Hydration Is Important in Winter Too

More and more athletes are becoming aware of the important role that nutrition and hydration play in performance. In fact, inadequate hydration on competition day can throw all the effort up to that point out the window, as there is ample evidence showing that excessive dehydration (whether starting dehydrated or not hydrating enough during the event itself) can negatively affect performance. For example, a recent meta-analysis showed that starting an event dehydrated (usually defined as a body weight loss of more than 2%) resulted in lower performance (-2,4%), with the effects being more pronounced the greater the dehydration (especially from a loss of 2,8% of body weight).¹

Dehydration in the absence of heat

One of the processes that contributes most to dehydration is sweating, which occurs to reduce body temperature. Thus, we take advantage of the evaporation of sweat on the skin to lower its temperature and, in doing so, also reduce our internal temperature. This is why sweating becomes especially important in hot environments. However, although we usually only pay attention to hydration in hot and/or humid environments, we must keep in mind that in cold environments dehydration can also have very negative consequences. For example, a study analyzed the isolated and synergistic effects (that is, in combination) of heat and dehydration, and observed that although the most negative effects occurred when both factors were combined, dehydration on its own resulted in a marked decrease in performance (a reduction of 5% in maximum oxygen consumption and 26% in time to exhaustion) even when body temperature was not elevated.² Therefore, in the absence of heat, dehydration can occur and limit our performance. But can we really become dehydrated if it is not hot?

Fluid loss depends on a multitude of factors, one of the main ones being the ambient temperature. However, during exercise our body also needs to dissipate the heat generated by the metabolic processes that are taking place, especially muscle contraction. In addition, another important route of fluid loss is urine, and in this regard, it has been described that in cold environments a phenomenon called “cold-induced diuresis” occurs; that is, a greater need to urinate, possibly caused by vasoconstriction and the need to reduce fluids to lower blood pressure. On the other hand, we also lose fluid through respiration, although in this case in the form of vapor.Therefore, although in smaller amounts than in hot environments, when exercising in cold environments we can also lose a significant amount of fluids.

The importance of salts

It is important to emphasize that during exercise, not only are fluids lost that need to be replenished, but also salts. In this regard, although fluid intake may become more important, especially in hot environments, salt intake could still maintain its importance in cooler environments. In fact, a study analyzed a group of people who exercised wearing the same clothing in a chamber simulating hotter conditions (30-35 ºC) or cooler conditions (15-20 ºC), and the results showed that, although the sweating rate was slightly higher in the hot environment than in the cold (0.47 versus 0.41 liters per hour), the sodium concentration in that sweat was higher in the cooler environment.³

Therefore, we see that in addition to replenishing fluids during exercise, in cool environments the replacement of those minerals (such as sodium) that are also being lost could be especially important. The loss of these minerals varies greatly between individuals, but it has been described that the loss increases the higher the intensity of the exercise. For example, a study in marathon runners observed sodium losses between 7 and 95 mmol/L, with higher concentrations the faster the running speed.⁴ That is why, during exercise, especially if it is high intensity and/or prolonged, it is advisable to replenish the losses with a drink containing electrolytes (and, if possible, also carbohydrates to reduce fatigue and improve performance).In fact, an excessive intake of liquid with a low amount of electrolytes (as occurs, for example, when consuming large amounts of water) could result in a condition called hyponatremia, which represents one of the main health risks in events such as marathons. So much so that a study published in one of the most prestigious medical journals in the world observed that 13% of the participants in the Boston Marathon had hyponatremia, especially the low-weight runners who had taken longer to complete the distance (more than 4 hours) and who had consumed more liquids during the race (more than 3 liters).⁵ Therefore, although one must stay hydrated, excessive hydration without the proper dose of minerals can be as harmful asdehydration.

Conclusions

Although most athletes only pay attention to hydration in hot environments, in cold environments we can also lose a significant amount of fluids (not only through sweat, but also through breathing or urine) and especially sodium, which can have negative effects on performance. That is why when exercising, especially if it is high-intensity and prolonged, consuming drinks with electrolytes (and, if possible, carbohydrates to delay fatigue) can be the most recommended strategy.

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References:

1. Deshayes TA, Jeker D, Goulet EDB. Impact of Pre-exercise Hypohydration on Aerobic Exercise Performance, Peak Oxygen Consumption and Oxygen Consumption at Lactate Threshold: A Systematic Review with Meta-analysis. Sport Med. 2020;50(3):581-596. doi:10.1007/s40279-019-01223-5
2. Nybo L, Jensen T, Nielsen B, González-Alonso J. Effects of marked hyperthermia with and without dehydration on VO2 kinetics during intense exercise. J Appl Physiol. 2001;90(3):1057-1064. doi:10.1152/jappl.2001.90.3.1057
3. Bates GP, Miller VS. Sweat rate and sodium loss during work in the heat. J Occup Med Toxicol. 2008;3(1):1-6. doi:10.1186/1745-6673-3-4
4. Lara B, Gallo-Salazar C, Puente C, Areces F, Salinero JJ, Del Coso J. Interindividual variability in sweat electrolyte concentration in marathoners. J Int Soc Sports Nutr. 2016;13(1):1-8. doi:10.1186/s12970-016-0141-z
5. Almond CSD, Shin AY, Fortescue EB, et al. Hyponatremia among runners in the Boston Marathon. N Engl J Med. 2005;352(15):1550-1556. doi:10.1056/NEJMoa043901