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Could Year-Round Heat Training Improve Performance?

A 2022 study on cross-country skiers found that 5 weeks of 5 weekly heat training sessions increased hemoglobin mass and red blood cell volume, adaptations which should have beneficial effects for training. I think the recent literature may be pointing toward lessons that can change how we think about heat training outside of specific heat protocols.

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My co-coach Megan and I get to see a lot of blood work. And while we love to dig into the science on the most obscure biomarkers, the first place we look is always the simplest: hemoglobin. It’s measured in every blood panel, a major determinant of athletic potential staring us right in the face.

Hemoglobin is a protein in red blood cells that carries oxygen to working muscles. It’s a load-bearing pillar of athletic performance and aerobic development, influenced by genetics, iron levels, training history, altitude, and stress. Hemoglobin is so important that endurance sports have faced existential crises from athletes trying to hijack their physiologies through illegal, immoral performance-enhancing drugs like EPO, which stimulates red blood cell production and increases hemoglobin levels. 

A disturbing 2013 study in the PLOS One journal found that 4 weeks of EPO injections increased hemoglobin mass by 19.7%, which corresponded to a 6% improvement in 3k time trials with no training changes. That study was funded by the World Anti-Doping Association, which seems similar to the DARE program conducting a study on whether or not heroin feels good. Interestingly, that study also tried to optimize for iron levels to improve hemoglobin mass increases, having athletes take 100 mg of elemental iron a day (550% of the recommended daily value). Unethical, arguably evil use of EPO and blood doping has risked ruining Olympic endurance sports over the last 30 years, and it all gets back to that simple biomarker. Increased hemoglobin mass improves oxygen transport during exercise and thus has a profound impact on performance.

Cheating to raise hemoglobin mass is both vile and dangerous. But what if I told you that there are healthy, legal interventions that can improve hemoglobin mass? You can tell they are healthy because they often correspond with reduced stress and improved cardiovascular function–think optimizing iron intake, balancing stress, and living at altitude. You can tell they are legal because the interventions work over longer time horizons and only budge hemoglobin mass by a couple percent at most. It’s the difference between feeling good after eating a serving of dark chocolate versus feeling good after snorting a bunch of cocaine. 

RELATED: 3 Heat Training Strategies, From “Beginner” To “Advanced” to “Western States”

New Study on Heat Exposure and Hemoglobin Mass

A 2022 study in the Scandinavian Journal of Medicine and Science in Sports asked a fascinating question: how does heat training via heat suits affect hemoglobin mass in elite athletes? To evaluate the question, the study split 25 elite cross-country skiers into a heat group and a control group that were matched for intensity and duration of training. We’re not just talking normal athletes, but national and international competitors at the pointy end of human performance. So any changes in hematological variables would be coming on top of relatively optimized physiologies.

Participants in both groups completed normal training in the AM. In the PM, the heat group did 50 minutes of low-intensity indoor cycling on a stationary bike 5 times a week for the 5-week intervention. But there was a very hot twist. They were wearing ​​a “wool layer on both the upper and lower body, a wool hat, nylon rain jacket, down jacket, and nylon pants with poor evaporative capacity.” While they followed the study instructions well, they fully ignored Nelly’s instructions, because it got hot in there and they put on all their clothes. We need a follow-up study to test whether they do, in fact, want scrubs.

Despite the added double sessions in the heat, there was no difference between the groups in training duration or intensity across the 5 weeks. Now is the time when we take a step back to hypothesize what we think happens next. The participants’ baseline attributes are relevant to your guesses. The heat group had a starting point hemoglobin of 16.1 g/dl and hematocrit of 45.7%, already very high (likely due to some combination of training and genetics). For comparison, in that dastardly EPO study, the end point for hemoglobin levels was 15.2 g/dl even after the 20% increase from undergoing the Armstrong protocol. Moving a pebble with a bulldozer is one thing. But this study was looking at moving a boulder with a finger flick. Could a flick of heat exposure with low-intensity training move that boulder?

The answer: a resounding yes. The boulder didn’t move far, but it left an unmistakable trail, kind of like one of those mysterious moving rocks in Death Valley. And any movement of hematological variables through healthy means at this elite level of performance is a BIG FREAKING DEAL. 

Relative to the control group, the heat group’s hematocrit increased by 1.58% and hemoglobin mass increased by 30 grams. While there was no change in the performance metrics tested, those blood changes should have beneficial impacts on performance and recovery over longer time horizons. 

RELATED: Saunas: Hot Or Not?

So, Should I Just Hit the Sauna?

Here is where things get complicated, interesting, and… frankly, a bit annoying. A 1997 study in the International Journal of Sports Medicine found no hemoglobin mass changes with 4 weeks of training in hot environments between 19 C and 32 C, with numerous studies over the years showing small or negligible changes. That is contrasted by a 2022 study in Medicine and Science in Sports and Exercise that found 2.4-2.6% increases in hemoglobin mass from training in a heat suit or heat chamber (see also this 2020 study in the journal Experimental Physiology and this 2019 study in Frontiers of Physiology showing hemoglobin mass increases from heat acclimation protocols). And now is where we have to start questioning the exact mechanism at play, which will help determine how you put these findings into practice.

One theory is that with heat exposure, the body increases plasma volume (the liquid content in the blood), reducing hematocrit (the percentage of red blood cells within a blood sample) through dilution. As the kidneys sense an offset, the body increases natural EPO production to raise hematocrit levels to baseline, but with a higher denominator from the increased plasma volume, thus increasing the numerator: red blood cells and total hemoglobin mass. In that formulation, even after the plasma volume goes down later, the body will be left with more hemoglobin mass to play with.

If the driver is just the plasma volume-hematocrit-hemoglobin mass pathway (for more, the “critmeter” theory is outlined in this 2003 conference paper), it shouldn’t take heat suits or heat chambers to spur the adaptations. We should have seen them in the 1997 study, or other studies finding increased plasma volume from training interventions. Maybe it doesn’t have to just be kinda hot, it needs to be REALLY hot. As the authors of the main 2022 study, we are looking at say: “The rate of EPO synthesis is mainly regulated by the hypoxiainducible factor (HIF) system, which, however, also has been shown to stabilize with increased heat shock protein expression.” (Check out this 2005 research article in Physiological Genomics to go deep on the subject.) Heat shock proteins that incite the HIF system may require higher temperatures than normal training in warm ambient air.

RELATED: What Heat Acclimation Looks Like for Masters Runners

Detail from buckets and white towels in a sauna, wellness accessories.

Context and Implications of Heat Training

And that’s just the tip of the iceberg (the hotberg?). A recent study on the topic of natural EPO synthesis made me concerned, and it has nothing to do with heat at all. Published online in November 2022 in the American Journal of Physiology, Endocrinology, and Metabolism, it found that post-exercise ingestion of exogenous ketones increased natural EPO production by 20% over a control group. While the mechanism is uncertain, it’s possible that post-exercise ketones incite the HIF system, which may be part of the reason why it’s reported that more than 60% of the Tour de France peloton takes them. 

I’m bringing up that preliminary study on ketones to illustrate why I’m such a proponent of heat for athletes probing the margins of performance physiology: studies seem to indicate that it’s a natural, healthy way to increase hemoglobin mass and improve performance without probing the gray areas of what else incites the HIF system. Heat exposure has been practiced by some cultures for centuries, with plenty of positive impacts that have nothing to do with endurance performance (though based on the recent success of sauna-loving Norwegians, maybe there’s a connection there too). 

There are tons of unanswered questions that will be addressed by future studies. How do these adaptations unfold over longer time horizons, as plasma volume stabilizes? Will it work with passive heat exposure, like the sauna or a hot bath? Could it be counterproductive in athletes with limitations on hemoglobin production, like those with low iron stores or naturally low genetic setpoints? Would it be less effective at altitude? Could there be hormonal impacts that reduce the efficacy over time, or prevent it from working in female athletes at all? That last question is the biggest, since all of the studies I have seen do not include female participants, likely because the menstrual cycle causes blood volume fluctuations beyond those seen from the interventions. 

RELATED: The Science And History Of Heat Training

Conclusion: So, How Much Heat Training Do I Need?

The longer I coach, the more I support year-round heat exposure because these study findings seem to be backed up in the training data for athletes Megan and I coach, even for performance in temperate conditions. I think there might also be a health benefit in winter, with increased blood volume assisting with healing and recovery at a time when blood volume can naturally contract by more than 10%. 

My very loose suggestion: talk to a doctor. After the doctor approves, consider a background heat stimulus year-round if you enjoy it (possibly conducted in a manner that can spur heat shock proteins, though always being safe). Examples include a sauna, hot bath, hot tub, or heat suit. Don’t overdo it–heat is a major stress, and it can be dangerous. Please be careful.

I personally like to aim for 2 heat exposures a week, with one on my rest day and the other one whenever I have time, usually consisting of 15-25 minutes in the sauna or hot bath (getting out whenever my heart rate gets above 120, and being extra careful in hot baths since I seem more sensitive to them). A typical week when not doing specific heat training for a hot event might look like this:

 

Mon Tues Wed Thurs Fri Sat Sun
Training rest Easy and strides Workout and optional double Easy and optional double Extra easy Long run Easy and strides
Heat Sauna or hot bath none none Double may be in warm clothes Sauna or hot bath none none

 

The jury is still out on whether the hemoglobin mass increases from heat training seen in studies will apply to everyone (or anyone in the real world who isn’t doing 50-minute bike sessions in their entire wardrobe). So most of all, consider this intervention if it’s FUN, a way to add a bit more sweat to your day, and only if it’s approved by your doctor, lawyer, family, and life insurance salesperson. 

There are some cool adaptations at the limits of our physiology. And the coolest part about heat: you may be able to explore some of those adaptations while soft pedaling in a couple layers of clothes, sitting on a wood bench, or taking a relaxing bath. 

 

David Roche partners with runners of all abilities through his coaching service, Some Work, All Play. With Megan Roche, M.D., he hosts the Some Work, All Play podcast on running (and other things), and they answer training questions in a bonus podcast and newsletter on their Patreon page starting at $5 a month.