Sweat loss
Sweat loss is the amount of fluid you lose through sweat during an effort, usually expressed in millilitres per hour (your sweat rate). It varies strongly from person to person and condition to condition, and it determines how much you sensibly drink during longer or hot sessions.
What is sweat loss?
Sweating is your cooling system: when you exert yourself, you shed heat by letting fluid evaporate. How hard you sweat for that — your sweat rate — depends on intensity, duration, heat, humidity, clothing, body size and how well you’re used to the heat. The differences between athletes are large: from a few hundred millilitres to well over a litre per hour.
Heavy fluid loss without replacement drops your performance and, on long, hot efforts, raises the risk of overheating; you also lose salts with your sweat, sodium above all. At the same time, drinking more isn’t automatically better — drinking too much can dilute the sodium in your blood dangerously. The goal is to roughly match your intake to what you actually lose.
How do you find your sweat loss?
The practical measure is a weigh test around a session: weigh yourself (ideally dry and with as little clothing as possible) just before and just after exercising. The weight you’ve lost is fluid; add the fluid you drank during the session, and you have your gross sweat loss. Divide by the duration for your sweat rate per hour. A single measurement says little — conditions vary — so repeat the test over several sessions and average the results.
Don’t translate that loss one-to-one into a drinking plan. During exercise you usually don’t need to, and can’t, replace everything; leading guidelines (such as the ACSM’s) actually advise not drinking so much that you gain weight. Build up higher intakes and extra sodium gradually and test them in training — never for the first time in a race.
How Trimetra personalises this
Trimetra learns your personal sweat rate from your own sessions: weigh yourself just before and after a session and enter how much you drank along the way, and the app combines that into a measured loss — corrected for the fluid and mass you lose through breathing, so the number truly reflects sweat — and averages it over enough reliable sessions. The per-session drinking advice then aims at a realistic share of that loss — you deliberately don’t replace everything during exercise — and is topped up on hot days as long as you’re not yet used to the heat. Without enough measurements the app falls back on a safe general guideline; you stay in charge of what you actually drink.
Frequently asked questions
How much should you drink during exercise?
Roughly match your intake to what you actually lose — and you measure that with a weigh test. You don’t need to replace everything during exercise; leading guidelines even advise against drinking so much that you gain weight.
How do you measure your sweat loss?
Weigh yourself right before and right after a session, add what you drank along the way to the weight you lost, and divide by the duration: that is your sweat rate per hour. One measurement says little — repeat the test over several sessions and average the results.
Can you drink too much during exercise?
Yes. Drinking far more than you lose can dangerously dilute the sodium in your blood. So more is not automatically better: match your intake to your measured loss and build up higher intakes gradually in training.
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