Body mass index and disease duration are predictors of disturbed bone turnover in anorexia nervosa. A case-control study.

Abstract

To characterize the influence of body mass index (BMI), body composition, hormonal factors and disease duration on bone metabolism markers in anorexia nervosa (AN) patients. Case-control study with 51 AN patients and 51 controls matched for age, sex and body height. Assessment of anthropometric and bioimpedance data, and of biochemical serum parameters and of oral contraceptives use. Patients had a lower BMI, lower fat mass, lower fat-free mass and lower muscle mass (MM) compared to the controls (all P values <0.001). Moreover, serum levels of osteocalcin (bone formation marker) were lower while serum C-telopeptide concentrations (CTx; bone resorption marker) and the CTx/osteocalcin ratio (an index reflecting the balance of bone remodeling) were higher in the AN patients compared to the controls (P<0.01-0.001). In addition, patients had enhanced serum calcium and cortisol levels and reduced serum levels of thyroid hormones, insulin, and leptin (P values <0.05-0.001). Mean disease duration was 91+/-13 months. In a multiple regression analysis, BMI (P<0.0001), MM (P<0.005) and duration of the disease (P<0.005) were independent predictors of the CTx/osteocalcin ratio in the AN patients. There was a nonlinear association between BMI and the CTx/osteocalcin ratio of r=-0.72 (P<0.001) in the AN patients, but only a weak relation of r=-0.27 (P<0.05) between these parameters in the control subjects. Use of oral contraceptives had no effect on the CTx/osteocalcin ratio, neither in AN patients nor in controls. Data indicate an uncoupling of bone formation and bone resorption in AN, which is primarily the result of a low BMI and influenced by the duration of the disease.