1. An estimate of the mass of fat-free tissue in the body can be calculated from body weight and skinfold thickness; this estimate is called the ‘fat-free mass'. Total body potassium and nitrogen are alternative estimates. Factor analysis of data for healthy subjects has defined relationships between the true values of these three quantities and estimated the random component of the variance of each, i.e. the component independent of variations in the mass of fat-free tissue. The results indicated that all three were reliable measures of the mass of fat-free tissue. However, it is not known whether these findings are valid for patients who have lost weight.

2. We have measured the same three quantities in 104 wasted patients with heart disease or disorders of the gastrointestinal tract. The patients’ mean values were significantly less than corresponding values for healthy volunteers. The patients had a mean ratio of total body nitrogen to fat-free mass similar to that of healthy subjects, but lower mean ratios of potassium to fat-free mass and nitrogen. These findings suggest that the potassium content of the patients’ fat-free tissues was abnormally low.

3. Factor analysis of the patients’ data gave relationships between the true values of the three quantities similar to those for healthy subjects; however, total body potassium was 100-300 mmol lower in patients than in healthy subjects with the same fat-free mass or total body nitrogen.

4. Factor analysis also showed that the random components of variance of fat-free mass and total body nitrogen were similar to those in healthy subjects. Therefore, in the patients as in healthy subjects, fat-free mass was as valid a measure of fat-free tissue as the more complex measurement of total body nitrogen. The random component of total body potassium was twice as big as in healthy subjects; however, it formed no greater a proportion of total variance than did the random components of the other two quantities.

5. Total body nitrogen, and hence body protein, could be estimated from measured fat-free mass with a standard error of approximately 136 g (compared with 139 g for healthy individuals), and from total body potassium with a standard error of 129 g (compared with 91 g in healthy subjects).

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