In 30 seconds
- Dehydration of more than 2% of body weight usually impairs aerobic performance in the heat (above 25 °C), but not in the cold.
- In outdoor cycling time trials, losing up to 4% while drinking to thirst did not worsen the result.
- Drinking too much is the main cause of hyponatraemia in athletes. Drinking to thirst is the safest strategy.
- To rehydrate, drink more than you have lost and have some sodium with it.
For years, the rule was “drink before you get thirsty”. Today the science is more nuanced: dehydration matters, but it depends on the heat, the type of effort and how you drink, and drinking too much carries risks as well. This is the state of the evidence.
Physiology
What happens when you sweat without replacing fluid
What dehydration does to your body
When you sweat without replacing fluid, you lose water from your blood. With less blood volume, your heart has to beat faster to move the same amount of oxygen, and it is harder for you to get rid of heat. That loss of volume is what explains the drop in endurance performance 1.
Level of evidence by outcome
| Outcome | Evidence | What the data show |
|---|---|---|
| Dehydration of more than 2% in the heat | High | Usually impairs aerobic performance above 25 °C. |
| Dehydration of more than 2% in the cold | No clear effect | Does not affect aerobic performance between 2 and 10 °C. |
| Time trials drinking to thirst | Moderate | Losses of up to 4% did not worsen outdoor cycling performance. |
| Muscle strength and endurance | Moderate | Losses of 2-4% are associated with lower strength and endurance. |
| Mental performance | Low | Small effect, linked mainly to discomfort and distraction. |
| Overdrinking and hyponatraemia | High | The main cause of hyponatraemia and of the deaths known to be caused by it. |
The rating is our own, based on the type of studies, their number and the consistency of the results.
The key studies
| Study | Type | Sample | Outcome |
|---|---|---|---|
| Cheuvront and Kenefick, 2014 | Review | Physiology of dehydration | A threshold of around 2% body weight loss for impaired endurance performance, due to the loss of blood volume. Small effect on mental performance. 1 |
| Sawka, Cheuvront and Kenefick, 2015 | Review | Effect of the environment | In the heat, dehydration usually impairs aerobic performance; in the cold it does not; at mild temperatures, it varies. The hotter the skin, the greater the effect. 2 |
| Goulet, 2011 | Meta-analysis | 5 studies, 39 cyclists | Outdoor time trials: losses of up to 4% with no change in performance. Drinking to thirst was better than drinking below thirst (+5.2%). 3 |
| Barnes et al., 2019 | Observational study | 1,303 athletes | Average sweat rate of 0.83 to 1.51 L/h depending on the sport. Greater need to plan hydration in endurance sports and American football. 4 |
| Hew-Butler et al., 2015 | International consensus | 17 experts, 4 countries | Overdrinking is the main cause of hyponatraemia. Drinking to thirst limits excess intake and prevents excessive dehydration. 5 |
| Evans et al., 2017 | Review | Rehydration | You need to drink more than you have lost. Water on its own is quickly passed as urine; sodium, carbohydrates and protein help retain it. 6 |
| DGE, 2020 | Position statement | German Nutrition Society | 5-10 ml/kg before; to thirst and max. 80% of sweat losses during; 400-1,100 mg/L of sodium; 1.5 L per kg lost afterwards. 7 |
Dehydration over 2%
In the cold or in the heat, the effect is not the same
Heat changes everything
The 2% rule applies mainly in the heat. Above 25 °C, dehydration usually impairs aerobic performance; between 2 and 10 °C, it does not affect it; and at mild temperatures, the results vary 2. The hotter your skin, the greater the effect 2.
Meta-analysis of 5 studies
Drinking to thirst in cycling time trials
Is drinking to thirst enough?
In a meta-analysis of outdoor cycling time trials, losing up to 4% of body weight did not change performance compared with staying hydrated, and drinking to thirst gave better results than drinking below thirst 3. The intensity and duration of the effort mattered much more than dehydration.
The international consensus on hyponatraemia also recommends drinking to thirst: it limits excess intake and provides enough fluid to avoid significant dehydration 5.
Rehydrating afterwards
You need to drink more than you have lost, because your body keeps getting rid of fluid through urine. Water on its own is quickly eliminated; sodium helps retain it, and so do carbohydrates and protein 6. The practical reference is around 1.5 litres per kilo of body weight lost 7.
What we still do not know
- Lab studies versus real life: many trials are carried out without the wind and airflow of the outdoors, which can exaggerate the effect of dehydration 3.
- Individual differences: sweat and its sodium content vary enormously from person to person 4. That is why general recommendations need to be adjusted to each individual.
- Team sports and strength: there are fewer studies than in cycling and running.
What is in Nitrus
Nitrus Isotonic is our hydration drink: each 25 g serving in 400-500 ml provides 195 mg of sodium, 148 mg of potassium, 15 g of carbohydrate, all 13 vitamins, BCAAs, glutamine and taurine. The studies on this page are about hydration in general, not about any specific product.
Frequently asked questions
Does losing 2% of your body weight always impair performance?
Not always. In the heat it usually does; in the cold, it does not; and in outdoor cycling time trials, drinking to thirst, losses of up to 4% did not impair it. The 2% figure is a useful reference, not a universal red line.
Is drinking to thirst enough?
For most training sessions and competitions, yes. It is the strategy recommended by the international consensus on hyponatraemia, and the one that gave better time-trial results than drinking below thirst. In very long events and in great heat, it is worth knowing your sweat rate and planning ahead.
How much should you drink after training?
More than you have lost, because you will keep losing fluid through urine. The German Nutrition Society recommends around 1.5 litres for every kilo of body weight lost, with some sodium.
Sources
- Cheuvront SN, Kenefick RW. Dehydration: physiology, assessment, and performance effects. Compr Physiol. 2014;4(1):257-285. 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
- Goulet EDB. Effect of exercise-induced dehydration on time-trial exercise performance: a meta-analysis. Br J Sports Med. 2011;45(14):1149-1156. View publication
- Barnes KA, Anderson ML, Stofan JR, et al. Normative data for sweating rate, sweat sodium concentration, and sweat sodium loss in athletes: an update and analysis by sport. J Sports Sci. 2019;37(20):2356-2366. View publication
- Hew-Butler T, Rosner MH, Fowkes-Godek S, et al. Statement of the 3rd International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015. Br J Sports Med. 2015;49(22):1432-1446. View publication
- Evans GH, James LJ, Shirreffs SM, Maughan RJ. Optimizing the restoration and maintenance of fluid balance after exercise-induced dehydration. J Appl Physiol. 2017;122(4):945-951. View publication
- Mosler S, Braun H, Carlsohn A, et al. Position of the working group sports nutrition of the German Nutrition Society (DGE): fluid replacement in sports. Dtsch Z Sportmed. 2020;71(7-9):178-184. 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. A varied and balanced diet and a healthy lifestyle are important.
