How Much Sodium Do You Lose in Sweat? Working Out Your Number Without a Lab Test

Two riders finish the same three-hour ride in the same conditions. One has a clean dry jersey and feels fine. The other has white tide marks across the shoulders, stinging eyes and a head that feels like cotton wool. Same effort, completely different salt loss. If you have ever wondered how much sodium you lose in sweat, the honest answer is that it varies enormously from person to person, and the average figure printed on a drinks tub may be nowhere near your number.

The good news is that you do not need a lab to get a useful estimate. A few simple tests, a set of bathroom scales and some attention to what your body is telling you will get you most of the way there. This guide covers the typical ranges, why they differ so much, and how to turn your own numbers into a practical fuelling plan for long training days and races.

How Much Sodium Do You Lose in Sweat? The Typical Range

Sweat is mostly water, but it carries sodium and chloride along with smaller amounts of potassium, magnesium and calcium. Sweat sodium concentration is usually quoted at around 900 mg per litre on average, but individual athletes range from roughly 200 mg/L to well over 1,800 mg/L. That is a ninefold spread.

Sweat rate matters just as much. Most endurance athletes lose somewhere between 0.5 and 2.0 litres per hour depending on intensity, temperature, humidity, fitness and body size. Multiply the two and you can see how wide the real-world range is:

• Light sweater, low concentration: 0.6 L/h at 400 mg/L is about 240 mg of sodium an hour.
• Average athlete: 1.0 L/h at 900 mg/L is about 900 mg an hour.
• Heavy, salty sweater in the heat: 1.8 L/h at 1,500 mg/L is about 2,700 mg an hour.

That is more than a tenfold difference in hourly loss. It explains why generic advice such as "drink to thirst" or "take one salt capsule an hour" works brilliantly for some people and fails badly for others.

Why Sweat Sodium Differs So Much Between Athletes

Genetics play the biggest role. Sweat sodium concentration is largely a fixed trait, so a salty sweater in winter is still a salty sweater in July. Several other factors shift your hourly loss around it:

• Heat acclimatisation: after one to two weeks of training in the heat, the sweat glands reabsorb more sodium, so concentration drops, although total sweat volume rises.
• Intensity and fitness: fitter athletes start sweating earlier and produce more of it, which raises total fluid loss at the same power or pace.
• Diet: a habitually high-sodium diet tends to produce slightly saltier sweat, though the effect is modest compared with genetics.
• Clothing and conditions: a windproof layer on a humid day can double your sweat rate compared with a breezy dry ride.

This is why a rider from the Highlands who spends the summer in Girona can find their old kit list suddenly inadequate. Your number is not a single figure; it is a range that moves with conditions.

How to Estimate Your Own Sodium Loss at Home

Start with sweat rate, because it is free and takes ten minutes of maths. Do this on a typical session of around an hour at race-like effort:

• Weigh yourself nude or in minimal dry kit just before you start.
• Record exactly how much you drink during the session (1 ml of fluid weighs 1 g).
• Towel off and weigh again straight afterwards.
• Sweat loss in litres equals weight lost in kilograms, plus the fluid you drank, minus any toilet stops.

If you lost 1.2 kg and drank 0.6 L, your sweat rate was about 1.8 L per hour. Repeat it in hot weather, cool weather and on a hard indoor turbo session, because each will give a different answer.

For the sodium side, there are three levels of accuracy. The cheapest is observation. White salt crust on your kit or hat band, a gritty feel on your skin, salt stinging cuts or eyes and a strong craving for salty food after long sessions all point to a high concentration. The middle option is a home sweat test patch, which gives a rough concentration reading. The gold standard is a laboratory or clinic test that collects sweat and measures sodium directly, and it is worth doing once if you race ultra-distance or in hot climates.

Turning Your Number Into a Fuelling Plan

Replacing 100 per cent of your sodium loss is neither necessary nor sensible. Most athletes aim to replace roughly a third to a half of what they lose per hour, and plenty of sports dietitians work in the range of 300 to 600 mg of sodium per hour for typical conditions, rising to 800 mg or more for salty sweaters in the heat. Start at the lower end and adjust according to how you feel and how the session goes.

Fluid matters alongside sodium. Drinking far more than you sweat, particularly plain water, dilutes blood sodium and is the route to hyponatraemia, which is a genuine risk in slow, long events. A sensible target is to replace around 60 to 80 per cent of your measured sweat loss, keeping total weight loss below roughly 2 per cent of body mass over the session.

Carbohydrate and sodium also work together. Sodium helps drive glucose absorption in the gut, which is part of why drinks that combine carbs and electrolytes can feel easier to process than carbs alone. If you are aiming for 60 to 90 g of carbohydrate an hour, plan the sodium as part of the same system rather than as an afterthought.

What We Use at FTP Endurance Fuel

We split the job into two parts. Carbohydrate comes from our energy pastilles, which are easy to chew, easy to portion and simple to count out by the hour. Sodium comes from whichever electrolyte drink or capsule suits the conditions, so we can change the dose without changing the carbohydrate plan. Keeping them separate means you can dial salt up on a scorching sportive and back down on a cool Sunday spin.

For long efforts we like the Tart Cherry Pastilles 1kg, as the tart cherry gives a sharper flavour that stays appealing when everything sweet starts to taste cloying. If you want variety across a long day, the Mixed Fruit Pastilles 1kg are the better pick. Whichever you choose, test the whole plan, pastilles and electrolytes together, in training before race day.

Putting It Into Practice

Run one sweat rate test this week, note the temperature, and write the result in your training log. Do another in different conditions next month. Within a few sessions you will have a personal range rather than a guess, and you will know whether you are the rider with the dry jersey or the one wearing a salt map.

Knowing how much sodium you lose in sweat takes the guesswork out of long days. If you want to build the carbohydrate half of the plan, the Taster Pack is the easiest way to try both flavours before committing to a full tub, and our bundles make stocking up for a training block simple.