When I read that; I thought perhaps you had made a typo. Turd transplants? So I looked it up.
Medical science can be absolutely amazing. And sometimes bizarre.
The gross history of the science of this stuff is, well, weird sh*t.
Germans developed a probiotic after learning from Arabs that eating fresh, warm camel dung would often abort an incipient case of dysentery. It had to be fresh. Eating the cold dry stuff did not work. The reason it worked was that the bacteria thereby gained would out-compete the dysentery bacteria.
Anyhow, the science for some of this is still in a crude state (to put it in a multi-meaning way).
It looks like I am likely right. The massive amounts of antibiotics that Americans take over their lives and the antibiotics given to farm animals have likely been a factor.
Please do a Google search on the words:
Ted talk bacteria in the gut obesity”
One more thing:
Our community college was once the area's high school. They currently have an exhibit of photos of the former high school showing scenes from the early 1930s through the 1950s. There is not even one slightly overweight student shown in any of the photos. I carefully searched.
My guess it that the students in these photos were not exposed to as many antibiotics.