How much to drink while cycling?

Riding a bicycle is the most efficient method of locomotion. By bicycle, one gets the furthest with a given amount of food (energy). This is true even though human is a very inefficient heat machine. Only about 25% of the energy ends up in useful movement, the rest becomes heat.

To get rid of the extra heat derived from food while cycling, the cyclist will usually sweat. Only in very rare circumstances, such as riding slowly in cold weather, other forms of heat transfer (convention, radiation, etc) are sufficient. So, the cyclist sweats. The hotter it is and the faster the cyclist rides, the more he or she sweats.

The cyclist is made of 60 to 70% of water, which makes sweating easy. There is plenty of water. Unfortunately, the human machine is rather delicate. Losing only 2-4 % of body weight as sweat both decreases efficiency and can lead to health problems, and if not replaced, may lead to heat exhaustion, and finally death.

To maintain efficiency and to stay alive, the cyclist must drink even more as he or she sweats, as digestion (turning food into suitable sugars) requires water as well. Now, the question is: how much should one drink while cycling?

I encountered this eternal question again when arriving in Girona and starting to ride in the mountains. The temperature was 26C, while I was used to +2C in Finland. I really did not know how much water to take with me to the mountains. I played it safe and ended up carrying 1.5 litres of water over the mountains, in my back bag. Better safe than sorry, I assume. I felt stupid, anyhow, cycling with a bottle of unnecessary water up a 10% hill for 5 km.

The traditional advice is: drink before you get thirsty. That is easy to say, as it assumes that water is always readily available. In most places it is not: given a new route, one cannot be sure where the shops, fountains, and springs are. One must carry enough water on the bike. But how much is enough per hour?

After this long introduction, I cannot provide any real, exact answers. I can only mention some extreme data points from my own experience:

Getting a bit too hot

• In Hangzhou, China, I went out cycling for two hours in 42C and no shade (sub-tropical area). I drank 3 litres and still lost 1 kg bodyweight. Thus, I was sweating a pint every 15 minutes. 
• In Latvia, I was cycle touring with Sirkku. Day temperatures were around 36C, no shade. During the second day, after spending the night in a hot tent, we rode 108 km in about 7 hours. The road was flat, but soft from heat. From midnight to midnight, we drank 20 litres (together, 10 litres per cyclist).
• When riding from Girona to Olot and back (125km, 6.5 hours in 26C and mostly sunshine),  I drank 4.5 liters and was clearly dehydrated when I arrived in back Girona. 

It pays to drink a lot. One cannot really drink too much. Any extra water comes out very naturally. In addition to drinking, taking care of salt intake is important lest any cramps appear. Crackers, energy bars, even an occasional beer help.

One should note that when air temperature exceeds the body temperature (e.g. 36C upwards), the wind does not cool the cyclist down. Sweating increases a lot. Cycling becomes really hard. If possible, it is wise to avoid any exercise when temperature is over 36C. 

Note also, that the human heat regulation system can break down even when it is not especially hot. One can suffer from heat exhaustion even in temperatures less than 20C, if one sweats a lot and does not drink. Be careful out there, as they say!

When really hot,
wet your shirt under a faucet (not pants!)