Levels, K., Cotter, J. D., Rehrer, N. J., Sims, S. T., & Hopman, M. T. (June 02, 2010). Pre-exercise sodium+glycerol loading aids athletes' fluid balance but not performance in the heat. Presentation 1684 at the 2010 Annual Meeting of the American College of Sports Medicine, Baltimore, Maryland; June 2-5.

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"Preloading with a glycerol- or high-sodium-containing fluid can improve exercise performance, but effects are equivocal for endurance athletes (who are most hypervolemic), and when fluid intake is permitted during exercise." This study determined whether preloading with a combined sodium and glycerol fluid enhances fluid balance, reduces endurance athletes’ physiological strain, and increases work capacity when fluid intake during exercise is allowed. Endurance-trained athletes (M = 7; F = 1) completed four trials. Ss ingested either no fluid, a low-sodium fluid, a high-sodium fluid, or a sodium+glycerol fluid before cycling to exhaustion at 60% of VO2max in the heat (32°C; 50% RH, Va ~4.5 m/s). Preload fluids were ingested in seven portions over 60 minutes, starting 90 minutes before the onset of exercise. Rehydration during exercise was 1.5 mL/kgBM of an isotonic sports drink every 15 minutes.

More fluid was retained after preloading and across the full trial for the sodium+glycerol drink treatment than for the low-sodium drink and the high-sodium drink conditions. Only the high-sodium fluid gave a significant plasma volume expansion 15 minutes after preloading, but no long-lasting differences were observed. Drifts in heart rate and core temperature were similar for all preloading regimes, as was time-to-exhaustion. No differences in final ratings of perceived exertion, body temperature, or thermal discomfort were observed.

Implication. Preloading with a fluid containing both high-sodium and glycerol-enhanced fluid balance before and during exercise but did not reduce thermal or cardiovascular strain or improve exercise capacity for well-trained athletes exercising in the heat, when partial rehydration was allowed. [This is an example of physiological factors being altered but not being accompanied by any change in performance. The faith of many coaches and sport scientists in physiological factors being absolute determinants of performance is not supported by this and a host of other studies.]

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