Calcium supplements & bioavailability

Townsend Letter for Doctors and Patients, April, 2005 by Jule Klotter

In marketing nutritional supplements, manufacturers often promote the superiority of their product by focusing on the supplement's bioavailability and ease of absorption. In a Journal of Nutrition article, Robert P. Heaney, MD, of Creighton University's Osteoporosis Research Center (Omaha, Nebraska) discusses six primary methods for measuring bioavailability, using calcium as a model. Results from the various methods do not always agree. When Dr. Heaney performed a comparative study of absorbability of calcium carbonate and calcium citrate, he found that citrate appears to have greater bioavailability when serum concentration or urinary excretion measurements are used. Citrate binds to calcium ions more readily than carbonate does, which skews the values in its favor. When Dr. Heaney checked bioavailability of the two salts using the tracer method, calcium carbonate was absorbed better than the citrate. The tracer method measures tagged nutrients. The binding of calcium ions in the blood or urine does not affect outcome. Of the six methods, Dr. Heaney finds the least fault with the tracer method. When the test nutrient is properly tagged, he has found tracer methods to be "highly sensitive and reproducible, and depending upon the tracer used, [they] can be very quick and inexpensive." It is important to recognize that bioavailability of pure salts is not the same as bioavailability of the pharmaceutical formulations. In Dr. Heaney's experience "the pure salt is often somewhat better absorbed than the pharmaceutical mixture."

In evaluating human studies of a preparation's bioavailability, Dr. Heaney says that subject factors (i.e., age, mucosal mass, intestinal transit time, and nutritional status) can affect outcome. People with calcium deficiencies are more likely to absorb supplemental calcium than those with diets high in calcium. He also notes that absorption increases when taking calcium supplements in divided doses throughout the day "by as much as 80% relative to the same intake ingested as a single bolus." Other factors that may increase calcium absorption, according to the National Nutrition Foods Association, include exercise, increase in vitamin D levels, intestinal acid, ingestion with a meal, acidic amino acids, and increased metabolic needs caused by growth, pregnancy, and lactation.

Although literature about calcium supplements is primarily aimed at supporting bone health, calcium and the salts have other positive effects. Unabsorbed calcium reduces the risk of kidney stones and colon cancer by combining with oxalic acid, bile acids, and other harmful substances left over from digestion. Carbonate, lactate and citrate salts may help neutralize a diet that is high in acid-forming foods. Dr. Heaney says that phosphate salts of calcium may be especially helpful for the elderly. Phosphorus is the second most abundant mineral, after calcium, in bone. Given the widespread distribution of the mineral in animal proteins, nuts, legumes, grains, and soda drinks, few realize that the elderly, particularly those receiving bone-strengthening drugs and supplemental calcium, may not be getting enough phosphorus. "Without adequate attention to meeting the phosphate requirements for bone building," Dr. Heaney warns, "therapeutic success in these patients may be limited."

Dr. Heaney questions manufacturers' pursuit of enhanced bioavailability for poorly absorbed nutrients like calcium. Focusing on bioavailability, according to Dr. Heaney, "seems inappropriate and misdirected from both cost benefit and nutritional considerations." He sees little point in buying a supplement that is absorbed at 40% efficiency that costs twice as much as one absorbed at 30% efficiency. Dr. Heaney, who is considered an expert on calcium, has not found a calcium supplement that meets manufacturer's claims of superiority that justify significantly higher prices.

While no calcium supplement is notably superior to any other, some supplements contain harmful metals. A study by E. A. Ross, N. J. Szabo, and I. R. Tabbett (JAMA, September 20, 2000) reports that some over-the-counter natural and refined calcium carbonate products, as well as bone-meal and dolomite, contain high amounts of lead. The authors recommend using supplements from reputable manufacturers that test for lead content.

Heaney, Robert P. Factors Influencing the Measurement of Bioavailability, taking Calcium as a Model. Journal of Nutrition. 2001;131:1344S-1348S. www.nutrition.org

May, Charles H. V., MD. Microcrystalline Hydroxyapatite. 1996. www.healthierwhey.com

McCormick, Charles C., PhD. Calcium & Osteoporosis--A Weak Link. Cornell Cooperative Extension www.cce.cornell.edu/food/

NNFA. Calcium. No date. www.nnfa.org/services/science/bg_calcium.htm

Ross EA, Szabo NJ, Tebbett IR. Lead content of calcium supplements. JAMA 2000 Sept 20.