Biological and Technical Sources of Variability in Bovine Carcass Lean Tissue Composition II. Biological Variation in Potassium, Nitrogen and Water

The use of whole-body K, N and H2O as quantitative indices of the fat-free body is based on the assumption that each of these constituents is maintained in a relatively constant concentration in the fat-free body. Two further assumptions—that each of these constituents is found predominantly within the lean muscle mass in a relatively constant amount and that amounts found outside this tissue vary in proportion to amounts found within the lean—lead to the conclusion that body K, N and H2O are also indices of lean muscle mass. Much chemical data has been reported emphasizing muscle-to-muscle and tissue-to-tissue differences in potassium content as important sources of error in predicting body composition from 40K. However, it is not necessary for K, N or H2O contents to be the same in each tissue of the fat-free body, or each muscle of the lean muscle mass, for the whole-body constituents to be valuable measures of the fat-free body or of lean muscle mass.