How Sweating Works: What Sweat Actually Tells You About a Workout

Sweat is a cooling system, not a scoreboard. Your body does not release it because you earned it with a good set — it releases it because your core temperature is climbing and the heat has to go somewhere. Once you understand the mechanism, you stop judging sessions by how wet your shirt is and start doing concrete things with fluid and salt.

Where the heat comes from

Muscle is an inefficient engine: only about 20–25% of the energy it burns becomes movement, and 75–80% leaves as heat. During moderate running or circuit work that is roughly 700–1000 watts of heat. With no cooling, your core temperature would rise about 1 °C every 5–8 minutes — twenty minutes in, you would be in real trouble.

So 2–4 million eccrine sweat glands switch on, densest on the forehead, palms, soles and upper back; a single palm holds several tens of thousands of them. Here is the part most people get wrong:

Sweat that drips off you never cooled you. Only the sweat that evaporates off your skin does any work.

Evaporating one litre of sweat removes about 580 kcal of heat. That is why a fan or moving air beats a towel — towelling off wipes away the thing that would have cooled you. Above roughly 70% humidity evaporation stalls, sweat just runs, and you keep heating up despite losing huge volumes of water.

How much you actually lose

Realistic numbers for an adult training indoors at 20–24 °C:

• Strength work, 60 minutes, 2–3 minute rests: 0.4–0.8 litres per hour
• Conditioning, rower or spin bike, 45–60 minutes: 1.0–1.8 litres per hour
• Running outdoors at 30 °C with humidity above 60%: 1.5–2.5 litres per hour, and over 3 litres for larger people

You do not have to guess your own number. Weigh yourself in underwear before training and again straight after, towelled dry. The difference in kilograms is roughly the litres of fluid lost; add whatever you drank. Example: 80.0 kg before, 78.6 kg after, 0.5 l drunk → total loss is 1.4 + 0.5 = 1.9 litres. Do it three times in different conditions and you have your summer range and your winter range.

Performance starts to slide at about 2% of body mass. For an 80 kg person that is 1.6 kg, so 1.6 litres. Past that point heart rate drifts up roughly 5–8 beats per minute for every 1% of mass lost, perceived effort rises, and technique on the last sets visibly falls apart.

Salt, not just water

Sweat is about 99% water, but the rest matters. Sodium concentration varies enormously: roughly 460 to 1800 mg per litre (20–80 mmol/l), which is 1–3 grams of salt per litre. If you leave white streaks on a dark shirt and sweat stings your eyes, you are probably at the high end.

A practical schedule:

• Sessions up to 60–75 minutes, losses under a litre: plain water, 400–800 ml during the session.
• Sessions over 90 minutes, or losses above 1.2 litres per hour: add sodium, 300–600 mg per litre. For reference, one level teaspoon of table salt is about 6 g of salt and about 2300 mg of sodium, so 1/4 to 1/2 teaspoon per litre covers it.
• After training: drink 1.25–1.5 litres for every kilogram lost over the next 2–4 hours, alongside a salted meal. Replace litre for litre with no sodium and your kidneys dump part of it, leaving you still short.

There is an opposite failure mode. Drinking large volumes of plain water while losing a lot of salt can drop blood sodium (hyponatraemia), and that is more dangerous than mild dehydration. It happens to marathoners and long-day hikers who drink at every station just in case. Drink to thirst and to your measured loss, not on the theory that more is always better.

Why some people pour and others barely glisten

The spread is wide and mostly outside your control: gland number and sensitivity, body mass (bigger bodies generate more heat), body fat percentage, sex, genetics, and how heat-acclimatised you are. A trained person sweats earlier and more than an untrained one — that is an adaptation showing better thermoregulation, not evidence that the session was harder.

Heat acclimatisation comes fast: 10–14 days of 60–90 minutes per day in the heat at 50–65% of maximum intensity. After that, sweating starts at a lower skin temperature, total volume goes up, and sweat sodium can fall by up to 50% because the glands reabsorb it better. If you have a July half marathon or a gym with no air conditioning, plan those two weeks in advance rather than the day before.

Odour and skin

Fresh eccrine sweat is essentially odourless. Smell appears when skin bacteria break down apocrine gland secretions (armpits, groin) into acidic and sulphurous compounds. That is why showering within 15–20 minutes and dry, breathable clothing beat everything else. Antiperspirant and deodorant are not the same thing: antiperspirant temporarily narrows the gland duct and reduces how much sweat reaches the surface, deodorant only targets bacteria and smell. Sitting in a soaked shirt for hours gets you both a rash and an odour — change it.

When to see a doctor

• Emergency, right now: you stop sweating in the heat, your skin is hot and dry, and you become confused or faint — suspect heat stroke. Equally urgent: cold clammy sweat with chest pain, breathlessness, or pain spreading to the jaw and arm.
• Night sweats that soak the bedding, together with fever, a persistent cough, or losing more than 5% of body mass over 6–12 weeks.
• Heavy sweating of palms, soles or armpits for more than 6 months, unrelated to heat or effort, that interferes with work — that is hyperhidrosis and it is treatable.
• Sweating on only one side of the body or face, or sudden new sweating with hand tremor, weight loss and a resting pulse above 100.
• Cramps, nausea or headache after every longer session despite drinking properly and salting your food.

If you recently started a medication and your sweating changed, mention it: antidepressants, some painkillers and thyroid medication frequently alter thermoregulation.

The short version

• Only evaporated sweat cools you — airflow and a fan do more than a towel.
• Weigh in before and after: 1 kg of difference is about 1 litre of fluid; keep losses under 2% of body mass.
• Water is enough up to 75 minutes; past 90 minutes add 300–600 mg of sodium per litre.
• Replace 1.25–1.5 litres per kilogram lost over the next 2–4 hours, with salt in a meal.
• A soaked shirt is not a result. Load on the bar, sets completed and time under tension are.