@ffswhatnext, there's been lots of studies done, a few of them televised, were they've had people, some athletes, eat different types of junk food. They mostly didn't put on weight, but their heart health etc suffered. I can remember a boxer having to stop because he became pre diabetic, even though he kept up his training.
It isn't just about weight.
I've lost nearly three stone and I'm on a few supportive weight loss forums. It is about not seeing this stuff as everyday food. Once you become health conscious, including limiting processed carbs/potatoes/rice, you only shop in a quarter of any supermarket.
I'd say that that's how it should be for anyone, for 3 weeks a month.
The food and farming industry has massively influenced our eating habits and I think we should address that in schools, as part of the critical thinking modules. It would fit in well with all the fake news about.