I read "The Big Fat Surprise" and it was eye opening about how much of the American Heart Association directives came from almost religious like fervor of one man's opinion, not backed up with hard data. One doctor, Ansel Keyes, single handedly made dietary fat public enemy number one starting in the 50s and up through the 70s. He used rigged studies to do it, rigged by cherry picking the populations he used to emphaisize those that fit his theory and leave out those that contradicted it (for example he explicitly cut the data from France and Switzerland, both of whom have very low heart disease rates but eat tons of saturate fats). In short, he lied and then used his influence to crush and bury the findings of anyone who challenged him.
And America followed his direction, issued via the AHA, AMA, and the USDA. The result. Americans got fatter, much much fatter in response. But heart disease didn't drop at all.
There's money to be made, selling diets that don't work.
Don't forget
WHERE they got these false premises :
" Now the Spirit speaketh expressly, that in the latter times some shall depart from the faith, giving heed to seducing spirits, speaking lies in hypocrisy; having their conscience seared with a hot iron;
forbidding to marry, and commanding to abstain from meats,which God hath created to be received with thanksgiving