When you eat 100 calories of white sugar, the work the body must do to absorb it burns up only 7 calories, so 93 usable calories remain. Thus, the SDA for carbohydrates is 7 percent.
When you eat 100 calories of butter or oil, assimilating them is a bit more laborious. The body burns 12 calories in absorbing them, leaving only 88 usable calories. Thus the SDA of fats is 12 percent.
Finally, to assimilate 100 calories of pure protein -- egg whites, lean fish, or nonfat cottage cheese -- the task is enormous. This is because protein is composed of an aggregate of very long chains of molecules whose basic links, amino acids, are connected to each other by a strong bond that requires a lot more work to be broken down. It takes 30 calories just to assimilate the proteins, leaving only 70 usable calories. Thus the SDA of proteins is 30 percent.
Find out more about this book:The Dukan Diet: 2 Steps to Lose the Weight, 2 Steps to Keep It Off Forever