Body composition of fasting obese patients measured by in vivo neutron activation analysis and isotopic dilution.

The masses of fat, water, protein and minerals in ten obese patients (seven women and three men) have been measured, by in vivo neutron activation analysis and isotopic dilution, before and after four weeks of starvation. Mass of skeletal muscle was estimated from measured total body potassium (TBK) and nitrogen (TBN). Before starvation the patients weighed, on average, 56.5 kg more than predicted from sex, age and height. The composition of their excess tissue was consistent with that of adipose tissue. During starvation the patients lost, on average, 12.1 kg of tissue containing more water and potassium than adipose tissue. Regression analysis of the measured changes in potassium and protein suggested that approximately 230 mmol of potassium was lost independently of protein, the remainder being lost with 'non-muscle' fat-free tissue. Because some potassium was lost without protein, the change in muscle mass calculated from the changes in TBK and TBN overestimated the true mass of muscle lost. After starvation the patients still had 44.5 kg of excess tissue. Their composition was consistent with an excess of adipose tissue and a deficit of potassium.