If you take a second to check out your latest tub of pre-workout or EAAs/BCAAs, you’ll find many common ingredients on there, most of which you probably know what they do. Caffeine boosts energy, gives you focus, motivation, etc. Beta Alanine enhances endurance and fatigue resistance. It also gives those delightful (or depending on how you look at it, loathsome) tingles. Citrulline improves blood flow and helps get a serious pump going. But, quite often, these ingredients have certain designations in front of them. The most common example is “L-” in front of various amino acids, such as L-Citrulline, L-Leucine, L-Glutamine, etc. What does the “L” stand for, exactly? Let’s find out.
What does the “L” mean?
With the exception of glycine, all amino acids are stereoisomers. Stereoisomers are a type of isomerism in which molecules have the same molecular formula and sequence of bonded atoms, but differ in the three-dimensional orientations of their atoms in space. You can think of it as having one molecule and its mirror image. The same makeup and arrangement of molecules, just flipped.
Scientists have labeled these two different forms as L-amino acids and D-amino acids. In fact, most (if not all) of the amino acids that are manufactured and used in supplements today are L-amino acids.
Why do brands only use L-amino acids?
It’s a matter of basic human physiology. Only L-amino acids are created in cells and incorporated into proteins. Based on this, it was thought that D-amino acids did not exist in nature. However, scientists later found evidence that D-amino acids do exist and perform a variety of functions.
You may also notice DL-amino acids (such as DL-Phenylalanine), which contain both L- and D-amino acids. DL-amino acids are also called racemic mixtures. As we mentioned at the outset, all amino acids, except glycine, have these left and right-handed isoforms.
The reason that glycine, the simplest amino acid, does not have two different forms is that it has two hydrogen atoms attached to the central carbon atom. It’s only possible to have different isoforms when all four attachments are different.
The “L-” designator in front of the various amino acids in your pre workout, post workout, and/or amino acid supplement simply refers to which “hand” the molecule is -- left or right. L-amino acids are the only ones that can be used and produced by our cells to construct proteins, repair tissue, and build muscle.
D-amino acids can be found in nature and, in certain cases, can be found in dietary supplements, such as D-Aspartic Acid (DAA).