I've run a number of races, and there are almost always a number of water stations. I don't usually stop for water on the short races, but I'm not sure how long of a race I should do that for - when should I start drinking water? How much and often should I drink it?
Hydration when racing has a lot of variables, all of which need to be taken into consideration for longer races - which I would say is anything longer than a 10k:
- difficulty of the course (hills, headwind, altitude)
- temperature and humidity
- and your acclimatization to those weather conditions
- your effort, this is somewhat of a double edged sword: run fast and you're done quicker, run slow and you're out there longer, and often you'll be out in the warmer parts of the day more than the early finishers.
- your body: everyone sweats different amounts. Some don't drink much during a 1/2 marathon, some drink a liter or more.
- for longer races, your nutrition intake also adds a fluid component: you need water for your body to digest the nutrition.
- pre-race hydration: it's not uncommon for racers of >4 hour races to adjust up their electrolyte and hydration in the days leading up to a race.
First - try to do a sweat test in an environment as close as you can to the race, and get a good idea of what you generally need, and then consider the other variables.
At the end of a race you will almost never hear people saying "oh dang, I drank too much", but go by the medical tent after a marathon, and you will see racks of people wishing they had taken hydration more seriously.
Ideally, you'd have a little straw follow you around and you could have a constant drip of hydration. Take a cup at every water station, and take a few sips. Depending on how far apart they are, you'll want to drink more or less, aiming for an ounce of water every 4 minutes since your last station. This isn't an exact calculation, but it's a good place to start from.
This is going to leave you with leftover water almost every time. I personally would use the remaining water to cool off and keep yourself alert with a good face splash. Unless it's gatorade, then it probably wouldn't feel so good on your face :)
It's really much simpler than this. The marathon doctors have been recommending the following for a few years now, with the risk of hyponatremia in mind:
If you're thirsty, drink.
- Runner's World magazine, 2005: "How much should you drink during a marathon?" (references, in particular, Tim Noakes, author of the widely-respected Lore of Running and author of the IMMDA statement below)
- Georgetown University Medical Center, 2007: "Runners: Let thirst be your guide"
- British Journal of Sports Medicine, 2013: "Current hydration guidelines are erroneous: dehydration does not impair exercise performance in the heat"
Additionally, this 2001 statement from the International Marathon Medical Directors Association (IMMDA), which concludes:
"Accordingly perhaps the wisest advice that can be provided to athletes in marathon races is that they should drink ad libitum and aim for ingestion rates that never exceed about 800 ml per hour."