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 skeletal muscle glycogen during a very low calorie diet (VLCD) was associated with decreased muscle potassium content. A comparison between different methods 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 after 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 composition 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 decreased (P<0.01), but muscle potassium increased (P<0.01). Muscle sodium decreased (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-40 and DEXA decreased by 2.7 vs 1.9 kg (P<0.001). Body fat measured with DEXA decreased by 1.1 kg (P < 0.01), but not body fat calculated from K-40. TEN 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 increased muscle potassium content in spite of decreased total body potassium. FFM contributed to the main part of body weight loss during short periods of severe energy restriction, but remained unchanged during long-term dietary treatment. Body fat became mostly responsible for the body weight loss during long-term LCD. Calculations of changes of FFM from K-40 and TEN seem to overestimate the FFM decrease associated with short-term VLCD.