Training in the Heat: Why You Slow Down, and What Your Body Builds Instead

There is a particular kind of disappointment that arrives in the first week of a heat wave.

You have been building all spring. The long runs got easier, the numbers got politely better, and then one morning in late July you go out at the pace you have run comfortably for two months and come home two minutes slower, breathing like you have done something reckless, wearing a heart rate that looks borrowed from someone else's workout. The obvious conclusion is that the fitness has gone somewhere.

It hasn't. What has changed is the environment your cardiovascular system is being asked to work in — and, if you keep showing up, what your body will quietly build in response.

The August slowdown is real, and it isn't fitness loss

When you exercise in the heat, your heart is asked to do two jobs with one output. Working muscle wants blood for oxygen. Skin wants blood to dump heat into the air. Both requests are legitimate, and the supply is finite.

The result is a well-described cascade that Julien Périard and colleagues laid out in a 2021 review in Physiological Reviews: as thermal strain rises, stroke volume falls, heart rate climbs to defend cardiac output, and endurance capacity is impaired through combined adjustments in cardiovascular function, the central nervous system, and skeletal muscle. Blood flow to skin, active muscle, and brain ends up in genuine competition.

So the watch is telling the truth. At the same pace, the heart rate is higher, because the job is harder. What's wrong is the interpretation — reading a hot-weather split as a fitness report is like weighing yourself in a winter coat and concluding something about your diet.

What your body builds when you keep showing up

Heat is also one of the more reliable adaptive stimuli in exercise physiology, and it works fast.

In 2025, McDonald and colleagues pooled 211 studies covering 2,587 participants in Comprehensive Physiology and ran Bayesian meta-regressions on the whole heat-acclimation literature — the closest thing we have to a settled answer on what changes, and how quickly. The typical protocol was about eight exposures of roughly 90 minutes. The averages across that literature: plasma volume expanded 5.6%, resting heart rate fell about 5 beats per minute, end-exercise heart rate fell about 17, end-exercise core temperature dropped 0.43 °C, and whole-body sweat rate rose by about 163 mL per hour. Time-trial performance improved around 3%.

Most of it lands inside 7 to 10 days. That is a remarkably short runway for changes of that size.

The more interesting finding is buried in the dose-response. For plasma volume, the meta-regression found that minutes per exposure mattered — about +0.4% per extra 15 minutes — while adding more days barely moved the estimate at all. Heat adaptation appears to be paid for in time under thermal load, not in calendar days. Which is a rather different instruction than "do this for two weeks."

The bath study, and why it made people put down their coffee

Last November, a small paper in The Journal of Physiology pushed the idea somewhere less comfortable.

Jenkins and colleagues took ten well-trained runners and put them through a counterbalanced crossover: five weeks of hot-water immersion, five sessions a week, 45 minutes each, in water at or above 40 °C — sitting in a very hot bath, alongside their normal training, with no extra running added.

Maximal oxygen uptake rose by 2.7 mL·kg⁻¹·min⁻¹. Haemoglobin mass rose 33 g. Blood volume rose 284 mL. The left ventricle's end-diastolic volume — how much blood the main pumping chamber holds when full — rose 10 mL. Haemoglobin mass was the strongest single predictor of the VO₂max change.

The reason that raises eyebrows is that the stimulus was entirely passive. Nobody trained harder. They sat in hot water and their oxygen transport chain remodelled itself at three separate links.

Now the honest half. This was ten people, nine of them male, already well trained, in a single study. Forty-five minutes in a 40 °C bath, five times a week, for five weeks is an unpleasant and substantial commitment — considerably more heat than most people accumulate by accident in summer. And a crossover design in ten athletes is a signal worth taking seriously, not a protocol worth copying wholesale. Read it as evidence that heat is a real physiological stimulus, which is a claim the wider literature already supports.

Heat is a stimulus, not a spa

This is where I part company with how heat usually gets sold.

The wellness framing files heat under recovery — the warm, restorative thing you do after the hard thing. The physiology files it under load. Your heart rate goes up, your plasma volume gets disturbed, your core temperature climbs. Those are the fingerprints of a stressor, and the adaptations are the body's answer to it. Something can be good for you and still be a cost on the ledger.

The evidence is fairly blunt about the difference. In a 2025 multi-arm randomised trial in Physiological Reports, Lee and colleagues put 38 sedentary adults through eight weeks of exercise plus post-exercise sauna, exercise alone, or nothing. Exercise improved heart rate variability, as exercise reliably does. Adding sauna afterwards added nothing to those HRV measures over exercise alone.

That is a useful, deflating result. Heat does specific things — to blood volume, to thermoregulation, to how a hot day feels. It does not do everything, and it is not a general-purpose upgrade you bolt onto a session. If you take a long sauna or a very hot bath after a hard workout, you have not added recovery. You have added a second stressor that happens to be adaptive. Budget for it accordingly. Our fuller read of the temperature evidence is in sauna and cold exposure.

What your wrist will show during a hot week

If you track anything, a heat wave will make itself visible, and mostly it will look like bad news.

Heart rate runs high at a given pace. Resting heart rate often sits a few beats above its usual line for several days. HRV frequently dips. Sleep degrades, partly because a warm bedroom makes the overnight drop in core temperature harder to achieve — the reason bedroom temperature is one of the few sleep variables genuinely worth controlling.

None of those numbers mean fitness has gone. They mean load has been added, and your body is telling you so honestly. The signal actually worth watching is the relationship between pace and heart rate rather than either one alone. As acclimation arrives over that first week to ten days, the same pace starts coming back at a lower heart rate. That reconvergence is the adaptation showing up in a number you already have — which is the whole point of measuring anything. Use the numbers to correct the fantasy, not to replace the experience.

How to spend a hot week

A few things follow from the evidence, none of them heroic.

Train by effort or heart rate rather than pace for the first ten days or so, and stop logging the pace hit as decline — it isn't one. Keep easy aerobic work in the heat if you want the acclimation, and move genuinely hard sessions to the cool end of the day, where the quality is still there to be had.

Take the sweat rate seriously. Acclimation makes you sweat more, not less, so fluid and sodium needs go up exactly when you have started to feel more comfortable — a quietly common way to have a bad afternoon. Our guide to hydration and electrolytes covers the practical side.

And think of it as seasonal. The adaptations decay within a couple of weeks of leaving the heat, which means there is no bank account here, only a habit. If you want the effect in September, you have to be in the heat in August.

One caution that matters more than the rest: heat is a genuine physiological stressor, and hot baths, saunas, and hard training in high temperatures are not appropriate for everyone. If you are pregnant, have a cardiovascular or blood-pressure condition, take medication that affects sweating or fluid balance, or are older, talk to your doctor before adding deliberate heat exposure. Stop and cool down for dizziness, nausea, chills, headache, or confusion — those are not signs of adaptation in progress.

The bottom line

Summer doesn't take your fitness. It taxes it, visibly and rudely, and then — if you keep turning up with some judgment — it pays part of the tax back as plasma volume, haemoglobin, and a ventricle that fills a little more completely.

The pace that returns in September always feels like a gift. It isn't. It was earned in July, in the worst conditions of the year, by someone who didn't quit because the watch said something unkind.