the thing is, it takes a decade of delay for this to set in because fast-food and other harmful food like substances are much more harmful in child hood. So, if you were raised descently well on normal food until your 20s, then started eating horribly, it's not nearly as bad as if you started eating horribly in your childhood.

We can see this clearly in China. The older generation seems to have some resistance to junk food in terms of weight gain: this is why asians have a reputation for remaining slim, even if they eat junk food. But, it's not due to their genes: they were simply lucky enough to not be introduced to those bad foods until their adulthood.

If you look at the newest generation in China in the cities, their obesity rate is skyrocketing at an extremely high rate.