Watch out, you can't easily assume that it's not caused by general bad nutrition and lifestyle.
Assuming that you're probably from the USA, where obesity is ramping up, from a (especially southern) European perspective, the eating culture of USA is very, very bad.
Please stop using the word epidemic in this way. Especially for a medical condition which is not contagious. I know what you mean but the wording makes it sound you can get obese by being around obese people. Thats tabloid vocabulary at best.
I know. That's why I mentioned clearly "especially for a medical condition" - because then you use a figurative word associated with a medical term, while you would expect in that context the other meaning of "epidemic". If you were talking about "epidemic unemployment" there would be no ambiguity, but "epidemic obesity" is a very improper use of the word, ambiguous in this context.
Anyway, most of the time if you use the figurative sense of the word, just replacing it with "growth/increase/spread" is good enough. Why use an overly dramatic word ?
But it's not incorrect or even unusual in a medical context. Epidemic has often been used for non contagious conditions including diabetes, ADHD and obesity. It's a dramatic word, but the obesity rate in the US is dramatic.
I have been reading a study arguing that people are more predisposed to obesity if they have fat friends, being a factor more important than genetic predisposition. I can't find the link right now.
Also, more and more people are getting fatter / obese every day. Clearly genetic predisposition can't explain this problem for such a large scale in such a small amount of time ... so this is in fact a social problem that grows by network effects.
So whether it is our lifestyle, or our diet, it's clearly spreading.
Therefore I don't think it's a stretch to call it an epidemic.
>Especially for a medical condition which is not contagious.
But the fact is, obesity is contagious. Habit and culture are huge factors in obesity, both of which are contagious to those who you share extended proximity with.