Cycling in Summer Heat

On many of my bike rides in warmer weather, I’ve observed that I sweat a lot more than the average person.  In extreme heat (>30 degrees Celsius plus humidity), I can’t keep enough fluids in me to keep up with the sweat.  This has prompted me to do a little research into the physiology of exercising in hot weather.

Here’s what I’ve learned:

1. During exercise (at any temperature), the core temperature of the body increases.  The rate of this increase depends on how hard the athlete is exercising and the air temperature (and it is different person to person).
2. The body’s cools its core temperature two ways: (1) sweating + evaporation of sweat (evaporative heat loss) and (2) heat flow from the body core to the skin via blood (convective heat loss)
3. At higher air temperatures, we rely more on evaporation of sweat to keep our core temperature lower (as we exercise, the sweat on our skin evaporates and when this happens, the skin surface cools).
4. Rate of sweating is highly variable between individuals.  In hot and humid weather, athletes have been tested to produce 2 to 3 litres of sweat per hour, and even moderate exercise can result in a sweat rate of 1 litre per hour.
5. Dehydration can result if sweat loss is not replaced.  Dehydration will result in a further increase in core body temperature.
6. During exercise in hot/humid weather, the body’s need to cool requires increased blood flow to the skin.  This blood demand means there’s less blood available for the muscles and there can be a reduction “output” (i.e.,  cycling power)

But what about hydration?  If we’re losing 1 to 3 litres of sweat per hour on hot rides, how do we replace it?  Other references state that healthy kidneys can filter about 0.8 to 1.0 litres of water every hour.  However… during exercise, the body releases an antidiuretic hormone that lowers the ability of your kidneys to filter water.

So if I’m losing 1-3 litres of sweat every hour, but can only absorb 1 litre every hour, what does that mean?  Some studies have shown that certain individuals can absorb more than 1 litre every hour (that’s potentially good news).  Also, we can acclimatize to the heat (also, good news) by slowly increasing the effort and duration of workouts in the heat.

Although I’ve never experienced it myself, apparently it is possible to over-hydrate.  In fact, one report states over-drinking water can be more dangerous than under-drinking (I don’t believe this…. but as I said, I’ve never had too much water on a ride).  My issue tends to be carrying enough water to stay hydrated on hot summer bike rides.  I suspect that my rate of sweating is on the upper end (i.e., more than 1 litre per hour).  I can drink well over a litre an hour on hot days and still keep sweating, so to my knowledge, I’ve never over-hydrated.

So from my experience, here’s what I try to do:

• Wear light coloured cycling clothing that wicks moisture away from my skin
• Try to carry 1 litre for every hour I’ll be riding, or plan a ride with a stop where I can re-fill water bottles such that I can consume 1 litre per hour. Cycling water bottles are typically 3/4 of a litre (the larger ones) so two bottles should last for about an hour and half.
• Make sure to include electrolytes in fluid consumption (I’m currently using Skratch because it’s easy on my gut.)
• Avoid the hottest time of day (the afternoon) if possible
• Reduce my expectations during hot rides.  Shorter, easier rides are preferred.

One final graphic about heat exhaustion and heat stroke:

I’ve had a couple of rides in the past week where I’ve had some of the symptoms of heat exhaustion, so I need to be careful!  Maybe I’ll try an ice vest!

Finally, here are a few links:

https://www.bikeradar.com/road/gear/article/8-tips-from-the-pros-for-staying-cool-when-cycling-in-the-heat-47563/

https://www.ncbi.nlm.nih.gov/books/NBK236240/

https://www.active.com/triathlon/articles/cracking-the-code-on-sweat-rates

https://www.hammernutrition.com/knowledge/hydration-what-you-need-to-know/

http://running.competitor.com/2014/07/training/heat-acclimatization-for-runners_12035

https://www.bicycling.com/training/a20011258/heat-acclimatization/