Learn · Fueling
How much should you drink on a gran fondo?
The short answer: drink enough to keep your fluid losses under about 2% of your body weight, and never more than you sweat. How much that is depends on you and the day: somewhere between half a litre and two litres an hour.
Why 2%?
The American College of Sports Medicine’s guidance is that drinking during exercise should prevent losing more than 2% of your body weight to dehydration, because beyond that performance suffers 1. For a 70 kg rider, 2% is 1.4 kg, which is about 1.4 litres of sweat you have not replaced.
Start well hydrated, so you are not behind before the first climb 1.
Why not drink it all back?
Drinking more than you lose is not just unnecessary, it is risky. Sustained drinking beyond what the body loses through sweat and urine is the main cause of exercise-associated hyponatremia, a dangerous drop in blood sodium 2. The expert panel on hyponatremia concluded that drinking when thirsty is the safest individual strategy 2.
So the aim is to limit losses, not to erase them.
How much do you sweat?
Sweat rates vary a lot. In athletes they typically range from about 0.5 to 2 litres an hour, rising with heat and with how hard you ride; a few people sweat more than 3 litres an hour in extreme conditions 3. The same rider can sweat twice as much on a hot afternoon climb as on a cool morning flat.
Because the range is so wide, any general number is a starting point at best. Your own sweat rate is easy to measure: weigh yourself before and after a ride 1,3. See Measure your sweat rate.
How the FondoLab planner handles it
The planner estimates your sweat for each leg of your ride from your sweat profile and that leg’s temperature. It then plans to drink back 75% of it, held to a maximum drinking rate you can set (1 litre an hour by default), and tells you what fluid deficit that leaves. The 75% and the 1 litre an hour are FondoLab defaults, chosen to keep you under about 2% without encouraging overdrinking; they are not numbers from a study.
References
- Sawka MN, Burke LM, Eichner ER, Maughan RJ, Montain SJ, Stachenfeld NS (2007). American College of Sports Medicine position stand. Exercise and fluid replacement. Medicine and Science in Sports and Exercise 39(2):377–390. doi:10.1249/mss.0b013e31802ca597
- Hew-Butler T, Rosner MH, Fowkes-Godek S, 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. doi:10.1097/JSM.0000000000000221
- 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. doi:10.1007/s40279-017-0691-5