Electrolytes and Hydration: How Much Salt Do You Actually Lose?

Hydration advice is unusually bad, and the reason is that it is almost always stated as a population average applied to an individual. "Drink two litres a day." "Take an electrolyte tab every hour." "Salt prevents cramps."

The actual variation between people is enormous — larger than for almost any other nutrition variable — which means the useful move is not finding a better rule. It is measuring yourself once.

How much you sweat

Sweat rates range from under 0.5 litres an hour to over 2.5 litres an hour between individuals doing similar work. That is a five-fold spread, and it means any generic fluid recommendation is badly wrong for most of the people reading it.

Measuring yours takes one session:

1. Weigh yourself naked, immediately before an hour of running
2. Note anything you drink during the hour
3. Towel off and weigh yourself naked again immediately after

Each kilogram lost is roughly one litre of sweat. Add back whatever you drank — if you lost 1.2 kg and drank 500 ml, your sweat rate was about 1.7 litres an hour.

Do this twice: once in cool conditions, once in the heat you actually train in. Those two numbers will differ more than you expect, and the hot one is the one that matters for planning. Heat and humidity both push the rate up, for the reasons covered in running in Indian heat.

How salty your sweat is

This varies even more than the volume, and it is the number almost nobody knows about themselves.

Sweat sodium concentration ranges from roughly 200 mg per litre to over 2,000 mg per litre. A ten-fold spread, largely genetic, and not something you can meaningfully change through diet or training.

Combine the two variables and the practical range becomes stark:

Those are the same activity. The person at the bottom of that table needs roughly fifteen times the sodium replacement of the person at the top, which is why generic advice fails so comprehensively.

Rough self-assessment, in the absence of a lab test:

• White salt residue on your skin, cap or dark clothing after a hot session
• Sweat that stings your eyes noticeably
• Persistent salt cravings after long sessions

Those suggest you are toward the salty end. Proper sweat testing exists and is genuinely useful if you race long in heat, but the informal version is enough for most people.

What this means for tablets

Here is the practical consequence, and it is where a lot of money is wasted.

Many popular electrolyte tablets contain 200 to 500 mg of sodium per serving. For the light dilute sweater in the table above, that comfortably covers an hour. For the salty sweater losing 3,000 mg an hour, it is a rounding error being sold as a solution.

So: read the sodium content per serving, and compare it against your own estimated loss rather than against the packaging's confidence. If you are a heavy salty sweater doing long hot sessions, you likely need a higher-sodium product, more of them, or simply added salt in your drink.

The other electrolytes — potassium, magnesium, calcium — are present in sweat at much lower concentrations and are far more readily replaced by ordinary food. Sodium is the one that actually needs attention during exercise. Products emphasising a long ingredient list are mostly selling reassurance.

The cramp question

This deserves a direct answer because the marketing is so confident.

The evidence that sodium prevents exercise-associated muscle cramps is weak.

The theory is intuitive — you lose salt, you cramp. But the research is not supportive: athletes who cramp frequently have normal electrolyte status, cramps often occur in a single muscle group rather than systemically as an electrolyte deficit would predict, and the strongest correlations are with neuromuscular fatigue — going harder or longer than your training supports, particularly late in a race.

Which fits what most people actually experience: cramps arrive at the end of an effort that exceeded what you had trained for, in the specific muscles doing the most work.

This does not mean skip the sodium. Replacing it matters for plasma volume, for how you feel, and for avoiding the genuine risk below. It means that if you cramp regularly, the more productive investigation is your pacing and your training volume, not your salt intake.

The risk on the other side

Hyponatremia — blood sodium diluted to a dangerous level — is much rarer than dehydration and much more dangerous. It happens when someone drinks large volumes of plain water over a long event, often while sweating heavily and replacing no sodium.

Confusingly, its early symptoms resemble dehydration: headache, nausea, confusion. The instinct is to drink more, which makes it worse.

The protective habits are simple:

• Drink to thirst plus a little, not to a fixed schedule
• Include sodium in your fluid on anything long or hot, rather than plain water only
• Do not aim to finish a session at the same weight you started. Losing up to about 2% of bodyweight across a hard hot session is normal and fine

If you are gaining weight during a long event, you are drinking too much.

The practical protocol

Sessions under an hour: water, to thirst. Nothing else required regardless of what the shelf suggests.

One to two hours, or hot conditions: 400 to 800 ml per hour as a starting point, adjusted to your measured rate, with 300 to 700 mg of sodium per litre — more if you are a salty sweater.

Over two hours, or racing in heat: plan it properly against your own numbers. This is where getting it wrong has real consequences, and where the measurement session pays for itself.

Daily, outside training: eat normally. If you train hard in heat, do not aggressively restrict salt — athletes in hot climates with high sweat rates frequently need more than general population advice assumes, and general population advice is written for sedentary people with high blood pressure.

The summary

Two numbers replace all of the guessing: your sweat rate, which takes one session and a set of scales, and roughly how salty you are, which you can estimate from the residue on your cap.

Get those and every product decision becomes obvious. Skip them and you are choosing between marketing claims aimed at an average person who does not exist.