Some kind of plastic might be a risk factor, even if not everyone who is exposed to it becomes obese. Cigarettes were once practically universally present in American life, but that did not make lung cancer universal.

But blaming "plastic" is ridiculously general; it's not very far from blaming "matter", "energy", or "technology". To take one example, there's a fairly plausible causal link between halogenated flame retardants in polyurethane foam for furniture and feline hyperthyroidism. Polyurethane is a plastic (WP: "Polyurethane is a commodity plastic."), or, better, a large family of plastics, but even if the link turns out to be real and strong, it would not be very accurate to say "Plastic causes feline hyperthyroidism."

As for "incredibly strange", since most people have no idea what is causing the obesity pandemic despite many researchers looking, anyone who does figure it out will unavoidably have an incredibly strange idea. There's nothing wrong with an idea being incredibly strange; in this case it's a necessary precondition to being correct!

Still, as I said, I suspect it's not a plastic, but one or more food additives. There have been promising preliminary results from fecal transplants for obesity treatment that suggest that the intestinal microbiome is a significant factor (see de Groot, Frissen, de Clerq, and Nieuwdorp 2017). So my attention is particularly drawn to things that affect the intestinal microbiome. Unfortunately, everything you eat affects it.

Some of my own incredibly strange ideas, which probably are not correct, are:

· Titanium dioxide nanoparticles: these are commonly used as a white food coloring (E171), and there's some evidence they migrate to the pancreas and stay there, where conceivably they cause damage in ways that larger titanium dioxide particles do not.

· Propionate salts: these are commonly used as fungicides, for example, in bread and on peaches. Many bacteria produce them as a fungicide, including human gut commensals, but it wouldn't be surprising if adding them in effective quantities changed the ecological equilibrium in the intestinal microbiome. These are GRAS (E280, E281, E282, E283) and an experiment apparently showed behavioral difference in children fed calcium propionate (Dengate and Rubin 2002). (Many propionate esters are also used as artificial flavorings, but in much smaller quantities, and I don't think those flavorings are effective fungicides.)

· Xanthan gum (E415), a slime produced by the broccoli black rot bacterium, commonly used as an emulsifier or thickener in salad dressings, egg substitutes, and oil-well drilling mud. Representing a significant source of energy that most gut commensal bacteria cannot digest, it has been shown to produce changes in the human intestinal microbiome.

· Widely used artificial sweeteners of various kinds (E950, E951, E954, E955); there have been studies suggesting that, although these do not provide a significant number of calories, they may still provoke significant changes in the human intestinal microbiome. This might also be true of things like steviol (E960) or cyclamate (E952) but those clearly aren't in wide enough use to cause the obesity pandemic by themselves.

· High-fructose corn syrup has been suggested by many people, but in my view it is an unlikely candidate, because sucrase turns sucrose into high-fructose corn syrup in the human small intestine.

· Livestock is commonly dosed with significant amounts of hormones and antibiotics in order to promote rapid weight gain, because meat is sold by weight. Some of these hormones and antibiotics are present in the resulting meat. It wouldn't be very surprising if, under some circumstances, they also produced weight gain in the humans who eat it. (But, as I said above, people on fad carnivore diets tend to lose weight.)

But most likely it'll be something nobody has even suspected previously.