It’s no secret that you need to stay hydrated when racing and training, not only for the performance benefit, but because serious medical complications can occur if you don’t. Let’s cover the basics of dehydration. According to a study by the University of Capetown, the average runner will lose 40oz of fluid per hour at 77% F, with relative humidity of 55%. You can easily double that on a hot day to over 80oz per hour. By the way, for our readers in the UK, I’m using US fluid ounces and not UK fluid oz. I’m sure our sharp European listeners already know that 1 liter is about 34 US fluid oz, so as I give these figures you can convert them on the fly in your head. So, losing at least 40 oz per hour when running, and about 32oz per hour when cycling on a cool day. I couldn’t find any information on fluid loss when swimming. My guess is that it is lower than cycling, but certainly some significant fluid loss takes place during swimming, and I’m not talking about peeing in the pool. That is just wrong.
Now that you know approximately how much fluid you lose when training or racing, what’s the big deal? The primary consequence of becoming dehydrated is that it literally makes your blood thicker, forcing your heart to work harder, raising your heart rate as high as 15 beats per minute higher than if you were hydrated. The clincher is that for every percent of body weight you lose when exercising, your performance decreases 2%. Is a 2% performance decrease really a lot? It is! Let’s give a more detailed example. If you are a 180 pound athlete, running in relatively cool environment for 1.5 hours, without taking in fluid, you would likely lose 60 ounces in fluid during that 1.5 hour run, which is 3.75 pounds, or 2% of your total body weight. Losing 2% of your totally body weight would translate to a 4% decrease in performance, which would be a loss of 3.5 minutes over that 1.5 hour run. Now, this is a crude example, because your fluid loss would be only 20oz after the first half hour, 40oz after the second half hour, and 60oz at the end of the 1.5 hour run, and so the performance loss would increase throughout the run. A better example would be if you were doing a half-iroman, and started the run portion with a 2% loss in body weight, your half-marathon time would be at least 3.5 minutes slower. In any case, even small amounts of dehydration will decrease your performance.
So, is the solution as simple as drinking an equal amount of fluid to replace what is lost? Should you drink 40-80oz of fluid per hour when running? Unfortunately, dehydration is a vicious enemy when racing or training. Although you will lose 40oz per hour or more when running, your stomach can only process up to 34oz of fluid per hour. In short, it is almost impossible to stay ahead of dehydration when running.
Now, when cycling in relatively cool weather, you can get closer to maintaining your body weight, since you lose less fluid when cycling, and can also take in a little more fluid when cycling than running. I believe that it is possible in a triathlon, to hit the run as hydrated as when you started the race, due to your hydration strategy on the bike. In fact, I have actually done some brick workouts where I have weighed more after the workout than before because of high fluid intake. Now, throw this all out the window if you are working out in the heat.
So, specifically, how much fluid should you take in when exercising? There are 2 schools of thought. One is to drink when you are thirsty. The other is to drink a certain amount per hour. The problem with drinking when thirsty, is that according to the University of California, thirst is probably not perceived until an individual has incurred a water deficit of approximately 2 percent of body weight. Using our example I shared earlier, that means you would be performing 4% slower before you ever even felt thirsty. For long distance racing, I recommend trying to taking in between 30-40oz per hour on the bike, and 20-30oz per hour on the run. These should be taken in gulps of 6-8oz every 15-20 minutes. It is possible to take in too much fluid. The same study from the University of Capetown I mentioned earlier showed that 2 of 8 runners who were taking in more than 32oz per hour could not finish the run test due to gastrointestinal discomfort.
For short distance racing, I’d say 2 hours or less, you can afford to take in less fluid. 2 separate studies from the University of Wolverhampton in England and Creighton University both showed that cyclists who were fully hydrated prior to a 1 hour time trial, did not improve performance by consuming fluid during the 1-hour ride. Even the University of Capetown study showed that in that cool environment of 77 degrees, there was no performance improvement in a 2-hour run between the runners who took in 16oz per hour compared to those who took in 33oz, and again, 2 of the 8 runners taking in 33oz had to drop out from gastrointestinal problems.
For long distance racing, remember that you are not drinking to address your current level of thirst or hydration, you are drinking to address your hydration 2 hours later, and so taking in higher levels of fluid on the bike becomes more important.
A few more notes on hydration. First, when I say fluid, I don’t mean water. Any exercise longer than an hour should include a drink with sufficient sodium and magnesium and electrolytes. Second, practice your hydration strategy when training for your race. You need to find out how much fluid you can stomach, which may be more or less than 34oz per hour. Third, if you are concerned about urination when racing, it is likely that the performance loss you will suffer from dehydration will be greater then the time you spend in the port-a-potty. Also, if you are have to pee more than every hour when racing, it may not be fluid intake alone, it may be the kind of nutrition you are taking in. For example, the dyes used in some sports drink are a diuretic, and may make you feel like you need to pee all the time. Experiment with different fluids. Fourth, I reviewed a scale in Tri Talk Episode 16 that measures your hydration levels. This is a great gadget from Tanita. I use it to check that my hydration levels are good before I go to bed each night, and this has allowed me to personalize proper fluid intake for me, and ensures I am properly hydrated before I train. Finally, in my quest to find out all I could on dehydration, I found out that moderate levels of dehydration impairs bowling accuracy but not bowling velocity in skilled cricket players. I thought I’d throw that in for our international listeners.
This topic is one of several topics covered in Episode 23 of the Tri Talk Triathlon Podcast.