Abstract
Weight reduction involves loss of both Lean Body Mass (LBM) and adipose tissue. Total Body Potassium (TBK) has been used to estimate LBM, but is an unreliable predictor of Total Body Nitrogen (TBN), and therefore, of body protein in the undernourished. 17 obese adolescents, 12.5-17.5 yrs, weight >150% Ideal Body Weight (IBW), (70-126 kgs) were followed over a 3-month period on a diet of lean meat, fish and poultry (2.5 g protein/kg IBW/d, and 900 kcal/d). Body composition was assessed by estimating Fat Free Body Mass (FFBM) from 4 skinfold thickness, and LBM as derived from 40K, or from TBN by neutron activation.Potassium supplements, 75-100 mEq/d, did not reduce TBK loss, which was biphasic, with significant falls at 2 wk and at 10 wk. There was a significant correlation (p<0.005) between TBK and TBN. There was no relationship between change in TBK and change in body weight nor in LBM as estimated from TBN. The N/K ratio was 12.0-17.4 g/g. A substantial fall in TBK with relative sparing of TBN was demonstrated. The biphasic nature of the TBK loss may explain the late and unexpected deaths reported on Liquid Protein Diets. The wide range of N/K ratios confirm that TBK cannot be used to predict TBN in a changing nutritional state.
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