Diets may lead to bone loss

Men and women who lose weight by cutting calories also may be losing bone density, but weight loss through exercise does not seem to produce the same effect, according to a report in the December 11/25 issue of Archives of Internal Medicine, one of the JAMA/Archives journals.

Most U.S. adults are either overweight or obese, according to background information in the article. The primary treatment for these conditions is lifestyle modification, including exercise and low-calorie diets. However, decreasing body weight is associated with decreased bone mineral density, which increases the risk for osteoporosis (weakening of the bones) and hip fractures in older men and women.

Dennis T. Villareal, M.D., Washington University School of Medicine, St. Louis, and colleagues studied the effects of weight loss on bone loss in 48 adults (30 women and 18 men, with an average age of 57). Nineteen were assigned to follow a calorie-restricted diet (to decrease energy intake by 16 percent for three months, then by 20 percent for nine months), 19 to eat the same number of calories and begin an exercise program (to maintain energy intake, but increase energy expenditure by 16 percent for three months and 20 percent for nine months) and 10 to receive information on healthy lifestyles only when requested. All participants were weighed at the beginning of the study and again after one, three, six, nine and 12 months. Bone mineral density was measured every three months using a technique known as dual-energy x-ray absorptiometry. At the start of the study and after six months and 12 months, blood samples were taken to test for hormones and chemical markers that show whether bone tissue is being absorbed and regenerated.

Forty-six of the participants completed the study. After one year, those in the calorie restriction group lost an average of 8.2 kilograms or 18.1 pounds, those in the exercise intervention group lost 6.7 kilograms or 14.8 pounds and those in the healthy lifestyle group maintained their weight. Individuals in the calorie-restriction group also lost an average of 2.2 percent of their bone density in the lower spine, 2.2 percent at the hip and 2.1 percent at the top end of the femur—all high-risk fracture sites. There were no significant changes in bone mineral density in the exercise or healthy lifestyle groups. In both weight-loss groups, bone turnover—which occurs when old bone is broken down—increased, as indicated by the markers of bone turnover in the blood.

"A common explanation given for the bone loss induced by weight loss is reduction in mechanical stress on the weight-bearing skeleton (i.e., hip and spine)," the authors write. "Accordingly, the preservation of bone mineral density in the exercise group could be mediated through exercise-induced bone loading." In addition, although both weight-loss groups experienced an increase in bone turnover, this was only detrimental to the calorie restriction group. Muscles pulling on bones during exercise is thought to produce strains in the skeleton that stimulate new bone production. "Our results are consistent with an osteoprotective effect of exercise-induced mechanical strain on the skeleton and consequent increase in bone turnover," the authors continue.

"These findings have important implications in designing an appropriate weight-loss therapy program in middle-aged adults, particularly in the subset of patients who may already be at increased risk for bone fracture," they conclude.