“Calorie dense” is the opposite of “nutrient dense.” Simply put, calorie dense foods contain a lot of calories relative to their weight and size.
Foods that are calorie dense are often high in fat. Coincidence? Not really. When we consider the three macronutrients (protein, carbohydrates, and fat), there are 4 calories in every gram of protein and carbohydrate you consume, and 9 calories in every gram of fat you eat. Every additional gram of fat in a snack or meal represents a spike in calories that is more than 2x greater than the same thing with protein or carbs…It’s why so many foods and brands market themselves as “low fat” or “fat free.” Get rid of the fat, and you’ll reduce the overall calories much faster than if you tried to remove one of the others.
Calorie density and the differences in the macronutrients also contributes largely to hunger and feelings of fullness. If I eat 100 calories worth of lean chicken (protein source), I might feel pretty satisfied and full. Or if I eat 100 calories worth of veggies (carb source), I might not need another snack for a while, as the pure volume of that would take up most of my stomach. However, 100 calories worth of butter, or sauce (fat source) is barely enough to satisfy my hunger at all, and thus I can easily over-do it, calorically speaking. And that is your introduction to the density of calories.