Define measuring body composition- Total body potassium?

It applies the' concept that body cells contain a constant amount of potassium and that its measurement is therefore an indicator of lean body mass. The body potassium can be measured by injecting a radioisotope, K42, allowing time for its equilibration within the cells, and then withdrawing samples for analysis. Another technique involves the measurement of the naturally

Occurring K40 in the body with a low level whole body scintillation counter. Since K40 accounts for 0.012 percent of naturally-occurring potassium, the total Body potassium can be calculated.