Carnitine is often categorized as an amino acid, yet it is not truly an amino acid. Carnitine is considered a vitamin-like and amino acid-like compound that is related to the B vitamins. More specifically, it’s an ammonia-containing compound that is formed in the liver and kidneys from the amino acids lysine and methionine. Although it is formed in these organs, it is stored primarily in muscle (including the heart), as well as the brain and even sperm. Carnitine exists in two forms, D-carnitine and L-carnitine. The L-form is the one found in nature and is biologically active. The D-form, on the other hand, is biologically inactive and may actually have negative effects on the body.
The most critical and understood role that carnitine plays in the body is in helping to transport fat, particularly long-chain fatty acids, across the mitochondria of cells. The mitochondria are essentially a cell’s power plant where the majority of ATP (adenosine triphosphate) is derived for energy. Once the fatty acids pass into the mitochondria, they can be oxidized (“burned”) to generate ATP. Without adequate carnitine, most dietary fats cannot get into the mitochondria and be burned for fuel. An interesting note here is that this is one reason why medium-chained triglycerides (MCTs) and coconut oil, which contains MCTs are not as readily stored as fat in the body as most other dietary fats are. They don’t need to be transported into the mitochondria and instead have direct access in. So they are more readily burned for fuel.
Several research studies support the notion that supplementing with carnitine enhances fat burning, not just during exercise, but also at rest. Its ability to increase the amount of fat burned at rest means that this supplement has solid potential to aid fat loss and prevent fat gain during bulking periods. Due to its ability to increase fat burning during exercise, supplementing with carnitine spares muscle glycogen and delays muscle fatigue, allowing you to train harder for longer.
Intense exercise reduces the amount of free carnitine in muscle, which may lead to quicker fatigue due to a limited amount of fat being burned for fuel. Increasing muscle carnitine content could alleviate the decline in fat burning and enhance performance. In fact, studies confirm that supplementing with carnitine enhances athletic performance.
In a recent study, University of Nottingham Medical School researchers had athletes consume 2 grams of carnitine along with 80 grams of Vitargo (a high-glycemic or fast-digesting carb supplement) or just 80 grams of Vitargo first thing in the morning and four hours later for 24 weeks. They found that during low-intensity cycling, the subjects taking carnitine burned 55% less muscle glycogen, while increasing fat burning by 55%. During high-intensity cycling the subjects taking carnitine had lower levels of lactic acid and higher levels of creatine phosphate. When they measured their ability to resist fatigue during a cycling test, they found that those taking carnitine were able to go more than 25% longer than those supplementing with just carbs. This is likely due to the fact that they burned more fat while preserving more muscle glycogen, as well as having lower levels of lactic acid and higher levels of creatine phosphate.
The UK researchers also found that despite adding 160 grams of fast-digesting carbs (640 calories) to their daily diet, the subjects taking carnitine gained no additional body fat. The group taking the 160 grams of fast carbs without carnitine gained over 5 pounds of body fat during the 24 weeks. This may be due to carnitine’s ability to keep blood glucose levels low even following a large carbohydrate meal. It does this by enhancing glucose uptake at the muscle cells. This was shown in a fairly recent study from Scotland. It appears that carnitine enhances insulin’s actions at the muscle cell to aid glucose uptake. This makes carnitine a great supplement to take after workouts with fast carbs to aid glycogen repletion. Carnitine’s ability to help carry more fat into the mitochondria of muscle cells where it is burned away for good also likely helped to keep body fat off despite consuming all those fast carbs every day.
Carnitine’s performance benefits not only stem from its ability to increase fat burning and decrease glycogen use, but to other factors as well. One of these has to do with enhanced blood flow to muscles. Research shows that carnitine can enhance blood flow, which enhances nutrient, hormone and oxygen delivery to muscle fibers. This not only enhances energy during workouts, but enhances muscle recovery following workouts, as research shows. This property of enhanced blood flow appears to be due to its ability to increase nitric oxide (NO) levels by reducing oxidative damage to NO, as well as its ability to enhance the activity of a key enzyme involved in NO production.
In addition to boosting athletic performance, carnitine also has anabolic properties that can help to increase muscle growth and strength gains. Research shows that carnitine influences testosterone. University of Connecticut researchers found that carnitine increases the amount of testosterone receptors, known as androgen receptors, inside muscle cells. The more of these receptors you have in your more cells, the more of your testosterone that can bind to them and stimulate muscle growth. In addition, research shows that carnitine supplementation facilitates the action of insulin-like growth factor-I (IGF-I), a critical growth factor for stimulating muscle growth, by boosting levels of one of its binding proteins that enhances its anabolic actions.
Carnitine has even been found to be able to hep you “under the belt” in more ways than one. Because it boosts NO levels, as discussed above, this can help you get more blood flow down there. In fact, one study from Italian researchers reported that carnitine was more effective than testosterone therapy for treating male erectile dysfunction. There is also research that suggests that supplementing with carnitine enhances male fertility, mainly by enhancing sperm mobility.
In addition to physical benefits, carnitine, particularly acetyl-L-carnitine, also provides mental benefits. Acetyl-L-carnitine can be found throughout the central nervous system, where it plays a role in producing energy, as well as producing 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. Research suggests that acetyl-L-carnitine may provide restorative and protective actions against aging processes and neurodegeneration, as well as helping to maintain brain function and enhance memory.
One recent study postulated that carnitine supplementation may increase the risk of atherosclerosis. However, the study design has been met with much criticism from the science community and many medical experts point to the numerous studies showing a reduction in the risk of cardiovascular disease with carnitine supplementation. You can read my personal take on the study here.
It is imperative that you supplement carnitine at a time when you are consuming high GI carbs to spike insulin levels, such as postworkout. This is one of the reasons why I included 2 grams of carnitine in Post JYM. The early studies that showed no benefit of carnitine failed to adequately raise muscle carnitine levels. That’s because it was not supplied at the right time when insulin would be spiked and muscle uptake would be adequate. The newer research now shows that insulin levels must be quite high for adequate amounts of carnitine to enter muscle cells, where it performs the majority of its work. On the flip side, taking carnitine enhances insulin’s actions at the muscle cells, helping to deliver more gluose into the muscle cells as well as more carnitine.
The three main forms of carnitine are L-carnitine, acetyl-L-carnitine, and proprionyl-L-carnitine. L-carnitine either as straight up L-carnitine or L-carnitine L-tartrate and propionly-L-carnitine definitely should be taken with high carb and high protein meals. Acetyl-L-carnitine, however, may be better taken up by the intestines and the muscle cells, not to mention the brain, in the absence of food. Therefore, this form of carnitine may be stacked with other ingredients that enhance fat burning, such as caffeine and green tea and taken between One strategy that I have found to work well for both fat loss and performance benefits is to take 2-3 doses of acetyl-L-carnitine with other fat-burning ingredients without food between meals, and to take 1 dose of L-carnitine L-tartrate with your postworkout meal, such as in Post JYM.
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