Training In Hot Conditions May Enhance Fitness And Performance

One of my favorite running memories is from July 2014. I was run commuting from my downtown office in Washington DC through Rock Creek Park, and I felt like a pot of chunky mashed potatoes. It was yet another uncomfortable run in a series of uncomfortable runs. Who decided to build our nation’s capital on top of a swamp? The room where it happened must have been sticky.

So I would slog. Squish squish squish. Stop for a breather. Squish squish. Get home and collapse on the couch. Squishhhhhhhhhh.

But not that day. Suddenly a few miles away from home, a breeze kicked up, pushing around 100-degree air that was thicker than gravy. Clouds rolled in and a few thunderclaps boomed. And just that moment, I could breathe again. The cold front blew through, and to the eternal disappointment of Snoop Dogg, the dew point dropped it like it was cool. I went from running underwater to effortlessly flying. It turned into what is still one of my fastest tempo runs ever.

It was obvious that running in the heat did something to my underlying physiology. But why did I feel transformed by heat training?

That summer of afternoon training in DC led to many of my best races ever. From the climbs of New Hampshire to the altitude of Colorado, every mountain effort felt easy compared to the sunny jogs on that gently sloping city path. It was obvious that running in the heat did something to my underlying physiology. But why did I feel transformed by heat training?

New studies may have pinpointed the answer.

(Shout out to Alex Hutchinson, who published an article on the same studies in Outside last week. I would be frustrated if he weren’t so darn amazing. Because Alex’s article focuses on the science and statistics of the studies, this article will try to focus on coaching/training implications and complications to consider.)

First, a step back. We have known for decades that exposure to heat causes rapid expansion of blood volume. As outlined in a 2000 review article in the Medicine & Science in Sport & Exercise journal, total blood volume is primarily composed of the sum of erythrocyte volume (red blood cells) and plasma volume (mostly water plus some dissolved proteins, glucose, clotting factors, electrolytes, hormones and other components). Most short-term studies focus on the plasma volume element. For example, a 2015 study in the European Journal of Applied Physiology found a 17.8% increase in plasma volume in highly trained cyclists after just four exposures to post-exercise sauna. Plenty of other studies back that up, and it may explain why heat acclimation doesn’t take long when done right.

As outlined in a 2014 review article in the journal Comprehensive Physiology, that rapid plasma volume expansion combines with more efficient and dilute sweat, improved vascular function, decreased metabolic rate from heat stress and increased thermal tolerance at the cellular level (at least partially explained by changing genetic expression). A 2010 study using an acclimation protocol of 10 days found that those adaptations can all improve performance even in temperate conditions, a finding backed up by some other studies, but controversial.

Many of those adaptations are connected to increased plasma volume. But what about the other main blood volume input: red blood cells? That’s the million-dollar question, since an athlete with higher quantities of red blood cells will usually be able to go faster in any conditions.

A substantial portion of what we call “fitness” is the ability to transport oxygen-rich hemoglobin in red blood cells to working muscles.

That’s a big reason why athletes train hard or train at altitude. A 2016 review article mentioned the theory that many physiologists and doctors have held for a long time—plasma volume expansion caused by heat stress could cause secondary stresses that spur increased production of red blood cells and hemoglobin mass. But those adaptations may take longer, as outlined in this 2017 review article in the American Journal of Physiology. In a past article, I coyly referred to these theories without having the courage to commit to them, saying “researchers are now looking more at whether it can spur natural EPO production too.” In the last year, studies made some breakthroughs that may confirm the theory.

A November 2019 study in Frontiers in Physiology had 21 male cyclists (emphasized to flag gender differences for future discussion) split into two groups. The first group consisted of 12 cyclists that would train easily for one hour in 100-degree F temperatures, in addition to normal training. The other nine cyclists just trained as usual, which amounted to an average of 67 minutes more per week, so any positive changes in the heat group are not explained by increased training.

In the six-week intervention, the heat group increased plasma volume as expected. But here’s the really interesting finding. Hemoglobin mass increased by 34 grams in the heat group, and 2 grams in the control group.

In the six-week intervention, the heat group increased plasma volume as expected. But here’s the really interesting finding. Hemoglobin mass increased by 34 grams in the heat group, and 2 grams in the control group. An important note is that the error bars on hemoglobin mass were large—plus or minus 33 and 36 grams respectively, due primarily to individual variance in responses. As the researchers summarized: hemoglobin mass “was slightly increased following prolonged training in the heat and although the mechanistic link remains to be revealed, the increase could represent a compensatory response in erythropoiesis secondary to [plasma volume] expansion.”

Thus, heat training may cause fitness gains related to EPO production in a similar manner as altitude training, just over a slightly longer time scale.

Past studies had not seen similar results, but had failed to go beyond three weeks. For example, a 2017 study in the American Journal of Physiology found increased plasma volume and increased EPO concentration after 10 days of heat acclimation, but no associated changes in hemoglobin mass. (See here for a 2017 study that looked at heat and hypoxic training over three weeks). So a big question remained. Was the 2019 study seeing a physiological signal based on more prolonged exposure, or was it just statistical noise? We needed more data.

And that data came in May 2020 with a study in the journal Experimental Physiology. In a similar study design, 23 elite male cyclists were split into two groups, with the heat group adding one hour easy training in 100-degree F chamber set at 60% humidity five times per week. Hemoglobin mass in the heat group increased by 42 grams, compared to 6 grams in the control group. Again, there were big error bars, but the increased red-blood-cell totals were backed up by better lactate threshold power output.

Putting it all together: based on biology and applied studies, it seems likely that prolonged training in hot conditions can improve fitness via increased hemoglobin mass.

Mechanistically, the theory is that increased plasma volume decreases hematocrit, or the percentage of red blood cells that make up total blood volume. Simplifying it to a fine paste, the kidneys sense perturbation, increasing natural EPO production and subsequently hemoglobin mass. Based on that theory, if the 10-day study cited above were extended, it likely would have seen increased hemoglobin mass too. Putting it all together: based on biology and applied studies, it seems likely that prolonged training in hot conditions can improve fitness via increased hemoglobin mass.

There are lots of unanswered questions for training. 

Does it apply in the same way to female athletes? Average hematocrit and hemoglobin are lower in female athletes, and the menstrual cycle and ferritin also provide potential obstacles. Anecdotally, I have seen female athletes with a more variable heat response, which is backed up by some research on how the menstrual cycle affects thermal tolerance. In addition, I have seen hemoglobin go through the floor in multiple female athletes during summer, which may be unrelated, but is important to monitor. More research is needed, but I imagine that if the same principles apply, it’s important that an athlete has high enough iron and energy availability, which are essential for hemoglobin production. Thus, for female athletes in particular, I suggest paying close attention to feelings during and outside of runs, rather than relying on these studies to justify approaches to heat training.

Does this apply to passive heat training, like the sauna? Based on the biology, it seems somewhat likely. And that backs up altitude-acclimation protocols for sea-level athletes that use post-exercise sauna to increase blood volume. I wrote about the protocol that athletes I coach use here. I’d love to see extended-duration studies on athletes using the sauna or hot tubs during periods of heavy training.

Does it apply to runners generally, rather than just cyclists? It probably applies across endurance sports, though foot-strike hemolysis unique to runners may provide a roadblock in hemoglobin mass for some athletes. I have seen higher baseline hematocrit levels in elite cyclists than in elite runners, so perhaps the findings don’t apply in quite the same way.

Do lower temperatures get a similar stimulus? Is it related to dew point to factor in humidity, or just the temperature? If we are assuming any increase in plasma volume is the main driver, then it’s likely that lower temperatures would still do the trick. However, if the extra-large changes from very hot temperatures are needed to drive the adaptations, it may not apply outside of the lab (or Phoenix). And if it’s related to heat shock proteins or another complex physiological pathway, it may get even more complicated.

Will the adaptations apply to athletes living at altitude? A 2017 study in the Medicine & Science in Sports & Exercise journal found that heat and altitude training may be less than the sum of its parts, but that could be due to excess stress on athletes in the specific study protocol. My guess is that if stress is optimized (a big ‘if’ given the combined demands on the body), the blood-volume adaptations would have positive long-term effects. This element may be especially important for female athletes since the excess stress could present increased hormonal risks.

How do energy availability and iron stores play a role? I imagine that high enough iron and surplus energy availability are important, since those may be limiting factors in secondary adaptations, particularly hematological ones. Eat lots, hydrate well and supplement as indicated by a doctor or blood tests.

Will the adaptations reach equilibrium over longer time horizons? Plasma volume increases rapidly, but it also oscillates over time and levels off. I’m really curious what would happen over an entire summer, particularly in an uncontrolled environment. Interestingly, the sauna protocols I have heard about for athletes competing at altitude from sea level often last four to six weeks, similar to the studies. Whether that’s a coincidence or detecting an underlying signal or something else is uncertain. Plus, the individual variability of responses could be magnified (or smoothed out, we’re throwing darts at a guess board now).

And who let the dogs out? It has been 20 damn years, and researchers really need to get on that question.

Training takeaways

But for now, I think it’s safe to assume based on the mechanisms at work that prolonged increases in blood plasma volume likely can increase hemoglobin mass after a few weeks in healthy endurance athletes training consistently (with the caveat that there is individual variance and it may apply differently for female athletes or athletes with low hematocrit/ferritin levels). There are three big points to remember that apply to almost all athletes, particularly if you are slogging through the summer like I did in 2014.

First, stay consistent, since blood-volume stress needs to be reinforced to avoid contraction. Just as plasma volume expands rapidly, it contracts rapidly as well. An extended roadblock in consistency may theoretically reduce natural EPO production. The studies used at least five weekly sessions, so that may be a good guideline to start. One option is to do short, light doubles on the run or bike in the heat of the day.

Second, make sure you have healthy blood biomarkers, particularly related to ferritin. Hemoglobin mass likely requires optimization of health and hormones, so it’s key to avoid limiting factors that may halt adaptations. Health is the most important part of the game, and these hemoglobin mass changes are playing at the margins.

The third and final point is the big one. For the purposes of this article, there is one major lesson: embrace the heat.

You don’t need to seek out the highest temperature each day, just don’t shy away from it either. The studies did a protocol of 5 x 1 hour sessions of low intensity biking, so it’s likely that your normal easy runs can do the trick unless you’re dodging the summer sun like a vampire.

Yes, training in the heat and humidity can be hard. I would personally rather do a speed workout at 14,000 feet elevation than an easy run along the Florida coast. But that difficulty comes with great benefits. My guess is that these studies reveal one of the “whys” behind a truth that runners have known since the sport began.

Summer training creates superstars.

David Roche partners with runners of all abilities through his coaching service, Some Work, All Play. With Megan Roche, M.D., he just started a 30-minute podcast on running (and other things), and new episodes are available now.