Calcium - Protecting Your Bones: Osteoporosis

Content provided by the Faculty of the Harvard Medical School

Calcium

If you don't supply your body with the calcium it needs, it will respond by raiding calcium stores in your bones, weakening them. Consuming adequate amounts of calcium limits these unhealthy withdrawals and safeguards your bones. Calcium alone can't protect you from bone losses caused by certain medications or diseases, a sedentary lifestyle, smoking, alcoholism, or a lack of estrogen. But getting enough calcium plays an essential role in the health and strength of your bones.

Most people already know this, but that knowledge doesn't always translate into action. Only 45% of adult men and 22% of adult women get the recommended amount of calcium, and the numbers are even worse for youngsters. Among 9- to 17-year-olds, just 25% of boys and 10% of girls get enough calcium.

Since your body's calcium demands shift with age, you should adjust your calcium intake as necessary. How much calcium is enough? That question is at the center of a vigorous debate. The National Academy of Sciences has established recommended intakes by age (see Table 1). These guidelines are supported by the National Institutes of Health and the National Osteoporosis Foundation. However, some experts believe that the recommendations may be too high, particularly for men.

A report from the Harvard School of Public Health points out that the average daily calcium intake in countries such as India and Japan is as low as 300 milligrams (mg), and that fracture rates are low. But it goes on to say that the low fracture rates may be the result of differences in other bone health factors, such as the amount of exercise or exposure to sunlight (which spurs production of vitamin D). Critics of the current calcium recommendations also say that there's little evidence that high intake has more than a marginal effect on bone density and fracture prevention. And they cite Harvard studies of male health professionals and female nurses that found that the participants who drank one glass of milk or less a week weren't any more likely to break a hip or forearm than those who drank more milk.

What's the drawback of getting high amounts of calcium? For women, there doesn't seem to be any disadvantage to getting 1,000–1,300 mg of calcium a day. The story may be different for men, though. Some studies have found a connection between calcium and prostate cancer. The Health Professionals Follow-up Study found that men who drank two or more glasses of milk a day were almost twice as likely to develop advanced prostate cancer as those who didn't drink milk at all. At first, researchers thought the connection was due to the saturated fat in dairy products. But the results from this study, as well as more careful analyses of other data, suggest that calcium is the culprit. Great Britain set its daily calcium recommendation at 700 mg, which Harvard researcher and professor Walter Willett believes is probably closer to the amount that men should be consuming.

But proponents of the current calcium recommendations say that dozens of studies have shown that high calcium intake builds bone and prevents fractures. According to a report from the National Institutes of Health, randomized clinical trials have found that adequate calcium intake — from either diet or supplements — increases bone mineral density in the spine and reduces vertebral and other fractures. One study found that calcium supplements lowered the risk for fracture by 25%–70% in women who had already gone through menopause.

Another study of 389 men and women ages 65 or older found that the participants who took 500 mg of calcium and 700 IU (International Units) of vitamin D daily lost significantly less bone over the course of three years than their counterparts who did not receive additional calcium and vitamin D. In some cases, men taking the combination actually increased their bone density. In addition, the group taking supplements had fewer nonvertebral fractures. This is in keeping with a study of 3,000 elderly French women in nursing homes, which found that among women taking 1,200 mg of calcium and 800 IU of vitamin D, hip fractures dropped by 43% and nonvertebral fractures decreased by 32%.

Calcium may also boost the power of osteoporosis treatments, such as hormone therapy. An analysis of studies of women on hormone therapy found that bone mineral density was significantly greater in those who got nearly 1,200 mg of calcium a day as compared with those who got about 600 mg a day.

Proponents of the higher calcium recommendations also cite calcium's other possible benefits, such as modest protection against colon cancer.

More information on the relationship between calcium and fractures is on the way, and it may help settle this debate. As part of the ongoing Women's Health Initiative, nearly 40,000 women are participating in a study comparing the effect of calcium and vitamin D supplements against that of a placebo. Results, which are due in 2005–2006, should shed more light on the matter.

In the meantime, calcium continues to be an important tool in the fight against osteoporosis. While scientists debate the optimal amount and sources of calcium (see "Sources of Calcium"), it's clear that calcium plays a crucial role in bone health and that you should not neglect it in your diet.

Last updated:	January 23, 2007