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Am Fam Physician. 2007;76(1):128

Background: Obesity is a significant problem in the United States. However, body weight also has an impact on bone mineral density. Some studies have suggested that weight loss through caloric reduction may be associated with a loss of bone mineral density, but these studies did not evaluate specific bones at high risk of osteoporotic fractures. Exercise is a recommended intervention for the prevention and treatment of osteoporosis, as well as for reducing weight. No studies have assessed the effects of an exercise-based weight-loss program on bone mineral density at sites at high risk of osteoporotic fractures. Villareal and associates compared the impact on bone mineral density of caloric restriction versus increased exercise-based weight-loss programs in nonobese adults.

The Study: The study recruited adults 50 to 60 years of age who were overweight but not obese (body mass index [BMI] of 23.5 to 29.9 kg per m2). Other criteria for participation included having a stable weight for three months and being a nonsmoker.

Participants were randomized into three groups: caloric restriction, exercise, or healthy lifestyle (the control group). The caloric restriction group reduced their energy intake by 16 percent for three months and then by 20 percent for the next nine months. The exercise group maintained their energy intake but increased their energy expenditure by 16 percent during the first three months and then by 20 percent for the next nine months. Participants in the healthy lifestyle group received a supplement, as well as information on healthy eating if they requested it. Body weight was measured at one, three, six, nine, and 12 months. Bone mineral density was measured using dual energy x-ray absorptiometry (DEXA); sites included total body, lumbar spine, and proximal femur. Biomarkers and hormone levels were measured at baseline and again at six and 12 months. Diet and exercise data were collected on all participants.

Results: There were 48 participants in the study. The mean age was 57 years, and the mean BMI was 27 kg per m2. There was no significant difference in the amount of weight loss between the caloric restriction and exercise groups, but the lifestyle group had a lower decrease (1.2 percent) in body weight. The caloric restriction group had a significant decrease in bone mineral densities at the total hip, femur, and lumbar spine compared with the lifestyle group. The exercise group had no bone mineral loss. Loss of body weight was correlated with loss of bone mineral density in the caloric restriction group. Bone turnover markers and hormone levels were the same in the caloric restriction and exercise groups.

Conclusions: Weight loss through caloric restriction was associated with a decrease in bone mineral density at clinically important sites; this did not occur with weight loss through exercise. The authors suggest that exercise programs should be a part of any weight-loss strategy to offset the negative impact of caloric restriction on bone mineral density.

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