We tend to eat the same volume of food every day. If you try to lose weight by reducing the volume of food that you eat, you are setting yourself up for failure. You need to eat the same amount of food that you’re used to but choose lower calorie foods.
The calorie density of an apple is 0.40, but the caloric density of a slice of apple pie is 2.60. You would have to eat six apples to get the same number of calories that are in one slice of pie. You would be very full after eating half a dozen apples, but might still be hungry after eating a slice of pie.
Another way to think of this is how much water is in your food. The more water there is, the fewer calories will be in a serving of that food. Drinking a glass of water will not fill you up the way that having a bowl of vegetable soup or eating a fruit salad will. If the water is bound to fiber in food, it will stay in the stomach longer while the food is being digested. Fruits and vegetables are about 90% water.
Celery, cucumbers, asparagus, strawberries, watermelon, oranges, blueberries, and green peas are very low or low-calorie density. You would expect this. However, whole-wheat spaghetti is also a low-calorie density food. Does that surprise you?
Likewise, granola bars, almonds, and hard pretzels are higher density (they have no water in them and will not leave you feeling full).
It’s not just huge portions that are leading to our obesity epidemic. Specifically, it’s significant portions of very calorically dense foods. You can eat 955 spears of asparagus or six pieces of chicken pot pie for the same number of calories.