Hydration & Aerobic Performance: Impact of Environment (2015)
Michael N. Sawka, PhD., Samuel N. Cheuvront, PhD., & Robert W. Kenefick, PhD
This paper was written for The Gatorade Sports Science Institute’s Sports Science Exchange and discusses the impact hypohydration has on aerobic performance in certain environmental conditions. Hypohydration can be defined as a body water deficit of >2% of body mass or approximately 3% of total body water for the average athlete. A change in body mass has proven to be the most sensitive as well as simplest measure to determine acute changes in body water for all types of dehydration. Body water deficits of >2% of body mass signify a threshold where compensatory fluid regulatory actions occur. When we exercise in the heat our bodies most significant physiologic burden is to support high skin blood flow for heat dissipation. The warmer the skin, the greater the skin blood flow response and therefore the greater the heart rate elevation. This heart rate elevation reduces cardiac filling and stroke volume, thereby providing a challenge to maintain blood pressure. When exercising in high heat our sweat rates are increased at the same time our plasma volume is reduced from dehydration in addition to elevated skin blood flow requirements. This is an important physiological strain on our cardiovascular system that contributes to impaired aerobic performance. In the 1940s experiments were done involving military personnel who exercised in hot environments. It was concluded that the replacement of fluids resulted in better sustainment of their endurance. Subsequent studies have continued to confirm this. Hypohydration has a negative impact on aerobic performance during high heat stress whereas during cold weather environments hypohydration did not alter aerobic performance, and during temperate climates hypohydration was shown to may or may not alter performance. Also, when skin temperature exceeds 81 degrees fahrenheit hypohydration impairs aerobic performance by an additional 1% for every 1.8 degree fahrenheit skin temperature elevation. It is also noted in the paper that at high altitudes sweat rates are comparable to those at sea level, but respiratory water loss is elevated and plasma volume decreased. Therefore, hypohydration at altitude is related to both sweat loss and environmental adaptations.
This paper is interesting and a lot of information can be gathered for both the endurance coach and endurance athlete. Considering athletes who drink ad libitum tend to under consume fluids it is important to remain well hydrated throughout the day, especially on days when it is hot outside and we will be exercising in the hot environment. Being aware of our thirst, urine volume and color are helpful, but like most things in endurance racing and training it also important to train and race with a plan. A hydration and fueling plan that has the athlete consuming fluids on a set schedule will greatly reduce if not eliminate hypohydration and its adverse effects. This can be formulated by knowing one's sweat rate. By performing a sweat rate test and knowing this information we can replace our sweat losses and make sure we do not dehydrate >2% of our body weight. Although, conditions might alter this as we train and race it is the right place to start and should be practiced in every training session as well as utilized on race day. If we are training or racing in hot environments we need to also minimize our skin temperature elevations. Techniques such as acclimitizing, using sunscreen,
pouring cool water over our skin and head, wearing white or light colored fabrics and wearing a white hat are all helpful methods.
Paying attention to the details and following a plan have been proving to ensure success in this sport.