The effects of glycerol ingestion on fluid balance and cardiovascular hemodynamics in males during hypobaric hypoxia

Acute altitude exposure causes a loss of total body water and plasma volume. Glycerol ingestion has been shown to be rapidly absorbed by the intestine, leading to increases in serum osmolality, plasma osmolality, and total body water. The purpose of this study was to determine the effects of glycerol ingestion on fluid balance and cardiovascular hemodynamics at rest and during exercise during hypobaric hypoxia. Twelve men (33 ± 8 yr, 177 ± 3 cm, 75 ± 11 kg, 52 ± 10 ml O2·kg -1·min-1) each completed 4, 3.5 hr trials in a randomized design: (A) 6% carbohydrate/electrolyte drink at 1524 m, (B) 5% glycerol drink at 1524 m, (C) 6% carbohydrate/electrolyte at 3659 m, and (D) 5% glycerol at 3659 m. The 3659 m trials were 2 weeks apart. Subjects consumed 30 ml/kg lean body mass of each drink in 5 equal volumes at minutes 0, 30, 60, 90 and 120. Exercise intensity was 50% of the ventilatory threshold at 1524 m. Plasma volume change, free water clearance, osmolar clearance, and body mass change were measured every 30 min. Cardiac output was measured at min 0 and 150 during rest and cycling by CO2 rebreathing. Glycerol ingestion (GLY) resulted in significantly lower free water clearance compared to carbohydrate/electrolyte (CHO) at min 90, 120, and 150 (p < 0.05) across altitude. Osmolar clearance was significantly higher for GLY vs CHO at min 60, 90, 120, and 150 (p< 0.05). No significant differences were found for plasma volume change, stroke volume, or heart rate. The body mass change and fluid retention were significantly higher for glycerol than for carbohydrate/ electrolyte (p< 0.05). Even though a 5% glycerol solution offset the diuresis of acute hypobaric hypoxia, there did not appear to be any beneficial effects on cardiovascular hemodynamics