> Saw a movie back in I think the seventies that involved a time traveler coming back to present day America. He's happily eating what was regarded at the time as a very unhealthy diet and is horrified at all the people eating what we all thought was the healthy choice. It was a comedic punchline throughout the movie.

You are probably talking about Woody Allen's "Sleeper".

https://en.wikipedia.org/wiki/Sleeper_(1973_film)