Calorie Counting Fails
Despite the insistence of the DGAC that calories in must equal calories out, if your weight is to remain constant, we all know that's not true.
First, a calorie is measured in a physics lab, and it's a measure of the heat a substance creates when burnt.
Calories in the Body are Metabolized
The food you eat is broken down into components before it's absorbed into the blood stream.
Even then the pancreas and the liver, and other organs respond to the presence of that food by producing hormones or enzymes that use the food available or store it in some way.
Your Metabolism Rate Changes
When people are starving, their metabolism rate slows down, and they use calories very efficiently.
If calories are abundant, the metabolism rate rises, and the body tries to burn off as many as possible.
That's just the process of homeostasis working normally.
The Body Can Eliminate Some Calories
I've never seen this explained, but in extreme conditions, some of the calories eaten are not processed as food.
For instance, Sam Feltham did this three-week experiment on himself.
On a Ketogenic diet, Sam ate an average of 5794 calories a day for three weeks. He should have gained about 7.5kg. His actual weight gain was 1.7kg.
Maybe because the extra calories were all fats, the body has a way excrete them in the stool.
Different Macro-nutrients Are Metabolized in Different Ways
The quickest way to deliver calories to the body, is to drink them in the form of glucose or sucrose drinks.
You can also deliver large numbers of calories by eating cake, bread, or potato chips. These carbohydrates convert to glucose very quickly, and within two hours you are craving for more. Carbohydrates make you feel hungry.
The slowest way to deliver calories to the body is to eat protein with very little fat. Explorers forced to kill their horses for food, found the horse meat (horses also starved of course), had no fat on it, so they couldn't get any energy although they had food.
Eating fat delivers a lot of calories to the body, and they make you feel full quickly, and they process slowly. Fats make you feel full. There may be no hunger for four to six hours.
Over Eating Experiments
Morgan Spurlock did an experiment eating 5000 calories a day for 30 days at McDonald's. This was a high-carbohydrate and high-fat diet.
Spurlock gained 11.1kg. But that's still somewhat less than he might have predicted.