Total Body Nitrogen and Its Relation to Body Potassium and Fat-Free Mass in Healthy Subjects

Abstract

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