Most of what heat training does for your heart happens in the first week. After that, extra days buy you very little. That’s the main message from a 2025 review of 211 studies and 2,587 people.
The review isn’t the last word. But it’s the biggest pile of data we have on how much heat you need.
What the average heat protocol delivers
The paper is McDonald et al., in Comprehensive Physiology. It’s a Bayesian meta-regression, a method that pools many studies and tests which protocol details move the results. The average study used 8 sessions of 90 minutes at 39°C. Those are averages, not a recommended dose.
After that average protocol, athletes showed these changes:
Core temperature fell 0.43°C at the end of exercise and 0.19°C at rest. Plasma volume rose 5.6%. Sweat rate rose 163 mL per hour. Time-trial performance in the heat improved 3.1%.
In short: your body runs cooler, pumps less hard, and sweats more. Time trials got about 3% faster.
How many days of heat training does each marker need?
Different markers move on different clocks. Think of your body as a house with a thermostat and a sprinkler system. The thermostat settles fast. The sprinklers keep getting stronger.
| Marker | After the average protocol | Per extra exposure day | Per extra 15 min of session |
|---|---|---|---|
| Resting heart rate | -5 bpm | -0.3 bpm | about -0.1 bpm (not meaningful) |
| End-exercise heart rate | -17 bpm | 0.0 bpm | -1.1 bpm |
| Resting core temperature | -0.19°C | -0.01°C | not reported here |
| End-exercise core temperature | -0.43°C | not reported here | -0.04°C |
| Plasma volume | +5.6% | -0.1% (flat) | +0.4% |
| Sweat rate | +163 mL/h | +9 mL/h | not reported here |
Read the middle column. Past the first week, another day of heat did almost nothing for end-exercise heart rate. Seven extra days lowered resting core temperature by only about 0.07°C.
Translation: the quick wins come early. Sweat rate is the one marker that keeps creeping up.
Heat lies about how hard you’re working. A few early sessions teach your body to answer honestly.
Session length, temperature and humidity
If days add little, what else helps? Per extra 15 minutes in the heat, end-exercise core temperature fell another 0.04°C and plasma volume rose 0.4%. Each hotter 5°C of ambient temperature lowered end-exercise heart rate by 2 bpm.
Humidity helped one thing: sweat. Each extra kilopascal of water vapor pressure added 37 mL/h to sweat rate.
Here’s our own arithmetic from those numbers. Going from 60 to 90 minutes should give roughly 0.08°C less core temperature, 0.8% more plasma volume and 2 bpm lower heart rate. Treat that as an estimate, not a tested result.
The style of session barely mattered. End-exercise heart rate fell 16.8 bpm with constant effort and 16 bpm with controlled hyperthermia (holding core temperature up). Controlled heart rate gave 15.2 bpm. Intermittent work gave 17.3 bpm. A steady, easy effort in the heat is enough.
What the evidence can’t tell you
Be careful here. The authors flag real limits. About 60% of the papers had no control group. Only 19% were randomised. Just 8.9% of participants were female. The models assume straight-line effects, and few studies went past 15 sessions.
So no one has proven that 8 sessions is optimal. Only that it’s typical, and that it works.
Hemoglobin mass takes weeks, not days
Hemoglobin mass is the total oxygen-carrying protein in your blood. The review’s model suggests no gain at about 13 days and roughly +20 g at 25 days. Call it 3 to 5 weeks. That rests on only 12 papers, and just 4 found an increase.
The evidence is mixed. Ronnestad’s team gave elite cyclists 5 weeks of heat sessions, 5 times a week. Their hemoglobin mass rose 42 g, against 6 g in controls. Oberholzer’s group saw +34 g versus +2 g, but it missed significance (p = 0.061).
Don’t plan a race around this one. Treat it as a possible bonus.
How fast you lose it, and how fast it comes back
A 2018 meta-analysis found you lose about 2.5% of your heart rate and core temperature adaptation per day away from heat. Re-acclimating is 8 to 12 times faster than the loss.
Picture a phone battery that drains while the phone sits in a drawer. A short top-up charge brings it back fast. So a refresher block in the final two weeks beats one huge block a month out.
A case study: Dana’s 70.3 in Texas
Take a runner and triathlete I’ll call Dana. She’s 41, and she was training for a late-summer 70.3 in Texas. She lives somewhere cool.
Dana did 10 easy sessions of 60 minutes in a warm room, then 2 short refreshers before race week. She tracked her drift ratio, the change in heart-rate-to-pace efficiency between the first and second half of a workout. Pros call it Pa:Hr or aerobic decoupling. At a fixed easy pace, her drift ratio fell from 8% to 4.5%.
Her heart rate stayed steadier in the second half. She raced the 70.3 without the late-run fade she’d had the year before. This is an illustrative example, not a study result.
Hot baths and saunas
Passive heat is popular. A 2025 review of 10 studies and 199 people found a trivial effect on hot-weather performance. Certainty was low to very low. If you can’t train in heat, it may help a bit. Don’t expect it to match real sessions.
Putting heat training into your plan
Aim for the first week, then add length or warmth before you add days. Keep the sessions easy so the block doesn’t wreck your key workouts.
Here’s an illustrative two-week block for a runner with a race three weeks out. The numbers sit inside the ranges the review studied. They aren’t a prescription.
| Week | Sessions | Length | Effort |
|---|---|---|---|
| 1 | 5 | 60 min | Easy, steady, in a warm room or hot outdoor run |
| 2 | 4 | 75 to 90 min | Easy, steady, same conditions |
| Race week -1 | 2 | 45 to 60 min | Easy refresher |
That’s 11 sessions in total. It’s close to the 8 sessions in the average protocol, with the extra time spent on length instead of more days.
Translation: front-load the first week, then make sessions longer rather than more frequent. Use the last two weeks to top up what you’d otherwise lose.
Our look at running economy and VO2max in the heat covers what to expect from the fitness side. Racing in the heat? Pacing matters as much as acclimation.
You don’t need a pill to tell if it’s working. Core temperature wearables are shaky, so a falling drift ratio is a practical signal. Gut trouble in the heat is its own problem, covered in the blood-flow mechanism behind race-day stomach issues. For a full comparison of apps, see the best app for a heat acclimation block.
AthleteOS adapts your training plan as your workouts come in and shows your heart-rate drift, labeled aerobic decoupling, after each workout. You can start a 14-day trial and see it in your own data.