What is the difference between L-Taurine and Taurine, or between L-Glycine and Glycine?

Neil E. Levin, CCN, DANLA
Board certified clinical nutritionist  

The natural forms of amino acids are typically the “L form”, as in L-arginine, L-cysteine, etc. Synthetic forms are denoted as “D forms”, such as D-Methionine and D-Carnitine. 

But there are 2 aminos that have only one form without these variations: Glycine and Taurine. These two aminos are sometimes called L-Taurine or L-Glycine, but are more properly called just “Taurine” and “Glycine”. Regardless of the name used, they are always natural amino acids. Technical explanation: Most aminos have a property that, when the molecule is put into a solution, it will polarize and rotate light either to the left or right. The differing L and D forms are called stereoisomers. 

For amino acids that polarize light, the L form is the natural form. However, Taurine is an amino acid that does not polarize light. It thus is properly called just “Taurine”, without L or D configurations. While some label Taurine as “L-Taurine”, that name is not technically correct. “Taurine” is the same exact molecule and form as what is commonly mislabeled as “L-Taurine”. There is another amino acid that lacks a potential optical rotation. Glycine is a very simple molecule that comes only as “Glycine”, also lacking different L or D stereoisomer forms. 

The D forms of amino acids sold commercially are considered to be synthetic. However, D forms of amino acids are not always synthetic. There are several D forms that exist in nature. Amino acids can be racemized by the body and go back and forth between the D form and the L form quite easily. However, only L forms can be incorporated into proteins. 

For the purposes of dietary supplements, the L forms are natural and the D forms are synthetic. DLPA and DL-methionine are actually racemic mixtures of both L and D forms. But there is no such thing as D-Taurine or D-Glycine; in other words, no synthetic forms exist of these two aminos since each only comes as one isomer that doesn’t polarize and rotate light to the right. Nor are there really L forms of these, since they do not polarize and rotate light to the left, either. There are simply single, natural isomers of just plain Glycine and Taurine. 

Don’t assume that all D or L forms of molecules are good or bad, since it really depends on the individual substance concerned. For example, the D isomers of vitamin E are the natural forms and the L isomers are synthetic; just the opposite of amino acids. Thus the terminology and forms of what is natural or synthetic will vary by substance. Some natural molecules exist as L form, some as D form and some have only one form, whether in food or if synthesized. Look for companies that only sell natural form amino acids and Vitamin E and use the correct scientific names for substances and compounds on their labels.