1. The amount of lean tissue in the body can be assessed by measuring total body nitrogen, total body potassium or fat-free mass. To compare these techniques we have measured total body nitrogen, total body potassium and fat-free mass in 91 healthy subjects (62 males, 29 females).
2. Total body nitrogen in the women and civilian men agreed closely with the few values reported previously and was closely related to total body potassium and fat-free mass.
3. The simplest estimate of total body nitrogen in a subject whose body content has not been measured is the mean value for healthy people of the same sex. The standard deviation of individual values about this mean is 253 g. The precision of the estimate can be improved considerably by predicting body nitrogen from fat-free mass (156 g) and somewhat more by predicting it from body potassium (115 g). The error of measuring total body nitrogen directly is approximately 76 g.
4. When an individual's total body potassium is measured in a search for potassium depletion, the observed value must be compared with the value expected if the subject were healthy. The standard deviation of the healthy values about the group means is 408 mmol. The precision of the estimate can be improved by predicting total body potassium from fat-free mass (sd 237 mmol), and rather more by predicting it from total body nitrogen (sd 186 mmol). If gross body composition is normal, measurement of total body nitrogen has little advantage over measurement of fat-free mass by the anthropometric technique.
5. These results suggest that the simpler measure of fat-free mass from body weight and skinfold thickness has a major role in the assessment of total body nitrogen, and thus lean body tissue, in the individual.