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Toolbox: Cooling from the Inside Out
August has been sweltering for much of North America and Europe, and we know that hyperthermia can have a major negative impact on our performance and even health. Many different methods have been suggested for pre-cooling prior to exercise, but some are limited by their practicality in the field. One simple solution may be to cool from the inside out by ingesting cold drinks, ice, or ice slurries. Pre-race slurpee, anyone?

Hot Enough for You?
This past Sunday, I rode our local Pedal 100 century ride in support of the Ontario Heart and Stroke Foundation. Besides a great ride and cause, my birthday is in mid-August so it also serves nicely as my birthday “big ride.” Being southern Ontario in August, the entire summer has been like riding our bikes underwater, with both July and August featuring very high combinations of heat and humidity.

During the ride itself, the high heat itself is not always the main concern, as the high speeds of road cycling, especially on our very flat course, makes for high rates of convective cooling. This was further aided by the strong wind all day. Overall, this combination of high cycling and wind speed is analogous to high wind speeds causing higher wind chill and cooling in the winter, or a fast running stream taking away more body heat than a still lake at the same water temperature.

Convective cooling is not very effective, however, when you’re moving relatively slowly. This can be the case in running races, such as the Flying Chicken Run I volunteered at last Saturday. Despite generating high amounts of heat buildup, the much slower running speeds of most participants meant minimal cooling via convection. This can also be the case in slower cycling speeds, such as while mountain biking or climbing on the road.

What is often the bigger concern is the humidity, as the high water vapour content in the air means that the sweat produced by your body cannot evaporate into the air to dissipate the heat you are producing. As a result, much of that sweat drips off your body and doesn’t do a thing to cool you, or else stays on your skin and greatly adds to your discomfort level.

I have written much about the potential health dangers of hyperthermia, and also its impact on performance in Toolbox and of course in my own professional career. While many mechanisms may play a role, what is clear is that the body is designed to function optimally within a narrow range of body temperatures. This range of safety and effectiveness may be higher in fit athletes, who may also be capable of temporarily and voluntarily over-riding discomfort from hyperthermia. However, in general, the cooler you can keep the body during exercise, the better your performance.

Pre-cooling Before Exercise
The concept of keeping the body cool for optimal exercise has led to the concept of pre-cooling prior to training or racing, with the general idea of increasing the amount of heat that can be built up in the body before performance decreases. Many methods have been attempted, including the ice or cooling garments first popularized by rowers in the mid-1990s and now commonly employed by many top cycling teams. These work pretty well in allowing some degree of mobility, such that you can multi-task with a warm up on the trainer while keeping the body as cool as possible. Thus, it’s usually much more practical than the initial pre-cooling methods of cool baths, where you can’t prepare for competition and you may end up cooling your legs too much.

Cooling garments are nice, but they still often require a power supply or, in the case of ice vests, a way to replace the ice as it melts. Therefore, unless you’re a very well-funded pro or individual with a large support infrastructure, it may still be impractical to implement. The same is true in many work applications where the site is remote or the work is highly mobile, hence recent work on other alternative pre-cooling methods.

Drink Your Way Cold
Rather the cooling from the outside in as with ice vests or cool baths, the other potential is to cool from the inside out. This can be done by drinking cold fluids, as first demonstrated with some of my doctoral research (3). In our case, we were looking at exercise in the heat while wearing military chemical warfare clothing, obviously a very high combination of heat stress. What we found was that drinking even a moderate amount of warm (35oC) water before and during exercise, which would offer pretty much no pre-cooling effect, can still significantly increase the total heat storage capacity of the body. In effect, the extra mass of fluid acts as an additional heat sink for all the metabolically generated heat from exercise.

So If Warm Water is Good, is Cold Water Better?
One followup study from what we tested was performed by my former post-doctoral advisor’s research group in the UK. Lee et al. (2) looked at the possibility of rapidly cooling individuals prior to exercise by simply drinking cold fluids

The basic experimental plan:
– 8 males cycled moderately (66% VO2peak) in a hot and humid (35.0oC, 60% relatively humidity) environment to voluntary exhaustion.

– Pre-exercise, subjects drank 300 mL of either cold (4oC) or body temperature (37oC) water, followed by 100 mL of that drink each 10 min during exercise. 30 min rest occurred between fluid ingestion and exercise.

The primary results:
– The cold drink was able to pre-cool subjects by 0.5oC. This is well in-line with the magnitude of pre-cooling achieved in previous studies using cooling garments or cold baths.

– Exercise time at the fixed pace to the point of voluntary exhaustion was much longer with the cold drink (63.8 min) than with the warm drink (52.0 min).

– Heart rate and skin temperature was lower in the initial half of exercise with cold drink consumption, while overall sweat rate was less in the cold drink condition.

– Thermal sensations and ratings of perceived exertion were lower with the cold drink condition.

So if you combine my initial study with that of Lee’s, the picture appears to be that simply ensuring that you’re adequately hydrated before exercise is beneficial in sustaining and prolonging performance in the heat. Furthermore, actively drinking a small amount of cold water can bring physiological and perceptual benefits prior to exercising in the heat.

Overall, such a simple and practical recommendation has been adopted in many occupational settings, including the Canadian military that funded my doctoral research, and also the Toronto firefighters that my doctoral lab has worked extensively with in developing heat stress policy.

For cyclists, a simple countermeasure prior to exercise in the heat may be to simply have a supply of cold drinks in a cooler at your events for both pre- and post-event hydration. The key to remember that it’s not necessary to drink a massive amount of water beforehand, and that 300 mL of cold water was sufficient in Lee’s study.

If Cold Water is Better, Then is Ice Best?
If cold water is better than body temperature water, and if the name of the game is extracting heat from the body, then the next logical step would appear to be to lower the drink temperature even further down to the level of ice, crushed ice, or some form of ice slush/slurry. This too has received interest in occupational settings. While power and refrigeration requirements may increase over that needed to maintain cold water, one inherent advantage is that it is very hard to consume massive amounts of water as ice. Therefore, the risk of over-consumption of water or hyponatremia is minimized.

The use of crushed ice on cycling time trials was published this past spring by Ihsan et al. (1). Like the Lee study, the experimental design was fairly straightforward:

– Seven trained subjects were familiarized with the 1200 kJ cycling TT, done indoors on an ergometers.
– 30 min before the TT, subjects consumed 6.87 g/kg body mass of either crushed ice or tap water.

Major results:
– The ice ingestion dropped body temperature (measured using gastrointestinal core pills) to 36.74oC, compared to 37.27oC with tap water. Again, similar 0.5oC pre-cooling as targeted in many pre-cooling studies.
– The core temperature difference was maintained until about 200 kJ of work was completed, before the core temperature profiles converged and remained similar for the rest of the TT.
– No difference was found in skin temperature, heart rate, or ratings of perceived exertion throughout either condition.
– Performance-wise, TT was faster for ice (5011 s) compared to tap water (5359 s). Split times for each 100 kJ interval tended towards being faster for ice throughout every interval, becoming statistically significant in the final 3×100 m of the TT.

Overall, the combined studies suggest that effective pre-cooling does not necessarily have to involve expensive and logistically demanding technology. Rather, clear performance improvements may be achieved very simply by making sure that you’re adequately hydrated to begin with, and furthermore by ingesting a small amount of cold water or crushed ice before training or competing in the heat.

Nothing beats experimenting on yourself, and there’s no better guinea pig than yourself. So try it out in training to see if it works for you, and let me know how you respond.

Have fun and ride safe!

1. Ihsan M, Landers G, Brearley M and Peeling P. Beneficial effects of ice ingestion as a precooling strategy on 40-km cycling time-trial performance. Int.J.Sports Physiol.Perform. 5: 2: 140-151, 2010.
2. Lee JK, Shirreffs SM and Maughan RJ. Cold drink ingestion improves exercise endurance capacity in the heat. Med.Sci.Sports Exerc. 40: 9: 1637-1644, 2008.
3. McLellan TM and Cheung SS. Impact of fluid replacement on heat storage while wearing protective clothing. Ergonomics 43: 12: 2020-30., 2000.’

About Stephen:
Stephen Cheung is a Canada Research Chair at Brock University, and has published over 50 scientific articles and book chapters dealing with the effects of thermal and hypoxic stress on human physiology and performance. He has just published the book Advanced Environmental Exercise Physiology dealing with environments ranging from heat and cold through to hydration, altitude training, air pollution, and chronobiology. Stephen’s currently writing “Cutting Edge Cycling,” a book on the science of cycling, and can be reached for comments at .


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