Osteoporosis Accelerated by High Protein Diet

Study shows for the first time how acidic blood robs bones of calcium

November 05, 1999

A change in blood acidity caused by a high-protein diet accelerates osteoporosis by depleting bones of their calcium, say researchers at the University of Rochester. Their study, which appears in today's issue of the American Journal of Physiology, reveals for the first time how bones sacrifice themselves to compensate for the acid-producing foods we eat.

"When we eat, we generate acid," explains David A. Bushinsky, M.D., lead author of the study and professor of Medicine and of Pharmacology and Physiology at Strong Memorial Hospital at the University of Rochester. "These acids are ultimately excreted by the kidneys, but as we age, our kidneys don't function so well. If the kidneys can't keep up with our appetite, the bones step in and absorb the excess acid. That's good in the short term, but in the process the bones surrender calcium, phosphorus, sodium and everything they should be keeping to stay strong."

The process is called metabolic acidosis and it can become a problem for a middle aged or older person whose kidneys are not working as efficiently as those of a younger person's. Not only does the bone trade off its calcium for the blood's acid, but the acid environment hinders osteoblasts, the cells that naturally rebuild damaged bone.

"An older woman with kidney trouble should definitely watch how much protein she eats," says Bushinsky. As much as 30 percent of post-menopausal white women, the group at greatest risk, have osteoporosis. Protein generates more acid than other foods, and the proteins in red meat generate more acid than those in fish or poultry. Vegetable proteins give rise to the least amount of acid.

Though scientists have suspected that high levels of acid in the blood contribute to osteoporosis, this is the first time researchers have been able to confirm that the bones are actually deteriorating. Using a prototype machine called an "ion microprobe," Bushinsky was able to zoom in and identify what made up each layer of a mouse bone. Up until now, researchers had to test the entire bone, and so would easily miss the loss of calcium that occurred only at the surface. The ion microprobe strips away layers of bone like a leaf-blower whisking away the top dry leaves while the wet ones stay on the lawn. Then it's just a matter of collecting the blown fragments and analyzing them.

Bushinsky found that in just seven days, mice experiencing metabolic acidosis had measurable depletion of calcium in their femurs. "But the calcium depletion probably started just days, even hours after the blood became acidic," he explains.

Some medications and diseases can hinder the kidneys' ability to keep the blood pH at healthy levels. The two most common diseases that limit kidney function are hypertension and diabetes.

This study is funded entirely by the National Institutes of Health. The ion microprobe was developed by Riccardo Levi-Setti, Ph.D., of the University of Chicago.

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