The dairy debate: Does milk build stronger bones?
Bones need calcium. Doctors, dietitians and researchers agree on this point.
Conventional wisdom holds that dairy foods are the best source of calcium, and that American adults need to pump up their dairy intake to get the large amount of calcium their bodies need every day. Not everyone, however, believes the conventional wisdom.
Researchers are even raising questions about whether children need as much milk as guidelines recommend. A review article in the current issue of the journal Pediatrics concludes that there is “scant evidence” that increasing dairy intake is the right way to promote bone health in children.
Lately a small but highly respected band of scientists has been speaking out. They say Americans need less calcium than dietary guidelines recommend, and that drinking cup after cup of milk is not the best way to get it.
On one side are the federal government, the dairy industry and the majority of the nutrition community.
Milk plays a big part in the dietary guidelines recently released by the federal government. Anyone older than 8 is urged to drink three cups of low-fat or fat-free milk or eat an equivalent amount of yogurt or cheese each day. The thinking behind this recommendation is that the calcium in dairy products helps build strong bones and wards off osteoporosis, a disease in which bones become porous and break easily.
On the other side are nutrition researchers from Harvard and Cornell universities who say that when it comes to dairy, the U.S. dietary guidelines have gone too far. They believe that exercise, heredity, hormone levels, smoking, protein intake and intake of vitamins D and K matter more than milk.
The debate over dietary calcium is occurring because of rising concern over osteoporosis, or low bone mass. An estimated 10 million Americans older than 50 -- most of them women -- have osteoporosis, and 34 million are at risk for developing it.
By 2020, one in two Americans older than 50 will be at risk for fractures from osteoporosis or low bone mass, according to the U.S. Surgeon General, who issued a report in October that sounded an alarm on bone health. Bone health is so important that President Bush has declared 2002-11 as the “decade of the bone and joint.”
As for the link between dairy products and osteoporosis, “there’s no solid evidence that merely increasing the amount of milk in your diet will protect you from breaking a hip or wrist or crushing a backbone in later years,” says Walter C. Willett, chairman of the Department of Nutrition at the Harvard School of Public Health.
Willett bases his calcium conclusions on research that he and his team at Harvard have done during the last 25 years. He is one of the principal investigators of the Nurses’ Health Study, which has looked at the diet and health of tens of thousands of nurses since 1980, and of the Health Professionals Follow-Up Study, an all-male study underway since 1986.
When Willett and his colleagues investigated the milk-drinking habits of 72,000 women in the Nurses’ Health Study, they found that milk consumption was not associated with a lower risk of hip fracture, a measure of bone strength. In fact, women who drank milk twice a day were as likely to suffer a bone break as women who drank it once a week.
Likewise, the Health Professionals Follow-Up Study failed to find a relationship between calcium intake and bone fractures in more than 43,000 men. And a 2003 Swedish study of more than 60,000 women, which was published in the journal Bone, found no association between dietary calcium intake and fracture risk.
“We do need some calcium -- it’s essential -- but the question is, how much?” says Willett, author of the 2001 book “Eat, Drink, and Be Healthy.” He believes the body needs 500 to 700 milligrams of calcium daily rather than the 1,000 to 1,500 milligrams a day recommended by the dietary guidelines.
T. Colin Campbell, professor emeritus of nutritional biochemistry at Cornell University, also questions dairy’s place in the dietary guidelines. “I like dairy. I grew up on a farm. But one has to look at the facts,” he says. “Dairy has been considered a health food, and that’s an unfortunate myth.”
Campbell’s views come from observations he and his colleagues made during a series of nutritional studies that began in 1983 and are collectively known as the China Study. In these studies, Campbell found that Asians, who consume far less dietary calcium than Americans, have one-fifth the bone fracture rate of Americans.
“Those countries that use the most cow’s milk and its products also have the highest fracture rates and the worst bone health,” Campbell says. He details the results of his work in a new book called “The China Study.”
In Asian countries, people can get all the calcium their bodies need from plant sources such as leafy green vegetables, Campbell says.
Americans have weak bones not because they drink too little milk but because they drink too much, Campbell says. Animal protein, such as the protein in milk, makes blood and tissues more acidic, and to neutralize this acid, the body pulls calcium, which is a very effective base, from the bones. Because dairy products contain substantial amounts of animal protein, drinking milk actually robs the bones of calcium, he says. The more meat and milk Americans eat, he says, the more calcium they need to consume to process that protein.
That’s ridiculous, osteoporosis researchers say. Although they agree that eating excessive amounts of protein may leach calcium from the bones, they see moderate amounts of protein-rich dairy foods as an excellent way to keep bones strong.
“There is a growing number of studies that have shown an association between higher protein intake and less bone loss,” says Bess Dawson-Hughes, director of the Bone Metabolism Laboratory at Tufts University.
To be sure, many studies do point to a connection between dairy and bone health.
A research review of 138 studies exploring the relationship between bone health and calcium intake, including numerous studies that used dairy products as the calcium source, found overwhelming evidence that lifelong calcium intake is one of the most significant factors for determining risk of an osteoporotic fracture, says Deanna Segrave-Daly, a registered dietitian and spokeswoman for the National Dairy Council.
The review was published in the American Journal of Clinical Nutrition in 2000.
But that same review reported that not all dairy foods boost bone health. “Foods such as milk and yogurt are likely to be beneficial; others, such as cottage cheese, may adversely affect bone health,” the review states. “The high calcium content of processed cheese products may be offset by the high sodium, polyphosphate, and protein contents of these products, which can be expected to increase calcium losses.”
Researchers say there are several possible reasons why milk study results vary so.
Most clinical trials -- studies in which one group of people increases calcium intake and another group does not -- have shown that adding calcium to the diet increases bone density. But most clinical trials last for less than three years, says Diane Feskanich, an investigator for the Nurses’ Health Study. “It could be that bone density does not continue to increase in the long run -- in fact, a study that went on for three years found that after an initial increase in bone density, it did not continue to increase in the third year.”
Observational studies such as the Nurses’ Health Study “are usually run over many years and in this way better suited to determine the long-term effects of high calcium intakes,” Feskanich says.
It is also possible that vitamin D is as important or more important than calcium for maintaining bone density into adulthood.
Researchers don’t understand exactly what role vitamin D plays, but there is a growing belief in the scientific community that the poor state of the nation’s bones has something to do with a widespread shortage of vitamin D. The body gets vitamin D from food and sunlight, and as people cover up to avoid the cancer-causing rays of the sun, they may also send vitamin D levels plummeting. “Most Americans are short on vitamin D,” Willett says.
The rest of the diet may play a part in bone health too, in ways researchers don’t yet understand. Other nutrients in the diet may either help or hinder calcium absorption. “We are overfed, but are we eating the right things?” asks Lori Hoolihan, nutrition research specialist with the Dairy Council of California. “We are a fat nation, but in some ways we are malnourished.”
Even those researchers who agree with the three-glasses-a-day recommendation say there is a limit to what dairy calcium can do. “The gene pool accounts for most of your risk,” Dawson-Hughes says.
During the years in which people build bone mass -- from birth to about age 20 or 25 -- bone density is determined 80% by genetics and only 20% by lifestyle factors such as exercise and diet. Bone loss, which starts to occur after age 25 or so, is determined half by genetics and half by lifestyle choices, Dawson-Hughes says.
Finally, there is an emotional side to this issue. The dairy debate is conducted in large part by two groups who accuse each other of twisting science and letting money or ideology cloud their views: the dairy industry and vegetarians.
The dairy industry accuses the anti-dairy camp of promoting an animal-free diet whether it makes nutritional sense or not. Dairy critics charge the dairy industry with bankrolling pro-dairy research and influencing the government’s dietary recommendations.
One thing both parties agree on is that exercise helps to build bones and maintain bone density throughout life.
