Determination of water and electrolytes in muscle biopsies in the nutritional assessment of clinical disorders.

Nutritional inadequacies profoundly affect the content and distribution of water and electrolytes in muscle tissue. However, most changes in tissue water and electrolytes are nonspecific and reflect the degree of sickness; they cannot be used for evaluation of malnutrition, except to indicate whether there is a loss or excess of intracellular ions, such as potassium and magnesium. In patients without disturbances in potassium metabolism, muscle cell potassium and protein are strongly correlated, but this relationship does not hold true in patients with potassium depletion. Accumulation and loss of potassium occur in parallel with changes in muscle glycogen, independent of other muscle constituents, eg, magnesium. Potassium depletion and excess, as well as changes in muscle glycogen, must be considered when evaluating the relation of lean body mass and total body protein content to total body potassium.