I’ve come to understand that the reason humans shouldn’t drink seawater is that it requires more water to get rid of that salt through urine. But if salt is a necessary nutrient, why would the body need to get rid of it at all? If your body were low on salt, what it be good for you to drink seawater?
Edit: added follow-up hypothetical question
In: Biology
What the body needs is to get rid of toxic and unnecessary substances that come from the metabolism of other substances. We generate a lot of molecular waste during the metabolic processes inside our cells. The way our body has to get rid of all that molecular waste, is to eliminate it dissolved in water, that’s urine basically. So in order to eliminate that molecular waste, we need to eliminate water.
That water is depleted from our blood. But if we were to only eliminate that waste and water, then all the salts and ions dissolved in the blood would increase their concentration (potassium, sodium, calcium, magnesium, etc). So we need to eliminate part of those ions in the urine to keep the concentrations within the normal range. So to replenish everything, we need to consume new water, new salts, new ions, everything, but in the right proportions.
Sea water for example, has way too much salt in relation to water; and within the intestines, water will be sucked from the inside of the cells towards the cavity of the intestines via osmosis, producing a diarrhea.
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