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Can You Take the Heat?

By Mackenzie Madison | Aug. 25, 2009, 6:28 p.m. (ET)

It’s that time of year when the summer heat can really push you over the edge when it comes to training and just as important – racing. You don’t need a thermometer or to check the weather to know that it’s hot and humid. It’d be quicker to step outside and notice the immediate beads of sweat start to roll down your face along with your sunglasses fogging up. Some of you dread the heat, some of you don’t mind.

But here’s why you should mind. Recently, in a group of elite cyclists completing just a 30-minute time trial in moderate heat of 90 degrees Fahrenheit, their overall power output was lower by 6.5% than when performing a time trial in a thermoneutral environment of 73F. (Tatterson) So yes, even in elite cyclists and moderate heat exposure, your performance decreases.

So what can you do to minimize these performance decrements? Get used to it – literally – and get heat acclimated. Besides improving your ability to perform at a high level in the heat, heat acclimation also starves off serious health risks when you are exposed to the heat. Not being heat acclimated exposes you to several risks including muscle cramps, feeling faint, inability to coordinate muscular movements, headache, nausea, vomiting, dizziness, hyperthermia, shortness of breath and overall general fatigue.

How the Heat can affect Your Performance: Did You Know?

Heat Acclimatization reduces overall lactate accumulation in both warm and cool environments – meaning that in general, you’ve improved your lactate threshold by purely heat acclimating. (Young) Studies are still needed to further prove that heat acclimation is beneficial in the cool environment and lowers lactate thresholds in the cooler environment as well, but current research is telling us otherwise.

It’s not uncommon that athletes have lower than normal blood mineral concentrations, meaning that yes, you do need that extra helping of table salt… and calcium… and copper, iron, magnesium and zinc. In fact, mineral losses in sweat can be up to 40% of your Daily Required Intake for Calcium, 200% of Copper, 80% of your Iron, 10% of Magnesium, and 50% Zinc.  Once your complete heat acclimation status is achieved, it will conserve whole body sweat losses of these mineral concentrations. (Chinevere)

Those who are fitter and have a higher VO2 max are able to tolerate the heat more easily than those who are not aerobically trained. (Sawka) Individuals with higher VO2 max tend to gain the benefits from heat acclimation quicker than those with lower aerobic fitness levels. As far as heat tolerance and heat acclimation, younger or older individuals do not have any reduced ability to heat acclimate as long as the individual is in shape. However, just because you are training, it doesn’t mean that you are already heat acclimated. Further improvements can be continually seen with proper heat acclimation. It’s also important to keep in mind that some heat acclimate quicker than others depending on how you deal with heat stress.

How to Heat Acclimate
Last summer at the University of Oregon in Eugene, researchers had marathoner Dathan Ritzenhein, who placed 9th in the Beijing Olympics Marathon (1st American) with a time of 2:11:59, heat acclimate in the environmental chamber in order to prepare for the race that was claimed to have the most humid and hot climate exposure in the Olympics yet… along with the smog. The sole purpose of why he was heat acclimating was to minimize the effects of hyperthermia (over-heating of the body) and dehydration that are exacerbated during heat exposure while exercising. Heat acclimation improves exercise performance, heat tolerance and reduces physiological strain and the incidence of some forms of heat illness. (Young)

When you exercise in the heat, many physiological compensations occur. In the heat when you begin exercising, your body starts to produce extreme amounts of heat. As a matter of fact, when you begin exercising approximately 70-80% of your total metabolic contribution is produced as heat – signifying how easily your body can heat up and the importance of dissipating the heat that is produced. 

In the heat, your blood is pumped more superficially to your skin and as you sweat, it further cools that blood flow into the skin, resulting in cooling your core body temperature. Since a greater amount of blood is directed away from the central part of your body, less blood is returned to the heart and reduces your cardiac output – or how much blood your heart is pumping throughout your body each minute. Cardiac output is equal to stroke volume multiplied by heart rate. So, in order for your body to maintain your cardiac output during exercise, with the decreased stroke volume or blood volume returning to your heart, it causes an even greater rise in heart rate to compensate for that lower blood volume. This is essentially why your heart rate is 5+ beats per minute higher in the heat at the same workload in a cooler environment. Your heart rate is also bumping you out of your training zones because your body is trying to cool itself down due to overheating and this requires extra metabolic work from your body to reduce the thermoregulatory and cardiovascular strain on your body.

Your body further cools the superficial blood and skin by sweating creating an evaporative cooling system. However, there is a point when your body can no longer sweat, causing increased heating. This often marks the point of when your body starts to become dehydrated. In order to try and continue to sweat and maintain your plasma or blood volume that you sweated out, water starts to be taken from your muscles and skin at an expense to maintain your blood volume. One of the major significant things that heat acclimation does is increases your total plasma volume or blood volume and increasing your total body fluid available for sweating and blood volume maintenance. The increase in plasma volume increases your stroke volume, and remembering that your total cardiac output is stroke volume multiplied by heart rate, this results in helping decrease your high heart rate in the heat. (Fortney)

Here’s how to do it

You need at least 7 days and preferably up to 14 consecutive days of moderate exercise intensity for at least 90 minutes in a fairly hot environment. Acclimation will only occur when the core body temperature is elevated with moderate to profuse sweat rates – so expect to be in an environment of +100F with 30% humidity. (Wenger) At first, it might be difficult to complete a full 90 minutes of exercise, but work up to completing the full 90 minutes by resting or sitting in the heat for the rest of the duration. If it’s not as warm as it needs to be to get you to really sweat, wear additional clothing that is sweat-wicking. You also have increased heat stress during the noon hours and during the afternoon making for better times for your heat acclimation sessions. DO NOT complete your quality intensity sessions during the heat. You will not get as much of a benefit from the workout and they are best completed in a cooler environment or in the morning.

Heat acclimation Benefits:
  • Decreased core temperature
  • Decreased heart rate 
  • Decreased psychological perception of perceived effort (RPE) 
  • Increased exercise tolerance time
  • Increased plasma volume
  • Increased sweating rate with decreased sodium chloride (loose less electrolytes)
At first, you can expect cardiovascular adaptations through reduction of heart rate and overall cardiovascular strain. (Wenger) Decreasing heart rates occur in the first 4-5 days of heat acclimation and complete heart rate adaptations after 7 days. (Machle) Plasma volume expansion also occurs rapidly in the early days of heat acclimation and then continues to slowly increase after 7 days resulting in a 12-27% increase in blood volume and a 6-16% increase in your extracellular fluid. After the first week of heat acclimation, the secondary benefits start to occur. This is when you begin to sweat earlier and you are able to maintain a higher sweat rate. Additionally, your sodium sweat concentration can decrease as much as 59% despite an increase in sweat rate by 12-15%. (Sawka) The increased sweat rate further helps to decrease your overall skin and body temperature. All in all, heat acclimation lowers the total metabolic cost of exercise in the heat, allowing for you to race at your potential instead of wasting excess energy cooling yourself. Your body becomes more efficient. Increases in sweat rates and further reductions in the total metabolic cost of exercise can take up to a month for complete adaptations. (Horvath, 1946) The lesson here is the longer you expose yourself to heat, the more you maximize your benefits, but on a time crunch, 10 days is optimal.

The Importance of hydration

Exercise in the heat can cause sweat output to easily exceed water intake, resulting in a body water deficit. (Sawka) Once you get to greater than a 2% decrease in total body water lost during exercise, huge performance decreases are seen. Dehydration alters many cardiovascular, thermoregulatory, central nervous system and metabolic functions in your body- in other words, multi-system shut-down. Dehydration causes greater heat storage and reduces your ability to tolerate heat strain. It delays the onset of sweating and when you do sweat, you lose an even greater amount of electrolytes and salts. Dehydration also lowers the core temperature that can be tolerated during heat stress due to these multiple factors. In fact, dehydration alone can completely eliminate the advantages of being heat acclimated. Monitor your total sweat loss (TSL) during heat acclimation can be accurately estimated from changes in body mass by using: 

TSL = Δ Body Mass (pre ex- post ex) + Fluid Intake

Make sure to take off sweaty clothes before you calculate how much you sweat per hour. Use this as a guideline of how much fluid you should intake to prevent dehydration.  Fluid replacement is limited by the maximal gastric emptying rate which is 1.0-1.5 liters per hour and is further decreased as you exercise harder or if you are dehydrated. Make sure you are hydrated before, during and after training or racing. Also, don’t skimp on the salt – your body needs it to maintain its body fluid volumes as athletes already tend to have lower than normal salt mineral concentrations.

During the race
  • Take water while you’re on the bike and run and pour it over your head in order to maintain a wet, convective evaporation between you and the environment. You’re sweating for your body without losing your own and cooling your body.
  • Take ice whenever you can and place it under your hat, hold it in your hands, or place it in your uniform making sure you have contact with it to your skin for further cooling.  
  • Make sure to hydrate with a sports drink or water at least every 10-15 minutes and take sips often. Your body can not absorb fluids faster than your gastric emptying rate, so smaller, more frequent sips of water will help speed the rehydration process more than larger quantities will. Plus excessive fluid intake produces gastric discomfort and will hinder your performance.
  • Along with drinking water, make sure you are eating and drinking items that have calories – meaning that they will make water digestion easier. Your body can absorb more fluid if you have a mixture of fluid and food items. Key lesson here: drink a variety of fluids and gels. Always try these out before you race and see what does and does not work for your body – perfect for trying out during heat acclimation. You’ll also find out how much your gut can handle when you have even greater reduced blood flow to digest items due to the heat. 
  • The sun rays are the most intense from 10 a.m.-1 p.m., and ultraviolet rays from the sun can hinder your performance. Take into consideration how long your race will last and how intense the heat will be – half Ironman or Ironman distances really start to count on your body's ability to heat acclimate in order to complete the race
  • Sunburn to the skin hinders sweat gland function by inhibiting the bodies ability to cool itself down, so take that extra time in T1 and T2 to slap on some sunscreen and be sure to prepare before the race as well
Things You Should Know About Heat Acclimation: The Importance of Maintenance 
It is possible to completely eliminate the benefits you’d normally get from heat acclimation for that “A” race by making a couple small, but major, mistakes. The benefits of heat acclimation are undone by sleep loss, infection, alcohol abuse, salt depletion and most importantly – dehydration. Also, as quickly as you gained the heat acclimation adaptations, just as quickly they can be gone. Heat acclimation is transient and lost if not maintained by repeated heat exposure. You can expect to lose complete heat acclimation adaptations ranging from a total of 7-17 days depending on the person – but if I were you, I wouldn’t count on the effects lasting 17 days. The adaptations of heat acclimation that occurred first are lost first – so decreases in heart rate, total plasma volume, and easier perceived exertion is gone within days.  However, partial losses of a few days’ lapse are easily made up; but for a week without heat exposure? Again, don’t count on it.

And for those of you wondering about caffeine, it has not been shown to cause significant levels of dehydration or greater heat production from the body during exercise in the heat. (Coso, 2009) So having your race day caffeine jolt or pre-workout pick-me-up won’t affect your thermoregulation or fluid-electrolyte balance during moderate caffeine consumption.

Lastly, if you can’t escape the heat, it’s better to train in the heat in the morning than in the evening. Recently, Hobson found that endurance exercise capacity in the heat at the same temperature was significantly greater in the morning than in the evening. This is possibly due to a lower initial body core temperature instead of going out into the heat later in the day after your core temperature as risen a fair amount. Good thing you’re typically starting your race in the morning. 
Remember to train safe and be smart – tell others when you are heat acclimating, always be close to contacting other people and have close water access.

Mackenzie Madison of Zoom Performance Endurance Coaching has a B.S. Exercise Physiology, a minor in Coaching, and is a Masters-Doctoral student at the University of Oregon. In addition, she is a USAT Level I Coach, USAC Level III Coach, and a Professional Triathlete. You can reach her via email at or visit her website at

The views expressed in this article are the opinion of the author and not necessarily the practices of USA Triathlon. Before starting any new diet or exercise program, you should check with your physician and/or coach.

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