This would be sort of a que sera sera thing except for the pervasive role of government today coupled with the speed of information. The impact of a fad-belief on a single population will normally be along a distribution curve, probably directly relateable to how much it contradicts 'current' or 'conventional' wisdom:
- some will believe it wholeheartedly and take it as gospel
- some will guardedly believe it
- some will reject it ...with the end result being a distribution of results. If over time it seems to be beneficial, it becomes universalized.
Vaccines "might" be dangerous? An uninformed (but pretty) celebrity makes a public statement and the next day thousands follow it like unthinking sheep.
Eggs are (believed to be in the fad of the moment) bad? Some bureaucrat swipes a pen and eggs are expunged from every school meal program and officially 'frowned on' across the public, leading to a decrease in consumption of what may be a perfectly healthy food, replaced by high-sugar, high-fat breakfast 'snacks'.
PERSONALLY (I am not a dietician) being alive since 1967, my observation of the (somewhat sudden) increase in obesity across American society seems to dovetail with the whole 'cholesterol' thing - the early-80s crusade drove out what seems to be to have been a relatively unprocessed staple of civilized human consumption. But I'm aware too that the latter-20thC saw the unprecedented industrialization of food production so there could be a host of chemicals at fault that happened at the same time.