Hey everyone and welcome back to my weekly newsletter. For those of you who it is your first time reading my newsletter, I’m happy to have you. I hope you gain some useful insight.

In last week’s newsletter, I discussed the acute effects of heat exposure on exercise tolerance, and a few tips to reduce the effects of heat as we progress through the summer. This week’s newsletter will discuss heat acclimatization, how long it takes, and whether it really is poor man’s altitude.

Growing up in western Pennsylvania, I wasn’t too familiar with the effects of acute or chronic heat exposure on running performance. The highest we would get in the summers would rarely crest 90 degrees Fahrenheit, and most of my runs were done after work when the sun was setting. As such, I was really confused when I started training in graduate school and heard all my friends and training partners referring to summer running as “poor man’s altitude”. 

This didn’t make sense for several reasons, namely because the mechanisms aren’t even remotely the same. Altitude exposure increases red blood cell mass from the lower partial pressures of oxygen that signal the kidneys to produce erythropoietin. Heat exposure simply increases stress on the kidneys and heart due to fluid loss and the redirection of blood flow to the skin. 

Well, it turns out I should have given my friends more credit…

Practically, we see people perform better in the heat when they’ve been training in the heat. However, there are studies that show enhanced performance in cooler weather as well. While there are many responses, the most important to me are:  increased plasma volume, increased stroke volume, increased skin blood flow, lower core temps and increased sweat rate.

The increase in plasma volume is the first adaptation in response to exercise training, occurring within the first few days, and we know that this is driven even further when exercising in warm conditions. Because VO2 = Heart rate times stroke volume (amount of blood pumped per beat) times the avO_2diff (the amount of oxygen extracted from the blood by muscle), the increase in plasma volume subsequently increases the amount of blood that can come into the heart, which stretches the heart to let it beat harder, that then further increases stroke volume and blood flow to increase oxygen consumption. 

The increase in sweat rate is also important in both hot and thermoneutral conditions because core temperatures around 40 degrees Celsius are dangerous for protein structure. Keep in mind that core temperature will rise even in colder conditions due to the inefficiency of metabolism (75% of its energy production is lost as heat L ). As such, the increased sweat rate permits the endergonic effects of evaporation due to the energy required to break hydrogen bonds in liquid water. 

Functionally, this manifests as lower heart rates at submaximal workloads, progressively lower VO2s, and lower blood lactates as the body can prevent full redistribution of blood to the skin for cooling purposes. If you look at figure 2 for example, you can see how increasing heat tolerance and plasma volume would permit a higher amount of cardiac out flow to muscle as opposed to skin during intense exercise. This figure also demonstrates nicely how much lower total cardiac flow is in hot environments with quantitative values for a-vO_2diff and stroke volume. The consequences of this redistribution of blood are shown in figure 3.

The good news is that heat exposure really is poor man’s altitude. Specifically, a meta-analysis assessing 96 studies on heat exposures averaging 7-14 days showed that heat training like saunas increased exercise performance by about effect sizes of 0.5 to 0.75, which we consider as a “medium” effect  [3]. Consider figure 4

There are stepwise increases in exercise performance (time trial) and exercise capacity (time to exhaustion) with increasing volume of heat exposures. This figure pretty much sold me on the principle though with some caution. Some of the mean differences were small while others were large. Moreover, some studies showed no difference at all. One thing to consider as well is that you can’t blind participants in a lot of these conditions. They know they are being exposed to heat, so that inherently adds some bias. Also consider that when heat will increase time to exhaustion more than time trial performance. The ambiguous end of capacity tests like this have seemingly more of an effect than on time trial performance with a hard endpoint. As such, we can infer that the cognitive side plays a substantial role when it comes to heat acclimatization.

Overall, it seems to work, and protocols as short as 7 – 14 days are seemingly enough to see meaningful changes with the first benefits (plasma volume) coming within a few days. Keep in mind, these studies aren’t on the effects of sauna but rather exercise in warmer conditions. When it comes to the rate of decay, figure 5 shows that you can expect about a 20% decay every 5 days with complete loss of benefit by day 20. Interestingly, some of the studies showed prolonged benefits, though this could just be from continuing regular exercise and progressively increasing intensity.

One thing I will point out is that while the benefits dissipate by 20 days, you are going to see increased intensities at thermoneutral temperatures prior to the loss of plasma volume. As such, your overall training load will be higher. Thus, you will retain the benefits from the heat acclimatization from increased training load. This is a similar mechanism as taking bicarb or Nomio for workouts. They increase your overall workout quality which enhances your training load and long term adaptation when stacked week over week.

If you’re hoping to implement this practically, here is what the literature says. Disclaimer: I am not recommending you do this without expert guidance. Heat illness is very dangerous and can result in death. Consult your physician before considering implementation.

·      Adaptation happens at temperatures between 80 and 100 degrees Fahrenheit.

·      Exercise or heat exposure (sauna or warm bath immersion) can last 30-100 minutes.

o   Post exercise sauna can be effective

·      Acclimatization happens between 3 and 14 days.

o   Longer duration increases exercise capacity

·      Generally, a wet-bulb globe temperature ≥30°C provides sufficient environmental stress to induce HA

·      Missing 1–3 days of exercise-heat exposures during a 10–14 day acclimation period will not likely impede HA. When heat exposure occurs every 2–3 days, it can take a full month to induce HA

·      I wouldn’t recommend working out vigorously in the heat. This will reduce exercise tolerance, functional VO2max, and reduce workout intensity.

o   ”Robbing Peter to pay Paul”.

If you don’t know, I offer person coaching for whatever running goals you may have, and the entire team is crushing it. Below are some highlights from this past week. I’m looking for dedicated runners to join the squad, so if you’re ready to take it to the next level, you can check it out on my website and book a free consultation call with me.

https://www.endurancelaboratory.com

In this week of Endurance Lab Athletes, we had some big wins. First, we have Ryan Monohan and Quinn Kilkenny of New Canaan in CT. The two of them raced in their state open meet and both had personal bests. Ryan ran a 4:08 in the 1600 while Quinn ran 4:14 for second and third, respectively. Well done lads.

On the roads, our very own Florencia Kah ran a 10k PB of 62 minutes!! This was a massive improvement, and she is poised for even more improvements as we keep training.

Some of the smaller wins were for Jones Nauseef and Adam Hunt who have both hit an annual mileage PR. Jones was banged up for a bit, but he’s slowly been working back while Adam prepares to break 5 in the mile as a working professional with kids.

If you found this newsletter insightful, I think you would really appreciate some of the work content we have on the podcast, where I have discussed the science of running with incredible guests, including professionals, olympians, and top NCAA athletes. Additionally, make you follow me on Instagram for more insight. Kill it this week!!

1.         Pryor, J.L., et al., Application of evidence-based recommendations for heat acclimation: Individual and team sport perspectives. Temperature (Austin), 2019. 6(1): p. 37-49.

2.         Périard, J.D., T.M.H. Eijsvogels, and H.A.M. Daanen, Exercise under heat stress: thermoregulation, hydration, performance implications, and mitigation strategies. Physiological Reviews, 2021. 101(4): p. 1873-1979.

3.         Tyler, C.J., et al., The Effects of Heat Adaptation on Physiology, Perception and Exercise Performance in the Heat: A Meta-Analysis. Sports Med, 2016. 46(11): p. 1699-1724.

4.         Daanen, H.A.M., S. Racinais, and J.D. Périard, Heat Acclimation Decay and Re-Induction: A Systematic Review and Meta-Analysis. Sports Med, 2018. 48(2): p. 409-430.