I am glad you wrote this. In our conversations about BMI, we forget how multi complexed this topic is, and it isn't just about nutrition or exercise. Eating healthy takes money, which explains a correlation between income levels and rates of obesity.

My understanding is that there is good evidence that gut bacteria affects weight in humans. A woman, who was healthy BMI all her life had a colon cancer, and was irradiated and given a fecal transplant. The donor was family, and someone who had been obese all her life. Recipient of the donor began to gain weight. Changing diet and increasing exercise did nothing to prevent the weight gain.

https://www.bbc.com/news/health-31168511

maybe we should stop shaming people, cause clearly that is not an incentive for people to work on BMI.