I think a lot of it has to do with what we eat here in the U.S. Engineered flour and sugar in everything. I am not an MD, but have my own observations.
My grown children are addicted to processed food, fast food, things that are not good for them. They are just like drug addicts. They can't help themselves and don't want to change, no matter the consequence.
Every year, I spend about 1 month in Africa doing some work for national parks and game preserves. I typically stay at a nice lodge or someone's farm for the trip. I am thin, in shape and eat like a horse. Typically lose 8 to 10 pounds while there. The flour is ground in the back yard for the breads, puddings and cookies. The sugar cane is harvested nearby. The milk and butter are from the cows on the farm. The preserves are made in camp, and all of the meat was killed that day or the day before. Ate eggs, toast, potatoes for breakfast, some sort of lasagna like meal with bread for lunch, and some grilled meat, potatoes, bread and cake/pie/pudding for dessert for dinner. No processed foods were consumed during the trip. Some of it can be accounted for being very active and outdoors while there, but unprocessed food has to play a large part.
My next door neighbor's son wanted to work in Africa as a summer job. I got him a job for 3 months in the summer at one of the preserves. Typical US kid, freshman in college, over 6' and 40 pounds over weight. When he got home, he looked like a different person. Had lost all of the excess weight. I asked the preserve owner if they did not feed him and their response was they had to increase their harvesting/food planning to account for how much he ate. He did not work that hard. Food had to be important in the weight loss.