Fiber reduces the absorption of many minerals. However, high-fiber diets also tend to be high in minerals, so the consumption of a high-fiber diet does not appear to impair mineral status. However, logic suggests that calcium, magnesium and multimineral supplements should not be taken at the same time as a fiber supplement.

Bran, which contains insoluble fiber, reduces the absorption of calcium enough to cause urinary calcium to fall.² In one study, supplementation with 10 grams of rice bran twice a day reduced the recurrence rate of kidney stones by nearly 90% in recurrent stone formers.³ However, it is not known whether other types of bran would have the same effect. Before supplementing with bran, people should check with a doctor, because some people—even a few with kidney stones—do not absorb enough calcium. For those people, supplementing with bran might deprive them of much-needed calcium.