Calcium in the Body: Almost all of the calcium in the body is in the bones. There is a tiny amount in the blood stream which is responsible for muscle contraction, maintenance of the heartbeat, and transmission of nerve impulses and other functions. Hormones control the amount of calcium in the blood. Everyone constantly loses calcium through urine, sweat, and feces, and it is renewed with calcium from the bones. In the process, the body constantly breaks down and rebuilds bones. Ultimately, the body's calcium is replaced by calcium from food.

Reducing Calcium Loss: Since the 1920's researchers have known that diets that are high in protein, especially animal protein, cause calcium to be lost through the urine[1]. In nations with high rates of osteoporosis, protein intake is generally high~usually more than twice the U.S. Recommended Daily Allowance. Vegetarians have lower rates of osteoporosis than meat eaters. This may be due to the lower protein intake of vegetarians. Different types of protein also affect this loss. Meats are overly high in protein and are high in a particular kind of protein building block, called sulfur-containing amino acids. These cause increased calcium loss[2]. Caffeine and sodium also increase the rate at which calcium is lost through urine. Alcohol inhibits calcium absorption and may also be toxic to bone[3]. Vitamin D, copper, zinc, manganese, fluorine, and boron are all essential for good bone formation, and weight-bearing exercise also increases bone mass and helps to prevent osteoporosis[4]. Boron appears to help stop the loss of calcium. The best way to get boron is through fruits, vegetables, and beans.

The Need for Calcium: Throughout life, people's calcium needs change. Until about age 35,people consume more calcium that their bodies lose. But around age 45, the body begins to slip into "negative calcium balance"~slowly the body loses more calcium than it takes in. As shown above, how rapidly calcium is lost depends, in part, on how much protein is in the diet, and the kind of protein it is. The loss of too much calcium can lead to "soft bones," or osteoporosis.

Fighting Bone Loss: Most studies have shown that high doses of calcium do not slow bone loss. In fact, many populations with high intakes of calcium also have high rates of osteoporosis[5], probably because their high protein intake causes significant calcium loss. Some African cultures consume no dairy products and typically get only 175 to 475 milligrams of calcium per day (800mg is the U.S. RDA), but they have low rates of osteoporosis. Rates of hip racture among different populations is one way researchers measure the prevalence of osteoporosis. One such study of ten nations revealed that as calcium intake increased, so did the number of hip fractures. Such studies have also led researchers to believe that exercise and other factors have more to do with preventing osteoporosis than calcium intake does.

Absorbing Calcium: The body carefully regulates its calcium absorption. The average person absorbs 30 to 70 percent of the calcium she or he eats, but the more calcium taken in, the less the body will absorb. This is to protect the body from overdosing on calcium. At the U.S. RDA of 800mg, the body may absorb as little as 15 percent of the total amount. This may be one reason that high calcium intake does not generally prevent bone loss. While milk is a source of calcium, it certainly is not the ideal way to get your daily dose. Dairy products, with the exception of skim products, are loaded with saturated fat. Fat is directly related to heart disease and cancer. Dairy products are also high in protein. There are other reasons to worry about milk, too. Cows are routinely fed antibiotics. These are then passed directly on to the milk drinkers; antibiotics are detectable in one out of three cartons of milk. Many people are also allergic to milk, and over three-fourths of the world's population is lactose-intolerant, which means their bodies lack the enzymes necessary to digest milk.

Great Sources of Calcium: Dark green vegetables, such as broccoli and collard, mustard, and turnip greens are much better sources of calcium than milk. A single cup of broccoli contains almost a fourth of the U.S. RDA of calcium. Another good source is calcium-fortified orange juice. Beans and tortillas are also good sources of calcium. Some people do need hormone treatments and/or calcium supplementation for varying conditions. The risks and benefits should be discussed with one's doctor.

Conclusion: Calcium is an essential nutrient and is needed for healthy bones particularly during childhood and adolescence. While it is uncertain how much calcium is actually needed, it is certain that diet affects calcium balance. Calcium supplements are not the best way to control osteoporosis for most people. A diet that is modest in protein, complemented by exercise, is much more effective. Green leafy vegetables and beans are good sources of calcium that are also moderate in protein and very low in fat.

References:

1. Hegsted M, Schuette SA, et al. Urinary calcium and calcium balance in young men as affected by level of protein and phosphorus intake. J Nutr 1981;111:553-562.

2. Marsh AG, Sanchez TV, et al. Cortical bone density of adult lacto-ovo- vegetarian and omnivorous women. J Am Diet Asso 1980;76:148-150. 3. Rivlin, RS. Women's health: osteoporosis. Public Health Reports 1986;131-135.

4. Schaafsman F, van Beresteyn ECH, et al. Nutritional aspects of osteoporosis. Wld Rev Nutr Diet 1987;49:121-159.