly decreased (e.g., 17%) calcium transport across the cells and uptake of calcium (e.g., 24%), in comparison to the fiber-free control. However, this inhibitory effect on calcium

uptake was removed when the wheat bran was dephytinized, 82 again indicating that

Functional Food Carbohydrates

the presence of phytate, rather than wheat bran itself, had a major role in inhibiting calcium absorption.