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For healthy bones, fat’s good, dieting’s bad and exercise is complicated

January 6, 2010 |  5:46 pm

Exercise Don’t inhale that bag of chips just yet. Tuesday, we reported research suggesting that, at least for girls, fat can be a very good thing for bone mass. Today, we’ll be more specific. Some fat can be a very good thing for bone mass.

Lead study author Jonathan Tobias elaborates – even as he points out that the line between “fat” and “too much fat” is less than precise.

He and fellow researchers pegged down the basic relationship -- 5 kg of fat (about 11 pounds) correlates with 1 mm extra of bone on the tibia.

That fat's especially important for girls, for whom fat mass seemed to be about 50% more effective at building bone mass than in boys. While muscle mass has a direct, strong relationship to stronger bones, girls have comparatively less of it than boys do, Tobias said. Thus, their bodies might be reacting more to the fat mass to build up bone, in the absence of that muscle mass.

That's not to say an extra 50 kg (about 110 pounds) on your frame is a good idea. "Being obese doesn’t necessarily protect your bones," Tobias said in an interview. 

Dieting for weight loss' sake is not good for your bones either, Tobias said, pointing to earlier research that found that post-menopausal women who went on a diet regimen lost bone density.

Since bone size doesn't change over time (unlike bone density, which depletes as you age), increasing size during the teen years could help maintain bone strength long term.

"Puberty is an important time of bone growth... thickness of the bone is still continuing to develop," Tobias said. It's also the time when girls start dieting to control weight and cease to engage in rigorous physical activity, Tobias said -- a dangerous combination.

And even though exercising is theoretically a good thing, the scientist pointed out, "it's complicated. If you do exercise, you lose fat" -- fat that would otherwise be helping to build bone mass.

"Reducing obesity has lots of health benefits, but if one overdoes it, there is this slight risk," Tobias said. The solution? "Include regular exercise -- and as part of that doing regular exercise, do regular weight-bearing exercise -- running and jumping and so on."

Earlier studies on female athletes have found similar results.

Want to design your own weight-bearing, high-impact workout? First, know your own limits – don’t do anything you think your body can’t handle. Determine your fitness level, and then try one of the workouts (level 1, 2 or 3) suggested by fitness blog Stumptuous. Also, check out these helpful suggestions from the National Osteoporosis Foundation. Some of this information is geared toward an older crowd, but young folk take heed – these exercises work at any age.

-- Amina Khan

Photo credit: Robert Gauthier / Los Angeles Times