We all need energy to function and we get this energy from the foods we eat. The most efficient way for cells to harvest energy stored in food is through cellular respiration, a catabolic pathway for the production of adenosine triphosphate (ATP). ATP, a high energy molecule, is expended by working cells.
An Apple a Day. The calories in a McIntosh apple depend on its size. According to the U.S. Department of Agriculture, a medium-sized apple with its skin has about 95 calories. Eating an unpeeled apple is moderately higher in calories than a peeled apple, but don't rush to peel your apple.
An apple contains less than 200 calories. This leaves a lot of room for eating considering the amount of calories recommended by daily calorie intakes. That is why it is said that going on a 3 day apple diet cleanse will not only flush your system but help you lose weight as well. In a 100g serving size there are only 54 calories in apples.