Eli5: Why do electrolytes hydrate you but saltwater dehydrates you?
It’s not strictly speaking accurate to say electrolytes hydrate you. When you sweat you lose both water and salt. Simply replacing that with water will result in your body not having enough salt which is unhealthy. Sports drinks just add electrolytes (salt) to replenish what you lost from sweat. Also usuaadd sugar since you’re burning that too when you exercise.
Because of the concentration of salt. When outside the blood vessels, will draw fluids out in an attempt to balance the hypertonic solution you drank… or whatever
TLDR: oceanwater has ~3.5% water, which is way higher than your cells 0.45%. Electrolyte drinks have 0.1 to 0.4% table salt and also a bunch of other stuff.
For a more in-depth explanation:
basic chemistry and biology first:
– Diffusion. If you have two liquids with different salt concentration they will equalize over time, until it is one liquid with evenly distributed salt
– the water inside cells has a salt concentration of ~0.45%
– the membrane of a cell can let some things through and others not. More precisely, it can let water through but not salt
If you drink ocean water – which has 3.5% salt – you‘re raising your bloods salt content. Now you have two liquids, your blood with high salt content and your cells with low salt content. Over time the salt concentration will equalize, but salt can‘t travel through the cells membrane. So instead of the salt moving, the water is moving out of the cell.
Sports drinks are nowhere near as high in salt concentration, so they don‘t have this water-sucking effect. Also, your body has far more salts than just Sodium Chloride (table salt) And so do electrolyte drinks.
It’s because saltwater from the ocean is saltier than urine. The saltiest that urine gets is about 10 grams of salt in each liter (and normally your urine is a lot less salty than that). Ocean water has about 35 grams of salt in each liter.
So if you drink a liter of seawater (yuck), your body needs more than two additional liters of fresh water, just to get rid of the salt that came with that one liter of seawater.
If your kidneys were more efficient at concentrating the salt in your blood, then you could drink seawater. For example, ocean fish do that and they’re just fine.
Because saltwater – assuming you mean ocean water – is 3.5% salt, which is way more than a saline solution at 0.9%, which is still more than an isotonic sports drink at maybe 0.5% salt.