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Variables in calcium absorption

PITTSBURGH, PENNSYLVANIA. An adequate calcium intake is important in reducing the risk of osteoporosis, hypertension, and colon cancer. It is, however, becoming increasingly clear that a supposedly adequate intake does not guarantee the absence of osteoporosis. The calcium must not only be ingested, it must also be absorbed and its excretion minimized. In other words, it is not the calcium intake per se that is important, but rather how much of it is actually retained in the body.

Researchers at the University of Pittsburgh now report that the intake of fat and fiber significantly influences calcium absorption. Their study involved 142 healthy pre-menopausal white women who had enrolled in the Women's Healthy Lifestyle Project in 1995-96. The women had blood samples drawn three hours after consuming apple juice containing labeled (isotope) calcium. The blood samples were analyzed for calcium, 1,25 dihydroxyvitamin D (the active from of vitamin D), and parathyroid hormone. The women also completed food-frequency questionnaires and answered questions about their lifestyle and the use of vitamin and mineral supplements.

The researchers found that about 35 per cent (17 to 58 per cent) of the labeled calcium had been absorbed. It was clear that women with a higher fat intake and a lower intake of fiber absorbed significantly more calcium than did women with less fat and more fiber in their diet. Women with high blood levels of vitamin D also showed increased absorption while women who consumed alcohol had decreased absorption. There is also some indication that a higher total calcium intake is associated with a lower rate of absorption. The researchers caution that it may only be certain types of fiber (eg. wheat bran) that inhibit calcium absorption. Fiber found in green leafy vegetables such as kale, broccoli, and bok choy may not be detrimental to absorption. They found no indication that genetic differences among the women were in any way related to calcium absorption. The researchers express the hope that their findings will encourage a second look at the current standard recommendation to emphasize a low-fat, high-fiber diet.
Wolf, Randi L., et al. Factors associated with calcium absorption efficiency in pre- and perimenopausal women. American Journal of Clinical Nutrition, Vol. 72, August 2000, pp. 466-71

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