Electrolytes in winter sports: why you lose fluid and salt in the cold too
TL;DR: In the cold you sweat less visibly and feel up to 40% less thirst. Yet you still lose fluid and salt, through sweat under your layers, your breath and extra peeing. A small fluid deficit matters less in the cold than in summer, but on long days you fall behind. Plan fixed drinking moments, keep your drink and bars usable in the cold and keep eating carbs.
In winter you lose fluid and salt too. You just don't see it. Under your layers you keep sweating, and your thirst drops sharply in the cold. Ski tourers, cross-country skiers and winter cyclists therefore often drink too little. The full fuelling overview is in my complete guide to energy bars for cycling. How much sodium you need per hour, you'll read in electrolytes during endurance sport: how much salt do you really need. This one is about what the cold changes.
Why are you less thirsty in the cold?
Cold dampens your thirst, by up to 40%. Kenefick and colleagues (2004) showed this: exercising at 4 °C, athletes felt much less thirsty, even when they were already short on fluid.
The cause lies in your blood vessels. In the cold, narrowing of your blood vessels in your skin and limbs pushes more blood to your core. Your body reads that as enough fluid and turns down the thirst signal. In summer, drinking to thirst is a good guide. In the cold, that guide works less well.
Where do you lose fluid in winter without noticing?
In three places at once:
- Sweat under your layers: ski touring or winter cycling is hard work. You sweat, but it disappears into your clothing.
- Your breath: cold air is dry. Every breath gets warmed and humidified in your lungs, and you breathe that moisture out. The harder you breathe, the more you lose.
- Extra peeing: cold and altitude make you pee more. Sawka and colleagues (2015) describe fluid loss from cold and altitude coming on top of sweat loss.
Does a small fluid deficit in the cold matter?
Less than in summer. I'll be honest about that. According to Sawka and colleagues (2015), a mild fluid deficit usually doesn't harm endurance performance in a cold environment. In the heat it does. So no reason to panic.
But a winter day is long. Six hours of ski touring or a long winter ride, often at altitude, with little thirst: that's when the deficit piles up. And whoever drinks less often takes in less sports drink, and so fewer carbs. You feel that on the last descent, when you need your focus most. How to avoid an energy dip, you'll read in preventing the bonk.
What makes ski touring, cross-country skiing and winter cycling different?
| Sport | What's going on | What you do |
|---|---|---|
| Ski touring and skimo | Long climbs, hard work under thick layers | Plan drinks and food at every stop |
| Cross-country skiing | Steady high pace, lots of breathing in dry air | Drink belt with warm sports drink |
| Piste skiing | Stop-and-go, long days, little thirst | Drink at every lift, eat every hour |
| Winter cycling | Wind and cold on your torso, sweat under your jacket | Thermal bottle, bars in your inside pocket |
In summer you have the opposite problem: heat demands more fluid and salt. Read about it in carbs when training in the heat. More on balancing fluid and salt is in hydration and salt during endurance sport.
How do you plan fluid, salt and fuel on a long winter day?
Not by feel. On a schedule, with small sips and without overdoing it:
- Before you start: eat a proper breakfast with carbs and a bit of salt, and drink an extra glass.
- Every hour during hard efforts: 500 to 800 ml of sports drink with 6 to 8% carbs, plus 1 to 2 fruit performance bars. Together 60 to 90 g carbs per hour.
- Easier piste or hiking days: less volume, but fixed moments: drink at every lift or break.
- Lunch break: soup or something savoury. Warm fluid and salt in one go.
- Last hours: keep eating and drinking, even without hunger or thirst. That's where you make the difference.
How long you're out determines how many carbs you need. Up to 2.5 hours, 30 to 60 g per hour is enough. Longer? Then 60 to 90 g per hour pays off, using a mix of glucose and fructose (Jeukendrup, 2014). What makes a good sports drink, you'll read in isotonic sports drink with carbs.
How do you keep drinks and bars usable in the cold?
- Thermal bottle: fill it with lukewarm sports drink. Warm is easier to drink in the cold, and it won't freeze.
- Inside pocket: carry your bars close to your body. Cold food is harder to chew.
- Small format: my bar weighs 25 g. Two bites, even with gloves on, without a long stop.
Why do I fuel with fruit in winter too?
Because your body needs the same fuel in the cold as in summer. My fruit performance bar delivers 20 g carbs per 25 g, in a 2:1 glucose/fructose ratio, no artificials. Vegan, gluten-free, lactose-free. Together with a warm sports drink, that's your base on the piste, on touring skis or on the bike.
No gel. No brick. Fuel during exercise. Fuel your flight.
Scientific background
During exercise in the cold (4 °C), the thirst signal dropped by up to 40%, even in athletes who were already short on fluid. This went together with lower levels of the hormone that retains fluid (Kenefick et al., 2004). Cold and altitude cause extra fluid loss via the kidneys, on top of sweat. A mild fluid deficit usually doesn't harm endurance performance in a cold environment, but it does in a warm one (Sawka et al., 2015). The recommended carb intake is 30 to 60 g per hour up to 2.5 hours and up to 90 g per hour for longer efforts (Jeukendrup, 2014).
Sources
- Kenefick R.W., Hazzard M.P., Mahood N.V., Castellani J.W. (2004). Thirst sensations and AVP responses at rest and during exercise-cold exposure. Medicine & Science in Sports & Exercise, 36(9), 1528-1534. doi:10.1249/01.mss.0000139901.63911.75
- Sawka M.N., Cheuvront S.N., Kenefick R.W. (2015). Hypohydration and human performance: impact of environment and physiological mechanisms. Sports Medicine, 45(Suppl 1), S51-S60. doi:10.1007/s40279-015-0395-7
- Jeukendrup A. (2014). A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports Medicine, 44(Suppl 1), S25-S33. doi:10.1007/s40279-014-0148-z
Frequently asked questions
Do you lose electrolytes in winter sports?
Yes. Under your layers you keep sweating when ski touring, cross-country skiing or winter cycling, and sweat contains sodium. You notice it less because the sweat disappears into your clothing and you feel less thirsty. On long days the fluid and salt deficit builds up unnoticed.
Why am I not thirsty when exercising in the cold?
Cold dampens your thirst by up to 40%. The blood vessels in your skin and limbs narrow, pushing more blood to your core. Your body reads that as enough fluid and turns down the thirst signal, even when you're already short on fluid. That's why drinking to thirst works less well in the cold.
How much should I drink on a ski day or winter ride?
During hard efforts like ski touring or winter cycling: 500 to 800 ml of sports drink per hour, together with 1 to 2 bars. On easier piste days less, but at fixed moments, for example at every lift. Small sips, without overdoing it. How much sodium you need depends on how much you sweat.
How do I stop my sports drink or bars from freezing?
Use a thermal bottle with lukewarm sports drink. Warm is also easier to drink in the cold. Carry your bars in an inside pocket, close to your body, because cold food is harder to chew. A small 25 g bar goes down in two bites, even with gloves.
Do I lose more fluid at altitude?
Yes. At altitude you breathe faster and the air is drier, so you breathe out more moisture. Cold and altitude also make you pee more. Combined with less thirst, that makes long days at altitude prone to fluid deficits. Plan your drinking and eating moments upfront.
