This article should be a doddle: google, copy, paste, publish! … if only
It turns out that people don’t agree that much, scientific opinion has evolved and there are still differing opinions. As usual, the smart thing to do is to read all of the theories, “ground truth” those with your own experiences, and decide what will work for you (1).
You can lead a runner to water but …
In this short history of running and water loss, the author’s personal thoughts look like this
The marathoners in the early and mid 20th century were all male – really tough macho types. The idea in those days was that you don’t drink during an event because it made you slow and “waterlogged” (2). You drank when you finished the event.
I’m way too weak for this approach, I need a drink.
The message changed in the 80’s to “hydrate, re-hydrate, and … do some hydration”. The race winners generally ran faster times, but some runners further back became ill, and a few died. The new disease was called hyponatremia (3), and it was caused by lack of sodium (salt) in the body. People were sweating out salty water and replacing it (and more (4)) with pure water, resulting in dangerously low concentrations of sodium.
Drinking too much can be worse than drinking too little.
There are still some adherents to the complete re-hydration theory, they typically urge you to use sports drinks to replace salts and water.
Trouble is, you can still over drink with those products. Often those articles are associated with a drink manufacturer, so I’m taking that idea with a grain of salt (bet you saw that one coming!)
Current scientific opinion pushes the thirst mechanism, and say drink only when you feel thirsty (5). You will finish the race dehydrated, but the aim is keep that dehydration within an acceptable range, less than 2% (6). Other articles point out that our stomach can only process water at a certain rate (7), roughly 4 cups per hour, which puts an upper limit on our rate of useful drinking.
I have definitely experienced excess water sloshing uselessly inside me. As for the thirst only idea, I’m a bit sceptical when people suggest that our bodies are so well designed they never give wrong signals. Also during the excitement and then fatigue of a marathon, I suspect some of my thirst messages might get lost. I’m also suspicious of absolute limits like 2%, each person may have a different number.
Don’t sweat it
One thing you should know, is how quickly your body loses water when you run. This is known as your sweat rate.
The procedure to work it out is simple:
- weigh yourself before and after a run, step on the scales wearing as few sweaty clothes as you can get away with
- keep track of the following
- run duration
- how much you drink – add this to the mass lost.
- distance, pace, weather conditions (these are all optional)
- The sweat rate calculation is (Finish – Start + Drank) * 60 / total-minutes
Below is my data. I also measured weight loss overnight, to test the implicit assumption that weight loss during exercise is due to sweat rather that breathing out water and CO2.
|Start (kg)||Finish (kg)||Drank (kg)||sweat rate (kg/hr)||Temp (C)||Humidity
- It seems that I lose about 1.5kg each hour, overwhelmingly due to sweating.
- The rate of moisture loss is faster in the first hour than in the third hour.
- This rate is extremely particular to the individual, I believe 1.5kg/hr is higher than most.
For my next marathon …
From the data above I expect to sweat about 4.5kg in 42km. The most my stomach could possibly process is 14 cups (2.8kg), but that’s just too much, I don’t get that thirsty.
We are not camels, we can’t store up water … just my normal daily routine of a cup of liquid an hour.
Drink when I feel thirsty, however expect to consume a cup at every 5km station. That’s 8 cups or 1.6kg in total. This is more than I’ve had in past marathons, but I’ll still be 3kg lighter at the finish!
… oh, and make sure I actually drink it
Eat some food containing salt (and protein) immediately, and work my way through one 750ml drink bottle each hour for the next 3 hours.
CODA – how the marathon went
As usual it hurt during the last 8km. My time was OK (1 minute PB), but I was very, very hungry and thirsty at the finish line. Since I was getting a cold, it’s difficult to know how much the extra fluid helped.