An evidence-based review of heat illness for runners.
Written by Dr. Josh Mularella, DO, FACEP, FAWM, DTM&H - Medical Director, G2G & M2M
The average core body temperature is 37C (98.6F) with an optimum range around 35C to ~ 40C. If your body gets too hot (~ > 41C or 106F) then the biochemical pathways that keep your body running start shutting down resulting in seizures, coma or even death.
There are only a few general reasons why this happens:
1. Too much endogenous heat (you are running too fast)
2. Too much exogenous heat (it is really hot outside)
3. Impaired mechanisms for heat loss (you can’t sweat or increase your heart rate)
4. A combination of the above
1. Our bodies are not terribly efficient. Approximately 70% of the energy produced by your muscles while running is released as heat. Overexertion is probably the #1 reason for heat illness in runners which explains why it is most common in 10k races. Those events allow participants to run at fast speeds but are still of long enough duration for the overall heat balance to increase to a dangerous level. 5ks are usually over too quickly and marathons (should) force runners to go slower thereby decreasing heat production and allowing adequate time for heat dissipation.
2. Many road races will cancel/postpone the event if the temperature outside is too hot since it is an independent risk factor for heat stroke. A better predictor than just outdoor temperature is the Wet Bulb Globe Temperature (WBGT - see chart below) which takes humidity into account as well. When the WGBT index reaches “BLACK” the US Military ceases all outdoor physical training. Similarly, the American College of Sports Medicine (ACSM) heat and humidity guidelines for races recommends cancelation or voluntary withdrawal in blag flag conditions.In ultras, heat is part of the fun! The course at G2G is unforgiving at times with little to no shelter from the sun. Temperatures vary year to year but when it’s hot… IT’S HOT! Under very hot conditions it is imperative to go slower and take breaks as needed.
3. Fortunately we have all evolved to handle heat stress rather well. There are four main thermoregulatory principles allowing us to off load extra heat:
● Conduction: requires direct contact, such as sitting on the cold ground
● Convection: losing heat through movement of air/water molecules across the skin, such as jumping in a river
● Radiation: loss of heat as infrared rays to the environment, which is how we get heat from the sun
● Evaporation: the most important for runners, is the conversion of water (sweat) to gas
Evaporation will be your best friend at G2G (aside from the occasional stream). I certainly recommend getting sprayed by your friendly volunteer staff at checkpoints before heading back out on the trail. Evaporation is much less effective in high humidity and thus explains the “web bulb” inclusion in WBGT readings. Fortunately the average humidity for Southern Utah in September is < 10%.
While running, if your core temperature starts to rise, your body shunts some of your blood to the skin getting it closer to the (hopefully) cooler surface. This allows for heat loss by convection/conduction. It also activates your sweat glands allowing for increased evaporation. Other compensatory mechanisms include a faster heart rate to keep up with the metabolic demand. Any preexisting injury/illness or medication that interferes with these pathways will increase your likelihood of getting heat stroke.
For example, many heart medications slow down your heart rate. Allergy medications (and sunburn!) can dehydrate you and inhibit sweating. Antibiotics could make you more sensitive to the sun. This is why it is very important to talk with your primary care provider about your medications when you have your medical screening appointment.
I am not going to get into much detail on heat acclimatization but this is something you should definitely research on your own and discuss on FB. Heat training not only improves your tolerance/adaptation to heat but, similar to training at altitude, it also improves your cardiovascular fitness and VO2 max. Optimum acclimatization takes around two weeks. If you can’t train in the heat locally then I recommend showing up in Kanab a few days early if possible.
Now we can finally talk about “HEAT ILLNESS”. Let me be clear that the ONLY heat illness is heat stroke. “Heat cramps”, “heat exhaustion”, “heat syncope”, etc are not true heat illnesses. There is no spectrum of heat illness that progresses through these various conditions culminating with heat stroke. You either have increased core temperature and signs/symptoms consistent with heat stroke or you don’t. A good review of this by Timothy Noakes can be found here. Nowadays you will likely see these conditions instead referred to as “Exertion associated muscle cramps (EAMC)” and “Exertion associated collapse (EAC)”.
To further complicate things, a core temperature > 40C (104F) by itself does not equate to heat stroke. A study by Byrne et al in 2006 followed the core temperatures of 18 heat-acclimated soldiers in Singapore during a half marathon. All 18 soldiers finished without any symptoms of heat illness. 12 of the 18 soldiers finished with core temperatures > 40C and 2 were > 41C. None had heat stroke.
Heat stroke is very complicated. Physiologically it is not all that different from a severe infection (sepsis) as shown in a review article in the New England Journal of Medicine in 2002. Continued elevated core temperatures inevitably cause an exaggerated thermoregulatory response leading to circulatory collapse and heat stroke. There is a point of no return where even rapid cooling is no longer helpful.
Heat stroke is diagnosed based on elevated core temperature (ie a rectal temperature) and signs of central nervous system involvement (ie confusion and unsteadiness). Clinically heat stroke can resemble low blood sugar (hypoglycemia), low sodium (hyponatremia) and even altitude sickness. The medical team only sees runners at checkpoints and at base camp therefore it is very important that you keep an eye on each other as well. If someone has a change in behavior, seems confused, not walking steady, etc then please let one of us know ASAP!
Take Home Points
G2G is going to be a very hot race. There is little relief from the sun especially during the first two stages. If you feel like you are getting too hot, then slow down. Seek shade when possible. There is no shame in taking breaks at the checkpoints!
Try to get acclimated to the heat in the weeks leading up to G2G. Maximize your heat tolerance by wearing appropriate clothing, staying hydrated, and discussing any injuries/illnesses and medications with your primary care provider. I will discuss electrolytes and hydration in a different post.
Keep an eye on yourselves and each other. And remember that if you fall victim to heat illness you may end up getting a rectal temperature at the checkpoint!
Good luck with your training and I’ll see you in Kanab!!!
If you have any questions/concerns please feel free to email me at