A.Robert M. Russell, MD, emeritus professor at Tufts Friedman School, answers: It is known that many foods that are high in fiber (particularly cereals) can inhibit the absorption of iron and zinc from the gastrointestinal tract. However, cereals that are high in fiber are also high in a substance known as phytate, which also has an inhibitory effect on these minerals absorption. So it is hard to pick apart how much of the inhibition of absorption of these minerals is due to phytate alone versus how much is due to fiber-or if they can act synergistically. What is clear is that both phytate and fiber can inhibit the GI absorption of iron and zinc to some degree. Possible effects on calcium absorption (at least with cellulose) have also been reported, but most of the inhibition for calcium and magnesium absorption is thought to be due to phytate rather than fiber.
At dietary ranges of total fiber intake (21-25 grams per day for women and 30-38 grams per day for men), no chronic adverse effects have been seen due to mineral malabsorption. It is only when eaten at very high levels that adverse clinical effects from phytate/fiber have been reported.