Heat Acclimation and Blood Volume In Running

One of the subjects Jonathan Savage (the FellRNR running guy) discusses at length is heat acclimation training, where you train specifically in hot conditions to either prepare yourself to race in hot conditions or to successfully race in less extreme, even more normal temperatures.

This is of course amusing to me as we in Las Vegas (and most of the United States, admittedly) are currently suffering through a bout of extreme heat. And in Vegas, we’re used to high heat, with summer days topping 100° Fahrenheit (38° Celsius), but the 110-120°F heat we have now is even above our typical pay grade.

Thus, those of us who run in Vegas get to practice heat acclimation training whether we want to or not!

Of course, Savage refers mainly to winter training for a spring marathon. In the winter temperatures fall below freezing in most parts of the country. If eventually you have to run a marathon in 60-80°F weather (15-25°C), you’re going to get hit hard by relatively warm conditions, even though you’d love to have those conditions in the middle of August. Your body will have acclimated to the other extreme of those cold conditions.

On top of that, Savage typically runs ultra-marathons in more extreme conditions like the hot and dry Badwater 135 race. So he’s not just dealing with a slightly warm marathon in London or Boston. He’s dealing with potentially 100°F heat with doubly dehydrating dry conditions. So even if it’s negative celsius or fahrenheit outside he needs to bundle up to prepare for running in 100°F weather.

Now, all that said, just because you or I have no intention of attempting such a race coming out of winter doesn’t mean that heat acclimation isn’t valuable.

Even with no races on the horizon, running in summer heat and the resulting heat acclimation (within healthy reason: Don’t go taking extended runs once the temperatures are over 100° without an abundance of cooling resources and support)… has one additional key benefit.

Because the body responds to hot training by expanding blood vessels to facilitate cooling, the body over time correspondingly adjusts by improving the volume ported into your muscles and organs. Your heart rate and core temperature remain steadier in subsequent workouts.

As you develop and improve your body’s mitochondria with aerobic training, your body in kind improves its ability to transport oxygen, the key to maintaining your performance in aerobic exercise.

It is this improvement in oxygen and blood transportation that drives your overall improvement in running performance. The more liters of oxygenated blood per minute your heart and body can circulate, the better you will run… contingent as always on proper running form, muscular strength and endurance, staying injury and illness free, proper diet, etc etc.

But, all other things equal, your running performance will improve as your blood volume improves. This is a key component behind the sometimes-mythical VO2max statistic.

Some allege that your VO2max can’t improve. However, many who claim this tend to train/diet/live with the same habits over a lifetime with minimal deviation or troubleshooting.

My VO2max has improved, even after periods where it’s declined, and so has many others. In the years I’ve been able to track it, my VO2max has been as high as 56, dropped as low as 42, and currently sits around 47. The reason many vouch you can’t improve it is because it’s very hard to improve it if you do not vary your training habits… and most don’t vary theirs substantially much at all.

But, as I do, I digress. While VO2max is a relevant part of this topic, the key point I’m making is that training responsibly in hot weather can improve your overall blood volume, which will improve your running once you run and race in more normal conditions.

I probably don’t need to tell most of you this, but obviously you want to run when the hot weather is at its reasonably coolest. I avoid running after 8-9am, and if I run at all after 12 noon it’s at sundown, though even at that time temperatures in Las Vegas are around 100-105°F (37-40°C). But at least during sunset I can run under building and tree shadows as the sun sets.

Typically, if I run at all this time of year, it’s in the morning around 7am. Even then, temperatures in Las Vegas are typically in the 85-90°F range (29-32°C). The runner’s heat index in those conditions is somewhere around 120-125°F (49-52°C). That’s equivalent to running in 80°F (27°C), 50% humidity sunshine, which would be fairly brutal conditions at a race.

To run closer to evening would give you temperatures around 105°F (40°C) with about 15-20% humidity. This gives you a runner’s index of about 127°F (53°C). It’s definitely tougher, plus towards the end of the day you don’t have as much energy and will tire and get hot more quickly.

Running easy in those start/end-of-day conditions will definitely test and improve your body’s blood flow, heat acclimation, and mitochondrial function. Your body will respond to training in those conditions by slowly improving its oxygen intake/usage, blood flow, mitochondria, and ability to maintaining statis in warm conditions.

The other key to running in these extreme heat conditions is you don’t want to do any terribly long runs. I’d keep any single outdoor summer workout to 60 minutes max, because along with the difficulty of running in the heat, the higher UV index of summer sunlight vastly increases the risk of sun damage, from mere sunburns all the way to developing skin cancer. If you want to run longer in these conditions, I’d recommend doing part or all of your running indoors on a treadmill or indoor track.

However, be advised, with most buildings climate controlled to about 75°F, 24°C, and indoor air humidity typically being around 40%, you will have a runner’s heat index around 122°F, 50°C, with no air blowing past you which exacerbates the conditions, and the differing experience of running on a treadmill having some side effects to your running… not to mention treadmill running is monotonous and more psychologically difficult per mile, which offsets a lot of the benefits over running outside.

Also, I’d take it easy on doing any speedwork. I would generally save most if not all the speedwork for spring or fall. At most, maybe fast-finish your runs, going easy and then doing the last 5-15 minutes hard. As hot as summer conditions are, your body will have enough to deal with just running any easy pace in these conditions without having to manage and recover from the stress of a hard running session on top of it.

Unless you’re an elite young talent that can quick bounce back from just about anything, you likely will burn yourself out or get hurt trying to do regular speedwork and any substantial volume of running during an extreme summer… if you don’t suffer heat exhaustion or heatstroke first.

The only possible beneficial reason to do speedwork in this heat is if you have to run competitive races in early fall. In most years I can see considering it. And, again, I would just stick to fast finish runs as your only speedwork, unless your speedwork is just walking to a course, doing a brief warmup followed by a few 400-800 meter repeats (and I mean 4-8, with the workout over in 20 minutes), and then immediately calling it a day, with no other running aside from a very brief warmup.

But this year, with no races on the horizon, there’s not much reason to do more than easy running or light speedwork on a treadmill. Indoor cross training can provide you a venue for that higher intensity you are looking for in a more productive environment.

Now I’m starting to lean into a deeper topic for another time, so I’ll leave this for now. The key point here is that regular easy distance running in this heat can benefit your training down the road in more normal conditions.


So, what about the fall and winter, when it cools down and you may lose that heat acclimation?

Going back to the original point, Jonathan Savage’s winter heat acclimation techniques are best if you have a need to stay heat acclimated for a long spring race in hot conditions.

Otherwise, if once summer cools into fall you (wisely) decide to expand your outdoor training times and scope (e.g. re-introducing more speedwork) into the middle of the day… staying on that schedule as spring warms up can help you re-acclimate to the heat before summer gets hot enough to hen your training back into the early mornings and sunset evenings.

That said, while you may de-acclimate to hot weather during weather, you won’t lose most of the summer improvements to blood volume and oxygen usage if you continue to maintain and improve your training out of summer into the fall. Aerobically you will have likely improved a lot, and now you can open up into tempo runs, longer runs, and speed workouts in the cooler weather. You can expand your training volume and intensity, with your now-expanded aerobic fitness.


After getting away from endurance training for a bit to focus more on strength training, I’m now going to run more regularly with controlled possibly daily easy runs of 30-45 minutes in the morning or during sunset. And I will probably do this through next month.

The added heat acclimation should benefit me substantially once the 110° heat becomes 80ish° heat, and once again even more when whatever people in this desert call “winter” cools everything down to more human temperatures. From there I can ramp back up to a more runner-like training volume, and will be ready to attack my training.

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One thought on “Heat Acclimation and Blood Volume In Running

  1. […] of course, you can train to handle this, and training in hot conditions has all sorts of fundamental long term benefits. But once you get to race day, all the heat is doing is making your race tougher. And the threshold […]

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