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Sweetness is a taste sensation that requires interaction with receptors on the tongue. Many sugar substitutes, such as saccharin and acesulfame K, also known as Sunette, do not provide any calories. This means that they are not metabolized as part of the normal biochemical process that yields energy in the form of adenosine triphosphate, or ATP. In some cases, small quantities of additives such as lactose are present to improve the flow characteristics or to give bulk to a product. But the amounts are so small that they do not represent a significant source of energy.

The low-calorie approach of the sugar substitute aspartame, also called NutraSweet, is more interesting. This synthetic compound is a dipeptide, composed of the two amino acids phenylalanine and aspartic acid. As with most proteins, which are chains of amino acids, it can be metabolized and used as an energy source. In general, we obtain energy in the amount of four calories (more correctly termed kilocalories) per gram of protein. This is the same value as the number of calories acquired from sugars or starches. (In contrast, each gram of fat consumed provides more than twice that amount, or about nine calories a gram.)