Andy Blow of Precision Hydration can fill you in. “The first thing to say is that for a lot of people a lot of the time, water is the best thing to drink. And that goes for athletes a lot of the time.
“When you talk about the importance of hydration for athletes, the obvious point relates to replacing what you lose in sweat. Your sweat is predominantly water, so most of the time drinking water to a lesser or greater extent is all you need to do to replace that.
“You also lose electrolytes, salts, but though you lose potassium and magnesium and calcium, the major electrolyte you lose is sodium chloride.
"You lose more of that than the others, so in terms of the hierarchy of importance in hydration, first you have water, and then, if you sweat a lot, you might have to replace salt.”
‘Might’ is the key word. Blow points out that one size doesn’t fit all because of training circumstances, for one thing.
“The reason there’s confusion, I think, is that people feel they need to replace electrolytes all the time — and they do, but they do that through the food they eat.
“The difference is that for athletes who are training for ironman events or marathons, or training every day, or in very hot weather — they may not have enough electrolytes in their food, so if they’re drinking plain water they may start to dilute the body.
“That’s not really common — not among most people training from day to day — but among athletes training very hard in hot weather it’s more common than many people realise.
"A study on the European ironman championships, from 2005 to 2012, taking blood samples from competitors, found 10% of the finishers were hyponatremic — they’d either sweated out too much salt or drunk too much plain water.
“That can be pretty dangerous, so it’s an interesting statistic.”
The sheer differences between people is another side to hydration. Everyone sweats differently.
“Being crude about it, we’d feel that those who don’t sweat a lot — simply because they don’t sweat a lot or aren’t doing a lot of exercise and so on — then water should be adequate for them,” says Blow.
“It’s the people doing a lot of training, sweating a lot, day after day in high temperatures, then arguably putting some electrolytes in their fluid helps them maintain better overall hydration.
"The way your body balances fluid levels, it’s always a balancing act between the total amount of fluid on board and the level of salt in that fluid.
“If you get the ratios very wrong then that’s when you run into trouble. I sometimes describe it as being like the car windscreen washer — you start off with an optimal mix but as you use more and more it gets more and more diluted, and eventually you have something too diluted. That’s what can happen to your body if you drink loads and loads of water.
Ultra-trail champ @lucybartholomoo came down to PH HQ recently to work on her hydration/nutrition strategy with @andybl0w & even managed to get in some training in our epic backyard. Here's what she had to say about how she stays fuelled and hydrated... #hydration #TeamPH pic.twitter.com/iIziUVoh6g— Precision Hydration (@thesweatexperts) December 9, 2017
“The other point is that hydration is very individual. Two different people will have different rates of sweating and different composition of sweat.
"I learned this the hard way, because as an athlete I have a very high sweat rate and I also lose a lot of salt in my sweat. That’s genetics.
“But it meant replacing electrolytes in marathons in hot weather was very challenging. I got it wrong sometimes and ended up in the medical tent.”
Blow’s company works with professional athletes in rugby and baseball as well as Olympians, rally drivers, and ultramarathon runners.
“We run tests to see how much body weight they lose in exercise when they sweat and we measure the electrolyte/sodium content in their sweat so we can help them work out what level of replacement they need.
“For amateur athletes we have a free online sweat test — we ask a lot of questions and run the answers through a logarithm to work out how much salt you lose in exercise, and we can work out if you need more electrolytes in your drink.
“There’s a big issue in hydration in that people are looking for a one size fits all answer: Do I need a sports drink or not?
"Unfortunately, the answer’s a lot more nuanced and depends on your answers to certain questions.
"Then you have the sports drink industry. Sports nutrition marketing can be very confusing.”
What difference can good hydration make, or is it just one part — however vital — of an overall training and preparation regime?
Given how well elite athletes know their bodies, can they judge their own level of hydration?
“In terms of the difference hydration makes, it’s one component in a lot of self-care things. The whole marginal gains argument, which originated with Team Sky, has somewhat overplayed the marginal gains rather than having people focus on basics like good nutrition, adequate rest, properly programmed training.
“With hydration, imagine there’s a band within which you can operate successfully. If you under-drink considerably it affects your performance, though quantifying that precisely is very difficult.
“There have been lots of studies on hydration, and while they don’t all agree with each other, their general point is that if you allow people to dehydrate, their capacity to perform exercise, particularly in heat, drops off. That’s not surprising to anyone.
On the other hand, if people over-drink, that can be a problem also.
“So there’s a sweet spot in the middle of that band I mentioned, but it’s not a matter of hitting a bullseye as much as a barn door.
“The point about elite athletes is correct. They spend years perfecting their physical performance which makes them naturally curious about how thirsty they are, the colour of their pee — they’ll build up their own database which is ‘when my pee was this colour I performed badly, so I need to drink more’, or vice versa.
“They’re good at that, but it’s when someone doesn’t have those parameters they can get the balance wrong.”