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I used to do a lot of long distance running but have since switch to biking for lower impact but when I ran I would never carry water even on hot summer days doing long, hard runs and have kept this habit through my biking.

Now I am not one to drink very much water in general but it was always emphasized by my trainers that if you wanted to be hydrated for a race, most of your hydration was done 12 to 24 hr before the race and chugging water right before won't do you any good. So to me this begs the question;

Does drinking water during my workout actually benefit me or is it more of a comfort thing?

Also I have referenced this post but didn't quite answer the question and it is worth noting that I don't crave water during workouts like it seems a lot of people do. Any advice on this would be handy.

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    How long is long? How many miles/km/hours? How hot is hot? Humid? Dry? Shade? – LShaver May 15 at 15:54
  • Agreed - There are many studies showing that even a small % of dehydration will affect performance, but we need more information. Also your personal sweat rate might be helpful, as that differs between people as well. – JohnP May 15 at 17:37
  • My workouts are generally 1.5hr at anywhere between moderate tempo to race pace and mid summer here in Texas can get to ~105°F. Thats a good point, I did not think about that; I generally do not sweat much if any though. – JoeBo May 15 at 23:42
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There is no doubt that hypohydration negatively affects performance, both independently of thirst and independently of knowledge of our hydration status. Recent blind studies (here, here, here, and here) confirm the results of earlier research, whilst addressing its proposed methodological issues. There has long been, and remains, considerable question about the precise mechanics behind the loss of performance observed, with cardiovascular, metabolic, neuromuscular, and temperature regulatory factors being considered. The latter remains the prominent theory due to our knowledge of the relationship between core temperature and muscular exhaustion, but it is likely a combination of all of those factors.

We begin to feel thirst only once our dehydration has reached around 2% of our body mass. By that time, the data suggest that we have already lost approximately 10% potential power output. And by the time we have lost 5% of our body mass, our performance has suffered by more than 30%! There exists variation between the differences of mean and peak power output, as well as between different studies. However, the trend is clear. It is recommended, therefore, that hydration be programmed or otherwise done ad libitum to match fluid losses. And this has become standard practice amongst elite athletes in all endurance sports.

Figure: decrease in peak power output with dehydration

Rates of dehydration vary enormously based upon the individual, their physical output, and environmental conditions. One of the easiest and most practical ways of monitoring this is by calculating our own sweat rates during different exercise bouts. (These rates will vary with the type of exercise we are performing, typically being the greatest with those in which we are most proficient.) This is done by taking our scale mass immediately before and after the bout of exercise, and monitoring our total fluid intake during the course of the bout.

Sweat rate (l) = ( pre-exericse mass (kg) + fluid intake during bout (l) − post-exercise mass (kg) ) × duration of exercise (min) / 60

Typical values range between about 0.5-3.0 litres per hour. Ideally, we would replace our fluid loss with at least the same quantity being lost, gradually and at intervals during the course of the bout.

I hope that is clear and helpful.

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  • The links you give don't really support the claims you're making. The evidence suggests that performance is associated primarily with core body temperature... I don't see anything in the article about performance. this temperature is the primary cause of complete muscular exhaustion The link is to a web site called hyperthermicwellness.com, which looks kookish to me. Fatigue is a very complicated physiological phenomenon that is not well understood and includes a huge component from the central nervous system. Trying to reduce fatigue to nothing more than one factor, temperature, is wrong. – Ben Crowell May 18 at 2:39
  • Thank you for your very valid comment, @BenCrowell. I have made an effort to improve my answer with better sources. Although the graph may be from a questionable URL, the same data were presented to me at the Victorian Institute of Sport in Australia some years ago. And they closely approximate both the wealth of research in this area and my own training data. Of course, we should understand that precise, unequivocal figures are never going to be found for phenomena that rely on so many variables, but it is the trend that is important. – POD May 19 at 15:18
  • @BenCrowell: I should mention, also, that I was not suggesting that fatigue was a product of a single factor; I was only trying to keep my answer brief and practical. The OP is asking whether water is of benefit, so I am focused on that question. So my question to you is, do you believe that hydration is of no benefit, and if so, on what basis do you hold that belief? – POD May 19 at 15:31
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You're asking a seemingly straightforward question, and most people would probably imagine that it had a well-known answer. The reality is that this sort of thing is controversial and difficult to test scientifically. Historically, there have been all kinds of varying and inconsistent scientific claims about the need to drink fluids during endurance sports. A lot of the perceived need to take fluids during exercise is basically the result of advertising that started when Gatorade was introduced.

Studies with large samples of athletes in real-world conditions have found that post-race body temperatures were not reduced by drinking more water (Noakes et al., "The danger of an inadequate water intake during prolonged exercise," European Journal of Applied Physiology and Occupational Physiology 57 (1988) 210).

Here is a nice article written for EMTs that references a lot of interesting scientific papers. Some studies have found that people who won running races were likely to be the ones who tested as being more dehydrated at the end. Evidence does not seem to support the notion that fluids protect against heat stroke. Pop culture has convinced many people that they should drink huge amounts of fluids, and in some cases this leads to hyponatremia, which is extremely dangerous.

Fatigue is a very complicated physiological phenomenon that is not well understood and includes a huge component from the central nervous system. There are various scientific models to try to explain observations about fatigue, but a lot of the underlying mechanisms are still not known.

For endurance sports in the hot temperatures you describe, you should be careful about heat stroke, which is really dangerous and insidious. It affects your brain, so you may not realize anything is wrong, and then you die. As described in the emsworld.com article, hydration won't protect you. Basically you can do two things: (1) not do the activity if it's too hot, and (2) be with someone else who will be able to tell if you start acting loopy, stumbling, etc., and take appropriate action.

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  • Your claim that “this sort of thing is controversial and difficult to test scientifically” is plainly untrue. The benefits of water to exercise, which is the question, are easily tested scientifically, and the answer is well established. Yes, there have historically been varying and inconsistent claims, but whilst interesting, that is irrelevant to the ongoing and objective truth behind the question. So too your reference to “huge amounts of fluids”. The question of how much water is necessary is independent of the fact that it is. – POD May 19 at 6:43

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