Changes in total body nitrogen during weight reduction by very-low-calorie diets.

Body composition measurements, including total body nitrogen (TBN) by in vivo neutron activation analysis, were made on 11 female volunteers before and after an 11-wk very-low-calorie diet (VLCD) [1695 kJ (405 kcal) 6.7 g N]. Mean body mass index (BMI) changed from 32.1 to 26.2 kg/m2, corresponding to a mean weight loss of 16.2 +/- 2.4 (SD) kg. The mean loss of TBN was 125 +/- 57 g, equivalent to 781 +/- 356 g protein. The fat-free mass (FFM) component of the weight loss was calculated by two different methods as 23.5% (+/- 3% SEM) and 22.8% (+/- 2.7% SEM), respectively, thereby demonstrating the improved protein sparing of ketogenic VLCDs. FFM loss was not clearly related to BMI.