Clinicians treating obesity may hesitate to use fast weight loss via very-low energy diets due to perceived adverse effects on musculoskeletal integrity relative to effects of slow weight loss, but there is no direct evidence for this. We compared the long-term effects of fast versus slow weight loss on body composition and muscle strength in the randomized controlled TEMPO Diet Trial (ANZCTRN12612000651886).
101 post-menopausal women (BMI: 34.5±2.5 (SD)kg/m2, age: 57.5±4.2years) were randomized to either 4 months of FAST weight loss (60-69% energy restriction) followed by 8 months of slow weight loss (24-33% energy restriction), or 12 months of SLOW weight loss. Both diets had a prescribed protein intake of 1g/kg body weight per day, and physical activity was encouraged but not supervised. Body weight, fat mass, lean mass and bone mineral density (BMD) at the hip (DXA) and muscle strength (hand dynamometry) were measured at baseline and 12 months.
The FAST group lost more weight and fat mass than the SLOW group (FAST: 15.0±6.7kg, 11.1±5.6kg of fat mass; SLOW: 8.9±7.1kg, 6.1±5.5kg of fat mass; P<0.001 and P<0.05). Compared to baseline, both groups lost lean mass, with no significant difference lost between groups (FAST: 3.4±1.7kg, SLOW: 2.4±2.9kg). Muscle strength decreased in both groups at 12 months compared to baseline (FAST: 4.6±9.5kg, SLOW: 3.3±9.3kg, P<0.01), and BMD decreased from baseline in the FAST (0.03±0.03g/cm2, P<0.01), but not SLOW group (0.01±0.03g/cm2), with no significant difference in muscle strength or BMD between groups.
These findings suggest that when protein intake is adequate there is no greater adverse effect of fast weight loss relative to that of slow weight loss on lean mass or muscle strength, despite fast weight loss inducing a 1.7-1.8-fold greater weight and fat loss over 12 months. However, there may be a greater reduction in BMD during fast weight loss.