If carbs and proteins are both 4 cals/gm and fats are 9 cals/per gm, how come the calories listed on some food labels are different than if you total up the macronutrients? Example, the bread I usually eat says it has 60 calories per slice. But yet it has 14 gms of carbs, 2 gms of protein and and 1 gm of fat. I come up with 73 calories when I do the math. That’s over 20% more than the label claims! Why is that?
I believe it is because some of the carbohydrates are in the form of insoluble fiber, which your body doesn’t digest and thus does not have a caloric value. Look at a book of All-Bran Extra Fiber cereal. It has something like 24 grams of carbs and a sprinkling of protein and fat, but only 50 calories because of the high insoluble fiber content. When this is subtracted there is only 7 grams of carbs per serving and then the calorie value comes out on target.