In endurance sports, like Ironman, open water swimming, road cycling, marathon, or cross-country skiing, the most likely contributors to fatigue are dehydration and carbohydrate (especially liver and muscle glycogen) depletion.
Due to sweat loss needed to dissipate the heat generated during exercise, dehydration can compromise exercise performance.
It is important to start exercising in a euhydrated state, with normal plasma electrolyte levels, and attempt to maintain this state during any activity.
When an adequate amount of beverages with meals are consumed and a protracted recovery period (8-12 hours) has elapsed since the last exercise, the athlete should be euhydrated.
However, if s/he has not had adequate time or fluids/electrolytes volume to re-establish euhydration, a pre-hydration program may be useful to correct any previously incurred fluid-electrolyte deficit prior to initiating the next exercise.
If during exercise the nutritional target is to reduce sweat loss to less than 2–3% of body weight, prior to exercise the athlete should drink beverages at least 4 hours before the start of the activity, for example, about 5-7 mL/kg body weight.
But if the urine is still dark (highly concentrated) and/or is minimal, s/he should slowly drink more beverages, for example, another 3-5 mL/kg body weight, about 2 hours before the start of activity so that urine output normalizes before starting the event.
It is advisable to consume small amounts of sodium-containing foods or salted snacks and/or beverages with sodium that help to stimulate thirst and retain the consumed fluids.
Moreover, palatability of the ingested beverages is important to promote fluid consumption before, during, and after exercise. Fluid palatability is influenced by several factors, such as:
- temperature, often between 15 and 21 °C;
- sodium content;
Hyper-hydration, especially in the heat, could improve thermoregulation and exercise performance, therefore, it might be useful for those who lose body water at high rates, as during exercise in hot conditions or who have difficulty drinking sufficient amounts of fluid during exercise.
However there are several risks:
- fluids that expand the intra- and extra-cellular spaces (e.g. glycerol solutions plus water) greatly increase the risk of having to void during exercise;
- hyper-hydration may dilute and lower plasma sodium which increases the risk of dilutional hyponatraemia, if during exercise, fluids are replaced aggressively.
Finally, it must be noted that plasma expanders or hyper-hydrating agents are banned by the World Anti-Doping Agency (WADA).
“Pre-hydrating with beverages, if needed, should be initiated at least several hours before the exercise task to enable fluid absorption and allow urine output to return toward normal levels. Consuming beverages with sodium and/or salted snacks or small meals with beverages can help stimulate thirst and retain needed fluids” (Sawka et al., 2007, see References).
Jeukendrup A.E. Nutrition for endurance sports: marathon, triathlon, and road cycling. J Sport Sci 2011:29;sup1, S91-S99. doi:10.1080/02640414.2011.610348
Sawka M.N., Burke L.M., Eichner E.R., Maughan, R.J., Montain S.J., Stachenfeld N.S. American College of Sports Medicine position stand: exercise and fluid replacement. Med Sci Sport Exercise 2007;39:377-390. doi:10.1249/mss.0b013e31802ca597
Shirreffs S., Sawka M.N. Fluid and electrolyte needs for training, competition and recovery. J Sport Sci 2011;29:sup1, S39-S46. doi:10.1080/02640414.2011.614269