Does heat training build a better engine?

Summer is here. For those of us who train outdoors—running, cycling, whatever gets us moving—the heat can be unavoidable. It’s tempting to treat it as pure adversity: something to endure, hydrate against, and survive. But a growing (if still small) body of research raises a more interesting possibility. Could the heat itself be giving us an edge?

The idea hinges on the concept of hormesis: the principle that a stressor, in the right dose, can trigger a beneficial adaptation. In other words, it’s the scientific principle that, sometimes, what doesn’t kill us makes us stronger.

Exercise itself is the classic example—you damage muscle and deplete energy stores, and your body rebuilds stronger. Altitude works the same way. Thin mountain air starves the body of oxygen (hypoxia), and in response the body ramps up red blood cell production to carry more oxygen with each breath. 

VO2 max, the maximum rate at which an individual can use oxygen during exercise, is arguably the strongest physiologic predictor of both endurance performance and longevity. Altitude training improves VO2 max by stimulating an increase in hemoglobin mass (Hb mass), the total amount of oxygen-carrying hemoglobin in the blood. More hemoglobin allows more oxygen to be delivered to working muscle and, all else being equal, increases the ceiling for aerobic performance. This is why endurance athletes have spent decades chasing altitude. 

The question we’ll explore today is whether heat—a completely different stressor—might pull on the same lever.

Does heat training raise Hb mass?

The short answer is yes, at least in nearly every published study examining the question so far.¹ 

But before we go further, one caveat deserves top billing. This field is young—nearly all of this research comes from a single collaborative group of scientists working with Scandinavian athletes in the last ten years. When findings cluster in one lab, we can’t yet rule out that something specific to their methods, population, or protocols is driving the results. Independent replication is how science separates a real effect from a local quirk, and that replication is largely still missing here. The studies span from elite professional athletes to active, non-professional cyclists—so there is some generalizability across this axis, but no indication of how heat training would impact truly amateur or untrained athletes.

With that framing in place: their heat-training studies converge on a similar protocol. Roughly five weeks of training, six sessions per week, about 50 minutes per session, in an environment around 32°C (90°F). Under these conditions, participants consistently show meaningful increases in Hb mass.

How big? The effect sizes are comparable in magnitude to what altitude-training studies report: typically an increase in the range of 2–5% of total Hb mass. One study put heat and altitude head-to-head in the same elite athletes and found significant increases in Hb mass in either condition, with no significant difference between conditions (Altitude: 3.5 ± 2.0%, Heat: 5.4 ± 3.9% p = 0.801).² That said, the study was tiny (only 7 cyclists did both heat and altitude training), so if there are any small advantages for Hb mass between heat and altitude protocols, this study may have been underpowered to find them.

One more encouraging detail: the effect doesn’t seem to require punishing effort in each session. To stimulate the Hb mass response, participants trained for about an hour per session at around 60% of VO2 max—a moderate, conversational intensity. The researchers designed it this way on purpose, so that heat sessions wouldn’t cannibalize an athlete’s normal, harder training—while still stimulating hematological effects comparable to a few weeks of training in the mountains.

A lighter dose can maintain the gains

This is where the story becomes practically interesting.

Both heat- and altitude-induced gains in Hb mass are frustratingly fleeting. Once you return to normal conditions—sea level, thermoneutral training—Hb mass decays quickly, largely returning to baseline within about 10 to 14 days. For an athlete timing a peak to a race, that’s a narrow and unforgiving window.

But researchers found that a lighter maintenance dose—three heat sessions per week instead of six—was enough to prevent that decay, and held the Hb mass gains in place over the three week study period. This is perhaps one of the most optimistic points within the heat training literature: not only can 5 weeks of 6 heat training sessions per week increase Hb mass, but once the benefit has taken hold, the increases can be maintained while dropping session frequency. 

And many of these athletes opted to wear wool clothing while training at normal temperatures—simulating a heated environment wherever they trained, still finding the same benefit.

This study specifically measured prolongation of Hb mass increases that were initially stimulated by heat training. If this translates to altitude training as well—with heat maintenance sessions performed at sea level, no mountain required—heat training may provide a massive logistical advantage over extended training sessions spent chasing altitude far away from home.

How does Hb mass translate to what we actually care about?

Now for the uncomfortable part. Does any of this raise VO2 max, or make people faster?

The honest answer is that it’s unclear within the available data. Increases in Hb mass are consistently observed—5 of the 6 trials performed in the last 10 years show a benefit for Hb mass. And Hb mass tracks closely with VO2 max—with tight correlations between VO2 max and Hb mass reported in other studies, on the order of r=.90.³

But performance metrics are significantly noisier than measurements of Hb mass, and are much more prone to variations in both measurement and day-to-day variability of the athlete. This is unfortunately where we rarely see an effect in these small heat training studies—5 of the 6 trials fail to identify a clear payoff in the metrics athletes actually care about: lactate threshold, VO2 max, or sport performance.

Why the disconnect? There are a few possibilities worth separating.

First, statistical power. These are small studies, and performance is notoriously noisy to measure—a real benefit could exist while the studies remain too small to see it. Only one trial recruited more than 20 athletes to both the heat training and control arms—notably, this was also the one trial where lactate threshold, VO2 max, and 15 minute all-out power actually moved.⁴

We might be tempted to conclude, then, that the lack of consistent performance effects is a feature of noise and statistical power, more so than an actual absence of benefit. This seems like a reasonable hypothesis—but it’s hard to be terribly confident when it comes from a single study in a sea of negative data. We need replication in different, sufficiently-powered cohorts to really be confident in this explanation.

A second, more fundamental explanation could be due to the fact that Hb mass is a surrogate endpoint—a stand-in for the things we truly care about, which happens to be easier and more reliable to measure in the lab. 

Hb mass varies far less, both between people and within the same person over time, than VO2 max or race performance, making it an attractive variable for researchers. But this is exactly why surrogate endpoints can mislead us. They are easier to measure than the outcomes we actually care about. Oxygen-carrying capacity may simply not be the limiting factor for a given athlete—it sets the ceiling, but it doesn’t tell us about the engine underneath. Endurance performance also depends on lactate threshold, cardiac stroke output, mitochondrial density, and power economy. If those systems are the bottleneck, adding more oxygen in the form of Hb mass may do little to a saturated system, at least in the short term.

Which takes us to the third point: timing. We don’t know all the effects of heat training on the body. A five-week study may be too short to capture the full downstream benefit of a higher Hb mass—especially if it turns out that Hb mass is essentially the only lever being pulled by heat training. Even if the engine’s fuel line got bigger, in just a few weeks, the athlete may not yet have learned to use it.

The bottom line

So where does this leave us?

In hematology, the signal is reasonably consistent: heat training appears to reliably raise Hb mass in the people studied, whether elite athletes or fit volunteers. And in observational studies of trained athletes, Hb mass reliably correlates with VO2 max.

That raises an intriguing possibility: perhaps heat training works like creatine. The benefit may not be a dramatic improvement over five weeks, but rather a subtle increase in training capacity that compounds over months. Better training today enables better training tomorrow, eventually producing gains too gradual for a short study to detect. 

Hb mass is changing, and Hb mass changes the endurance systems we care about. Over a long enough period, if we’re pulling the right levers, it seems likely the right adaptations would appear.

But the list of genuine unknowns is long. We don’t fully understand the mechanism that drives Hb mass expansion in the heat. We don’t know what it does to other performance-relevant systems like lactate threshold. And we don’t know, with real confidence, whether it translates into being faster—we have a reasonable mechanism and one clearly positive study, but they are nestled within a landscape of suggestive-but-not-conclusive pilots that fail to find performance benefits. 

So this leaves us with cautious optimism. For people thinking of adopting heat training: there’s reason to think heat training may help, but no smoking gun indicating this should be a cornerstone of every elite training protocol. There are simply too many open questions—and the researchers themselves say as much: this is an active area of research in a young, evolving field.

Still, this is the time of year when many of us have little choice but to train in the heat. If you’re already logging miles through the summer, the evidence suggests you’re doing more than building mental toughness. You’re likely inducing a measurable hematological adaptation, even if we don’t yet know how much that adaptation ultimately translates into better performance.

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References

1. Lundby C, Robach P. Altitude or heat training to increase haemoglobin mass and endurance exercise performance in elite sport. J Physiol. 2025;(JP287700). doi:10.1113/JP287700

2. Cubel C, Klaris MB, Larsen JV, Faiss R, Nybo L, Lundby C. Haematological adaptations to high-altitude and heat acclimation training in elite male cyclists. Exp Physiol. 2026;111(3):820-833. doi:10.1113/EP092968

3. Goodrich JA, Ryan BJ, Byrnes WC. The influence of oxygen saturation on the relationship between hemoglobin mass and VO 2 max. Sports Med Int Open. 2018;2(4):E98-E104. doi:10.1055/a-0655-7207

4. Lundby C, Hamarsland H, Hansen J, et al. Hematological, skeletal muscle fiber, and exercise performance adaptations to heat training in elite female and male cyclists. J Appl Physiol. 2023;135(1):217-226. doi:10.1152/japplphysiol.00115.2023