I don't know how long that old adage has been around, but from experience, I know it's been longer than 60 years!
Way back, people most likely were eating an apple a day ~ in season, anyway ~ because fruit was a luxury. (My mom said one Christmas when she was a child she got an orange, and thought it was a wonderful gift!)
But we're finding more and more that if wealth is health, then fresh produce is a gold mine. Another bit of evidence ~ I read an article linked to Dr. Oz's website about how bad microbes in our gut may contribute to obesity, and an abundance of the good bug can slim you down. According to the article, we're talking trillions of these things, the key being the proportion of good bugs to bad.
A study showed that obese people are three times more likely to have a virus called Ad-36 in their digestive tracts. (Chickens that have it get fatter!) It seems to trigger bigger appetites and store more calories as fat.
But the good bugs that work in your favor are those probiotics ~ and apples have it! So do bananas, asparagus, onions and garlic, to name a few other sources.