Sweat equals fat loss

The claim, as heard: "Sauna, sweat suit, cling film round the waist — sweat it out and the fat goes with it."

What the data show. Nothing, because there is nothing to study: sweat is water with some salt, produced to cool the body, and it leaves the body's water stores, not its fat stores. A hard hour in a sauna or a plastic suit can lose a kilogram of water, which the scale duly shows and which returns as soon as you drink — as it must, because a body that stayed that dehydrated would be in trouble. Fat leaves the body as carbon dioxide you breathe out and water you excrete, in proportion to the energy deficit, and sweating does not change the deficit; heat does raise expenditure slightly, by an amount that does not matter. This site cites no source for the page because there is no level of evidence to grade: it is a definition, and the grade below reflects that the claim has no support, not that the physiology is uncertain.

Verdict: false. No source; physiological context only.

The grain of truth. Exercise that makes you sweat is exercise, and exercise counts — for the reasons on the exercise page. The sweat is a by-product of the work, not the result. And a sauna is a pleasant place to be; it just is not a weight-loss method, and dehydrating deliberately before a weigh-in is a practice from weight-class sports that has no place in ordinary life.

One sentence to remember: what you sweat out, you drink back; fat leaves through the lungs, in proportion to the deficit, however dry your shirt is.