In 30 seconds
- In a classic study, the same cyclists lasted 93.5 minutes at 10.5 °C and only 51.6 minutes at 30.5 °C.
- In seven marathons, the fastest men finished 1.7% off the course record in the coolest conditions and 4.5% off in the hottest.
- Heat acclimatisation is the tool with the greatest effect: in 135 studies it improved performance in the heat with a moderate to large effect.
- Cooling down before or during exercise improved performance in the heat by 6.7% on average across 28 studies.
Heat is one of the things that takes most away from long efforts: in the laboratory, the same cyclist lasts almost half as long at 30 °C as at 10 °C. The good news is that the studies point to three tools that work: acclimatising, cooling down and drinking sensibly. Here is the research; the practical plan is in exercising in hot weather.
How much heat takes away
In the laboratory and outdoors
How much heat takes away
The classic study had 8 men cycling at 70% of their maximum capacity at four different temperatures. At 10.5 °C they lasted 93.5 minutes; at 30.5 °C, only 51.6 minutes 1.
The same happens outdoors. An analysis of seven major marathons, with up to 36 years of results, compared the times of the fastest runners with the record for each race according to the heat index 2:
| Heat index (WBGT) | Men | Women |
|---|---|---|
| 5-10 °C | 1.7% | 3.2% |
| 10-15 °C | 2.5% | 3.2% |
| 15-20 °C | 3.3% | 3.8% |
| 20-25 °C | 4.5% | 5.4% |
Source: Ely et al., 2007. WBGT combines temperature, humidity, sun and wind. In women, the differences were not significant.
And slower runners were even more affected than the leaders 2. The longer you spend running, the more heat you build up.
Not all efforts suffer equally
Heat mainly penalises endurance. According to the international consensus on exercise in the heat, short efforts such as jumps and sprints are barely affected, and may even improve 4.
In team sports, the body protects itself: in the heat, footballers cover less distance at high intensity. With a heat index of about 34 °C, tennis players shorten the points or rest longer between them 4.
Why it happens
In the heat, blood has to be shared between the muscles and the skin, which needs it to give off heat. If you are also dehydrated, you have less blood volume and the heart works harder for the same output. That strain on the cardiovascular system is the main mechanism 3.
That is why dehydration matters much more in the heat. Above 25 °C, losing more than 2% of body weight usually impairs aerobic performance; between 2 and 10 °C, it does not change it 3. In a study at 27 °C, losing 4% of body weight cut performance by 17% 3.
Level of evidence by outcome
| Outcome | Evidence | What the data show |
|---|---|---|
| Aerobic performance in the heat | High | Gets progressively worse as temperature and humidity rise. |
| Short efforts: jumps and sprints | Moderate | Barely affected, and may even improve. |
| Heat acclimatisation | High | Improves performance in the heat (effect size of 0.7) in 135 studies. |
| Cooling before exercise | Moderate | 5.7% better performance on average. Combining methods is the most effective approach. |
| Cooling during exercise | Moderate | 9.9% better performance on average, with fewer studies. |
| Dehydration in the heat | High | Losing more than 2% of body weight usually impairs aerobic performance above 25 °C. |
The rating is our own, based on the number of studies, their quality and the consistency of the results.
The key studies
| Study | Type | Sample | Outcome |
|---|---|---|---|
| Galloway and Maughan, 1997 | Trial | 8 men | Cycling at 70% of their maximum capacity, they lasted 93.5 minutes at 10.5 °C and 51.6 minutes at 30.5 °C. 1 |
| Ely et al., 2007 | Observational study | 7 marathons, 6 to 36 years of data | Performance gets worse as the heat index rises from 5 to 25 °C, in men and women. Slower runners are the most affected. 2 |
| Tyler et al., 2024 | Updated meta-analysis | 135 articles | Heat adaptation improves performance in the heat (0.7), lowers resting temperature and heart rate, and increases sweating and plasma volume. 5 |
| Bongers et al., 2015 | Meta-analysis | 28 studies | Cooling beforehand improved performance by 5.7% and cooling during exercise by 9.9%. Combining several methods was the most effective approach. 6 |
| Siegel et al., 2010 | Trial | 10 men, at 34 °C | With an ice slushy before running, 50.2 minutes to exhaustion, compared with 40.7 with cold water. 7 |
| Racinais et al., 2015 | Expert consensus | International conference | Acclimatisation over 1-2 weeks with sessions of at least 60 minutes. The benefits last 2-4 weeks. 4 |
Acclimatising to the heat
When the changes from acclimatisation arrive
Acclimatising: the tool with the greatest effect
An updated meta-analysis in 2024 brought together 135 articles. Heat adaptation improved performance in the heat with an effect size of 0.7, between moderate and large 5. It also lowered resting temperature and heart rate and increased sweating and plasma volume 5.
The expert consensus puts it into practice like this 4:
- How much: sessions of at least 60 minutes a day that raise your temperature and make you sweat.
- When it arrives: most of the changes, in the first week. Those in the heart and in sweating are complete in 6-10 days, and for aerobic performance about 2 weeks is ideal.
- The example: in a 9-12 day protocol at 40 °C, the time to exhaustion went from 48 to 80 minutes.
- How long it lasts: most of what you gain is maintained for 2-4 weeks, and the second time you acclimatise sooner.
Trained athletes can acclimatise in up to half the time of untrained people 4.
Cooling down
Before or during exercise
Cooling down before and during
A meta-analysis of 28 studies found that cooling down improves performance in the heat by 6.7% on average: 5.7% if you cool down beforehand and 9.9% if you do it during exercise 6. For cooling down beforehand, combining methods was the most effective approach 6:
| Method | Improvement |
|---|---|
| Several methods combined | 7.3% |
| Cold-water immersion | 6.5% |
| Cold water or ice slushy | 6.3% |
| Ice packs | 4.3% |
| Cooling vests | 3.4% |
Source: Bongers et al., 2015, meta-analysis of 28 studies.
Interestingly, the improvement was not related to how much body temperature dropped 6. And the expert consensus recommends it for long efforts, not for sprints or jumps: cooling the whole body just beforehand can impair a one-off sprint 4.
The ice slushy
It is the easiest way to cool down from the inside. In a trial at 34 °C, 10 men had 7.5 g per kilo of an ice slushy or of water at 4 °C before running. For a 70 kg person, that is about 525 g 7.
With the ice slushy they ran for 50.2 minutes to exhaustion; with the cold water, 40.7. They also finished with a higher core temperature, a sign that they tolerated more heat 7. The expert consensus suggests around 1 litre of crushed ice at 4 °C or below, before or during exercise 4.
Drinking in the heat
In the heat it is common to sweat 1-1.5 litres per hour, and some people exceed 2.5 4. Drinking only to thirst can leave you with a 2-3% deficit when you sweat a lot 4. That is why, in long sessions in the heat, it is worth knowing your sweat rate.
You will find everything the studies say about fluids in hydration and performance, and about salt in how much salt you lose in sweat.
And caffeine?
The International Society of Sports Nutrition considers caffeine well supported for endurance in the heat, at 3-6 mg per kilo 8. We cover doses, safety and its effect on sleep in caffeine and performance.
What we do not know yet
- Few participants: many trials have 8-10 people, almost always young men 1 7.
- How long to acclimatise: protocols of 8-14 days are the most studied; longer ones might work better, but data are lacking 5.
- Cooling during exercise: there are far fewer studies than for cooling beforehand 6.
Nitrus for hot days
Nitrus Isotonic provides 195 mg of sodium, 148 mg of potassium, 15 g of carbohydrate and all 13 vitamins in each serving. Make it up with 400 ml of very cold water, or with crushed ice, and keep it to hand during your long sessions.
Riboflavin (vitamin B2), vitamins B6, B12 and C, niacin, pantothenic acid and folate contribute to the reduction of tiredness and fatigue. And it contains no caffeine, so you can also have it in afternoon and evening sessions. A varied and balanced diet and a healthy lifestyle are important.
Frequently asked questions
How much does heat lower performance?
It depends on the effort. In one study, the same cyclists lasted almost half as long at 30.5 °C as at 10.5 °C. In the marathon, the fastest runners went from finishing 1.7% off the record in cool conditions to 4.5% off in the heat.
How long does it take to acclimatise to the heat?
Most of the changes happen in the first week and the rest over the following two, with sessions of at least 60 minutes in the heat. What you gain lasts about 2-4 weeks, and the second time you acclimatise sooner.
Does drinking an ice slushy or very cold water help before exercising in the heat?
Yes, for long efforts. In a meta-analysis, having cold water or an ice slushy before exercise improved performance in the heat by 6.3% on average. In a trial at 34 °C, the ice slushy gave almost 10 more minutes of running than cold water.
Does heat affect strength training?
Much less than endurance. According to the expert consensus on heat, short efforts such as jumps and sprints are barely affected, and may even improve. What does suffer is long sessions.
Sources
- Galloway SD, Maughan RJ. Effects of ambient temperature on the capacity to perform prolonged cycle exercise in man. Med Sci Sports Exerc. 1997;29(9):1240-1249. View publication
- Ely MR, Cheuvront SN, Roberts WO, Montain SJ. Impact of weather on marathon-running performance. Med Sci Sports Exerc. 2007;39(3):487-493. View publication
- Sawka MN, Cheuvront SN, Kenefick RW. Hypohydration and human performance: impact of environment and physiological mechanisms. Sports Med. 2015;45(Suppl 1):S51-S60. View publication
- Racinais S, Alonso JM, Coutts AJ, et al. Consensus recommendations on training and competing in the heat. Br J Sports Med. 2015;49(18):1164-1173. View publication
- Tyler CJ, Reeve T, Sieh N, Cheung SS. Effects of heat adaptation on physiology, perception and exercise performance in the heat: an updated meta-analysis. J Sci Sport Exerc. 2024;6(3):195-217. View publication
- Bongers CCWG, Thijssen DHJ, Veltmeijer MTW, Hopman MTE, Eijsvogels TMH. Precooling and percooling (cooling during exercise) both improve performance in the heat: a meta-analytical review. Br J Sports Med. 2015;49(6):377-384. View publication
- Siegel R, Maté J, Brearley MB, Watson G, Nosaka K, Laursen PB. Ice slurry ingestion increases core temperature capacity and running time in the heat. Med Sci Sports Exerc. 2010;42(4):717-725. View publication
- Guest NS, VanDusseldorp TA, Nelson MT, et al. International society of sports nutrition position stand: caffeine and exercise performance. J Int Soc Sports Nutr. 2021;18(1):1. View publication
Information based on published studies. It does not replace the advice of a doctor or registered dietitian. The results describe what was measured in each study, not the properties of any specific product.
