Q. In books on nutrition, I'm told that to lose one pound of fat it's necessary either to reduce my food intake by 3500 calories or to exercise so that 3500 calories are burned. How is the value of 3500 determined? If fat contains 9 calories per gram, and there are 454 grams in a pound, then there should be 4086 calories in a pound of fat-- not 3500.
A. The nutrition books are correct-- 3500 calories per pound of fat is not an absolute amount, but it's very close. However, your math is correct, too. Here is the story.
When we burn fat, or other nutrients, heat is produced, which is measured in calories. As you note, each gram of fat generates 9 calories, and 454 grams equals one pound. But a pound of fat is not all fat. It's about 10% water. All of our body tissues--fat, muscle, bone, skin--contain some water. And water has zero calories.
In addition, not all the nutrients we eat are completely absorbed from the digestive tract to meet metabolic needs. In the case of fat, roughly 5% is eliminated in the feces. This 10% water content and 5% non-absorbed fat accounts for the 15% difference between your calculated 4086 calories and the actual 3500 calories in a pound of fat.
Two other minor points: The calories can be reduced when the meal is high in fiber. Fiber speeds the movement of food through the digestive system, there is less time for the nutrients to be absorbed before they are eliminated. So the non-absorbed fat could be a bit higher. Also, the amount of heat generated from fat differs a little for various foods (depending on fatty acid content). For example, there are 9.50 calories in one gram of meat compared with 9.30 calories for vegetables and fruits and 9.25 calories for dairy products.
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