[gradi3nt]

There is at least one fundamental truth: obesity is on the rise. Why?

I think it is safe to assume that a rapid change in the human genome is not responsible for skyrocketing obesity rates around the world. From what I have read it seems that dietary shifts towards more processed and simple-carbohydrate rich food are the main culprit. Increasingly sedentary lifestyles are accomplices.

These effects are on a statistical and societal level. Also, they don't imply that and individual can just change their diet and easily loose weight. It is possible once someone gains weight their body irreversibly changes so that losing weight becomes much more difficult (I mean irreversible to imply hysteresis, not impossibility).

[nevinera]

That direction of thought has produced quite a few useful insights, and continues to drive significant research. I'd never call it a 'fundamental truth' though - it's a question, and a type of question which allows for many interacting answers, none of which may be fundamental. In particular, it can be extremely difficult to trace the links between societal (non-physical) changes and the mechanisms by which they affect the physical.

We are eating more sugars - is that because more sugars are supplied, because we can afford more sugars, because sugars are snuck into all our foods, or because we like sugars more than we used to?

We are moving around less - is that because we have more sedentary jobs, because we have less physical labor to do at home, because the interesting hobbies that don't involve moving around are expanding, or because some quirk of the childrearing techniques in the 70s had significant impact on our interest in athletics?

You see how complicated these things get when you start talking about 'fundamental' causes and reasons.