system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do I feel like I'm losing weight by eating 2000 calories of grilled meat and vegetables, but not when I consume the same amount in ice cream, fried foods, and fast food?
gpt
Biologist here!

Our body gets calories from three main sources; fats, carbohydrates and proteins. While one calorie from a healthy food will provide the same amount of energy as one calorie from an unhealthy food, they will have different effects on the body. 

* Fats

There are three main types of fats; saturated, unsaturated and trans fats. Saturated and trans fat can increase your cholesterol levels which can lead to heart disease and strokes (found in a lot of your junk food). Unsaturated fats lower your cholesterol levels which can prevent heart disease and strokes (vegetable oils, nuts, avocados, olives, seafood).

* Carbohydrates

Carbs are made up of sugar molecules. The more sugar molecules are linked together, the more complex the carb becomes. The two main types of carbs are simple and complex. Simple carbs are unhealthy. They are digested and absorbed quickly leaving you hungry soon after eating them (cookies, donuts, candy). Complex carbs are healthy. They are digested and absorbed slowly which keeps you full and prevents cravings (legumes, vegetables). 

Certain diets, like Keto and other low carb diets, are healthy and most of the carbs are coming from vegetables. You don't actually need a lot to keep your energy up.

* Proteins

Proteins are made up of amino acids. While our body can create some of them, it can't create all of the necessary amino acids. There are two types of protein; complete and incomplete. If a protein contains all of the essential amino acids, it is complete. If it lacks one or more of the essential amino acids, it is called an incomplete protein. While you can get complete proteins from fast foods, they come with the lousy carbs and fats.

See it's not the calories that matter as much, it's what's in those calories. By eating junk you are also missing out on essential nutrients, vitamins and minerals that are found in non-processed foods (vegetables, meats, fish, nut, etc.).