1. The excretion of electrolytes was studied in six obese subjects before, during and after a 7 day fast.

2. Despite a constant sodium intake throughout the study, there was a marked but transient increase in sodium excretion during the fast, and a sharp reduction in sodium excretion on resuming the carbohydrate diet.

3. Fasting was associated with an increase in the excretion not only of sodium but also of potassium, magnesium, calcium, ammonium, titratable acid, chloride, phosphate, sulphate and organic acids. Refeeding was associated with a prompt reduction in the excretion of all ions other than chloride.

4. The patterns of electrolyte excretion indicated that a major factor responsible for the increased excretion of sodium on fasting and for the retention of sodium on refeeding was a failure of ammonium excretion to keep pace with the changing output of organic acid.

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