What you need to know about calcium

Calcium is billed as the bone-building nutrient. But some experts argue that we should pay more attention to exercise and vitamin D.

Starting on your 51st birthday, current government guidelines say you're supposed to consume 1,200 milligrams (mg) of calcium daily. With advancing years, both men and women begin to experience a decline in the density of bones that makes them weaker and more likely to break. In essence, your bone becomes more porous, and calcium supposedly fills in the holes.

But the amount of calcium adults need continues to be debated. The critics say there's little evidence that high intake has more than a marginal effect on bone density and fracture prevention. They say exercise and reversing vitamin D deficiency are not promoted enough and are more important for bone health. Professor Walter Willett, chair of the nutrition department at the Harvard School of Public Health, is one of the leading lights in the critical camp.

Naturally, the proponents see the evidence quite differently — and they set the government recommendations, so they're hardly a fringe group. They say dozens of studies have shown that high calcium intake builds up bone and prevents fractures. And they cite calcium's other possible benefits, such as modest protection against colon cancer.

So what should you do? For women, 1,200–1,500 mg of calcium daily doesn't seem to have any drawbacks. For men, though, it may. Studies have shown a possible connection between calcium and prostate cancer. Great Britain set its daily calcium recommendation at 700 mg, which Professor Willett believes is probably closer to the amount that men should be consuming.

Perhaps the most important thing you can do is change how you think about calcium. High intake is not the surefire ticket to bone health that it has been made out to be.

Food sources high in calcium

Most of the calcium in the American diet comes from dairy products. Not only do they contain a lot of the mineral, but it's in a form that's easy to digest and absorb. An 8-ounce serving of plain yogurt provides about 400 mg of calcium; an 8-ounce glass of milk, 300 mg; and a slice of cheddar cheese, 200 mg.

Vegetables are another food source, although figuring out how much calcium you're actually getting is tricky. If a vegetable contains oxalic or phytic acid, then the calcium may be poorly absorbed because of the acids. For example, a cup of frozen spinach contains almost as much calcium as a cup of milk, but only a tenth as much is absorbed because of the oxalic acid.

Calcium fortification of everything from orange juice to waffles is making it easier to get a great deal of calcium through diet alone. Breakfast cereals have been fortified for a long time; three-quarters of a cup of the breakfast cereal whole-grain Total contains 1,000 mg.

Problems with calcium supplements

Acid rebound. Calcium carbonate may cause acid rebound: the stomach overcompensates for the high dose of calcium carbonate, which is alkaline, by churning out more acid. For that reason, people with a history of stomach ulcers are advised that they may not tolerate it and may have to switch to calcium citrate.

Constipation. Calcium supplements can have a mild binding effect but by themselves don't usually cause serious constipation. But if you're taking another supplement or medication that binds the stool, the addition of calcium supplements could cause a problem.

Too much calcium. Although it doesn't happen often, some people have taken so much calcium that it causes hypercalcemia, an above-normal level of calcium in the blood. Hypercalcemia may cause nausea, vomiting, confusion, and other neurological symptoms.

Drug interactions. Large doses of calcium interfere with the absorption of a variety of drugs. You should avoid consuming large amounts of calcium — either in food or as a supplement — within 2–4 hours of taking a tetracycline or quinolone antibiotic. After taking alendronate (Fosamax), risedronate (Actonel), or another one of the bisphosphonate drugs for osteoporosis, you should wait at least 30 minutes before consuming a large amount of calcium.

Potential increased health risks. Excessive calcium supplement intake has been associated with a higher risk of kidney and possibly an increased risk of cardiovascular disease and aggressive prostate cancer.

Other ways to keep your bones strong

Exercise. Weight-bearing exercise includes any activity that pits you against gravity: not just lifting weights, but walking, climbing stairs, even dancing. It's tremendously important to bone health and preventing fractures. Your muscles get stronger and more coordinated, which helps prevent falls. There's also a direct effect on bone. Working muscle stimulates bone into becoming stronger. If you want to lower your risk of osteoporosis, a short, brisk walk every day might better serve the purpose than a big calcium pill.

Figure 1: Vitamin D and calcium absorption

Vitamin D. Getting enough vitamin D may be the most important variable in preventing osteoporosis. Vitamin D's main function in the body is to aid calcium absorption. An analysis of data from the Nurses' Health Study found that study participants who consume 500 IU of vitamin D daily are 37% less likely to have broken a hip than women who consume 140 IU. (IU stands for International Units, a measure of biological activity.) Neither total calcium nor milk consumption was associated with a lower risk for hip fracture.

The Institute of Medicine recommends 600 - 800 IU of vitamin D daily. Studies have shown that up to 50% of older Americans don't get enough vitamin D. There are several reasons for this. The vitamin's biologically active form is metabolized when the skin is exposed to the ultraviolet radiation in sunlight. Theoretically, sun exposure can give you all the vitamin D you need. But north of about 40 degrees latitude — the latitude of Philadelphia, Indianapolis, and Denver — the winter sunlight is too weak to produce significant amounts of vitamin D. Even in sunnier climes and times of year, older people tend to spend a lot of time indoors. Moreover, older skin is less effective in making the vitamin even when it's exposed to sunlight. Sunscreens are another problem: they filter out much of the ultraviolet radiation that produces vitamin D.

Theoretically, you could make up for a shortage of sunshine-generated vitamin D with diet. The problem is that precious few foods contain the vitamin. For practical purposes, it's limited to several types of saltwater fish. So decades ago, health officials in many northern countries decided to fortify foods with vitamin D. In the United States, milk — but not other dairy foods — was chosen. An 8-ounce glass of milk is supposed to contain 100 IU, although surveys have shown that the actual amount can be a great deal less.

Vitamin K. Your bones also need vitamin K, which is found in green, leafy vegetables.

Tips for increasing your vitamin D intake

• Eat more swordfish, bluefish, salmon, mackerel, or sardines. But swordfish and some species of mackerel are on the list of fish with high mercury levels, so don't go overboard.
• Take a vitamin D supplement or a multivitamin. Multivitamins are a safety net for many nutrients that might otherwise fall through the cracks. Most brands contain 400 IU of vitamin D.
• Spend more time outdoors. This is a balancing act, because you don't want to risk skin cancer in pursuit of more vitamin D production.

To learn how to prevent and treat osteoporosis through diet, exercise, and medication, buy the Harvard Special Health Report Osteoporosis: A guide to prevention and treatment.