Endurance performance in the heat

The human body functions the best at a stable temperature of around 97.7-98.6°F
Thermoregulation is the maintenance of internal core body temperature by balancing heat generation with heat loss. 

When I was participating in XTRI ICON in Livigno, Italy on September 1st, I experienced symptoms of hypothermia after the swim portion of the race. The air temperature was 32 degrees F and the water temperature was 52 degrees F. Although I had on a thermal swim cap, gloves and booties, my gloves filled up with water and I was forced to remove them. Spending almost an hour to cover 3.8K caused my body temperature to drop - I was losing heat faster than I could produce it. After I exited the water, I was shivering uncontrollably. The brain recognized that my body temperature had dropped and it made me shiver - producing heat through muscle activity. Thankfully, Karel and a few others athletes helped me out by getting me out of my wet clothes, putting on dry clothes (and lots of layer) and making me drink warm tea. and after almost 30 minutes, I was able to continue with the race. Thankfully, my body allowed me to perform for over 14 more hours. 

Hypothermia is extremely scary. 
And so is hyperthermia. Unfortunantly, athletes often overlook the risks of exercising in the heat, all in the name of performance. Whereas many athletes are afraid of getting too cold, some athletes don't have the same fears in hot conditions. 

Hyperthermia is the opposite of hypothermia. It's an abnormally high body temperature - or overheating. Hyperthermia occurs when your body absorbs or generates more heat than it can release.

Heat is produced as a byproduct of metabolism (any reaction that occurs in the human body). However, the body is very inefficient during exercise. You lose ~75% of energy as heat from the working muscles. And for that heat to removed from the body, it can be lost through the processes of conduction, convection, radiation and evaporation. Evaporation is the primary method of losing heat from sweating.

It is well known that exercising in the heat induces thermoregulatory and other physiological strain, negatively impacting the endurance performance and physical health. When exercising in the heat, skin blood flow and sweat rate increase to allow for heat dissipation. Sustaining adequate hydration is essential for proper body functioning in the heat. Otherwise, there is great risk for dehydration and experiencing a heat illness. And dehydration will further exacerbate thermal and cardio strain.

Although fluid intake, sodium and carbohydrate are very important during endurance exercise in the heat, athletes tend to make the following mistake.....

Consuming an excessive amount of fluid, sodium and carbohydrates while/as the body is overheating.
 

If your body is overheating because your muscles are creating too much heat in order to move you forward, there's a great risk for GI issues, fatigue and heat stress. Reduced blood flow to the intestines during exercise is one of the primary contributors to GI symptoms. GI issues are exacerbated when the body is unable to regulate body temperature and when dehydrated. Because blood is diverted from the gut and to the working muscles, the lack of blood flow in the gut slows gastric emptying. As a result, the delivery of nutrients and fluids to the muscles gets delayed (which brings on early fatigue) and may cause gut discomfort (and many other uncomfortable side effects). Reduced blood flow to the intestines may also contribute to increased permeability of the gut wall, which may trigger an inflammatory response resulting in prolonged GI distress.

In other words, if your body is creating too much heat because of your effort, fatigue or poor economy, no amount of extra nutrition, sodium or fluids on top of what your body can actually digest and absorb (which at this point may be minimal) will help your body perform better in the heat. 

Instead, to optimize performance in the heat, you need to focus on ways to reduce internal heat build up. This will require reducing your effort, including more stop/walk breaks throughout the race, being disciplined to not overconsume products beyond what you've trained your gut to tolerate and to pay attention to the warning signs of heat stress.