Currently the American College of Sports Medicine (ACSM) advise that sufficient fluid should be ingested to ensure that body mass (BM) loss during exercise does not exceed >2 % of starting BM so that exercise-associated medical complications will be avoided. The safety and efficacy of these recommendations have been questioned
Having considered the general gains and losses of water throughout the human body we have argued that water may be released from internal stores during exercise. This would add to the body’s overall hydration status even in the face of reductions in BM. Potentially large internal body water stores exist in the gut and perhaps in association with liver and muscle glycogen stores. These ideas should establish a more physiologically appropriate model for fluid replacement during exercise.
Current evidence establishes that it is normal to lose 2 % BM, and that BM losses within this range are not necessarily associated with clinically relevant reductions in TBW. It seems that performance may also not be compromised despite even large BM losses since athletes losing the greatest BM tend to perform better than their counterparts who lose the least or even gain BM during exercise.
It should be acknowledged that fluid replacement guidelines have evolved for the better since the 1996 ACSM Guidelines for Exercise and Fluid Replacement. Drinking to thirst (ad libitum) during exercise in races as short as 21.1 km or in more punishing ultra-endurance races lasting up to 12–24 h ensures that athletes will maintain body fluid allostasis and achieve their race goals without encountering avoidable medical complications.
Full text and ACSM recommendations
- Tam, Nicholas, and Timothy D. Noakes. “The quantification of body fluid allostasis during exercise.” Sports Medicine 43.12 (2013): 1289-1299.
- Sawka, Michael N., et al. “American College of Sports Medicine position stand. Exercise and fluid replacement.” Medicine and science in sports and exercise 39.2 (2007): 377-390.