I remember my mom taking me to the dr. when I was 17 because I was upset about not being able to gain weight...I only weighted 98lbs...But my dad never weighed over 130 his whole life so dr. just said I had high metabolism...Told me to drink a shake before I went to bed each night..
Now of course I still never weighed over 110 until I hit my mid 30's then I started gaining weight due to a medication I was taking..I am no longer on that medication but have fought my weight ever since then...My highest was 180...I now try to stay under 130..
My girls and nephews are all thin ...But one of my siblings really struggles with her weight while the other one stays small...