In very hot weather, the first threat to a cyclist isn’t the bonk, it’s dehydration. You can lose more than a litre of sweat an hour, and with it sodium, which your muscles need to work properly. How much to drink, water or electrolyte drink, how much sodium per litre: here’s the complete guide to riding in the heat without blowing up.
In short: in hot weather, aim for 500 ml to 1 litre of fluid per hour depending on how much you sweat, without ever drinking more than you lose. Beyond 90 minutes, switch to an electrolyte drink with 500 to 700 mg of sodium per litre, and up to 1,000 mg/L on rides of more than 3 hours if you’re a heavy, salty sweater. Weigh yourself before and after a ride to find out what you really need.
What heat does to your body on the bike
When it’s hot, your body gets rid of the heat your muscles produce by sweating. Some of your blood is sent to the skin to cool you down, at the expense of your muscles. Your heart beats faster for the same power, and the effort feels harder.
If you don’t replace your losses, dehydration sets in. The reference guidelines (Sawka et al., 2007) state that losing more than 2% of your body weight, or 1.4 kg for a 70 kg cyclist, can already reduce endurance performance, especially in the heat. Beyond that, the risk of heat illness rises.
Sweat losses vary enormously between cyclists: from under 0.5 litres to more than 2 litres an hour depending on heat, humidity, intensity and your own profile. The sodium content of sweat varies too: on average around 1 g per litre of sweat, but some people lose two to three times more than others (Baker, 2017). That’s why there’s no single number that works for everyone.
Water alone or an electrolyte drink?
| Length of hot ride | What to drink | Sodium per litre |
|---|---|---|
| Under 90 min | water, or a standard sports drink if the intensity is high | no need to add any |
| 90 min to 3 hours | electrolyte or sports drink | 500 to 700 mg/L |
| Over 3 hours, or heavy sweater | electrolyte and energy drink | 700 to 1,000 mg/L |
For reference, commercial sports drinks often contain 300 to 600 mg of sodium per litre: on a long hot ride, an extra electrolyte tablet can help. Read the labels: some list salt rather than sodium (1 g of salt contains about 400 mg of sodium).
Do you come home with white salt marks on your shorts or helmet? You’re probably a “salty” sweater: aim for the top of the range.
How much to drink: the weighing method
The best way to find out what you need is to weigh yourself. On a one-hour ride in the heat, at your usual pace:
- weigh yourself naked just before you set off;
- note how much you drink during the ride;
- weigh yourself naked straight after, once you’ve towelled off.
Your hourly loss = weight before − weight after + volume drunk. For example: 70.0 kg before, 69.4 kg after, 500 ml drunk, so 0.6 + 0.5 = 1.1 litres lost per hour. Your aim is to cover much of that loss, without losing more than 2% of your body weight over the ride, and without ever finishing heavier than you started.
Before, during and after the ride
Before
- Drink 400 to 500 ml in the 2 to 4 hours before you set off, then a few sips just before.
- Before a long hot ride, a slightly salty meal the night before and in the morning helps you start with good sodium levels.
- Head out as early as possible in the morning and plan water stops on your route.
During
- Take a few sips every 10 to 15 minutes rather than a big volume in one go.
- Aim for 500 ml to 1 litre per hour, based on your weighing.
- Choose a cool drink (around 10 to 15 °C), which is more pleasant and helps you cool down.
- Wet your neck, arms and helmet at water stops.
- Don’t forget carbohydrate: your drink can provide some of it. On a long ride, combine drink, gels and bars.
After
- Drink about 1.5 litres for every kilo lost, spread over the following 2 to 4 hours.
- Include sodium (recovery drink, broth, a salty meal): it helps your body hold on to the water you drink.
Hyponatraemia: the opposite risk
Drinking too much plain water, more than you lose, can make the sodium level in your blood drop: that’s hyponatraemia. At the Boston Marathon, a study found hyponatraemia in 13% of runners at the finish, mostly among those who had drunk a lot and gained weight during the race (Almond et al., 2005). The risk also exists on long bike rides. The signs: headache, nausea, swollen hands, confusion. In severe cases, it’s a medical emergency.
Two simple rules to avoid it: don’t drink more than you lose (you shouldn’t gain weight during a ride), and add sodium to your drink once the effort lasts more than 2 to 3 hours.
Warning signs you must know
- Dehydration: intense thirst, dry mouth, dark urine, cramps, loss of power.
- Heat exhaustion: sudden tiredness, dizziness, nausea, headache, cold and clammy skin. Stop in the shade, drink and cool down.
- Heatstroke: confusion, incoherent speech, very hot skin, collapse. It’s an emergency: call 112 (or your local emergency number) and cool the person immediately.
Ograal calculates your drink for each ride
In Ograal, every ride added to the calendar, with its duration and intensity, gets a fueling plan: fluid volume per hour, bottles of water or sports drink, gels and bars, and intake rhythm. For details, see How our plans are calculated.
Create your free Ograal account
Key takeaways
- 500 ml to 1 litre per hour in the heat, based on your weighing.
- No more than you lose: you shouldn’t gain weight on a ride.
- Beyond 90 minutes, add sodium: 500 to 700 mg/L, up to 1,000 mg/L on long rides.
- After the ride, 1.5 litres per kilo lost, with sodium.
- Confusion or collapse in the heat: it’s an emergency, call 112.
Go further
- Cycling Fueling in Summer Heat: Adapting Carbs and Timing
- Kilian Jornet: Trail Nutrition in Summer Heat — Adapt Your Fueling Strategy for UTMB
- Kate Waugh and Heat Acclimatization Nutrition for IRONMAN Triathlon
- Running a Marathon Without Cups: How to Stay Hydrated (Flask, Belt, Simple Plan)
- Pickle Juice for Cramps: Myth or Real Solution for Endurance Athletes?
- Tove Alexandersson: Altitude Nutrition, Altitude Anorexia and Fueling Strategy in the Mountains
Sources
- Sawka, M. N., et al. (2007). American College of Sports Medicine position stand. Exercise and fluid replacement. Medicine & Science in Sports & Exercise, 39(2), 377-390. DOI: 10.1249/mss.0b013e31802ca597
- Baker, L. B. (2017). Sweating rate and sweat sodium concentration in athletes: a review of methodology and intra/interindividual variability. Sports Medicine, 47(Suppl 1), 111-128. DOI: 10.1007/s40279-017-0691-5
- Almond, C. S. D., et al. (2005). Hyponatremia among runners in the Boston Marathon. New England Journal of Medicine, 352(15), 1550-1556. DOI: 10.1056/NEJMoa043901
