Bone metabolism in obesity: changes related to severe overweight and dietary weight reduction

Abstract

A non-invasive evaluation of bone metabolism was performed in 44 morbidly obese patients before and after a mean weight loss of 22.4 kg (range 7.9–43.4 kg) after 2 months and a further weight loss of 7.3 kg after 8 months (0.8–20.0 kg). This weight reduction was obtained by a nutritionally adequate very-low-calorie diet. Before treatment the bone mineral content of the distal forearm was increased compared to normals (51.9 U vs 43.7 U, p<0.001). Bone formation was evaluated by serum alkaline phosphatase and serum osteocalcin. Serum alkaline phosphatase was increased (187.8 U/l vs 147.4 U/l, p<0.001) while serum osteocalcin was lower than in the controls (0.67 nmol/l vs 0.98 nmol/l, p<0.01). Bone resorption, as measured by the urinary hydroxyproline/creatinine ratio, was not increased in the obese patients (19.2 molar ratio × 10−3 vs 16.7 molar ratio × 10−3, NS). After 2 months, the bone mineral content had declined by 3.3%. Serum alkaline phosphatase remained unchanged (187.8 U/l vs 186.9 U/l, NS) but serum osteocalcin demonstrated a significant rise (3.94 nmol/l vs 10.53 nmol/l, p<0.001), parallel to changes in the hydroxyproline/creatinine ratio (19.2 molar ratio × 10−3 vs 25.2 molar ratio × 10−3, p<0.001). At 8 months, no further change in the bone mineral content was seen. The hydroxyproline/creatinine ratio did still increase (from 25.8 molar ratio × 10−3 to 30.1 molar ratio × 10−3, p<0.05), while serum alkaline phosphatase and serum osteocalcin remained unchanged. In conclusion, the bone mineral content is increased in morbid obesity. During weight loss obtained by a very-low-calorie diet supplying the recommended amounts of vitamin D and minerals, bone resorption is increased rapidly and bone mass reduced. Owing to linkage of bone remodelling processes, bone formation increases with a longer observation period and the bone mineral content is stabilized.