US Navy 070723-N-6138R-001 U.S. Navy SEAL, Petty Officer 1st Class David Goggins runs 135 miles through Death Valley, California in the Kiehl's Badwater Ultra Marathon

"A gym gotta be hot..." - Floyd Mayweather

You don't have to be a championship fighter or an ultrarunner preparing for Badwater to benefit from heat training. Even if you don't specifically need to acclimate to exercise in the heat there are benefits to be had:

  • Weight Management
  • Cutting Weight
  • Enhanced Aerobic & Anaerobic Endurance
  • Anti-Inflammatory
  • Anti-Catabolic
  • Improved Immune Function including Anti-Cancer
  • Psychological

Thermal Distress

Training & competing in the heat are not without risk & this is therefore the primary reason for heat training. It is our bodies inability to meet the simultaneous demands of exercise & heat dissipation that poses the major risk. Heat is primarily dissipated through the skin predominantly through sweating & radiation. Both require peripheral vasodilation (widening of the blood vessels). Plasma volumes decrease during exercise, more so as the intensity increases. This is compounded by loss of fluids through sweating. The increased peripheral vasodilation & plasma volume decreases can result in a decrease in central blood volumes. In un-acclimatised individuals heart rate increases may help maintain blood pressure during sub-maximal exercise. However as the intensity increases with possible further dehydration & consequent plasma volume decrease peripheral vasoconstriction occurs to maintain core volumes at the expense of heat dissipation.

As the body's core temperature rises the rate of the chemical reactions (metabolism) increases. The rate doubles for every 10°C increase in core temperature, known as the Q10 effect. Additionally the metabolic reactions themselves generate heat. So as the rate of the reactions increases even more heat is generated & the cycle becomes self perpetuating & the risk of heat injury increases.

Various degrees of thermal distress are identified:

  1. Dehydration: a fluid deficit of 1% of bodyweight causes thirst, by 2% athletic performance, concentration & cognitive function are already deteriorating. A 5% deficit results in lethargy, nervousness, irritability, fatigue & a loss of appetite. 7% is extremely dangerous & characterised by an inability to salivate & difficulty swallowing. By 10% co-ordination fails. 15% delirium & 20 % represents the upper limit before death occurs.
  2. Heat cramps:primarily due to muscle fatigue, sodium & potassium depletion through perspiration also plays a role.
  3. Heat exhaustion: is characterised by a weak rapid pulse, low blood pressure, profuse sweating & disorientation.
  4. Heat stroke: also referred to as sun stroke, is a major medical emergency. The core temperature has reached 41°C. Extreme confusion & unconsciousness are common along with a hot dry skin (sweat might still be present if the injury is exercise induced)

The body's physiological adaptations to 2 weeks of training in heat would result in lower heart rate, core and skin temperatures at rest & sub-maximal exercise. There is also more stability of increased blood pressure during prolonged exercise.

The adaptations include a decrease in blood flow to the skin & an approximate 12% increase in plasma volume. These adaptations help to restore the central blood volume thereby, crucially, maintaining central blood volumes & blood flow to the muscles during exercise. The increase in plasma volume is due to an increase in plasma protein. For each increase of 1g of plasma protein, plasma water is increased by 15g.

Along with these adaptations there is a substantial increase in sweating & evaporative cooling capacity. Sweating capacity increases from around 1.5 litres per hour to 4 litres per hour. The increase in sweating capacity is accompanied by more complete & even distribution of sweating across the skin & earlier onset of sweating due to a lowering of the skin temperature threshold at which sweating starts. Losses of sodium chloride are decreased through an increase in aldosterone secretion by the adrenal cortex adjusting the kidney's salt/water balance.

Heat acclimatization only occurs if the athlete trains in the heat for between 1-2 weeks. To mitigate the risks of thermal distress it is important to:

  • ensure the athlete is in good condition before heat acclimatization training starts;
  • scheduling training at cooler times & cancelling training if the wet bulb temperature or heat index is too high;
  • athletes should tank up with 400-600ml of water 30 minutes before training;
  • regular water breaks should start early in the session with a supply of cold drinks with less than 9g of carbohydrates per 100ml & electrolytes;
  • athletes should avoid salt pills.

Additional benefits

Weight Management

Heat training helps with weight management (long term sustainable change) in at least 3 ways:

  1. Training in the heat requires up to 5% more energy during the workout. Burning that many more calories per workout can shift the energy balance enough to make the difference in weight classes if you are a fighter.
  2. Heat training also shifts the fuel source choice away from carbs to fat during the workout making the weight loss even more effective.
  3. Heat acclimatised athletes have a 5-15% increase in their basal metabolic rate. That is, they will burn more energy even at rest with a consequent further shift in the energy balance.

Cutting Weight

In combat sports there is an advantage to being larger than your opponent on fight night even if you weighed the same at weigh-in. Some fighters are known to be able to gain 10-15% bodyweight in the 24-48hrs between weigh-in & the fight. This is accomplished by manipulating the body's water content; remember muscle is 74% water. So yes, dehydration.

There are many strategies employed to manage this feat, some more dangerous than others. By far the majority employ some form of heat training in the training building up to the fight & in the 24hrs before weigh-in. Remember that being acclimatised to heat training includes the ability to sweat as much as 4l i.e. 4kg in an hour. The fact that the body stores water to glycogen in a ratio of 3:1 (500g of muscle & liver glycogen equates to 1.5l of extra water stored in the body) is also used when cutting weight. More details on safe effective weight cutting strategies in a future post.

Heat Shock Proteins

Heat Shock Proteins are produced by the body under stress, in particular heat stress, as with heat training. These proteins fold & unfold other proteins & are beneficial in many ways. They:

  • cause genes that enhance aerobic & anaerobic fitness to be expressed;
  • have an anti-inflamitory effect so you actually recover more quickly;
  • prevent muscle breakdown, anti-catabolic;
  • improved immune function including anti-cancer.


The mantra "Train hard, fight / race easy" doesn't only apply to the physical demands of the competition but also the psychological. If you have adapted & experienced to extreme training conditions you are far less likely to overreact or even panic when confronted by them during competition.

- mec