Importance of hydration pre-exercise

ALAN MCCUBBIN: So let’s talk about the effect of hydration on gastrointestinal issues now. Starting off with pre-exercise hydration. So there’s been a small amount of research in this area. And what they do in that research generally is take people that are euhydrated and then deliberately dehydrate them prior to the exercise, typically using a sauna. And then, they get them exercising. And the research in this area suggests that when you do that and you replace some, but not all fluid, during that exercise bout that you do tend to get an increase in gastrointestinal symptoms during exercise. So the pre-exercise hydration is quite important, because otherwise you’re sort of trying to play catch up and drinking more fluid to try and compensate.

When the participants were dehydrated prior to starting exercise, they had significantly increased rate of gastrointestinal issues. Particularly nausea and epigastric cramping were the main two symptoms that were increased. They also measured gastric emptying in that study. And there was a marked reduction in gastric emptying. And that correlated very strongly with nausea. So as gastric emptying got worse, nausea tended to increase quite significantly. The other things that they looked that were oral sequel transit times, so the transit time through the gut. And that wasn’t changed in that study between those who started well-hydrated and those who started dehydrated. Interestingly, when they looked at intestinal permeability and damage, they found no changes in these markers.

But part of the reason for that may be simply the short duration of exercise. It was only one and a half hours of exercise in the study. And what they found was no change in plasma volume in that particular study. So it probably just wasn’t a long enough bout of exercise to accumulate enough dehydration during the exercise bout between the two different groups to then see any changes in that effect.

The other thing to consider in this study is that the participants were dehydrated prior to exercise, but then they were prescribed a very specific amount of fluid to drink during the exercise bout, whereas in the real world, people would be able to drink more or less, depending on how thirsty they felt. So it may be that those who started more dehydrated, if they just drank a bit more during the exercise, may not have seen the same GI issues, the nausea and the reduced gastric emptying, compared to those who started euhydrated, but may not have drunk as much during the exercise bout.