I hate to sound like a grumpypants old person, but I think a lot of this is excessive screen time (during which it's waaaayyyyy too easy to stuff chips and such in one's face) and inadequate activity. Plus soda, which is bad news whether it's sugary or diet (I guess I'm lucky I can't stand the diet stuff; gave up on high fructose corn syrup more than 10 years ago). I was quite thin as a youngster and ate a lot of crap; didn't catch up to me till I started yo yo dieting (when I should have just gotten some exercise, sigh....I was 130 pounds when I started all that). Now, if I tried to eat such garbage, I would feel horrible and it wouldn't be worth it to me (like a hangover). But for those who eat that way every day, I guess it's like "fish in water," they just have nothing to compare it to.
I worked for years as a writer for a health insurance plan. They kept coming up with all these "healthy" messages, acting like no one knew any of this. News flash--WE KNOW! And guess what, we are tired of being nagged and bossed around. Young people notoriously don't like being told what to do, especially by authorities, including parents and teachers.
I mean, at least in the U.S., if someone doesn't know that junk food is bad for them and can make them fat, and vegetables are good, they have been living under a rock. Some other issues are more confusing (coffee? chocolate? meat/protein/soy? you get my point) and not so clear cut. And that's another reason why there are so many fad diets; people want a quick fix and not to have to "be good" all the time. The irony about 5:2/fasting not yet having official health results is that MOST of these diets not only don't have such experiential evidence, they are obviously unhealthy! I for one, am thrilled with 5:2; I can literally have my cake and eat it too...5 days a week! Yep, weight loss can be slow, but since I'm just gonna eat this way for the rest of my life, heck, it's fine. Plus, in theory I'll have less extra skin once I get down to fighting weight. And I can keep an eye on my face (face or ass, remember) to make sure I'm not overdoing it (once I hit a healthy weight, I'll reassess my goals).