Maybe it’s confirmation bias, but it seems to me that the vast, vast majority of people with Down syndrome are overweight. I can’t think of ever seeing a thin person with that affliction.
Is there some reason for this, inherent in the condition?
People with Down Syndrome have poor muscle tone as part of the syndrome. This makes them look pudgy. also the poor muscle tone makes for a low resting metabolic rate. In addition, those folks often have cardiac problems, limiting their ability to be very active.
I had a sister with Downs, in those days it was called Mongoloid, and as I understood from my parents she did have a low metabolic rate. She was sweet and innocent all of her 57 years.