An improved calibration for the in vivo determination of body nitrogen, hydrogen, and fat.

Additional investigation of the authors' original technique for measuring total body nitrogen by prompt gamma neutron activation has demonstrated the need for certain changes in the calibration procedures in order to apply the method to studies of patients with abnormal metabolism. In the present technique, total body nitrogen, hydrogen, and fat were derived, simultaneously, from data obtained by neutron capture gamma-ray analysis combined with the measurements of body weight, total body water, and total body calcium. In this improved calibration technique total body nitrogen is more accurately measured, not only in normal subjects, but also in obese subjects and in patients with marked changes in hydration, such as cancer patients. The fat values calculated do not rely on a fixed relationship of total body water or total body potassium with lean body mass as in the previous studies, but are calculated as the difference between body weight and the sum of body water, protein and bone mineral ash. This improved technique has been applied to the study of three groups of subjects, the general population with a normal weight distribution and two extremes represented by obese and cancer patients.