Heat load stress and dehydration is a major performance factor in all endurance events. It has the most significant effect in events last longer than 20 minutes, but will also take its toll on events as short as a 1km running time trial.

Comparing 1km, 5km and 10km running time trial times in hydrated versus dehydrated athletes.

Heat load can come internally from our muscles as they generate heat and energy, or from the environment in the form of ambient temperature and relative humidity. The warmer it is outside, the quicker our core body temperature (CBT) rises inside.

An increase in CBT forces our bodies to initiate sweating which will lead to dehydration if not counteracted. This results in impaired performance due to the following:

Decreased cardiac output (the amount of blood circulated in one minute): Blood is made up of two components: haematocrit (red blood cells) and blood plasma. Dehydration reduces our blood plasma volume because we sweat it out. This reduces our total blood volume and increases blood viscosity (thickness), making our heart work much harder to circulate blood. Less circulation means our stroke volume (amount of blood ejected from the heart per beat) also reduces. Heart rate is increased to an extent to counteract this, but as we know our heart rate has a limit to how high it can go. Do you notice that your heart rate is higher during very hot days? This is one reason why.
Reduced oxygen delivery: As our CBT increases, our main defence to counteract this is to initiate sweating. In order to start sweating, blood (and therefore oxygen) is redirected from the working muscles to the surface of the skin. This stimulates sweat glands to begin secreting sweat, which then evaporates into gas which cools our body down. Having blood close to the surface of the skin also allows us to diffuse heat out of the body and into the surrounding environment. This is obviously beneficial as controlling our CBT is crucial for performance (and survival for that matter), but diverting oxygen away from the working muscles means our aerobic power output will decrease. Again, this is somewhat counteracted by an increasing heart rate, but as explained above, our blood thickens under dehydration and our heart rate can only increase so much.
Decreased central drive and increased rating of perceived exertion (RPE): Our body does not want to damage itself. If we increase our CBT above a normal range our brain will refuse to send strong signals to our muscles to contract maximally. A decreased central drive is a protective mechanism aimed at combating heat illnesses and which ensures we do not get to a stage where significant damage can occur. The brain increases our perception of effort and reduces the strength of the signals telling our muscles to contract. It forces us to slow down.

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How Do We Combat Hyperthermia and Dehydration?

Fluid replacement and/or acclimatise to the environment.

Stay hydrated: Replacing the fluid lost through sweat will go a long way to reducing the impact of heat load. The current guidelines are to consume an isotonic sports drink containing 4-8% carbohydrate which contains a mix of electrolytes. As the environment gets hotter, a shift towards a hypotonic solution is preferred. Individual sweat rates vary greatly, but around 600-1200ml of fluid/hour is a baseline, with more fluid required as the environment heats up. There are three types of fluid you can drink:

Hypotonic: a fluid which has fewer electrolytes and carbohydrates than inside the body’s cell. This is the best type of fluid for rehydration only. It has electrolytes but little to no carbohydrate, and easily diffuses into our cells along a concentration gradient. Think of low calorie sports drinks like Endura. These will rehydrate you effectively, but will not replenish glycogen stores.

Isotonic: a fluid which has a similar concentration of electrolytes and carbohydrates to the body’s cells. This type of drink provides a good balance between rehydration and glycogen restoration. Think of drinks which contain a moderate number of carbs such as Powerade and Gatorade.

Hypertonic: a fluid which has a higher concentration of electrolytes and/or carbohydrates than the body’s cells. This fluid is effective at combating glycogen depletion, but will be absorbed the slowest out of the three fluids and will not rehydrate as effectively as the above. Think of RedBull, cola, and fruit juices here.

Drink according to the environment and length of event. A very hot and long event will need to find a balance between rehydration (hypotonic) and glycogen replenishment (isotonic). You will probably be safe with an isotonic-only solution if the temperature is not too great. If your nutrition plan hasn’t work out as you’d hoped and you’re beginning to hit the wall, a hypertonic solution might get you back on track. The nutritional side of endurance performance will be discussed in depth next week.

Acclimatise to The Environment:

The relationship between energy output, heat production, and heat loss.

Rehydration can only do so much. As evident in the above graph, heat production still exceeds heat loss in hot environments. Acclimatising to the environment can bring this total heat loss value (3) closer to the heat production value (2), which will improve our thermoregulation.

Acclimatisation involves being exposed to the same/similar environment to what you will race in. This can be done at the event (ie deliberately flying over to Kona weeks before participating in the Ironman World Champs) or replicating the environment at home (using a heat chamber, or, more practically, turning the heater up and creating a hot, steamy environment in the bathroom to complete ergo sessions in). It takes up to 14 days to reap the benefits of acclimatisation, but will certainly take at least 6. During this period, 1 hour of exercise at 50% of VO₂ max or higher at least once in every three days is required to physiologically adapt. 1 hour is the minimum, with 1.5-2 hours shown to have the most benefit.

Physiological adaptations to heat acclimatisation. Source: Armstrong & Maresh 1991.

The main outcome of acclimatisation is that you begin sweating earlier in exercise and at a greater rate but with a reduced sodium loss.This allows your body to evaporatively cool more effectively, but without experiencing the dehydrating effects of sodium/electrolyte loss.

Monitoring fluid intake (including the type of solution) in conjunction with heat acclimatisation will give you the best chance of reducing any performance decrements associated with racing in warmer environments to what you’re used to.

Tune in next week where we will discuss sports nutrition for endurance events.

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Written by Luke McIlroy – Director of Sport Science at METS Performance Consulting

BEx&SpSci, ESSAM, AES