This myth drives me crazy. You hear it all the time: sit in a sauna or do “hot yoga” so you can sweat out all your toxins. I wonder what these people would say if I asked them what “toxins” they are referring to.

Here’s the deal: when you perspire a lot, you are just sweating out more sweat. Sweat is 99% body water. It also contains some sodium (which is why it tastes salty), and trace amounts of potassium and chloride. These things are not toxic, which is why we must replace them after exercise.

The sole purpose of sweating is for thermoregulation. It is extremely important that our body temperature stay within a few degrees. If the core heats up – either because the environment is warm or because we are producing heat internally with exercise – our primary means for cooling is sweating (evaporation).

The main mechanisms for ridding our bodies of toxins involve a toilet. The colon is the primary pathway for this, which is why eating fiber is so important;  it helps to really “scrub” the colon to remove all the toxins in there, ultimately reducing risk for colon cancer. Excess sweating (unless you’re referring to exercise) certainly isn’t on the list of activities to protect against disease!

Since excess sweating only rids ourselves of more body water, it can result in dehydration and overheating.

Also, people should know that purposely exercising in a hot environment merely results in more rapid muscle fatigue and higher (undesired) cardiovascular strain. See, when you’re exercising, your body requires a higher cardiac output – which is the product of stroke volume and heart rate. Since about 50% of our blood volume is water, excessive sweating results in reduced blood volume, thus a lower stroke volume. So for any given exercise intensity, heart rate must be higher (than it would be in a cool temp) to compensate for the lower stroke volume.

So often the fitness industry makes things more complicated than they really are, and this is a perfect example. We sweat simply to cool down…that’s it!

Update:

I’ve had a lot of firefighters ask me if this means that infrared saunas that are now being used to decon after fire suppression are a hoax. This article is not meant to comment on that. As a scientist, I couldn’t say for sure if those saunas work to remove carcinogens from your skin any faster than your skin shedding does. It certainly is plausible. There’s just no long term research yet to show that it actually does result in a reduced risk for firefighters developing cancer. I hope one day there is. But for now, what we do know is that profusely sweating does not remove any sort of metabolic waste, which is what proponents of hot yoga claim. We also know, as we have long term research to show it, that exercising, eating fruits and vegetables and maintaining a healthy weight do lower risk of developing cancer.

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