Q: I am a bit confused about the distinction between the ATP that can be produced from a gram of stored body fat versus the ATP that can be produced from a gram of stored carbohydrate. You have written that a gram of fat produces 3 x the ATP that can be produced from a gram of carbohydrate. I researched the issue in some biochemistry textbooks and discovered that a Mole of glycogen produces 0.20 Moles of ATP, and a Mole of stored body fat produces 0.49 Moles of ATP. The ratio between these two (~2.5) is almost identical to the ratio between the calories contained in a gram of carbohydrate versus a gram of fat (when using that actual amount of calories contained in each and not the approximation of 4 and 9 that we actually use). So, unless I am missing something, aren't calories derived from carbs and fats in our food an almost exact approximation of how much ATP can be generated from them?
A: The conversion of dietary calories into ATP is complex and never 100 percent of the theoretical conversion of dietary calories into an equivalent amount of chemical energy. Under ideal conditions, one molecule of glucose can make 36 molecules of ATP. Under less than ideal conditions, it can drop to as low as 2 molecules of ATP. One molecule of fat can make 108 molecules of ATP. This is because most of the ATP conversion takes place in the mitochondria and is dependent on the number of two-carbon fragments that previously entered into the Krebs cycle. Since a fatty acid has 9 two-carbon fragments versus 3 two-carbon fragments in the glucose, you can make a lot more ATP from fats versus carbohydrates under even the most ideal conditions. Even when you take into account the differences in calories, a calorie of fat will make 33 percent more ATP than a gram of carbohydrate.