In the largest genome-wide association study of its kind, catchily called STILTS (Study Into Lean and Thin Subjects), Sadaf Farooqi’s team in Cambridge have reported that slim people have a favourable genetic profile compared to those at the obese end of the spectrum.
Genetic architecture of human thinness compared to severe obesity
https://doi.org/10.1371/journal.pgen.1007603
Endodiabesity 