A better way to keep your cool when the heat is on

Talk to most any endurance athlete on the matter of staying cool in the heat, and you’ll hear the same advice: ice in the hat, ice on the neck, ice down the bra, even ice vests. Alas, it seems we’ve been doing it wrong.

First, there’s nothing magical about cooling the head – or major vessels leading to the head.  It is true that the hypothalamus regulates temperature, but this structure is deep within the brain and certainly won’t be impacted by some ice in the cap. It is more about cooling the circulating blood that passes the hypothalamus. When body temperature rises, the nervous system redistributes blood flow, sending a larger percentage of it to the skin. This shunting of blood to the periphery promotes heat loss through convection. The brain loathes major change, and you don’t see any such increase in blood flow here with exercise or heat (imagine the headache).  So, our physiology really doesn’t support any unique benefit of ice application to the head.

Neither does the research. Both neck cooling and head cooling have come up short in studies. Each approach has been ineffective in lowering body temperature while exercising in the heat (1,2).*

Another enduring notion around cooling stems around the misconception that the more surface area you target with cold, the more heat that dissipates from the body.**  This would be true if heat transfer were uniform across body surfaces, but that has not been supported by the research. Cooling vests provide a nice example here. They can provide some performance gains at shorter durations, but the research has not consistently supported their use as a means of actually lowering body temperature (3). Consider, too, that muscle and fat are good insulators. The direct application of cold to the skin is all about optimizing conductive heat loss, and it’s just not easy for heat to escape through these anatomical layers.  Ice down the bra or shirt hasn’t exactly been studied, but the same principles apply!

Importantly, overwhelming cold can undermine our goals. Very cold temperatures cause vessels to constrict. This slows the flow of cooler blood back to the core – the opposite of what we’re seeking.  Following tips like putting ice under your arms (eeek!) or near the groin may do you more harm than good. The reduction in blood flow triggered by cold may partially explain why ice baths after exercise may impair long-term gains (4).*** If anything, we’re restricting blood flow when it is needed most. And, why are athletes so intent on hindering the inflammatory process that our physiology has so masterfully designed to speed healing?! …closing that can of worms back up.

I’ll cut to the chase. How do we better work with our physiology to stay cool in the heat?

There is a convincing body of evidence showing the palms of the hands, the soles of the feet, and regions of the face are unique portals for dumping heat. The glabrous skin in these regions is anatomically adapted to accommodate large volumes of blood flow – and in turn to dissipate large quantities of heat (5,6,7). What makes these regions so unique is that they have a particularly high concentration of arterio-venous anastomoses (AVAs).  

Quick anatomy tutorial: arteries carry blood away from the heart and veins return blood to the heart. Typically – small, diffuse capillary beds in the tissues allow blood to transition from the arterial side of our circulation back to the venous side. But with AVAs, there is a direct connection between the small arteries and the small veins (see pic!). These AVAs really don’t play a role in nutrient transport, but they’re very adept at transporting heat (7)!

A team of Stanford researchers has published promising findings in this area. In one study, subjects wear a cooling glove on a single hand while exercising at both moderate and high workloads. With the cooling glove, subjects exercised 30-40% longer at high workloads and a striking 50-70% longer at moderate workloads (8). A subsequent study showed hand cooling to speed recovery from heat stress and found the effects to be additive. That is, when cooling was applied to both hands, body temperature recovered even faster (5).

Other teams of researchers have demonstrated that hand cooling between bouts of activity in the heat increases the duration of subsequent exercise and lowers body temperature (9). Yet another group had trained cyclists submerge their hands in water at various temperatures while riding a stationary bike. Once again, hand cooling allowed subjects to ride longer in the heat and keep their body temperatures lower (10).

So, how can we sensibly apply this knowledge? We get creative. At my last ultra in the heat, I ran with anything cold I could get my hands on – or in. I’d dunk my hands in creek crossings for 10-15s and pass frozen otter pops or crushed ice between hands as I left aid stations. That passing back and forth is key with items that cold! Remember, you don’t want to risk constricting the vessels in the AVAs and sabotaging your efforts. If those vessels narrow in reaction to the cold, then you’ve just lost a critical means of heat loss.

What about those small instant ice packs? I could even see them marketed in a similar fashion to the hand warmers used for cold-weather endeavors. Held between your hand and a water bottle, you wouldn’t even notice! Shuffling an ice-filled buff between hands brought some positive results for a friend who just tackled Western States 100. Even hand-held water bottles themselves have hidden potential. As long as you are grasping something cool, blood should travel back to your heart at a slightly lower temperature.

We’re more limited with the feet and face, but we can do our best to keep the cheeks cool. Rather than laying that ice-filled buff around the neck, try periodically pressing it into your cheeks or lips. I see the biggest potential for gains here in cold weather. Our thermoregulatory mechanisms work no different in winter. It may be time to embrace the balaclava. Thanks to the pandemic, I think we all have a healthy collection of buffs that we can tap into as well.**** ^{(last footnote – promise!)}

For now, hands are the easiest targets. I do look forward to seeing more practical approaches to AVA cooling roll out for athletes. Until then, our physiology and the research have convinced me to change my own approaches. If you’ve read this far, hopefully you may do the same!

Lastly, I can’t recommend the Huberman Lab podcast enough. This episode tuned me into the benefits of these glabrous ports, and I took a deeper dive into the endurance piece and thermoregulatory research here.

*Still, there’s no denying that some cold ice around the neck or in the hat simply feels amazing in the heat. Such perceptual effects are thought to be behind some of the temporary gains in performance that have been observed (2). So, these approaches are seemingly better than nothing.

*Surface area is a relevant factor when considering evaporative heat loss. We have our sweat to thank for this type, but its effectiveness is really limited by humidity and other factors. The direct application of ice to the skin targets conductive heat loss.

**Pre-cooling, like drinking ice cold drinks or soaking in cool baths before a short event, does have merit and support in the scientific literature. Still, the effects of this heat sink approach don’t appear to last through the hour. Unless you’re racing a crit or a 10k in the heat, the approaches are of little practical benefit. Athletes also have to be cautious not to overdo the cold-water immersion and demand high workloads from muscles that are still cold (2).
Cold baths or showers can be awesome for certain metabolic and nervous system gains, but I skip them after workouts now.

**** For reasons described earlier, we do not lose 40-50% of our body heat through our heads. This myth has been repeatedly debunked, but the advice from generations of well-meaning parents carries on.

REFERENCES

1. Ansley, L, Marvin, G., Sharma, A, Kendall, M., Jones, D., & Bridge, M. (2008). The effects of head cooling on endurance and neuroendocrine responses to exercise in warm conditions. Physiological Research, 57, 863-72.
2. Tyler CJ, Sunderland C, Cheung SS. (2013) The effect of cooling prior to and during exercise on exercise performance and capacity in the heat: a meta-analysis. Br J Sports Med, 49(1):7–13.
3. Bongers CC, Thijssen DH, Veltmeijer MT, et al. Precooling and percooling (cooling during exercise) both improve performance in the heat: a meta-analytical review. Br J Sports Med. 2015;49(6):377–384.
4. Broatch, J. R., Petersen, A., & Bishop, D. J. (2018). The Influence of Post-Exercise Cold-Water Immersion on Adaptive Responses to Exercise: A Review of the Literature. Sports medicine (Auckland, N.Z.), 48(6), 1369–1387. https://doi.org/10.1007/s40279-018-0910-8 https://pubmed.ncbi.nlm.nih.gov/29627884/
5. Grahn, D., Dillon, J., & Heller, C. (2009). Heat Loss Through the Glabrous Skin Surfaces of Heavily Insulated, Heat-Stressed Individuals. Journal of Biomechanical Engineering. 31,
6. Grahn, D, Makam, M, & Heller, C. (2018) A method to reduce heat strain while clad in encapsulating outerwear, Journal of Occupational and Environmental Hygiene, 15:8, 573-579, DOI: 10.1080/15459624.2018.1470635
7. Walløe L. (2015). Arterio-venous anastomoses in the human skin and their role in temperature control. Temperature (Austin, Tex.), 3(1), 92–103. https://doi.org/10.1080/23328940.2015.1088502
8. Grahn, D., Vinh C., & Heller, C. (2005). Heat extraction through the palm of one hand improves aerobic exercise endurance in a hot environment. Journal of Applied Physiology, 99, 972-978.
9. Goosey-Tolfrey, V., Swainson, M., Boyd, C., Atkinson, G., & Tolfrey, K. (2008). The effectiveness of hand cooling at reducing exercise-induced hyperthermia and improving distance-race performance in wheelchair and able-bodied athletes. Journal of Applied Physiology, 105, 37–43. doi:10.1152/japplphysiol.01084.2007
10. Ruddock, A. D., Tew, G. A., & Purvis, A. J. (2017). Effect of hand cooling on body temperature, cardiovascular and perceptual responses during recumbent cycling in a hot environment. Journal of Sports Sciences, 35(14), 1466–1474. https://doi.org/10.1080/02640414.2016.1215501