Salt is important for a hunter gatherer because it’s both directly involved in some signaling and makes your body retain water which is far more important than food for keeping you alive. So people whose brains trigger a very strong reward when salt is consumed survived to reproduce if water was even a little bit scarce.
This means salt still triggers some very important reward systems even though most people have very little danger of consuming too little salt.
Salt draws tasty chemicals out of food because it draws things out along with water, then makes those flavors easier to sniff out. A lot of how stuff taste to us is based on how it smells, so this greatly enhances flavor. It also just moves flavorful compounds to the surface of food where you can taste it on your tongue.
Salt plays an important role in a very large amount of biological processes (blood pressure, neuron depolarization, etc.) and is not storable in the body like how carbohydrates can be. This means there were evolutionary pressures for land creatures to have a desire for some salt intake. There’s a feedback loop pathway controlled by the hormones renin and angiotensin, which affects someone’s need for salt intake.
There’s various (though not well understood) mechanisms for increasing flavor. Salt can decrease water activity, thereby increasing the concentration of other molecules in food. It can also suppress the feeling of bitter tastes, which increases perceived taste.
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