Loss of total body potassium during rapid weight loss does not depend on the decrease of potassium concentration in muscles. Different methods to evaluate body composition during a low energy diet

OBJECTIVE: The aim of the study was to elucidate whether combustion of skel etal muscle glycogen during a very low calorie diet (VLCD) was associated w ith decreased muscle potassium content. A comparison between different meth ods was also performed to evaluate body composition during a VLCD and a low calorie diet (LCD). DESIGN: Dietary treatment of obese women by VLCD and LCD. Measurements afte r 1 and 2 weeks of VLCD and 6 months of LCD. SUBJECTS: Fifteen perimenopausal obese women aged 46.5+/-1.3 y and 15 of 48 .0+/-0.7 y of age. MEASUREMENTS: Skeletal muscle biopsies under local anaesthesia. Body compos ition measurements by means of deal-energy X-ray absorptiometry (DEXA), and measurements of total body potassium (K-40) and total body nitrogen (TBN). Measurements of electrolytes and glycogen concentration in muscle samples. RESULTS: In the first study (1 week of VLCD) skeletal muscle glycogen decre ased (P<0.01), but muscle potassium increased (P<0.01). Muscle sodium decre ased (P<0.01), while muscle magnesium was unaltered. Body weight decreased by 2.9+/-0.5 kg and K-40 decreased. Fat-free mass (FFM) calculated from K-4 0 and DEXA decreased by 2.7 vs 1.9 kg (P<0.001). Body fat measured with DEX A decreased by 1.1 kg (P < 0.01), but not body fat calculated from K-40. TE N decreased by 0.03+/-0.01 kg (P<0.05) and FFM calculated from TEN by 2.9+/ -0.5 kg (P<0.002). In the second study, 6 months on the LCD resulted in 17. 0+/-2.0 kg weight reduction and this was mainly due to reduced body fat, 14 .0+/-2.0 kg measured with DEXA and from K-40 (P<0.001). The decrease in FFM was slight. CONCLUSION: One week of VLCD resulted in muscle glycogen depletion but incr eased muscle potassium content in spite of decreased total body potassium. FFM contributed to the main part of body weight loss during short periods o f severe energy restriction, but remained unchanged during long-term dietar y treatment. Body fat became mostly responsible for the body weight loss du ring long-term LCD. Calculations of changes of FFM from K-40 and TEN seem t o overestimate the FFM decrease associated with short-term VLCD.