The lean body mass is composed of the body cell mass (BCM) and extracellular mass (ECM). The former can be estimated from the total exchangeable potassium (which is mostly intracellular), while the latter can be estimated from the total exchangeable sodium (which is mostly extracellular). If one of these is known, then the other can be calculated from the lean body mass.
lean body mass in kilograms =
= (total body water in liters) / 0.73
lean body mass in kilograms =
= (extracellular mass in kilograms) + (body cell mass in kilograms)
where:
• The total body water can be estimated from isotopic dilution of oxygen-18.
• Total exchangeable sodium can be estimated from isotopic dilution of sodium-22.
• Total exchangeable potassium can be measured using isotopic dilution with potassium-42, but it is usually estimated from total body water and sodium-22.
body cell mass in grams =
= 8.33 * (total exchangeable potassium in mEq)
where:
• Shizgal (1976) shows the total exchangeable potassium to range from 1,000 – 4,000 mEq (Figure 1, page 1187). Cohn et al show the body cell mass in adults to range from about 15 to 32 kilograms (Figure 4, page 1188). If the exchangeable potassium is in mEq, then the BCM must be in grams; if in Eq, then it would be in kilograms.
The extracellular mass can be estimated from sodium-22, as reported in Shizgal (1987). This might be the more practical approach.
