row2k: It appears that the study indicates the athletes did not slow down with .8 - .9 kg weight loss compared to very little loss; does the study suggest that staying hydrated is not as much a concern as is commonly thought?

Dr. Hoffman: I’d say it a bit differently. Drinking ad libitum, which resulted in a 1.0 kg (1.6%) loss in body weight, caused no alteration in the physiological responses or perceived effort associated with the exercise, compared with programmed drinking that resulted in only 0.4-0.5 kg (0.6-0.7%) body weight loss. I wouldn't say that staying hydrated is not a concern, but one needs to understand what staying hydrated actually means. If you look at the study, you will see that the calculated reduction in body water with ad libitum drinking was 0.7 L (which equals 0.7 kg). That is less than the loss in body weight (1.0 kg).

That's because we have this amazing process of generating water when we utilize fuel, and water is also released from glycogen stores as the glycogen is used. This discrepancy isn’t a big deal during short bouts of exercise, but is important during longer bouts (a few hours) of exercise. The point is that you can’t use change in body weight to define hydration status. So, hydration is still important. It’s just that you can’t say one is 2% dehydrated if they have lost 2% of their body weight.

row2k: It I noticed you have done other studies on hydration; is ad lib drinking a reliable strategy for most athletes under most conditions?
Dr. Hoffman: Yes. We have a meta-analysis nearing the end of the review process that compares endurance performance of ad libitum drinking vs. higher volume programmed drinking for 1-2 hours of running or cycling. Ad libitum drinking actually shows a very small, but statistically significant, advantage over drinking greater volume than with ad libitum drinking. We’ve concluded that either strategy results in similar endurance performance for this duration of exercise.

We've also done studies at a 161-m ultramarathon, with exercise durations of 15-30 hours, and demonstrated that drinking to thirst can adequately maintain hydration. Now, there may be some outliers who don’t manage hydration well with ad libitum drinking, but most do.

row2k: Do thirst triggers vary by individual such that this can be unreliable?
Dr. Hoffman: It’s possible. Aging is known to alter thirst sensitivity. And there may be other factors as well.

row2k: We have seen athletes in training almost compulsively (and sometimes immediately) pulling on a water bottle after every repeat, while others are far less likely to do so. Any thoughts?
Dr. Hoffman: There are variations in sweat loss that would account for variations in fluid needs. But, I suspect that a lot of the variation you are seeing relates to a learned behavior from being convinced that thirst is an inadequate stimulus to maintain proper hydration. The sports drink companies, and their ties with various scientific organizations and scientists, have done a very good job at convincing people that our bodies are not adequately adapted to tell us when we need to drink.