As a conscientious bodybuilder you've done your homework when it comes to supplements. You understand how important amino acids are for meeting your goals and you know that the amino acid carnitine is a great choice for fat burning.

Yet when you get to the supplement store and reach for a bottle of the stuff you find yourself faced with options: L-carnitine vs. acetyl-L-carnitine. Which to choose?

Carnitine is a type of amino acid synthesized by the body. But it's not like typical amino acids, which the body uses for building proteins. Instead, carnitine plays functional roles in the body, with one of the most significant being the transport of fats into the mitochondria of cells. There, the fats are converted into energy that the cells–including those of the heart, brain and muscles–can use. This is why carnitine has been found to have effects on all three systems.

The form of carnitine produced in the body is L-carnitine, also known as the L-form. This is the only form the body recognizes and can use. Supplement manufacturers produce L-carnitine to mimic the type produced by the body.

The body also makes some acetyl-L-carnitine; it does this by adding an acetic acid group to L-carnitine. The addition of the acetic acid allows the carnitine better entry into the mitochondria.

Studies show that L-carnitine can be effective for increasing the body's ability to use fat. Research on acetyl-L-carnitine indicates it has additional benefits, including enhancing cognitive function and even playing a role in testosterone production. Although the benefits of supplemental acetyl-L-carnitine are clear, there is still some debate over whether it is necessary to take that form as opposed to the much cheaper L-carnitine.

Those who argue that there is no difference suggest that when acetyl-L-carnitine is ingested, the acetic acid group is cleaved off in the cells of the intestines and then a certain amount of free L-carnitine is later reattached with the acetic acid. Those who argue for the use of acetyl-L-carnitine believe that since that was the form used in the research, then that is the one to take. Studies have shown that the additional acetic acid is important because it can be donated to produce acetylcholine, a significant neurotransmitter that enhances mental and muscular function.

FLEX recommends that you use what you can afford in accordance to the benefits you desire. If you are going for fat burning, try two to four grams of L-carnitine daily in two or three divided doses. If that doesn't seem as effective for you, or you wish to reap the brain-boosting and testosterone-sparing benefits of the supplement and can spend the extra moola, try taking two to four grams of acetyl-L-carnitine daily.