Actually, 6 foot and 90 pounds cannot be that healthy since that probably means that your frame and ligaments are very very light (candidate for osteoporosis?, no fat and little muscles!!
Even with a high metabolism, I'm sure my 150 pounds, with lots of muscle,
burns much more calorie than someone at 90-110 pounds. I can stay at a stable weight while eating 2800-3000 calories a day.
That's a LOT of calories. Even after 18 months HRT can still
bike at 35 mph for a mile, don't think someone with 90-110 pounds could do this.
My frame is small, but its covered with mucho muscles and
some fat, that's why
I fit in size 6 tops and size 4 bottoms even with all that muscle.
Some people need to starve themselves to fit into that size, if
I starved myself to 125 and lost muscles I'd fit in size 0.
But, at 6 foot tall, don't think I'd look too good!!! Muscles give curves too,
not just fat.
Still, its possible for me to gain weight in the right place even with that
high metabolism. Estrogen means that you'll strip the fat from the stomach
if you run any caloric deficit and if you got any and any supplement will
put in on the tigh, breast, face and buttocks.