The prescription of intravenous maintenance solutions - although widespread - lacks important data on the optimal sodium and potassium content, which has given rise to an important debate in the scientific literature. Our study compares two different infusion fluids in 12 healthy adult volunteers without renal failure in a single-blind randomized crossover design over two 48 hour periods during which subjects are not allowed to eat or drink. Fluid 1 is a premixed solution containing 54 mmol/L of sodium and 26 mmol/L of potassium; fluid 2 is sodium chloride 0.9% in glucose 5% with 40 mmol/L of potassium. Both solutions are administered at 25 mL/kg of ideal body weight, as recommended by current guidelines (NICE 174) and both solutions are widely used in daily clinical practice. The primary hypothesis is that isotonic maintenance solutions lead to more fluid retention than hypotonic fluids. Metabolism of both solutions is assessed by sequential analysis of urine and serum, clinical parameters and bioelectrical impedance analysis.