I’ve long wondered just how efficient the human body is at extracting energy from food. We’re told, for example, that every gram of fat contains nine calories, and that protein and carbohydrate each contain four calories per gram. The implication is that if we consume some amount of fat, protein, and carbohydrates, our bodies will extract all of that energy.

But is that true? Do our bodies really extract and store all of those calories?

We already know that individuals differ. Some exist on a diet of primarily carbohydrates, and gain weight quickly if they eat fat and protein. Others seem to turn every gram of carbohydrate into two grams of fat on their hips. (Yes, that’s an exaggeration.) Is is possible that some people metabolize fat more efficiently, and that others metabolize carbohydrate more efficiently?

I think it’s likely, but I’ve been unable to come up with solid numbers. What is the average efficiency? Has it been measured? If so, how widely does it vary among individuals?

I’d sure be interested if you could point me to solid research in this area.