Heat Training for Runners and Triathletes: What the Science Says (and How to Train in it)

Another UK heatwave, another week of runners and triathletes wondering whether to push through, back off, or write the whole thing off until September. It's the right question. Heat genuinely changes what a session does to you, and what it does for you. But the advice out there swings wildly between "don't be soft" and "don't run at all," and neither is much use when you've got a race in six weeks.

There are two things worth separating here: how much heat costs you today, and what training in it does for you long-term. Both get exaggerated online, so I decided to research some honest answers - and more usefully, find out how to keep training properly when it's 30°C outside.

Part 1: How Much Does Heat Actually Slow You Down?

Optimal running temperature is cooler than most people assume. The largest study ever conducted on this analysed nearly 1.8 million marathon finishes across Berlin, Boston, Chicago, London, New York and Paris over ten years, and found the temperatures producing maximum running speed sat between 3.8°C and 9.9°C, varying by ability level [1]. Air temperature was the single most influential environmental factor on performance - more than humidity, atmospheric pressure or pollution.

For the cost itself, the cleanest data comes from a US Army analysis of seven marathons [2]. Across rising temperature bands from 5°C to 25°C, elite men slowed progressively from 1.7% to 4.5% off course record pace, with women following the same trend.

The table below serves as a working guide for the recreational runner. For a 45-minute 10k runner, 4% is nearly two minutes at identical effort.

Conditions	Realistic pace cost at the same effort
15–20°C	2 - 4%
20–25°C	5 - 8%
25°C+ with humidity	6% +

Check the humidity, not just the temperature

Sweating isn't what cools you down - sweat evaporating is. When the air is already saturated, it can't, and you're left dripping without any of the benefit. The research bears this out: alongside air temperature, humidity was the second most influential environmental parameter on marathon performance [1].

In practice it means the forecast temperature alone tells you very little. A 26°C day at 40% humidity is pleasant training weather. A 26°C day at 80% humidity is a different sport, and it explains why some hot days feel fine and others destroy you. Before you plan a session, look at both numbers.

Part 2: Does Heat Training Actually Make You Fitter?

Here's where I'd push back hardest on what circulates online.

What's beyond dispute

Heat acclimation makes you better in the heat. Within 10 - 14 days you get plasma volume expansion: measured at 6.5% in one controlled trial [3], 7.1% in another [4], alongside earlier and more dilute sweating, lower heart rate and core temperature at a given workload, and reduced perceived effort. This is well replicated and not seriously contested.

The numbers, honestly

In hot conditions, well-designed time trial studies show improvements of roughly 5–8%. Post-exercise hot water immersion improved 5km time trial performance by 4.9% [5], and exercise-based acclimation improved cycling time trial performance by 8% [3].

You'll see far bigger figures quoted - 30% and more. Those come from time-to-exhaustion tests, where you hold a fixed pace until you stop, and they don't translate. The clearest example: a sauna study found a 32% increase in time to exhaustion, and the authors themselves calculated that this equated to about 1.9% in an actual time trial [4]. Treat the huge numbers with scepticism.

In cool conditions, the transfer is genuinely contested. A frequently cited trial found 10 days of acclimation improved VO2max, lactate threshold power and time trial performance by 5-6%, with no change in controls [3]. But well-trained athletes haven't replicated it: in a Danish trial of 28 heat sessions, acclimation was clearly achieved, yet VO2max was unchanged and time trial power rose almost identically in the control group [6]. The gain was training, not heat. Several other trials agree [6, 7], including the sauna study above, which improved performance in the heat but not in temperate conditions [4].

So, realistically:

• If performing in the heat: 4 - 6%. Well evidenced.

• If performing in cool conditions: 0 - 4%. Contested. Possibly real for some athletes, unreliable in already well-trained ones.

Is it "equivalent to altitude training"?

Not quite, and the comparison gets stretched. Prolonged heat acclimation raised total haemoglobin mass by 3% at three weeks and 4% at five weeks in trained athletes, and both returned to baseline after two weeks without exposure [8]. Comparable altitude work in similarly trained athletes reports 5 - 6% [9].

Heat's distinct advantage is plasma volume. Altitude actually contracts plasma volume initially, whereas heat expands it, which is why a growing number of coaches now run a heat block before an altitude camp, rather than instead of one. Sequencing heat first pre-loads plasma volume and improves cardiovascular efficiency, reducing the physiological strain of arriving at elevation [9, 12].

Plasma volume is the clear, liquid part of blood that makes up about half of your total blood volume. Regular cardio training expands this fluid volume, which helps your heart pump blood more easily, cools your body faster, and boosts your endurance performance.

The takeaway: treat heat as a legitimate, free, accessible training stressor with strong specific benefits if you are racing in the heat, and modest, uncertain general benefits.

Part 3: How to Train Effectively in the Heat

The organising principle: protect quality, use easy volume for adaptation

The mistake I see most often is athletes trying to do everything as normal, running their intervals 5% slower in 30°C, feeling defeated, and arriving at the weekend cooked.

Flip it:

• Hard sessions go in the coolest window you can find, or indoors. Quality work exists to hit specific paces and powers, and heat degrades both.

• Easy and aerobic work becomes your heat stimulus. Adaptation is driven by elevated core temperature over time, not by intensity. A 60-minute easy run at 28°C is a superb acclimation session and costs you almost nothing.

Hitting intensity when it's hot: five things that work

1. Train early - genuinely early. Both temperature and humidity are at their most forgiving shortly before sunrise, and a 6am session and a 7:30am session can be meaningfully different environments. Evenings are usually a trap: temperature drops but humidity climbs and the ground is still radiating heat.

2. Shorten the reps, keep the pace. If your session is 5 x 1000m at 3:45/km, don't run it at 4:00. Run 8 x 600m at 3:45 with full recovery. You preserve the actual stimulus - target pace, mechanics, neuromuscular recruitment - and simply limit how long you accumulate heat within each rep. Similar volume, intact quality. This is the single most useful adjustment in this article.

3. Extend and cool your recoveries. Add 30–60 seconds and use them deliberately: shade, water over your head and the back of your neck, keep moving gently rather than stopping dead. Longer recoveries in the heat aren't soft - they're what makes the work reps possible.

4. Pre-cool before quality sessions. Lowering core temperature before you start buys you a head start, and pre-cooling is well established in the review literature [10]. An iced drink 20–30 minutes beforehand is the most practical version; a cold shower, frozen bottles held in each hand, or a soaked cap all help.

5. Use the controllable environments you have. A turbo with a proper fan is often better than an outdoor ride for VO2max and threshold work - precise power control, no exposure. A treadmill in an air-conditioned gym does the same for run intervals.

Hydration and sodium, with actual numbers

Know your own number: weigh yourself before and after an hour's session in the heat without drinking - each kilogram lost is roughly a litre of sweat. Precision Hydration offer a more detailed breakdown of how to measure your sweat rate accurately. The below is just generic guidance:

• Before: ACSM guidance is 5–7ml per kg bodyweight at least four hours before exercise (325–450ml for a 65kg athlete) with another 3–5ml/kg two hours out if you haven't passed urine or it's still dark [11].

• During: there's no universal per-hour number, and anyone who gives you one is guessing. The actual target is to keep fluid loss under 2% of bodyweight, which means drinking to your own sweat rate.

• Sodium: ACSM recommends 0.5–0.7g per litre of fluid for sessions over an hour [11]. Salty sweaters (i.e. the ones who finish with white crust on their kit) may need more.

  
  

Passive heat: the low-cost option

If you can't get outside, post-session passive heating works well. The research protocols: a 40°C bath for around 40 minutes straight after an easy run [5], or roughly 30 minutes (no longer) of Finnish sauna between 80-90°C [4]. Slow rehydration is key; don't fully rehydrate immediately. The mild dehydration combined with heat exposure stimulates the kidneys to produce more EPO, which promotes red blood cell production.

When to Stop

Heat training only works if you respect the line. Stop immediately for: dizziness, chills or goosebumps while hot (a genuine red flag), confusion or difficulty concentrating, nausea, or if you stop sweating while still working hard.

Also back off if the cumulative signs stack up across the week - elevated morning heart rate, poor sleep, appetite loss, sessions that feel disproportionately hard. Heat is a real training stressor and needs to be budgeted for like any other. Something else in the week has to give.

And if it's too hot to run safely, that's a treadmill session or a strength session in an air-conditioned gym, not a lost day. Strength training for runners and triathletes is the most under-used injury prevention tool there is, and a heatwave is a good excuse to finally address it.

Want Support With Your Training?

I coach runners and triathletes online and across East London Hackney, Dalston, London Fields. Some are chasing a first 5k, some a 70.3, some a marathon PB. What they have in common is a plan built around their actual life: their schedule, their goals, and their history with exercise, injuries and niggles.

Get in touch to talk about coaching, or read what my clients say about working together.

Louise Whitting is a personal trainer and BTF-qualified triathlon coach based in East London, Head Coach at London Fields Triathlon Club, and a Team GB Age-Group triathlete.

References

1. El Helou N, Tafflet M, Berthelot G, Tolaini J, Marc A, Guillaume M, Hausswirth C, Toussaint JF. Impact of environmental parameters on marathon running performance. PLoS ONE. 2012;7(5):e37407. — Free full text: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0037407

2. Ely MR, Cheuvront SN, Roberts WO, Montain SJ. Impact of weather on marathon-running performance. Medicine & Science in Sports & Exercise. 2007;39(3):487–493. — Abstract: https://pubmed.ncbi.nlm.nih.gov/17473775/

3. Lorenzo S, Halliwill JR, Sawka MN, Minson CT. Heat acclimation improves exercise performance. Journal of Applied Physiology. 2010;109(4):1140–1147. — Full text: https://journals.physiology.org/doi/full/10.1152/japplphysiol.00495.2010

4. Scoon GSM, Hopkins WG, Mayhew S, Cotter JD. Effect of post-exercise sauna bathing on the endurance performance of competitive male runners. Journal of Science and Medicine in Sport. 2007;10(4):259–262. — https://doi.org/10.1016/j.jsams.2006.06.009

5. Zurawlew MJ, Walsh NP, Fortes MB, Potter C. Post-exercise hot water immersion induces heat acclimation and improves endurance exercise performance in the heat. Scandinavian Journal of Medicine & Science in Sports. 2016;26(7):745–754. — Full text: https://onlinelibrary.wiley.com/doi/full/10.1111/sms.12638

6. Mikkelsen CJ, Junge N, Piil JF, Morris NB, Oberholzer L, Siebenmann C, Lundby C, Nybo L. Prolonged heat acclimation and aerobic performance in endurance trained athletes. Frontiers in Physiology. 2019;10:1372. — Free full text: https://www.frontiersin.org/journals/physiology/articles/10.3389/fphys.2019.01372/full

7. Karlsen A, Racinais S, Jensen MV, Nørgaard BJ, Bonne T, Nybo L. Heat acclimation does not improve VO2max or cycling performance in a cool climate in trained cyclists. Scandinavian Journal of Medicine & Science in Sports. 2015;25(Suppl 1):269–276. — https://doi.org/10.1111/sms.12409

8. Time-course for onset and decay of physiological adaptations in endurance trained athletes undertaking prolonged heat acclimation training. Temperature. 2024. — Free full text: https://www.tandfonline.com/doi/full/10.1080/23328940.2024.2383505

9. Oberholzer L, Siebenmann C, Mikkelsen CJ, Junge N, Piil JF, Morris NB, Goetze JP, Meinild Lundby AK, Nybo L, Lundby C. Hematological adaptations to prolonged heat acclimation in endurance-trained males. Frontiers in Physiology. 2019;10:1379. — Free full text: https://www.frontiersin.org/journals/physiology/articles/10.3389/fphys.2019.01379/full

10. Périard JD, Eijsvogels TMH, Daanen HAM. Exercise under heat stress: thermoregulation, hydration, performance implications, and mitigation strategies. Physiological Reviews. 2021;101(4):1873–1979. — https://doi.org/10.1152/physrev.00038.2020

11. Sawka MN, Burke LM, Eichner ER, Maughan RJ, Montain SJ, Stachenfeld NS. American College of Sports Medicine position stand: Exercise and fluid replacement. Medicine & Science in Sports & Exercise. 2007;39(2):377–390.

12. Trolle J. Hot and High: The Shared Engine Behind Heat and Altitude Training. USA Triathlon, 27 April 2026. — https://www.usatriathlon.org/articles/hot-and-high-altitude-training

Further reading: Tyler CJ, Reeve T, Hodges GJ, Cheung SS. The effects of heat adaptation on physiology, perception and exercise performance in the heat: a meta-analysis. Sports Medicine. 2016;46(11):1699–1724.