High animal protein intake may increase risk of bone loss and fractures in elderly women, UCSF study finds

December 21, 2000

Elderly women who get a much higher intake of their dietary protein from animal products rather than vegetables have an increased risk of bone loss and hip fracture, a University of California, San Francisco study has found, suggesting women may be able to improve bone health by eating more vegetables.

"We should be encouraged to eat more vegetables and realize that our diets play an important role for our bones as we get older," said lead author Deborah Sellmeyer, MD, UCSF assistant professor of medicine and director of the Bone Density Clinic at UCSF Medical Center at Mount Zion. "There are lots of things we can do to improve bone health."

The study will be published in the January issue of the American Journal of Clinical Nutrition.

In the study, researchers gave 1,035 women enrolled in the Study of Osteoporotic Fractures a food frequency questionnaire, asking how much they ate of 64 different kinds of foods. They broke the food down into grams of protein, potassium, salt and other categories. They scrutinized the protein part more carefully, determining how much protein the women were getting from animal products compared with vegetables.

The women, ages 65 to 80, were grouped into three categories: those with a high ratio of animal to vegetable protein, a middle range ratio and low ratio, Sellmeyer explained. Researchers took the ratio and compared it with bone mineral density, bone loss and fractures in a seven year follow up period.

While there was no difference in initial bone mineral density among the groups of women, the high ratio category had three times the rate of bone loss as the women in the low group during the follow up period. The high group also had 3.7 times the rate of hip fractures compared to the low group. This is after researchers adjusted for age, weight, estrogen use, tobacco use, exercise, total calcium intake and total protein intake.

"We adjusted for all the things that could have had an impact on the relationship of high animal protein intake to bone loss and hip fractures," Sellmeyer said. "But we found the relationship was still there."

The most significant possible reason for this link between high animal protein and bone loss and hip fractures is because animal products have a high amount of acid, Sellmeyer said. Too much acid may be detrimental to bone health. While vegetables have some acid, they have much higher levels of base. Base is a bicarbonate, a substance that works to neutralize acid. The body works to achieve a balance between base and acid and gets rid of excess acid through urine.

"Our bodies don't like too much acid so our kidneys help us adjust by excreting the acid in urine," Sellmeyer said. "But as we get older, our kidneys are less and less capable of excreting the acid."

This causes bone-which is built of base and other components-- to step in to neutralize the acid. As a byproduct of this action, the bone dissolves over time-causing it to lose mass and calcium.

"We believe this happens very slowly, over decades," Sellmeyer said. Decreased bone mass makes fractures more likely.

While it appears that increasing vegetable protein intake and decreasing animal protein intake can decrease the risk of bone loss and hip fracture in elderly women, Sellmeyer stressed that the point of the study is not to recommend women give up eating meat or cheese.

"Protein is very important in maintaining strong bones and muscles. We don't want people to stop eating animal protein," she said. "But we do want people to work in more fruits and vegetables into their diets--not only because of the impact it could have on bone health, but also the impact it can have on lowering the risk of heart disease, diabetes and other illnesses. This study is yet another reason to eat more fruits and vegetables."
Other study authors are Katie L. Stone, PhD, specialist in the UCSF School of Medicine; Anthony Sebastian, MD, UCSF professor of medicine and co-director of the General Clinical Research Center, and Steven R. Cummings, MD, UCSF professor of medicine.

The National Institutes of Health funded this study.

University of California - San Francisco

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