people fail to recognize that people ate a lot less back in the days that salt was so precious. they also ate a lot more fat. eating less and having more protein and fat meant that they had less insulin in their blood and also more ketones in their blood. insulin tells your body to hold on to things including water and when you dont have a lot of insulin you lose more water and sodium with it. and ketones are a diuretic which promote the loss of water even more.

sodium replaces insulin because it also causes your body to hold on to water. so it was very important because you needed it to offset lower insulin and also because you were losing it much more. if you didnt have access to salt back then you would have become very ill.

a modern person who eats almost nothing but sugar in one form or another and in enormous quantities has so much insulin that they barely need any sodium unless they drink large amounts of water. we even tell people to have less sodium. but earlier people werent that way.