More on point--insulin resistance likely has more to do with personal fat threshold, this is an idea where once subcutaneous fat is sort of "maxed out" fat starts to gather ectopically, being stored in the liver and visceral area, contributing to insulin resistance. So by the classifications you gave, an endomorph would have a higher total body fat before this started happening than an ectomorph would-the ectomorph might actually get in trouble sooner, on a high sugar diet. There are lots of mouse studies where compromising subcutaneous fat storage leads to fatty liver and insulin resistance, a roadblock to developing a safe diet pill.