Salt plus Glycerol-Induced Hyperhydration Enhances Fluid Retention More Than Salt- or Glycerol-Induced Hyperhydration

Hyperhydration has been demonstrated to improve work capacity and cardiovascular and thermoregulatory functions, enhance orthostatic tolerance, slow or neutralize bone demineralization, and decrease postdive bubble formation. Adding sodium or glycerol to a hyperhydration solution optimizes fluid retention. Sodium and glycerol produce their effect through different physiological mechanisms. If combined into a hyperhydration solution, their impact on fluid retention could potentially be greater than their singular effect. We compared the effect of salt-induced hyperhydration (SIH), glycerol-induced hyperhydration (GIH), and salt + glycerol-induced hyperhydration (SGIH) on fluid balance responses during a 3-hr passive experiment. Using a randomized, crossover, and counterbalanced experiment, 15 young men (22 +/- 4 years) underwent three, 3-hr hyperhydration experiments during which they ingested 30 ml/kg fat-free mass (FFM) of water with an artificial sweetener plus either (a) 7.5 g of table salt/L (SIH), (b) 1.4 g glycerol/kg FFM (GIH), or (c) 7.5 g of table salt/L + 1.4 g glycerol/kg FFM (SGIH). After 3 hr, there were no significant differences in plasma volume changes among experiments (SIH: 11.3% +/- 9.9%; GIH: 7.6% +/- 12.7%; SGIH: 11.3% +/- 13.7%). Total urine production was significantly lower (SIH: 775 +/- 329 ml; GIH: 1,248 +/- 270 ml; SGIH: 551 +/- 208 ml) and fluid retention higher (SIH: 1,127 +/- 212 ml; GIH: 729 +/- 115 ml; SGIH: 1,435 +/- 140 ml) with SGIH than either GIH or SIH. Abdominal discomfort was low and not significantly different among experiments. In conclusion, results show that SGIH reduces urine production and provides more fluid retention than either SIH or GIH.