Resting metabolic rate, body composition and thyroid hormones. Short term effects of very low calorie diet.

27 obese women (relative body weight 191 +/- 27%), aged 35 +/- 11, underwent a 15 day very low calorie diet (VLCD: 1337 kJ/day). Mean body weight loss was 6 +/- 1.5 kg. Resting metabolic rate (RMR), measured by indirect calorimetry, showed a significant decrease both in absolute values (from 1793 +/- 225 to 1569 +/- 231 kcal/day; P less than 0.001), and in keal/kg body weight/day (from 17 +/- 2, to 16 +/- 2, P less than 0.001). Serum triiodothyronine (T3) significantly decreased from 1.1 +/- 0.2 to 1 +/- 0.2 microgram/1 (P less than 0.001). Before VLCD the differences between measured RMR and theoretical RMR (RMR-T) were not significant. After VLCD RMR values were significantly (P less than 0.001) lower than RMR-T. It was concluded that fat-free mass loss, combined with periferic thyroxin (T4) reduced monodeiodination in its more active form mainly due to low calorie intake T3 could be responsible for the important reduction observed in the resting metabolic rate.