system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How is it some people can consume whatever they want and still be lean?

I used to be fat and then I started working out and eating health. I work hard to look the way I do, nutrition is a big part of that. I have friends who can eat whatever they want and they are still fit/lean. Obviously eating pizza and donuts is not health but I am not asking whether it is health or not, just the physical aspect. Most of these guys rarely workout and they have poor diets however, they still maintain a single digit body fat or pretty close to it. With fitness they look even better while still eating like sh*t.
gpt
While there must be some natural variation in the degree to which people process and metabolise fat-- in fact, genetic markers for predisposition to weight retention have recently been identified-- it always comes down to the amount of calories ingested relative to the body's needs and ability to metabolise fats, carbohydrates, etc. It's very easy to under- or over-estimate how much a person is truly eating; a small amount of junk food, while likely poor in nutrients and nutrient-quality, will not enable a person to gain weight as much as an excessive amount of healthy food. Since you're not talking nutritionally, from a purely physical standpoint it's literally all about caloric intake. Regardless of what foods are being consumed, if the calories are there, the body will build muscle and store fat. If the calories aren't available, then people stay slim.