You may remember the process of osmosis from science class at school. Osmosis occurs across a semi-permeable barrier (our cell membranes). That barrier allows water molecules to pass in certain directions, depending on the concentration of water solutions on each side (imagine different concentrations of salt water).
In normal circumstances, the saltiness of water in your cells is the same as the saltiness outside your cells. This is called an isotonic state. But when your cells consume water, the solution becomes more concentrated, and the natural process of osmosis allows water from outside your cells to pass into your cells to achieve equilibrium (this is called osmoregulation). That’s how we absorb water as it’s needed; it’s a natural process. Your cells are isotonic to about 0.9% saline solution. If the salinity of the solution outside the cells decreases, your cells absorb more water to get back to an isotonic state. When marathon runners and other endurance athletes drink too much fresh water, the solution outside their cells drops rapidly in salinity, so osmosis allows water to pass into the cells as a part of osmoregulation. If they absorb too much water, the cells will swell and burst, which can lead to a quick death. This is why runners drink sports drinks that contain sodium and potassium, to help maintain an isotonic state (and it’s why they’re called isotonic sports drinks). The opposite happens when drinking sea (salt) water. The salinity outside your cells increases rapidly, so osmoregulation effects a movement of water from in your cells, to outside your cells, to achieve an isotonic state. So even though you may be dehydrated, your cells will actually release, rather than absorb, the water around them.
But why would your dehydrated body expel water when death is imminent? Well, this isn’t a conscious decision by your body. It’s simply molecular physics and osmosis at work. And in osmosis, water passes from low saline concentrations to high saline concentrations, end of story. So, you should absolutely not drink sea or salt water when dehydrated. It’s not just an issue of not absorbing the water, but an issue of osmosis accelerating the release of water and dehydration.
I said in the introduction, “You’ll die if you don’t drink something soon, so what’s the worst that can happen?” Well, what’s worse than dying? Dying much sooner, potentially before help arrives.
