Sugars are most readily absorbed into cells and oxidized to regenerate ATP for energy. Hence the quick but temporary energy boost from sugary soft drinks or other sweets. Starchy foods are close second as starch is quickly digested into sugars (see: molecular structure of starch). Fats and lipids take a bit longer to convert for oxidation but generate even more ATP per gram eaten. Proteins may be metabolized for energy but demand even more time and regenerate less ATP per amount eaten.
Junk foods: Have students observe that sugars, starches, fats, and lipids have no elements beyond the basic carbon, hydrogen, and oxygen (see molecular structures of each). Therefore, they do not and cannot provide building blocks for body maintenance, repair, or growth. Their only role is to be used for energy. This leads to their being referred to as “empty calories” or “junk foods”.