Effect of acute fasting in the rat on water and electrolyte content of serum and muscle and on total body composition

Abstract

Thirteen acutely fasted male adult rats were compared with six fed controls in respect to serum electrolyte levels (7 rats), total carcass (7 rats) and skeletal muscle composition (13 rats). Of the serum electrolyte levels only potassium was mildly depressed; there was no evidence of hypochloremic alkalosis or of hyponatremia. The loss of weight ranged between 25 and 37% of the initial value (mean = 32%). Total body fat was decreased most (mean = –97%). Lean body mass was relatively increased from 90 to 99% of the final body weight, but was absolutely decreased by 25% of the initial value; the ratio of water to fat-free solids was unchanged. In the total fat-free carcass, chloride was increased more than sodium, potassium was decreased relative to the total wet tissue, to total water and to nitrogen. In fat-free skeletal muscle, chloride and the chloride/sodium ratio only were increased. Either the extracellular fluid taken as the chloride space was expanded or chloride was sequestered or both. On the former assumption, intracellular water, sodium and potassium concentrations were unchanged.