[cjlars]

Standard advice in the nutrition community is to that most people should be substituting processed foods and meats in the diet with whole fruits, whole vegetables and whole grains (a "plant based" diet):

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4991921/

Healthy diets strongly correlate with both sufficient fiber and healthy BMIs. The point is that 95% of people don't have healthy diets. As long as diets remain so poor -- the phrase "standard American diet" purposely forms the acronym SAD -- there is no obesity mystery.

[SomewhatLikely]

If healthy diets have sufficient fiber and insufficient fiber is what's causing the obesity problem, then why do the overwhelming majority of people gain the weight back within a year on almost every diet?

[amanaplanacanal]

There was very little obesity in the 70s. There is a lot now. If you can show that diets are worse now than they were in the 70s, you might have something. But are they worse?

[cjlars]

"Eating five or more fruits and vegetables a day has decreased from 42 percent to 26 percent" ... "between the period 1988-1994 and then 2001-2006."

https://www.livescience.com/5435-american-diets-worse.html