Somewhere, some'one' company decided to change the way we think about food. Somehow there were messages sent to our brains that made us believe wheat, corn and potatoes (these things especially make you very fat) are foods you need to eat a lot of and that it is healthy for you. When behind the scenes, farmers had excess amount of the stuff and needed to make money off, so lets lie to you about the health aspect to make you buy it. Now we have an obesity problem, because people don't exercise enough. People are exercising more now than in the 50's but were much thinner back then, eating less wheat, corn and potatoes.
As a graphic designer, I enjoy seeing the persuasion techniques to make you believe and buy something you don't need. I remember in my forth year at university we had a lecturer who touched on this topic. He said we as designers should be taking a hypocrite oath, because we have as much effect on people as doctor do who operate, for what you hope both has the best intention for you and are going to make you better. (I was told, hope is not an option).
Here are some of the posters I found:
A poster from 1918 by Lloyd Harrison for the US Food Administration