Glucose is a ubiquitous sugar that is a source of energy for living organisms. There are two forms of glucose, however, which are differentiated from one another through use of the prefixes "D" and "L." D-glucose is the form your cells can use for energy, while L-glucose is exceedingly rare in nature.
Glucose
The glucose molecule is a carbohydrate, meaning it's made up of the elements carbon, hydrogen and oxygen on a 1:2:1 ratio. Specifically, glucose is a monosaccharide, which means it consists of a single sugar ring, explain Drs. Reginald Garrett and Charles Grisham in their book "Biochemistry." You'll find glucose on its own -- corn syrup, for example, contains glucose -- and you also consume it chemically combined with other monosaccharides. Table sugar contains glucose bonded to another monosaccharide called fructose.
D vs. L
Many biomolecules take either the prefix "D" or "L" before their names; the prefix gives chemists information about the absolute spatial orientation of the atoms in the molecule. In the case of monosaccharide molecules, including glucose, "D" indicates that one of the oxygen atoms on the molecule points to the right when the molecule is drawn in a particular way, while "L" indicates that the same oxygen atom points to the left. Simply stated, D-glucose and L-glucose are mirror images of one another.
Why D and L Matter
Your cells -- and those of almost all other living organisms -- can't burn L-glucose for energy, but they can burn D-glucose. The reason they can only burn one form of the sugar is that enzymes that assist in the reactions that result in burning of sugar can only bind to D-glucose; trying to get one of these enzymes to bind to L-glucose would be like trying to put your right foot into a left shoe. L-glucose, because it's useless to living organisms, isn't really found in nature.
L-Glucose
Even though you can't find it in nature, it's possible to make L-glucose in the lab. It tastes sweet, just like D-glucose, explains NASA. However, because your cells can't burn it for energy, it doesn't contain any calories. Researchers have explored using L-glucose as a low-calorie sweetener, but it's not cost-effective to produce it; there are other sugar substitutes that are much cheaper to manufacture, which is why you won't find L-glucose on a supermarket shelf.
