Why heat makes hydration harder
In heat, your body sends more blood to the skin to cool you down and sweats more. That leaves less blood for working muscles, and if you lose too much fluid, your heart rate climbs, your core temperature rises and effort feels much harder. Humidity makes it worse, because sweat cannot evaporate to cool you.
How much dehydration hurts performance?
Losing more than about 2% of body mass through sweat can impair aerobic performance and mental function, especially in warm or hot conditions. The goal during exercise is to prevent excessive loss, without drinking more than you sweat.
Source: Sawka MN et al. (2007). ACSM Position Stand, Medicine & Science in Sports & Exercise; Cheuvront SN, Kenefick RW (2014). Comprehensive Physiology.For a 70 kg athlete, 2% is just 1.4 kg, which a hard hour in the heat can easily cause.
Why water alone is not always enough
Sweat is not just water. It carries electrolytes, mainly sodium, plus smaller amounts of potassium, magnesium and calcium. Sodium helps your body hold on to the water you drink and keeps nerves and muscles firing.
How salty your sweat is varies hugely from person to person. If you finish sessions with white, salty streaks on your clothes or skin, or stinging eyes, you are probably a salty sweater and need to replace sodium, not just water.
Sweat rate and sweat sodium concentration vary widely between athletes, and also within the same athlete depending on fitness, heat adaptation and diet. Personal testing beats general rules.
Source: Baker LB (2017). Sports Medicine.Can you drink too much water?
Yes. Drinking far more than you sweat, especially plain water over long events, can dilute the sodium in your blood. This is called exercise-associated hyponatraemia, and in rare cases it is dangerous.
Drinking too much fluid is the main cause of exercise-associated hyponatraemia. The consensus recommendation is to drink to thirst rather than forcing fluids on a fixed schedule.
Source: Hew-Butler T et al. (2015). Clinical Journal of Sport Medicine.A simple hydration plan for hot days
Heat acclimatisation develops over roughly one to two weeks of training in the heat and improves sweating, heart rate and comfort.
Source: Racinais S et al. (2015). British Journal of Sports Medicine.Hydration is not about drinking more. It is about replacing what you actually lose.
GritxLabs Editorial Team
What we are working on
This is why GRIT. Electrolytes is in development: single-serve sachets you mix into a bottle of water, built for Indian training conditions. We will share the exact electrolyte profile at launch. No added sugar and 100% vegetarian, like every GRIT. product.
What this means for you
- Keep sweat losses under about 2% of body weight.
- Weigh in and out to learn your own sweat rate.
- Replace sodium if you are a heavy or salty sweater.
- Drink to thirst; do not force fluids.
Frequently asked questions
How much water should I drink during exercise in heat?
Drink to thirst, aiming to keep sweat losses under about 2% of body weight. Weighing yourself before and after training shows how much you typically lose.
Do I need electrolytes or is water enough?
For short sessions, water is usually fine. For sessions over an hour in heat, or if you sweat heavily and saltily, add sodium through electrolytes, ORS or salty food.
Can drinking too much water be dangerous?
Yes. Overdrinking during long exercise can dilute blood sodium, a condition called hyponatraemia. Drinking to thirst helps prevent it.
How do I know if I am a salty sweater?
White, salty marks on clothes or skin after training, or sweat that stings your eyes, are common signs.
References
- Sawka MN et al. (2007). American College of Sports Medicine position stand: exercise and fluid replacement. Medicine & Science in Sports & Exercise, 39(2), 377–390.
- Cheuvront SN, Kenefick RW (2014). Dehydration: physiology, assessment, and performance effects. Comprehensive Physiology, 4(1), 257–285.
- Baker LB (2017). Sweating rate and sweat sodium concentration in athletes: a review of methodology and intra/interindividual variability. Sports Medicine, 47(Suppl 1), 111–128.
- Hew-Butler T et al. (2015). Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015. Clinical Journal of Sport Medicine, 25(4), 303–320.
- Racinais S et al. (2015). Consensus recommendations on training and competing in the heat. British Journal of Sports Medicine, 49(18), 1164–1173.
This article is for general education and is not medical advice. Talk to a doctor or registered dietitian before changing your diet or supplements, especially if you are pregnant, nursing, under 18, taking medication or have a health condition.