In support of your comment, when talking about weight it's common to talk about "fast metabolisms" keeping people skinny. Studies have shown that metabolisms do

In support of your comment, when talking about weight it's common to talk about "fast metabolisms" keeping people skinny. Studies have shown that metabolisms don't vary much at all. What does vary is how hunger signals work with different people. The people who complain about being too skinny and can't gain weight simply find it difficult to eat enough to gain weight. Their body tells them they're not hungry. If you force feed them they'll still gain weight. I'm one of those people who gain weight easily due to always wanting to eat. I was morbidly obese at 19 despite coming from a household of healthy eating. I'm now 33 and have maintained a healthy weight for years but it isn't fun. For me to maintain a constant, healthy weight I need to be hungry. Despite 5+ years of maintenance, I am still hungry constantly.