Some populations have famously experienced strong detrimental effects on their health when switching away from more traditional diets. North Karelian heart attacks post-WWII are a notorious example.
Within my field of research, we know there's an enormous budding crisis in Southern US. A very significant fraction of all Alabama population is pre-diabetic. This is a massive public health problem. Corn syrup-driven diets are terrible.
Obesity as a result of poor diet is a point of pride for many people in the US.
I think it's in part a sociocultural issue. If your rent is low, you tend to avoid buying good food which is usually way more expensive and less satiating than cheap and simple carbohydrates.
Abusing soft drinks is also really problematic. These completely wreck your metabolism with their high concentrations of soluble syrups.
[1] www.cdc.gov/diabetes/pdfs/data/diabetesbelt.pdf
It was something said to everyone in high school.
Fortunately, almost everybody ignores the food pyramid. They just interpret it as "oh, I should try to eat from all the food groups". Fine.
The problem with the food pyramid is that it is based on obsolete science at best, and was probably designed by food industry lobbies.