Be Cool in the Heat
We will all exercise and compete in the Summer heat and cope with it with varying degrees of success. Hydration practices can be both helpful and damaging and we should carefully about when and what we ingest. Understanding a little about how your body regulates temperature will save you some discomfort and help improve performance. Temperature regulation during exercise is an often misunderstood area and one that can have fatal results if incorrect measures are not taken or if warning signs are ignored. As an interesting side note, there is a body of evidence to suggest that those of us who can sustain the highest core temperatures will have greater athletic success in endurance type events. Research supports that if you record body temperatures of individuals completing a marathon you could almost match their finishing place with their body temperatures. Higher finish position, higher body temperature!! In other words there exists great variation within individuals to not only produce heat but also to tolerate heat.
The human body is unique in than it really only possesses the ability to increase heat production and vary heat loss. That is to say we can’t really decrease heat production. As (exercise) intensity increases so does metabolic heat production. As we decrease intensity we simply produce less heat, but we still produce it. This is why recovering from heat disorders can be difficult. Individual body temperatures vary as a function of intensity and for the most part this will occur regardless of environment. You know this as you can just as easily feel hot and uncomfortable exercising indoors in the Winter as you can outdoors in the Summer. Yet the Summer is more problematic. Why so?
Heat loss in the body is via four main avenues: evaporation, convection, radiation and conduction. Maintenance of body temperatures is a interactive function of all four. Evaporation is perhaps the most important and versatile accounting for approximately 20-25% of heat loss at rest to upwards of 80% during exercise. It is important to know about evaporative mechanisms for several reasons. Firstly, it is the major heat loss mechanism during exercise, secondly, evaporation causes water loss, and thirdly, it is highly influenced by the way we dress and wear our equipment. Evaporation only occurs when liquid is dispersed as a gas. Therefore, sweat dripping from your brow etc. is ineffective water loss as it really contributes no cooling. Secondly, evaporation is maximal when the garment an individual is wearing is saturated. If it is not then the fluid tends to travel through the garment without noticeable heat loss. From this point alone one may question the science of team sport athletes changing uniforms at half time because they are wet. Science tells us that heat loss is in fact better achieved by soaking the garment before wearing it! Evaporation is also greater in less humid environments. High temperatures and low humidity actually present fewer problems with temperature regulation than slightly lower ambient temperatures accompanied with high humidity. Thus, working out not only at the cooler time of the day but also at the less humid time (early mornings) is also smarter. Obviously, if you choose to wear layers of clothing then you will inhibit this mechanism altogether and increase your risk for heat complication.
So, knowing that I lose fluid through cooling, how do I replace the fluid? Do not use the thirst mechanism as your main indicator of hydration, especially during exercise. When the thirst mechanism manifests itself you are already experiencing increased temperatures and reduced fluid levels. Drink regularly and often, i.e. before, during and after. Small cups about four ounces at a time are easily palatable and will not cause undue cramping or pain. Sport drinks are fine provided that the total carbohydrate is no greater than about 5-6% and they should contain sodium. Carbohydrate (or any other food source) concentration in excess of 6% causes the stomach to digest as opposed to absorb. This simply increases the transit time and will only lead to more complications. Water is the safest and simplest fluid during short duration exercise for the most part. However, as exercise times progress beyond 60 minutes commercially available beverages should be considered. One must also consider the prevalence of a condition called hyponatremia, which is characterized by low salt levels. This tends to occur when an athlete actually over-hydrates over a period of time diluting the body’s salt concentration. Exercise causes us to sweat and thus lose weight. Losing a little weight during exercise is not a bad thing and as an indicator is actually better than gaining weight. Athletes is exercise longer and this hydrate more are at higher risk for this condition.
Staying wet, if you can arrange it (easy for rowers) by spraying water or frequently wetting your head will also enhance convective cooling quite markedly and has been shown in laboratory tests to reduce both skin and core temperatures in athletes. Sunscreens can also offer added protection. Recent work from our laboratory demonstrated reduced skin temperatures when subjects performed the same bout of exercise wearing sunscreen.
Sweating is the major cooling mechanism during exercise and is a trained response. Sweat rates vary enormously during exercise and are affected by numerous factors such as intensity, duration, temperature and clothing. With a control of temperature and clothing, intensity of exercise becomes the most prolific factor. At exercise intensities of 65% VO2 max, a normal individual will sweat about 1 L/hour. Most individuals find this hard to believe as they really only view sweat as being produced from the forehead. We sweat all over our body, from the backs of our hands to the mid-belly of our thighs. It's just not as noticeable on the back of our hands as it is from our head. Sweating is also a learned response. As you get fitter you'll sweat more and earlier. Remember it is a cooling mechanism and will be initiated earlier in response to elevated body temperature changes and improved fitness.
As work rate increases, body temperature increases. As body temperature increases, so does sweat rate. Therefore, with increasing intensity (and duration), there is a greater need for fluid replacement. Fluid replacement during exercise directly affects body temperature. Over a two hour period at 70% intensity regular fluid ingestion can attenuate the rise in body temperature by more than 1° C. With an increase in body temperature comes an increase in metabolic cost due primarily to the occurrence of a phenomenon known as cardiovascular drift. Cardiovascular drift describes the slow rise in cardiovascular response over time, e.g. heart rate and oxygen consumption, even though work rate does not change. What this translates into is an increase in intensity with no change in work rate. Several researchers have reported a total prevention in cardiovascular drift (an increase in cardiovascular strain) with infusion of an 18% glucose solution over 2 hours of exercise. Supporting work by others using only water showed an increase of 5% heart rate versus a 10% increase with no fluid ingestion in cyclists over 120 minutes. It is interesting to note that differences in variables such as heart rate and cardiac output are more quickly visible than are body temperature differences when fluid is not ingested. These findings support the need for early and frequent hydration.
In general, it is recommended that fluid replacement take place before, during and after exercise. Using the thirst mechanism is unwise as it is typically a late warning sign of dehydration. Fluid ingestion should occur about every 15-20 minutes and at the rate of 8 oz every 15 minutes. Naturally, this could be higher if work rates require so. Fluid replacement should not be limited to distance events. Consider that the majority of people are normally dehydrated and fluid replacement during exercise is typically in response to thirst. Thus most athletes long or short distance need to consider active replenishment. It is estimated that each 1% decrease in body fluid levels results in a 10% decrease in performance measures such as reaction time or twitch time. This may not seem crucial in rowing, but imagine being a NHL goalkeeper having lost 6-7 lbs of sweat. How might this affect your save percent?
Thus, for the most part water is the most appropriate solution to drink, especially in events lasting less than 30 minutes. When events or training exceeds 60 minutes you might want to consider flavored drinks. But remember, as you increase the concentration of the fluid the transit time from the stomach is delayed. This is more evident when concentrations exceed 6%. For this reason most commercially available sports drinks are <6% concentration. Glucose drinks are beneficial in delaying fatigue, but as mentioned previously, their effectiveness is not really apparent until after at least 60 minutes of exercise. This reinforces the acceptability of water as a viable fluid replenishment. Of course, there is no arguing the taste preference for flavored drinks. Many times they are more preferable than water. However, from a scientific point, their benefits are documented only over longer durations and water will not compromise performance vs. a flavored drink over shorter durations.