Currently in the US, 8% of grown corn is the stuff we eat, and 11% is devoted to sugar production (and some other stuff) - with Ethanol production and livestock feed accounting for 60% of the total usage[1]. If there were droughts we'd probably see Ethanol become price inefficient first, then livestock, and it's that 11% of corn that gets transformed into sweeteners that is really driving obesity in the modern world.

The other issue that seems to be related to obesity is the weird chemicals we're imbibing - both knowingly through food consumption[2] and unknowingly by their entrance into the food chain[3]. The USA really needs to get it's act together about finding out wtf is poisoning all of the people there and fix that. I'd love it if they could get their stuff together about fighting obesity - there is something fundamentally wrong with the western diet that is more prevalent in cheap foods and making some greedy people a quick buck while setting society up for a really heavy cost in the near future. A recent decrease in activity certainly contributes, but it's more than just a lack of exercise.

[1] Corn usage... leaning on wikipedia https://en.wikipedia.org/wiki/Corn_production_in_the_United_...

[2] https://www.medicalnewstoday.com/articles/325034.php

[3] https://motherboard.vice.com/en_us/article/a3mzve/human-wast...