In the days before the Stop & Shop spice aisle, food was pretty lackluster, and SALT was so prized that people bartered with it. Roman soldiers were even paid their wages in salt -- hence the word salary and the phrase worth one's salt. Salt, of course, makes it easier to swallow food, so that's why suspicious-sounding stories must be taken with a grain of it.
Reader's Digest is great about packing in the facts! Many thanks!