Do meat and dairy harm aging bones?

With advancing age, our bodies experience an inexorable loss of bone. Two major studies in elderly populations now underscore the importance of dietary protein in this structural erosion, which can lead to osteoporosis. The reports come to sharply different conclusions, however, about the effect of animal protein on the rate of bone loss.

In one study, diets rich in animal protein correlated with greater bone loss

and fracture risk. In the other, animal protein appeared to protect bone.

Such contradictions, "which are not very satisfying," often emerge from observational studies where researchers record the natural habits and health of people, notes Robert P. Heaney of Creighton University Medical Center in Omaha, Neb. "That's not a criticism of such studies," he says, just a reminder of the uncontrollable variables rife in populations.

However, he notes, the new studies are important because they're in the vanguard of efforts to understand protein's impact on age-related bone loss. This condition underlies a growing U.S. epidemic of hip fractures. Treating them costs more than $10 billion annually.

The first of the new analyses computed bone loss over 7 years in some 750 elderly women, all taking part in a larger osteoporosis study that's been running since 1986 at four U.S. sites.

Women eating the most animal protein--roughly four times their vegetable-protein intake--lost 0.8 percent of the bone in their hips annually, notes endocrinologist Deborah E. Sellmeyer, director of the University of California, San Francisco-Mount Zion Osteoporosis Center. This is four times the rate of bone loss in participants who ate about equal parts animal and vegetable protein.

The risk of hip fractures in women eating the highest proportion of animal protein was 3.7 times that in women regularly downing equal quantities of animal and vegetable protein, the team reports in the January AMERICAN JOURNAL OF CLINICAL NUTRITION.

The findings make sense, Sellmeyer argues, because digestion of animal protein unleashes large amounts of acid that the kidneys must excrete. Previous Studies showed that when kidney function wanes with age and calcium intake drops, the body draws upon skeletal calcium to buffer the resulting buildup of acid, thus eroding bone. Most of the women in this study consumed only about half of the 1,500 milligrams of daily calcium recommended for people their age.

In a younger person whose kidney function is robust and calcium intake high, diets rich in animal products can help build bone, Sellmeyer notes.

In the recent study, the women eating the least animal protein also averaged only 70 percent as much total protein as did those eating the most animal protein.

Data from 600 senior citizens in the long-running Framingham (Mass.) Osteoporosis Study paint a picture quite different from Sellmeyer's results. Over a 4-year period, Marian T. Hannan of Harvard Medical School in Boston and her team measured changes in bone density among the elderly men and women.

Participants eating relatively little protein--from any source--lost about 1 percent of their hip and spine bone annually, whereas people eating the most protein held bone losses to about one-quarter of that. The scientists report their findings in the December 2000 JOURNAL OF BONE AND MINERAL RESEARCH.

In this study, the ratio of animal to vegetable protein was high and differed little among participants. Still, animal protein didn't seem deleterious. In several sites especially vulnerable in osteoporosis, seniors routinely downing the most animal protein showed bone loss at only about half the rate of those eating the least animal protein.

Two years ago, Ronald G. Munger of Utah State University in Logan and his colleagues reported similar evidence for a protective effect of animal protein in their study of hip fractures in Iowa women. But Hannan and Munger both say that ample consumption of meat and dairy products might also be a marker for a well-rounded diet that's good for bones.

One unambiguous take-home message from all these studies, Sellmeyer says, is that elderly people "need to maintain a good protein intake."