Measurements of total exchangeable potassium and total exchangeable sodium, total body water and extracellular fluid volumes were made in 69 patients aged 30–90+ years, using multiple isotope techniques. The patients were not suffering from any chronic disorder or having potassium or diuretic therapy and were considered to be having a normal hospital diet. The study showed a direct correlation between potassium and age, in that total exchangeable potassium decreased as age increased. The same relation was obtained when total exchangeable potassium was related to dry body weight and fat-free body water to age. Although total exchangeable sodium diminished with increasing age, the ratio of total exchangeable potassium was approximately 1 at 30 years of age rising to approximately 2 at 90+ years of age.

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