Dear 100 Hour Board,
So - I'm taking a nutrition class and it kind of talks about how if you eat healthy and live healthy you're skinny, and if you don't you're fat. But...that's not matching everything I see.
Like - I'm living with 2 roommates right now. One of them frankly lives on pizza, never exercises, but is super cute and all that. One of them has literally the healthiest lifestyle I've ever seen....all the green vegetables and healthy things and lots of exercise of lots of kinds. But she's kind of normal-larger. (Super great and awesome too - but not what I feel like the textbook says it should be.)
I'm aware that both of them have lived the same way for years, and in pictures of both of them, over the past few years, they haven't changed much.
-What gives? Eat pizza and be healthy?
A lot of it really comes down to genetics and what your body is predisposed to do. So maybe rather than thinking that everyone will end up a certain way if they eat healthy, think that they'll end up as their personal best self, based on their body's genetics.
Also, home girl who always eats pizza might not be eating enough other things to gain weight. To gain weight you need to have a caloric surplus, but the exact details of what counts as a caloric surplus for each individual depends on their body weight, muscle composition, and activity level. Furthermore, there are a lot of outside factors that affect weight gain/loss. We've (somewhat) recently talked about some of them here and here.
Skinny is also a pretty dang subjective term. Is it about what percentage body fat you have? Steve Cook is a body builder with very low body fat, but I wouldn't ever call him skinny. Is it about having a low BMI? My BMI is at the low end of normal, but I would say my looks are pretty standard, not really "skinny." Is it about fitting into size 2 jeans, and "looking skinny"? So often we have this mystical requirement of what a skinny person looks like, but it's so subjective it can be hard to actually achieve, because looks depend on genetics, developmental factors like Sheebs explains below, muscle mass, and bone structure.
Lastly, health and weight are two very different things. Oftentimes health goes along with a good body weight, but that's not to say that all thin people are automatically healthy, or all people with some extra chub are automatically unhealthy. It's better to look at overall trends and trajectories for people rather than their current body composition, and remember that although weight is one indicator of health, there are many many others, as well.
And beyond genetics, developmental factors are important in determining adult body weight. For example, adolescents who are obese because of poor personal or family habits will probably be larger than average even if they lose some weight in adulthood.
Also, just because someone is skinny doesn't mean they are healthy. They could have high blood pressure, high cholesterol, and be physically weak. For whatever reason our society conflates health with weight, probably to legitimize looking down on fat people.