would need to be dietary changes that don't also change BMI, since that was controlled for.

Which I think is a bit of a gaping hole.

As a male who has been around 70kg for all of my adult life, I have consumed many different "diets". All from eating whole foods to eating more processed foods and easy packaged meals/junk food, all not impacting my BMI.

I wonder if soil depletion of minerals and vitamins due to over farming couldn't also be a suspect here. It's a problem I haven't heard much about lately.