About 30 years ago, butter was bad for you and margarine became a healthy alternative. Left and right, healthy consumers tossed out the buttery saturated fat and replaced it with tubs of partially hydrogenated oils. The flavor was not as palatable, but the health benefits were worth it.
Then, all of a sudden (or so it seemed), the thinking changed. The medical community woke up and realized the health implications of eating trans fats, which largely come from these man-made partially hydrogenated oils.
Now the thinking goes that butter is a "healthy" fat and margarine is the worst thing ever.
This narrative is all too familiar to me. Daily I have discussions with patients about why it seems that medical or nutrition professionals "suddenly" changed their minds about the health benefits or drawbacks of food/supplements/medication/lab targets/diet trends. These patients are always so frustrated by this seeming lack of consistency.
Nutrition science is nuanced. It's not black and white, but rather all shades of grey. In order to understand why there are so many differences of opinion about nutrition, I want to review a few of the most common misunderstandings about scientific research in general.