Our expert says:
The problem with overdrinking during exercise is actually very simple, though it has been complicated by various factors, some of which I will mention. Very basically, what happens if you drink excessively is that you dilute the sodium content of the blood. When this happens, you can develop a condition known as hyponatraemia.
To understand the problem, you have to go back to the very early days of research on exercise and fluid. The first studies were always done in laboratories using nothing more than a fan to cool the guys, and so what happened was that researchers found two things. The first was that they found that if you did not drink, your body temperature rose a little higher than if you did drink. The second was that you needed to drink between 1 and 2 L/hour during exercise to keep your body weight relatively constant (because most people would sweat about 1.5L/hour under the conditions in the labs). So based on these studies, the advice started to be given that if you lost weight during exercise as a result of sweating, it was harmful to performance and to health. There are a couple of serious problems with this logic, the main one being that they do not use appropriate windspeeds. As soon as you increase the wind speed, the effect disappears and people no longer overheat. The second problem is that what was found in the lab was then applied to all people. In otherwords, the advice was given out that you need to drink to to replace sweat loss completely. But, because most people don't know exactly what their sweat rate is, the general guide was to drink about 1.5L per hour. Most people then, who are competing in marathons or exercising are not going all that hard (your 'back of the pack' runner is not going to sweat this much, unless it's seriously hot), will therefore need MUCH LESS that what is recommended.
So, while a lot of people will be able to drink this and have no problem at all (because their sweat rate is so high), in some people, who do not sweat as much, all you do when you drink more than you sweat is dilute the blood, and this is what causes the sodium levels to fall. So, basically, if you drink more than is required, you could be drinking yourself into trouble. Therefore, the safest guideline is to listen to your thirst, and you should be fine. You know that humans are the only mammals who have to be told what to drink and how often - all other animals drink when they are thirsty, and they never become dangerously dehydrated!
So, my advice is to drink if you feel the need. If you are doing an hour of running, and drinking to thirst, then it's quite conceivable that you would drink about 1.5 to 2L, and then with all the other light activity, you could easily drink more. As long as you are not forcing yourself to drink, then you will be fine and I don't think that you should cut back - the body is designed so well, it won't let you dehydrate yourself!
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