Because body potassium is proportional to body protein, measurement of total body potassium gives an estimate of lean body mass. However, to determine the lean body mass from the total body potassium, it must be assumed that the potassium concentration in the intracellular water is constant and that the ratio of total body potassium to lean body mass decreases linearly over time in both males and females. Because neither of these assumptions is strictly correct, the total body potassium does not provide an accurate assessment of lean body mass unless correction is made for sex and age. The most direct method for assessing lean body mass is to measure total body nitrogen using neutron activation analysis. A beam of fast neutrons is generated and captured by target atoms, leading to the creation of unstable isotopes.
Energy is measured as the isotopes emit gamma rays and decay to a stable form.
Spectrographic analysis identifies the element on the basis of the spectral energy, and the energy intensity corresponds to the element's abundance. Neutron activation analysis of samples has been used to assess changes in total body nitrogen in patients undergoing aortic repair, in those with gastrointestinal disease receiving parenteral nutrition, and in those with septic shock. The use of neutron activation is limited by its cost and requirement for skilled technicians. In practice, physicians are usually able to determine whether the patient is malnourished or at significant risk for malnutrition by using information gained by a detailed medical history and a careful physical examination.