The perils of drinking seawater
Ok, I get the concept that too much salt makes you pee more and peeing more hastens dehydration. However, I also understand that sports drinks and things like pedialyte work to rehydrate you by adding a little salt, which a) replaces salts lost through sweating, and b) tricks your body into thinking it is dehydrated by increasing the salt in your bloodstream, making you retain more water. I understand how that can help when you’ve just worked out and are really dehydrated, but what happens if you drink gatorade during a meal or something? Are you hydrating yourself, or are you actually doing the opposite by loading up on extra sodium that will just make you pee more later? It seems like sports drinks are great for immediate rehydration, but in the bigger picture, they’re a temporary fix; that extra sodium’s gotta come out sometime. Not that I’m afraid of dehydrating myself with gatorade (what a strange idea), but if I were that unlucky sailor on a ship stranded at sea, would I be in better shape if I had brought along a case of gatorade or a case of drinking water?
It’s a good question, and I’ve never really understood it well. In high school biology, we learned that salt outside your cell will dehydrate you, while salt inside your cells will help you retain water. When I asked how the salt gets inside your cells (since the famous semi-permeable mebrane only allows the passage of water), the biology teacher mysteriously answered, “through other mechanisms”. I never went much further with biology, so I don’t know what these mechanisms are. It will be interesting to see the responses to this question.
Allow me to introduce you to the “Sodium pump”
Almost forgot to mention Ion Channels
I think I might have figured it out, see if this makes sense. As long as you don’t start poisoning your body with sodium by taking in too much (like you would after a few glasses of seawater) increased sodium intake doesn’t make you pee more, it just makes your pee saltier (anyone care to test this theory?). So eating salt may make you thirstier, but it doesn’t necessarily dehydrate you; it’s only when you cross a certain threshold of sodium in your blood that the body goes into the flushing response. So as long as I don’t develop high blood pressure, I can keep drinking Gatorade just for the hell of it.
You are basically right. It also helps to understand how your kidneys work. Your kidneys filter the fluids out of your bloodstream by having a high concentration of salts within the kidneys. Thus fluid diffuses into your kidneys, from the low concentration of salts in your blood to the higher concentration of salts in your kidneys. If your salt concentration in your blood increases to a level that is greater than your kidneys (drinking significant amounts of salt water would accomplish this) then your kidneys would basically stop functioning. The salt concentration in Gatorade is not high enough, practically speaking, to raise your blood salt concentration to that level.
Then, is it a good idea for the unlucky sailor to stretch his water supply by adding some seawater to his fresh water?
Yes, in fact it’s recommended in most survival guides provided, and this is the important part, you have sufficient food. IIRC it’s one part salt to 3 parts fresh water, so you can increase your water supply by 25% in this manner.
The reason you need the food is that your kidneys can only deal with that amount of salt by upping the activity of the ion pumps in the kidneys. That causes a measurable increase in your energy requirements and increases your risk of starving to death or dying of hypothermia.