There are some theories. Most fresh food in a generic U.S. supermarket has something like 10-25% of the nutrients per pound than it used to a hundred years ago, thanks to soil depletion, so each generation has to consume more pounds of food to get the same amount of nutrients. There’s been long-standing corruption in the FDA “food pyramid” and “recommended daily allowance” systems to bias the U.S. population from recognizing that added sugar leads to obesity. And there’s the advent of chemical non-sugar sweeteners, which in recent decades are turning out to be just as harmful as sugar, only differently. Those may not fully explain obesity, but they certainly are known and understood explanations for obesity — and yet they remain wholly unaddressed.

I think the problem is not whether we’ll fully understand the causes, but more that every cause we have identified to date would require regulating corporations in profit-damaging ways to solve, and it is likely that any future causes we reveal will be the same. That’s anathema in the U.S.: profits are sacrosanct to the two primary political parties, discounting their occasional extremists who argue (correctly) that we should be regulating in favor of consumers, not profits. Typically, the desire for a ‘full’ explanation is used to delay or derail efforts to implement solutions to each single proven explanation, and so I tend to caution against pursuing a complete answer first, and instead recommend asking why we have not yet addressed the known causes while continuing to search for more.