The incidence rate of genetic issues generally doesn't change much over time due to how slow genes changes, so an increase in obesity rates from 5% to over 40% in just a few generations suggest the cause is very unlikely to be due to genetics.
As a kid in the 80s, I wondered how it made sense that my diabetic father was told to follow the same model as non-diabetics, which emphasized carbohydrates.
[1] - https://www.theguardian.com/science/2022/dec/09/academy-nutr...