Health Basics  /  4 min read  /  Aug 1, 2026

You don't sweat out toxins — here's what sweat really is

BIO 102 — Human Physiology I: Cell, Muscle & Cardiorespiratory Systems

Fifteen minutes into a sauna session, towel soaked, somebody always says it: "I can feel the toxins leaving." The feeling is real. The explanation is not.

What just left your body was water, sodium, chloride and trace amounts of a few other things. Your liver and kidneys handle the actual clearing, and they were doing it before you sat down. To understand why, it helps to know how anything gets across a cell wall in the first place.

Everything your cells need has to get through one thin wall

The membrane around a cell is mostly fat, which makes it a good barrier and a bad door. What crosses, and how, comes down to a few options.

Simple diffusion is passive drift from high concentration to low. Oxygen and carbon dioxide do this, which is why gas exchange needs no machinery, only a gradient.

Facilitated diffusion still runs downhill but needs a protein to get through. Channels are open pores and move things fast. Carriers grab a molecule, change shape and release it on the other side, which is slower and can be saturated. Glucose uses carriers — the GLUT family — and one of the most useful facts in this whole chapter is that muscle contraction itself moves more glucose transporters to the cell surface. That is a real, well-described mechanism, and it is part of why a walk after a big meal is not just folk advice.

The pump that never turns off

Some things have to move uphill, against their gradient, and that costs energy. The sodium–potassium ATPase — the pump sitting in every cell in your body — pushes three sodium ions out and pulls two potassium ions in, burning ATP every cycle, continuously, for your entire life.

That single pump accounts for a meaningful share of the energy you burn doing nothing at all. "Resting metabolism" is not idle. It is largely the cost of maintaining the difference between the inside of you and the outside.

That gradient is also a stored battery, and cells spend it. Secondary active transport uses the sodium rushing back in to drag something else along — glucose, amino acids, calcium. This is not trivia. Sodium and glucose share a transporter in your gut, which is precisely why oral rehydration solutions pair salt with sugar, and why a drink with both is absorbed better after a long, salty session than plain water is.

So what is sweat, actually

A sweat gland starts by secreting a fluid similar to the liquid part of your blood. As that fluid travels up the duct, active transport reclaims sodium before it reaches the skin. People who train regularly in heat tend to reclaim more of it, which is why a fit, heat-adapted person's sweat is usually less salty than a beginner's in the same July humidity.

What comes out is water and electrolytes, plus small amounts of urea, ammonia and other compounds. Small is the operative word. Sweating is a temperature-regulation system, not a filtration system, and nothing about sweating replaces the work of a kidney.

None of which makes the sauna pointless. The Finnish sauna cohort studies are real, they are large, and repeated sauna bathing has been associated with better cardiovascular outcomes in that population. That is an association worth knowing about. It is not a detox mechanism, and the studies cannot tell you it caused the outcome.

One more piece of honesty: the number on the scale after a sauna or a hot summer run is water, and it comes back with your next glass. It was never fat.

A word about drinking after a long, sweaty session

Water moves by osmosis, following salt. If you sweat heavily for hours and replace it with large volumes of plain water only, you can dilute the sodium in your blood. Confusion, a severe headache, or repeated vomiting after heavy fluid intake are not things to sleep off — get medical care immediately.