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Each day the human skeleton, that nimble framework of mineralized protein that holds us together and propels us into motion, is broken down and put back together again. The process, a complex, two-stage cycle by which new bone is created as old bone is removed, is called remodeling. When it works as it should, this dance of creation and destruction achieves a rough equilibrium, and our bones maintain their strength and integrity. When destruction outpaces creation, however, bones become brittle and fractures more common.

High-Impact Bone Health.Because this latter scenario has long been associated with aging, younger, active individuals -- or their physicians -- have seldom given bone problems a passing thought. This is changing, thanks to Pamela Hinton, an associate professor of nutritional sciences at MU, who co-authored something of a wake-up call in the February 2008 issue of the journal Clinical and Experimental Metabolism.

Hinton's study compared the bone densities of elite male runners to those of similarly accomplished cyclists. The bike riders were all relatively youthful, energetic men whose dedication to high-energy, low-impact exercise had, by all appearances, made them enviable physical specimens. Yet Hinton's findings told a different story, at least so far as subjects' bones were concerned. The cyclists, fit though they might be, consistently showed signs of osteopenia, a low bone mineral density condition that is a precursor to osteoporosis. It could double their risk of fracture.

"Unfortunately, some individuals who believe they are doing everything right in terms of their health might be surprised and upset by our finding," says Hinton. "On an individual level, I think the men in the study were surprised to learn they had osteopenia. They were somewhat disappointed and didn't know what to do about it. If anything, this study sends the message that osteoporosis doesn't only affect women. It's important for men to be aware that it's a disease they may have to someday deal with, in particular, if they don't get enough weight-bearing activity or are at risk for another reason."

Bone remodeling in its most basic form involves the work of two cell types, osteoclasts and osteoblasts. Osteoclasts are large, multi-nucleated cells that set in motion a process called "resorption," a secreting of enzymes and acids that dissolve bone mineral on a molecular level. As bone is dissolved, a second, smaller cell, called an osteoblast, simultaneously fills in what was lost with new bone tissue.

Around age 35, this process of remodeling can become imbalanced. By 50, the imbalance can lead to varying degrees of osteoporosis, a skeletal disease characterized by a microscopic breakdown of bone structure. Osteoporosis affects about 44 million U.S. women and men and leads to about 1.5 million fractures per year, according to the Washington, D.C.-based National Osteoporosis Foundation.

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