Discussion

I'm about 30 and still skinny. I eat at least 2,000 kcal a day, up to 3,000 kcal. I only get more weight by working out. Recently I've been injured and couldn't work out for several months. I dropped back to about 70 kg (I'm almost 1,90 m) - while eating the same amount of calories.

Maybe I need to space out my calories, I only eat two meals and don't snack at all. These just happen to be really big meals.

For the record, I've been checked for thyroid issues and other stuff. My body just really wants to stay at 70 kg, I guess.

Yep. I generally eat when I'm hungry and have always been underweight. I'm 5'11" and was 130 around age 20. I'm 150 now so still lanky but not skeletal. I think the fact that I'm accustomed to smaller meals helps. I find it genuinely difficult to eat more than 700 calories in one sitting unless it's something calorie dense like fast food.

Very broadly, yes.

Different people have different natural metabolisms, different builds, propensity toward muscularity, bodies that store fat in different areas and at different rates.

Of course, diet and physical activity play into this heavily as well, as does age.

A lot of people will be naturally skinny and more active when they are young, then you add a decade of sedentary office work... and they've never needed to learn how to eat well and exercise enough... and they get larger.

Other people can start off larger, learn a whole lot about healthy diet and exercise, and go in the opposite direction and become much more fit in that same decade.