How Much to Drink on a Bike Ride, Sweat Rate and Sodium

James. This is the WattPlan Podcast and I'm James. I ride, I've got a family and a job, I'm building WattPlan in whatever hours are left, and I read sports science the way other people read thrillers. Today's one sounds simple. How much should you actually drink on a bike ride? A bottle an hour, or is that nonsense? Lyra's our AI research voice, she's been through the sweat studies, and every one of them is linked in the show notes.

Lyra. It depends on how fast you sweat, and riders differ so much that one number can't work. Weigh yourself before and after a ride, and set your bottles so you finish no more than about two percent lighter, and never heavier. That one rule does most of the work.

James. Never heavier. Is that a thing? Who finishes a ride heavier?

Lyra. More people than you'd think, and we'll come back to why it matters. First the range, because it's the whole story. Lindsay Baker reviewed sweat testing in Sports Medicine in 2017, and she put whole body sweat rate in sportspeople at roughly half a litre to two litres an hour. That's a fourfold spread. A bottle an hour leaves the heavy sweater well short, and it has the light sweater drinking more than they lose. [1]

James. Okay, but where do cyclists sit? Am I a half litre person or a two litre person?

Lyra. You won't know until you measure, but there's a big dataset that gives you a starting guess. Kelly Barnes and colleagues at the Gatorade Sports Science Institute, funded by PepsiCo, pooled sweat tests from 1,303 sportspeople, published in the Journal of Sports Sciences in 2019. The endurance group sweated about one and a quarter litres an hour on average. It isn't split out into cyclists, and the spread around that average is wide, so treat it as a guess you replace with your own number.

James. Right. So how do I get my own number? Please don't say a lab.

Lyra. Bathroom scales. Weigh yourself in as little as possible before the ride. Ride for an hour or more and note what you drink. Towel off, get out of the wet kit and weigh again. Your sweat is the weight you lost plus what you drank, minus anything you left at a toilet stop, divided by the hours. One kilo is one litre. Baker calls body weight change the simplest and most accurate method there is.

James. Give me an example. I'm terrible at this kind of maths mid-shower.

Lyra. Seventy five kilos before a two hour ride, seventy three point nine after, and two bottles of five hundred mils drunk. That's one point one kilos lost plus a litre drunk, so two point one litres of sweat in two hours, about a litre an hour. And they finished one and a half percent down, inside the line. The number reads a little high, because breathing and the fuel you burn take some weight off too, but it's close enough to set your bottles.

James. And why two percent? Where does that come from?

Lyra. The American College of Sports Medicine's position stand on fluid replacement, from 2007. It sets the goal rather than a volume. Drink to stop yourself losing more than two percent of your body weight, and because sweat rates vary so much, build each person's plan from their own measured losses. For a seventy five kilo rider, two percent is a kilo and a half.

James. See, I've read that two percent is a myth. That you can lose way more and it doesn't slow you down.

Lyra. You've read Eric Goulet, and he's worth reading. He pooled fifteen studies in the British Journal of Sports Medicine in 2013 and split them by the kind of test. When riders held a fixed intensity until they stopped, dehydration did cut performance. When they rode a time trial at their own pace, it made no real difference to power. His conclusion was that losing up to four percent is very unlikely to slow you down in the real world, and that you should drink to thirst. [4]

James. So the scales are a waste of time and I should just drink when I'm thirsty?

Lyra. Not quite. Goulet's evidence is lab time trials, and they say little about the fifth hour of a summer sportive. So here's where we land. On a ride of a couple of hours in cool weather, thirst is a good enough guide. On a long sportive, a hot day, or a turbo session with no fan, the weigh-in plan earns its keep, because those are the rides where you can drift a long way off without noticing.

James. Why does the turbo matter?

Lyra. Wind. Baker lists it as one of the things that lowers sweat rate, along with cooler air. On the road you've got a breeze over you the whole time. In a back bedroom on a trainer you haven't, so the same effort can mean a lot more sweat. That's why it's worth doing the weigh-in once indoors and once outdoors, because they can give you very different numbers.

James. Okay, salt. Every drink tub I own says electrolytes in big letters. How much sodium are we actually losing?

Lyra. A lot, and it varies even more than the fluid. The same Barnes dataset had the endurance group losing a bit over a gram of sodium an hour on average, and the spread runs from about half that to about one and a half times it. Over a five hour sportive at the top end, that's several grams. So on the long rides and the hot ones, use a drink with sodium in it and check the label against that gram an hour.

James. Can I find out where I sit on salt, the way I can on fluid?

Lyra. Not with scales. Your sweat rate you can get at home, but how salty your sweat is needs a sweat patch test. Most riders start from the average and adjust. One useful detail from Baker's review is that sweat gets less salty as you adapt to heat, so you're likely losing more salt per litre in the first warm weeks than later in the summer.

James. Back to finishing heavier. You said it matters.

Lyra. It's the bigger risk on a long, easy day. Tamara Hew-Butler and colleagues define exercise associated hyponatraemia as blood sodium falling below a hundred and thirty five millimoles per litre during or just after exercise, and their 2017 review names drinking beyond thirst as a main cause. Gaining weight on a ride is the sign you drank more than you lost. And the part people don't expect is that salt doesn't protect you if you've drunk far too much. [5]

James. So more salt isn't the fix. Less water is.

Lyra. If you've finished heavier, yes. Drink less next time. Never set your bottles to beat the scales.

James. Make it practical for me, then. Well, not me. If you're new to this and your longest ride is a couple of hours?

Lyra. If you're new to this, drink when you're thirsty, take a bottle you'll actually use, and do one weigh-in on a normal ride so you know roughly where you sit. That's it for now. You don't need a sodium plan for a two hour ride in the spring.

James. And if you train with power, doing the long weekend rides?

Lyra. If you train with power and you're doing three or four hour rides, do the weigh-in twice, once on the turbo and once on a long outdoor ride. Set your long ride bottles so you finish between level and two percent lighter. In the WattPlan player you'll get reminders to drink through any longer session, but they don't know your sweat rate or whether you're indoors, so treat them as the cue and your weigh-in as the amount.

James. And if you're racing, or doing a big summer sportive?

Lyra. If you race, or you've got a hot five hour sportive coming, rehearse the event's drinking on the long rides before it, sodium included, so race day isn't the first time your stomach sees it. And weigh in after the hottest of those rides, because heat moves your number more than anything else you control.

James. What can't the research tell us?

Lyra. Quite a lot. The sweat data are observational, collected by one drinks company's institute, and the endurance group isn't broken down into cyclists. Goulet's trials were lab time trials. And nobody has tested a sodium dose for amateur cyclists, so the salt figures are averages from a range so wide your own losses could sit well outside them.

James. So, weigh myself before and after, finish a bit lighter but never heavier, thirst is fine for the short stuff, and salt on the long hot ones.

Lyra. And the scales tell you something no drinks label can, which is how much you lost.

Lyra. The full article and every study we mentioned are on the WattPlan blog, linked in the show notes. And if you've enjoyed this, follow the WattPlan Podcast wherever you're listening, so you don't miss the next one.

James. And if you want a plan built around the time you've actually got, get WattPlan on the App Store or Google Play. Thanks for listening.

Voices produced with ElevenLabs. James's voice is a clone of his own, Lyra is an AI voice, and James reviews every episode.