> and there are people who don't get fat no matter what they eat.

This is a myth.

When you actually track what these people eat, their daily caloric intake averages out to numbers you'd expect. At best, the difference between "fast metabolism" and "slow metabolism" people in studies comes out to a couple hundred calories per day.

You may witness someone consume 2 pizzas and a soda in one sitting without getting fat, but that person is consuming fewer calories for the rest of the day or the week to offset it.

I feel like the word metabolism is often misunderstood. My metabolism is just the number of calories I need to maintain my weight. If I start being more active my metabolism increases, if I become less active it decreases.

So, metabolism is really just an expression of how active we are. Some athletes eat 5-10,000 calories a day just to maintain their weight. Hard working laborers also need a lot more calories than office workers.

Meanwhile, people blame their metabolism as if it's just a trait they can't control like their height. You absolutely can increase your metabolism, just move more. This is also likely a big reason why many people gain weight in adulthood - in school we have weekly gym classes and we spend lots of time running around ad playing with friends etc. Then we grow up and our activity level plummets, but we don't adjust our diet to accommodate our much lower metabolism.

I think what they mean is "they eat whatever they want and however much they want" and still don't get fat.

Some people just only want to eat an amount that doesn't get them fat. They aren't using any willpower or anything, they just don't want to eat more than a healthy amount for them.

I am not one of those people (I am overweight when I eat as much as I want), but I am for other things. For example, I don't drink very often (only maybe 10 times a year) and don't get very drunk when I do. I just don't feel a desire to drink very often. It takes zero self discipline to not drink for an extended period of time because I don't have the desire for it.

On the other hand, many alcoholics struggle mightily to avoid drinking. It takes all their willpower to not drink.

Everyone is different in their ability to avoid different addictions.