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.