I guess I would rephrase the question this way: who causes obesity--and why? Who would most benefit from a population that is fat, sickly, out of condition and/or with heart disease, diabetes, and utterly dependent on drugs for its survival?
Could there be some people who stand to benefit from such a state of affairs? We could follow the money, and make a list--I bet it wouldn't take long to see some significant associations. As in any observational study, association may not equal causation. But it sure might imply it--and suggest the need for further research.
EDIT: The comment from "thhq" caused me to reflect further, and I decided instead of being coy I'd edit my response to explicitly reflect what I was implying.
It's way too long--DO NOT READ!!!!
Exactly What Causes Obesity: A half-baked, sophomoric manifesto I probably should have written when...well, when I was a sophomore. In high school.
The shift of over-eating and overweight from deviant to normal is a sociological problem. A nation full of skinny people did not one day decide, "It's now okay to eat too much and get fat, after all we won the war!"
The normalizing of over-eating and overweight occurred as more people became over-eaters and overweight--just the same way every other human behavior is normalized. The problems leading to our obesity situation were well underway by the time it stopped being okay to make fun of fat people.
And still these things aren't fully normalized. No one wants to be on camera, shoveling in two Big Macs, large fries, and a super-sized coke. Like our recreational drug use, we do a lot of our gluttonous food consumption behind closed doors. No one ever sees how much we eat--or what we eat. They just see more and more of us getting bigger and bigger, and that makes it more normal.
No one thing causes obesity. And no one thing turns individual obesity into a system-wide situation affecting half the population. The obesity problem is the natural result of a vicious cycle that, while perhaps not implemented purposely in the beginning, is now fueled--perhaps purposefully--by money.
The USDA ignores science (going all the way back to the 60's) and continues to recommend an unhealthful diet composed of foods directly refined from the products of federal farm subsidies (our taxes). And we all know what kind of "food" is made from these products. One can hardly use these products as recommended and not become obese. These foods are like automobiles and guns: they are among the few products that, when used for their intended purposes, kill people (in predictable numbers, year by year).
The government adopts new actuarial tables that suddenly categorize many more Americans as "obese," people who were once just "overweight." With their new diagnosis, these people are eligible for new, and more, medical interventions, especially drugs. Statins are prescribed for people who have no signs of CVD. Statins are prescribed for women over the age of 60--a group for which there is no evidence they help, and good evidence they harm. All of this gets the medical/pharmaceutical industry a sweet piece of the action, and you better believe they use it to lobby congress for more. I don't know if the pharmaceutical industry lobbied congress on those actuarial tables or not, but would it surprise you if it did?
Just an annoyingly lefty, half-baked theory here from me: I speculate that a population that is fat and sick with diabetes, heart disease, and cancer will tend to discharge its civic duties at diminishing rates. As we become obsessed with our illnesses (while continuing our consumption), bed ridden and bedraggled by an interminable medley of maladies, we will withdraw from public participation and become more and more attached to our technology, which will mollify our suffering. We, the electorate, whipped for so long by artificially low food and energy costs, will allow politicians the luxury of ignoring us, so long as things stay the same. Corporations (and unions, if any are left, but who am I kidding?) will be in charge. Making people stupid and sick, and persuading them to participate in their own subjugation, is a good first step for implementing control. This technique (hegemony) has been pondered endlessly in fiction, and demonstrated in real life countless times in the world's history of colonialism--long before technology, mega-corporations, and huge world markets were part of the picture.
Now add to all that crap all the many, myriad factors I can't even remember to include in my argument, and then start thinking about feedback loops and vicious cycles. Probability and inevitability.
It seems to me obesity (and all its associated problems) is about the only possible outcome to this system.
To stop obesity, this system must be interrupted somewhere. I am very cynical, so I predict the interruption will come only in the form of the dynamic duo of 1) skyrocketing energy costs, and 2) global climate change. I'm not sure which will happen fastest, or if they will dance slowly together for awhile, acting as force multipliers upon each other, but they are on the way sooner or later. The later they arrive, the fatter and sicker we'll all be. So that's the downside to putting it off. But a faster arrival means horror for all of us. So basically we're screwed either way.
However, we're not necessarily screwed forever. Barring catastrophic depopulation, we'll adapt, perhaps:
Once it's cheaper to grow food locally than to pump it full of fertilizer, pesticide, and ship it across the oceans, that's what we'll start doing. We'll be ripping out Big Box Store parking lots and turning them into fields of sweet potatoes.
We'll sell all our food at local farmer's markets that will dot the landscape the way liquor stores and pharmacies do today, because it's cheaper and easier to sell your food to the people who live within a few blocks of the farm.
When we can't afford gas for our cars (or coal for our electric cars), we'll live closer to where we work (perhaps on that farm), and bike and walk more.
When we can't afford heating fuel, we'll shiver more, and start living in smaller, denser communities with more people.
And we'll finally get skinny again because there just won't be enough of anything to get fat.
I can't decide whether this is dystopia or eutopia. Probably dystopia, on account of the roving bands of cannibals and people stealing your shoes in the winter and guns and zombies. But who knows.
So, that's what I think is the cause of obesity.