What Is L-Carnitine?

While it is often categorized as an amino acid, L-carnitine isn't technically an amino. It is considered a "vitamin-like" and "amino-acid-like" compound that is related to the B vitamins. When it was first studied back in the 1950s, L-carnitine was referred to as vitamin BT.

L-carnitine is formed in the liver and kidneys from the amino acids lysine and methionine. However, it is stored elsewhere in the body, primarily in muscle (including the heart), the brain, and even in sperm. In the diet, it mainly comes from meat and other animal products. You can get some from plant products like avocado and soybeans, but as a rule, meat is the best source—and the redder the better.

Carnitine exists in two forms: D-carnitine and L-carnitine. The L-form is the kind of carnitine found in nature and is biologically active. On labels, you'll see it listed as L-carnitine, L-carnitine L-tartrate, or Propionyl-L-carnitine. They're all similar, and similarly effective. The D form of carnitine, on the other hand, is biologically inactive and isn't sold as a supplement.

Acetyl L-carnitine, also known as acetylcarnitine or ALCAR, is another popular supplemental form of carnitine. It can be found throughout the central nervous system, where it plays a role in producing energy and produces the important neurotransmitter acetylcholine.

The acetyl group attached to the carnitine molecule enhances its ability to pass across the blood-brain barrier and enter the brain, where it acts as a powerful antioxidant. For this reason, some research suggests that acetyl L-carnitine may provide protective actions against aging processes and neurodegeneration.

What Does L-Carntine Do?

L-carnitine helps to transport fat, particularly long-chain fatty acids, into the mitochondria of cells. Once there, the fatty acids can be oxidized—used as fuel—to generate adenosine triphosphate, or ATP. L-carnitine does this cellular work both when you exercise and rest, but research confirms that it is especially effective during intense exercise.

Without adequate carnitine, most dietary fats can't get into the mitochondria and be burned for fuel. But for people with carnitine deficiency, it is a serious medical condition. It can lead to muscle weakness, stunted growth, an enlarged liver, and a number of other problems. This is one reason it's considered a "conditionally essential" nutrient: Your body produces it, but if it doesn't produce enough, your health can be seriously impacted.

A study out of Scotland concluded that in addition to its fat-transporting work, L-carnitine also enhances insulin's actions on muscle cells.[1] What this means is that it can help keep blood glucose levels low, even after a carb-rich meal, while also aiding glycogen repletion. This is one reason I advocate taking this supplement with a post-workout meal.

Some of the most interesting research into L-carnitine supplementation focuses on how it can enhance athletic performance.

What Are The Performance And Physique Applications Of L-Carnitine?

L-carnitine's bona fide role as a fat-burning supplement is well established. During bulking periods, it can help limit fat gains and make a "cleaner" bulk. If you're cutting, it can help transport the fat you have into your cellular furnaces to get burned as energy.

Burning fat as fuel: Some of the most interesting recent research into L-carnitine supplementation focuses on how it can enhance athletic performance. In one study, University of Nottingham Medical School researchers had one group of athletes consume 2 grams of L-carnitine along with 80 grams of a high-glycemic carb first thing in the morning and four hours later for 24 weeks. Another group only took the carbs.[2]

The researchers found that during low-intensity cycling, the subjects taking carnitine burned 55 percent less muscle glycogen while increasing the body's ability to burn fat by 55 percent. During high-intensity cycling, the subjects taking L-carnitine had lower levels of lactic acid and higher levels of creatine phosphate, one of the primary building blocks of ATP.

Fatigue resistance: When the Nottingham researchers measured the subjects' ability to resist fatigue during a cycling test, they found that subjects taking the supplement were able to go more than 25 percent longer. This was likely because they burned more fat while preserving muscle glycogen, in addition to having lower levels of lactic acid and higher levels of creatine phosphate.

Decreased Muscle Soreness, Improved Recovery: In a number of studies on human subjects, L-carnitine doses of just 1-2 grams per day have been shown to significantly reduce muscle damage from strenuous exercise, and improved recovery to exercise.[3-6] It has also been shown to decrease muscle soreness. The best part? It wasn't rats doing these workouts. It was healthy, active men and women.

Better blood flow and pumps: This supplement's performance benefits not only stem from its ability to increase fat burning and decrease glycogen use, but also from the way it can enhance blood flow to the muscles.[7] Increased blood flow means more nutrients and hormones go where they're needed most during exercise.

How does this work? For one, carnitine reduces oxidative damage in the body's nitric oxide (NO). But it also enhances the activity of a key enzyme involved in your body's NO production. The net result is higher NO blood levels, which not only enhance energy during workouts, but also muscle recovery following workouts.