Reconsidering Calcium
(Page 2 of 4)
December 2005/January 2006
By Walter C. Willett, M.D.
Real Calcium Needs
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About 99 percent of your bodys calcium is locked in bone. The rest is dissolved in your blood and the fluid inside and outside cells, where it helps conduct nerve impulses, regulate your heartbeat and control other cell functions. Although you would never know it from the milk-mustache advertisements, no one really knows the healthiest, safest amount of dietary calcium. Different scientific approaches yield different estimates.
Daily calcium requirements are traditionally calculated using a balance study. This is a relatively straightforward test you assemble a group of volunteers, put them on a diet of supplements containing different amounts of calcium for a few days or a few weeks, then measure the amount of calcium they excrete. Balance studies show that about 550 milligrams of calcium a day is an optimal level for the mythical average adult. Another route to estimate daily calcium requirements is called the maximal retention study. This approach, which was also used to help set the current recommendations, tries to determine the highest amount of calcium that the body (mainly the bones) can grab and hold on to. Yet another piece of evidence comes from measurements of bone density using X-rays before and after a year or so of calcium supplementation. All of these types of studies were used by the expert panel that set the current target recommendations for calcium intake. What these short-term studies fail to capture is the bodys remarkable capacity to adapt. A unique study of Scandinavian prisoners, all men, showed that their bodies were still adapting after several years on a lower-calcium diet (500 milligrams a day), mainly by excreting less calcium and using calcium more efficiently. In real life, broken bones are a better test of desirable calcium levels than the short-term flow of calcium in and out of the body or measurements of bone density. Studies comparing people who have broken their hips or wrists because of osteoporosis with people who havent broken bones have yielded mixed results. More importantly, the results from seven studies done in the United States, England and Sweden that followed large groups of people for long periods didnt show any important reduction in risk of broken bones with increased calcium intake.
Why Not Drink More Milk?
If no one really knows the best daily calcium target, then why not play it safe and boost your calcium by drinking three glasses of milk a day? Here are a few good reasons:
Lactose intolerance. All babies are born with the ability to digest milk. Some people, especially those of northern European ancestry, keep this trait for life. Most children, though, gradually lose this ability as their bodies stop making an enzyme called lactase that breaks down milk sugar (lactose). In fact, only about a quarter of the worlds adults can fully digest milk. In the United States, as many as 50 million adults arent equipped to digest milk. Half of Hispanic-Americans, 75 percent of African-Americans and more than 90 percent of Asian-Americans cant tolerate a lot of lactose. For them, drinking a glass of milk can have unpleasant consequences, such as nausea, bloating, cramps and diarrhea.
