Sunday, August 10, 2008

Negative effect of weight loss on bone may persist

NEW YORK (Reuters Health) - When people lose weight, they often lose some bone mass as well. Now a new study suggests that changes in bone metabolism may persist even after the weight loss stops.

In a study of obese adults who followed a very low calorie diet, researchers found that even after the dieters stopped shedding pounds and entered the "weight maintenance" phase, changes in their bone turnover remained.

Bone turnover, or "remodeling," refers to the normal process by which the body constantly breaks down old bone and builds up new tissue to replace it. It's known that even moderate weight loss can cause an imbalance in this process, leading to a dip in bone mass, but it has been unclear whether changes in bone turnover persist after a person's weight stabilizes.

Among dieters in the current study, researchers found that the balance between bone breakdown and bone formation was restored after they stopped losing weight. However, the overall rate of bone turnover increased during weight loss, and it stayed higher once their weight stabilized.

This is a potential concern, says lead researcher Dr. Pamela Hinton, because increased bone turnover can make bones more fragile, even when bone breakdown and formation are balanced.

"Accelerated bone remodeling is thought to be an independent risk factor for bone fracture," explained Hinton, an associate professor of nutritional sciences at the University of Missouri in Columbia.

However, she told Reuters Health, long-term studies are still needed to see how weight loss ultimately affects people's actual bone density and fracture risk.

Bone mass changes slowly, Hinton noted, and it takes six months to a year to see a shift. Moreover, bone fractures may not occur for many years after the weight loss, she said.

Her team's findings, which are to be published in the Journal of the American College of Nutrition, come from a weight-loss study of 37 obese adults age 50 and older.

For three months, the men and women followed a liquid-based diet that provided just 500 calories per day; in the end, they lost an average of 20 percent of their initial weight.

Hinton and her colleagues then followed the dieters through a nine- month maintenance phase where they increased their calorie intake to a level designed to maintain their weight or to continue a slower, gradual weight loss.

Periodically, the researchers measured the study participants' blood levels of certain proteins that serve as markers of bone breakdown and formation. Based on those markers, the average rate of bone remodeling sped up during weight loss and remained higher during the weight-maintenance phase.

It's not clear what people can do to alter their bone turnover rate after weight loss. Hinton said there is some evidence that a high calcium intake -- 1,500 to 1,800 milligrams per day -- lessens bone loss and adverse effects on bone turnover as people are losing weight.

Weight-bearing exercise -- such as jogging, stair-climbing and weightlifting -- helps build bone in people who are not shedding pounds, Hinton noted. However, she said, research has been inconclusive as to whether it can prevent bone loss in people who are losing weight.

SOURCE: Journal of the American College of Nutrition, in press.

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