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The Heat Will Slow You Down – but it Might Just Speed You Up!

Tom 'rehydrates' after some heat training.

We all know running in the heat is miserable – it's slower, sweatier, and takes more preparation with regard to clothing, sunscreen, route choices and hydration.

But what if, despite slowing you down, it could actually make you more fit? Science shows us that's exactly the case!

Benefits of Running in the Heat

It's been well-known for many years that running in the heat can have beneficial effects similar to altitude training (something almost all pro runners do, for good reason). Recently, further research has provided more evidence of the mechanisms behind this.

Blood Plasma

After just a few days of running in the heat, your body responds by significantly increasing the volume of plasma in your blood. This helps transfer heat more effectively. Remember, your muscles are generating heat as you run, regardless of external temperature. This will certainly boost performance in hot weather, but since plasma doesn't carry oxygen, how does it benefit your overall fitness?

For a long time, scientists were unsure of how that increase in plasma volume could lead to benefits at more reasonable temperatures, only that it did have benefits. However, a recent study in Experimental Physiology gives some clues.


Yup - that EPO. The hypothesis (the exact mechanism is still unproven) is that because increases in blood plasma lower hematocrit, the kidneys, always trying to keep hematocrit in a normal range, respond by producing EPO. Whether natural or 'augmented,' EPO triggers the production of more red blood cells. That leads to greater oxygen-carrying ability, and therefore more fitness.

The Study

This recent study involved 23 elite cyclists doing ten hours of training per week, 11 in the intervention group and 12 in the control group. As part of the study, they added five 50-minute afternoon sessions at an easy effort per week. The intervention group performed these in 100ºF and 65% humidity with only a half-liter of water, while the control group used reasonable temperatures and water intake. Athletes in the intervention group were encouraged to rehydrate immediately after each heat session.

The result after five weeks was a 4.7% increase in hemoglobin mass, which is statistically significant. There were other, smaller effects on performance tests, generally favoring the heat group.

The Bottom Line

Heat training – whether by choice or forced – is good for your fitness. Short-term effects happen in as little as a few days, and primarily benefit your running during hot weather. Longer-term effects may benefit your running in all temperatures, and happen in 5+ weeks, but the exact amount and mechanism is still unproven.

Should you deliberately seek out heat training to make you fitter, similar to altitude training? Probably not, unless you are training for a race in extreme conditions (Western States, Badwater, Marathon des Sables, etc). The effects are small enough that unless you are doing everything else to the best of your ability, it's probably not worth the trade-offs (being miserable and running slower).

If you are training for a race that you expect to be hot, you should definitely do more heat training, however there are a variety of ways to tackle this, from short doubles (like the study above) in the afternoon, to post-run sauna, to simply training normally if you live in a hot climate. I would recommend trying to keep your workout runs in more reasonable temperatures, so that you are running at a normal pace. (While pace doesn't matter for most training stimuli, it's helpful to give yourself the best chance of speed to ensure neuromuscular benefits.)

Regardless, what you should do is embrace the heat that summer weather brings you, and remember that despite slowing you down, it may just make you fitter!

Speaking of slowing you down – what should you expect?


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Adjustments for Running in the Heat

First of all, remember that running in hot temperatures isn't a 'mind-over-matter' situation. You must slow down. It's a physiological necessity. Yes, heat adaptations as mentioned above can help, and yes, your mental approach can also help, but there are still physiological limitations at play here and it's important to remember that.

Second, the performance impacts from heat start at 59ºF, or potentially lower. That's well-below where you will actually feel like it's hot. You don't just want to stay in the comfortable zone, you want to stay on the cool side of comfortable. Do whatever you can to do that! (See Other Considerations below).

If you are focused on your pace at all – read here why you shouldn't be – you need to adjust significantly for the heat! I recommend training by effort whenever possible. If you're not using pace as a guideline, you don't need to do much adjustment regardless of circumstances – very convenient!

If you're training by effort, the only adjustment I recommend making is erring on the slightly easier side for most runs, as core temperature will rise throughout the run. And unless you're running in the evenings, ambient temperature will either rise or stay static too. For intervals, you may be able to get away with running at a normal effort, but lengthening or making recoveries easier. This can be helpful to increase running economy benefits, because you don't have to slow down as much!

If you insist on training by pace, (seriously, you are holding yourself back here), below is an approximate chart of how much you should be slowing down, depending on the temperature and dew point.

I think you'll be surprised at just how much of a difference even conditions we think of as moderate will make on your pace at a given effort! It's common here in the Northeast USA to see temperatures in the mid-80's with dew points in the high 60's. As I write this, it's 87ºF with a dew point of 66º. Following the chart below, that means up to a 6% slower pace.

If you're normally running 8:00/mi, you should be running 8:28/mi. And of course, if it's an easy run – slower is even better, assuming your running form can stay the same.

Note that this chart has not been updated with recent studies on performance impacts of heat. It may be worth your while to add an 91-100 line up top and shift everything.

Other Considerations

For those of us that can't avoid running in the heat, here are some other things to keep in mind!


As the heat increases, be sure to stay hydrated. My recommendation to my athletes is to focus primarily on hydration throughout the day, rather than solely during the run. In fact, for shorter runs (less than an hour), if your overall daily hydration is perfect, you can get by with just pre-hydrating and post-hydrating. But a common theme I see among runners is to be really good at hydration during the run... but not so much outside it. If you're already starting at a deficit, no amount of fluid during the run is going to help you.

Sometimes, run without water (for shorter runs) or limit your water intake. This applies especially if you are targeting heat training by choice for a big race. This ensures very mild dehydration, which can boost the blood plasma volume benefits I mentioned earlier. This was an important component of the recent study. Just be sure not to do to this every run – as with many things, more is not better.


If you're sweating, you aren't just losing water, but salt, magnesium, potassium, etc. You don't need to go crazy with this, but be sure to be replacing electrolytes as well as fluids. Plenty of options out there: Salt Stick, S-Caps, Nuun, Skratch, Liquid IV, Hydrant and more. In addition, most liquid running fuels and some gels include electrolytes as well, so check your labels.

Don't have a go-to brand yet? Click here or use coupon NHFITVIBE for 20% off Salt Stick, courtesy of Miles & Mountains athlete Christine.

Adding a very tiny amount of salt to your daily water can help with absorption, without needing to spend money on commercial electrolytes. Not enough that it tastes salty, just enough that it 'feels smooth.'

Sun Protection

In low humidity, blocking the sun with white clothing can actually be more effective than exposed skin. At the very least be sure not to wear black, and don't forget to lube!

Wear sunscreen. Just do it. I have had good luck with Kinesys and Zealios sunscreens, but use what works for you! Also, wear sunglasses! I see so many runners out there squinting their way through their runs. It's terrible for your eyes and face muscles too!

Cool Off

On longer runs, try to get yourself wet periodically, which helps to lower your core temperature. Whether a hose, fountain, or dip in a river/lake, just a minute or two of cool water can make a significant and lasting impact on how you feel!

This is also vital during races, even races that don't feel that hot. Remember those performance impacts start at 59ºF. Keep yourself COOL not just comfortable. Douse yourself with water or ice if they have it.

Chill Out

Relax! Stressing about the heat, or how slow you're running, or whether you'll be ready for a race, or whatever reason is going to increase your heart rate leading to you getting warmer. The more you can just relax and let the heat envelop you, the better off you'll be.

Remember that running is a very, very long term sport. Some slow and steamy runs, perhaps interrupted by a dip in the river, or even resorting to the bike on particularly brutal days, will not affect your fitness in any meaningful way. (The bike helps because riding is at least double the speed, giving a lot of nice headwind!) In fact, science shows you may just get some medium-term benefits out of it!


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