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Supplement Breakdown: Carnitine

Supplement Breakdown: Carnitine

The lowdown on what I consider a "must-have" supplement.

Written By Jim Stoppani, PhD

Updated September 28, 2023

Depending on how old you are and how long you've been training, you may or may not remember carnitine when it broke onto the scene back in the 1980s as a hot fat burner. But its popularity fizzled out when sexier, stimulant-based fat burners hit the market. Plus, much of the early research on carnitine claimed that it did little to influence fat loss. Little did anyone realize that the research was all pretty much useless.

Today, carnitine is back and stronger than ever (and it's also found in Post JYM). We now know that it not only aids fat-burning, but it also enhances exercise performance and recovery. If it's not currently in your supplement repertoire, it should be soon. Here are all the reasons why...

Carntine Primer

Carnitine is often categorized as an amino acid, but that's not exactly accurate. Carnitine is considered a vitamin-like and amino acid-like compound that's related to the B vitamins. More specifically, it's an ammonia-containing compound that's formed in the liver and kidneys from the amino acids lysine and methionine. Although it's formed in these organs, it's stored primarily in muscle (including the heart) as well as the brain and even in 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.

Carnitine Benefit #1: Fat Burning

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 sidenote here is that 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 is that they don't need to be transported into the mitochondria. They instead have direct access in and are more readily burned for fuel.

But back to the long-chain fatty acids: Several research studies support the notion that supplementing with carnitine enhances fat burning – not just during exercise, but also at rest. Carnitine's 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 this decline in fat burning and enhance performance. In fact, studies confirm that supplementing with carnitine enhances athletic performance. In one study, University of Nottingham Medical School researchers had athletes consume one of two supplement protocols for 24 weeks, each taken twice a day, four hours apart: (a) 2 grams of carnitine along with 80 grams of Vitargo (a high-glycemic or fast-digesting carbohydrate supplement) or (b) just 80 grams of Vitargo.

The researchers 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 the subjects' 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 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 five 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, which was shown in a 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's burned away for good is also likely to help keep body fat off, even when consuming a large amount of fast carbs every day (ie, 160 grams).

Carnitine Benefit #2: Enhanced Athletic Performance

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 also 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.

Carnitine Benefit #3: More Size and Strength

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 muscle 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 growth, by boosting levels of one of its binding proteins that enhances its anabolic actions.

Carnitine Benefit #4: Sexual Enhancement

Carnitine has even been found to help you "below the belt." Because it boosts NO levels (as discussed above), it can help increase blood flow down low. In fact, one study from Italian researchers reported that carnitine was more effective than testosterone therapy for treating male-erectile dysfunction. There's also research that suggests that supplementing with carnitine enhances male fertility, mainly by enhancing sperm mobility.

Carnitine Benefit #5: Brain Power

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.

Carnitine and Cardiovascular Disease

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.

Supplementing with Carnitine

It's imperative that you supplement carnitine at a time when you're consuming fast-digesting carbs to spike insulin levels, such as post-workout. This is one of the reasons why I included two grams of carnitine in Post JYM. The early studies that showed no benefit of carnitine failed to adequately raise muscle carnitine levels – it wasn't supplied at a time when insulin would be spiked and muscle uptake would be adequate.

The newer research 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 muscles as well as more carnitine. While as little as one gram of carnitine can be effective, your best bet is to take 2-3 grams per dose for maximum benefit, along with at least 30 grams of high glycemic carbs and 20-40 grams of protein.

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 should definitely 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 meals. One strategy I've found to work well for both fat loss and performance benefits is to take 2-3 grams of acetyl-L-carnitine with other fat-burning ingredients without food between meals, and to take one dose of L-carnitine or L-carnitine L-tartrate with a post-workout meal such as Post JYM.

As I've illustrated by listing carnitines various benefits, this supplement should be a mainstay in any fitness-minded person's dietary regimen. It's a true super supp!

References:

Stephens, F. B., et al. New insights concerning the role of carnitine in the regulation of fuel metabolism in skeletal muscle. The Journal of Physiology 581: 431-444, 2007.

Buzzigoli, G. and Ferrannini, E. Effects of acute hypercarnitinemia during increased fatty substrate oxidation in man. Metabolism 42(5):594-600, 1993.

Muller, D. M., et al. Effects of oral L-carnitine supplementation on in vivo long-chain fatty acid oxidation in healthy adults. Metabolism 51(11):1389-91, 2002.

Karlic, H, and Lohninger, A. Supplementation of L-Carnitine in Athletes: Does It Make Sense? Nutrition 20:709 –715, 2004.

Spiering, B. A., et al. Effects of L-Carnitine L-Tartrate Supplementation on Muscle Oxygenation Responses to Resistance Exercise. Journal of Strength Conditioning Research. 22(4):1130-1135, 2008.

Stephens, F. B., et al. An acute increase in skeletal muscle carnitine content alters fuel metabolism in resting human skeletal muscle. J Clin Endocrinol Metab 91(12):5013-8, 2006.

Wall, B. T., et al. Chronic oral ingestion of L-carnitine and carbohydrate increases muscle carnitine content and alters muscle fuel metabolism during exercise in humans. J Physiol. 2011 Feb 15;589(Pt 4):963-73.

S. D. R. Galloway, Effects of oral l-carnitine supplementation on insulin sensitivity indices in response to glucose feeding in lean and overweight/obese males. Amino Acids 41(2):507-515, 2011.

Volek, J. S., et al. L-Carnitine L-tartrate supplementation favorably affects markers of recovery from exercise stress. Am J Physiol Endocrinol Metab 282: E474 – E482, 2002.

Spiering, B. A., et al. Responses of criterion variables to different supplemental doses of L-carnitine L-tartrate. Journal of Strength Conditioning Research. 21(1):259-264, February 2007.

Alvarez de Sotomayor, M., et al. Effect of L-Carnitine and Propionyl-L-Carnitine on Endothelial Function of Small Mesenteric Arteries from SHR. J Vasc Res 2007;44:354-364

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Bernard, A., et al. L-carnitine supplementation and physical exercise restore age-associated decline in some mitochondrial functions in the rat. J Gerontol A Biol Sci Med Sci. 2008 Oct;63(10):1027-33.

Bloomer, R. et al. Glycine propionyl-L-carnitine increases plasma nitrate/nitrite in resistance trained men. J Int Soc Sports Nutr. 2007 Dec 3;4(1):22.

Bloomer RJ, Tschume LC, Smith WA. Glycine propionyl-L-carnitine modulates lipid peroxidation and nitric oxide in human subjects. Int J Vit Nutr Res. In Press.

Ferrari, R., et al. Therapeutic Effects of l-Carnitine and Propionyl-l-carnitine on Cardiovascular Diseases: A Review. N.Y. Acad. Sci. 1033: 79–91, 2004.

Kraemer, W.J., et al. The effects of L-carnitine L-tartrate supplementation on hormonal responses to resistance exercise and recovery. J Strength Cond Res. 2003 Aug;17(3):455-62.

Cavallini, G., et al. Carnitine versus androgen administration in the treatment of sexual dysfunction, depressed mood, and fatigue associated with male aging. Urology. 2004 Apr;63(4):641-6.

Garolla, A., et al. Oral carnitine supplementation increases sperm motility in asthenozoospermic men with normal sperm phospholipid hydroperoxide glutathione peroxidase levels. Fertility Sterility 83(2): 355-361, 2005.

Malaguarnera, M., et al. L-Carnitine treatment reduces severity of physical and mental fatigue and increases cognitive functions in centenarians: a randomized and controlled clinical trial. Am J Clin Nutr. 2007 Dec;86(6):1738-44.

Lenzi A, et al. A placebo-controlled double-blind randomized trial of the use of combined l-carnitine and l-acetyl-carnitine treatment in men with asthenozoospermia. Fertil Steril. 2004 Jun;81(6):1578-84.

Stephens, F. B., et al. Insulin stimulates L-carnitine accumulation in human skeletal muscle. FASEB J. 20(2):377-9, 2006.

Stephens, F. B., et al. Carbohydrate ingestion augments L-carnitine retention in humans. J Appl Physiol 102: 1065–1070, 2007.

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