In the suggestions from one of the researchers referenced is that he thinks you don't need to try hard to hydrate. I think the truth is somewhere inbetween. There was a major consensus paper out recently where they basically decided that drinking to thirst will allow you to survive, but not necessarily perform optimally. The thirst indicator was less reliable in some runners than others.
I agree that over-hydration can be trouble, especially without sodium.
The interesting part for me was the idea that if you do NOT lose weight during a long run, you have overhydrated. We've been taught to believe that's a perfect outcome but this (controversial) study says otherwise. Losing body weight during a run does not just mean losing water.
Their study of Western States 100 finishers showed that runners should expect to lose 4-6% of their body weight just to maintain internal hydration status. Runners tend only to think about water they ingest but the burning of carbs, fat or protein as you run also releases water into your body. Maybe that helps to explain why some of us get away with drinking less than others. I'd like to see the actual study because all those runners ate on the course too, which would complicate the calculations.
I wonder how many kg of fuel you burn vs how much water you lose? Is it 1/4 the weight maybe, or 10%?
I know for a fact that drinking to thirst is big trouble for me. When you don't need to pee for 12 hours including hours after a race you're not overhydrated.
This explains why I have to drink so much and am always thirsty when I race... I'm so lean that there's no fat stored in my body to release water when it's burned. :-)
(for the portion of Bash' audience who know what FB stands for......)
If you deplete all the glycogen in your body, that's about 0.5 kg of carbs. Most sources say the body stores at least 3 g of water for every 1 g of carbs. So if you deplete your glycogen during a long run, that releases 1.5+ kg of water into your body, i.e. 1.5 L.
So if you're down 2 kg after a long run where you depleted all your glycogen reserves, your hydration status could be exactly the same as it was before the run.
This analysis is simplistic since it ignores inputs and outputs - how much you eat, drink, pee, etc. during the run - but the study's basic conclusion makes sense: that losing 2-ish kg in a long run doesn't mean you're dehydrated.
The study doesn't talk about drinking to thirst but one of the researchers points to some ultrarunning guidelines he has prepared where he includes that advice. That's a whole different discussion and definitely more controversial.
P.S. FB, it's fortunate you have no body fat because each gram of fat is only stored with 1 gram of water so it's a lot slower to lose body weight by burning fat!
Note- did a bit of edit as this wasn't part of the main article.
This is pretty interesting.
I started a different drinking protocol this year while riding and on a morning ride rarely drink (or eat) anything until I've burned about 1,000 calories. Then I'll drink a bottle over the next 500 calories. My body welcomes the drink, but I'm usually consuming it before my body is demanding the water. I do need to I take water regularly after that.
By contrast I need to drink far more spending the day outside in similar weather with a fairly low level of effort, and a degree of urgency in the need to drink appears quite quickly. I haven't really timed it, but my recollection over the last month is maybe 60% of the elapsed time when I choose to drink on a ride, I feel I have to drink with the lower effort.
Now I'm going to be watching for that!