The most likely culprit is processed food, where nutrients are not used in their natural form, but instead converted into something else. So using corn to make high fructose corn syrup, soy and cottonseed to make oil, or feeding animal protein to herbivores are a few examples of where the food supply is getting contaminated. For example, cottonseed oil is not only a relatively unhealthy oil, but it also contains one of the highest concentration of pesticides (because cotton was not traditionally a food crop).
Not to mention that plastics contain hormone-like chemicals like BPA, they use preservatives in things like canned food that don’t need them, and add animal byproducts like pork lard to foods like refried beans that don’t need them. So our immune systems are getting bombarded by substances we either have no evolutionary history with or simply don’t need to be eating in the quantities that we are. So that inhibits aspects of our immune systems and metabolism (triggering autoimmune disorders like allergies, asthma, arthritis and metabolic syndrome).
I could literally go on forever about this, so the gist of it is “it’s the food supply, stupid”. Yes, being more active helps a great deal, but without proper nutrition we’re looking at a modern form of the wasting diseases similar to the ones they used to get in the 1800s before they understood vitamins and minerals, except it’s masked by obesity. I think if we didn’t have the social stigmas around laziness and the political monkeying of the farm and retail lobbies, we would have recognized obesity as an epidemic the way we did with polio and done something about it decades ago. Nothing short of going back to the foods we ate before about 1980 is going to work for the vast majority of the population. And that’s going to start with campaigns to inform people about what’s in their food and reform the way we design cities so that food centers are near where people live.
TL;DR: They found medical evidence that the western diet disrupts metabolism via pathways other than caloric intake. Now let’s do something about it.