Eli5: Why is it okay to replace electrolytes (including sodium) after exercise, but high sodium consumption is bad for long term health?

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I can’t figure how it’s beneficial to consume what seems like a high sodium drink after a workout but putting too much salt on my steak will kill me.

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Electrolytes arent merely sodium. They include potassium (biggest one) along with calcium and magnesium.

“High” sodium consumption is relative. Your body needs sodium, but not too much. Eating too much of it at any time is bad. But your body loses some when you sweat, so you need to eat more of it if you exercise a lot.

Also, salt is added to a lot of foods, so it’s really easy to eat too much of it.

If you work out really hard, you will sweat a lot, and sweat contains salt. Our bodies need at least some salt to function, though there’s a common saying that “the dose makes the poison,” meaning that everything, no matter what it is, will be bad for you if you get too much of it.

So the idea is to stay at a relatively constant level of salt inside your body. This means if you lose a lot of salt from sweating, you should replace that lost salt by ingesting some more. However, sports drinks are made by companies whose goal is to make money, so they will tell you their product is good to consume even if you don’t actually need it. Most people don’t exercise hard enough to need to go out of their way to replace lost salt from sweat.

When you exercise, you heat up. To cool you down, your body sweats, creating a thin layer of water on your skin that will evaporate, taking heat with it. In order to get water to move out of your body as sweat, your skin secretes sodium. Because of osmosis, water automatically follows (to put it in simple terms, water molecules love sodium so they follow it wherever it goes). This is why your sweat is salty.

Because you’ve now lost sodium, it’s a good idea to replenish it, because sodium is also used by your nerves, muscles and a whole bunch of other things.

The problem with having too much sodium actually works in the same way sweat does. If you have too much sodium in your bloodstream or kidneys, then water moves out of your actual cells to hang out with the sodium. When cells lose water like this, they shrink, causing all sorts of damage.