Weeble, the problem is, that's how the
[whatever term applies here best] as a whole reacts to foods.
I could list a ton of examples of how things have been good for you, then bad for you, then good for you, then......
Remember when we were taught to never, ever, ever eat butter! It's horrible for you! You should only eat margarine, because it's much healthier! Then, in the not so distant past, that thinking turned completely around. We are to not use margarine, but use only butter! (Actually, the thought is more of using other oils and fats, but butter is preferred over margarine, at least.)
So I'm also sympathetic to DH's relative! He's probably just as confused about what's healthy or not as the rest of us by this point!
Editing, because it appears that the "scientific community" words are being focused on more than the point I'm trying to make! It doesn't matter what term you use! My point is that one week something's good for you, the next it's bad for you, etc.