One of those factors that has been drawing increased attention in recent years is L-carnitine, a naturally occurring amino acid biosynthesized from lysine and methionine. New research suggests that carnitine may play a role in a great number of positive physical outcomes of interest to athletes, including fat burning, muscle recovery, testosterone support, and more.
As a result, carnitine has been showing up in a variety of buzz-worthy supplements of late. More importantly, its addition to a supplementation regimen may have a synergistic effect when combined with superior-quality test-boosters, fat-loss catalysts, and such. Is carnitine worth adding to your supplementation arsenal? What exactly are the benefits of L-carnitine? Read on and chances are you may want to give this potential "super nutrient" serious consideration.
If you look in any standard biochemistry book (or read Wikipedia for those who don't own any textbooks), you'll learn that carnitine has a critical role in the fat burning process. That explains why you'll often see carnitine listed as an ingredient in fat-burning and thermogenic supplements, although usually the amounts are so low (less than 1 gram) there's little chance for a significant effect. However, higher daily doses of carnitine in the 2-3 gram range for more than a week could lead to enhanced oxidation of fat at rest and during exercise (1-3). What has not been investigated is whether carnitine would have an even greater effect in individuals consuming low carbohydrate diets where there is almost exclusive use of fat for fuel. In clinical practice, extra carnitine is often prescribed along with the use of very low carbohydrate/high-fat diets for seizure control. Although this traditional role of carnitine to support optimal fat burning has merit and continues to evolve, emerging research indicates a whole range of side benefits that extend beyond this normal function. In fact, these 'non-traditional' roles of carnitine (i.e., the ones you don't read about in Wikipedia) are the main reason you'll want to consider this commonly over-looked supplement.
Several lines of evidence support a positive role of carnitine in recovery from exercise.
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Consistent with carnitine's antioxidant effects, one of the most consistent findings was a protection from oxidative stress after high volume resistance exercise. We also showed positive effects on dilation of blood vessels, consistent with earlier work showing vasoactive effects of carnitine in animals and humans. In one study we used magnetic resonance imaging (MRI) to measure the extent of muscle tissue disruption and showed that carnitine supplementation resulted in less resistance exercise-induced muscle damage. Carnitine supplementation also resulted in less perceived muscle soreness after exercise.
Another exciting discovery has been the growing evidence supporting a role of carnitine in protein metabolism (12). For example, carnitine supplementation was shown to suppress muscle wasting in an animal model of cachexia (13). Researchers discovered that carnitine down-regulates genes involved in a specific pathway by which skeletal muscle proteins breakdown (14-15). This effect might be mediated by IGF-1, which consistently increases in animals and humans supplemented with carnitine. All these lines of evidence point to carnitine as having a positive effect on muscle anabolism and recovery.
Testosterone is a naturally secreted hormone that has a major role in promoting muscle growth and overall vitality. Low testosterone, defined as less than 10.4 nmol/L, may affect as many as one-third of men aged 46-89 years. Normal testosterone levels for adult men are between 10 and 34 nmol/L. In men the decline in testosterone usually begins in their late thirties and decreases at a relatively constant rate of 1-3% per year.
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One of the more provocative roles of carnitine relates to its role in maintaining healthy testosterone levels and functioning. Prior work has shown that carnitine is expressed in the testis (the major site of testosterone production in men), and contributes to sperm motility (16). In rats injected with a chemical that depressed testicular function, supplementation with carnitine enhanced the recovery of sperm count, sperm motility, and testosterone production (17). In a subsequent study, rats were subjected to a chronic cold-water swim protocol (two 10-min sessions per day for 10 days) that was designed to depress testosterone concentrations. The rats were either supplemented with carnitine or placebo (18). After 10 days of supplementation and cold-water swimming, testosterone concentrations were decreased two-fold in the placebo-supplemented rats. In contrast, rats supplemented with carnitine did not observe a reduction in testosterone (testosterone actually increased slightly). Researchers are not sure how carnitine preserve testosterone, but it's hypothesized that carnitine helps restore lutenizing hormone pulsatility (a hormone that controls testosterone production) or it may serve as a substrate for normal sperm production and biosynthesis of testosterone.
There is also promising work for a role of carnitine on testosterone in men. Carnitine administration was shown to be an effective treatment for erectile dysfunction in diabetic patients (19), and improve sexual dysfunction in aged men (20). A study just published in March 2013 showed that serum levels of carnitine were independently associated with free testosterone concentrations in men (21). In other words, lower carnitine levels translated into lower testosterone. These studies collectively point to the possibility that carnitine stimulates production of testosterone under certain physiologic conditions.
Carnitine might also positively impact the anabolic action of testosterone in muscle, which is under control by androgen receptors. In one of our carnitine experiments, we examined the effect of carnitine on testosterone responses to resistance exercise (10). In addition, we also took muscle biopsies from the leg in order to determine the level of androgen receptors. Testosterone transiently increases after a bout of resistance exercise, but paradoxically the response is lessened when food is provided post-exercise. We hypothesized that the provision of calories after exercise was increasing the expression of androgen receptors on muscle that bind testosterone. Indeed we showed that feeding after exercise decreased the testosterone response but that androgen receptors in muscle were upregulated. What was really intriguing was that that 3 weeks of carnitine supplementation increased resting androgen receptor content in muscle. Thus carnitine was having a direct effect in muscle to enhance its response to testosterone circulating in the blood.
Carnitine Technologies to Naturally
Boost Testosterone Function
Given these various roles of carnitine, there is reason to believe that carnitine might act in synergy with other nutrients to promote healthier testosterone metabolism. AndroCept from BioQuest includes 2 grams of L-Carnitine (the amount shown to increase androgen receptors and speed recovery) plus extra zinc (15 mg) and vitamin D (400 IU). Many people are marginally deficient in zinc and vitamin D, which negatively affects testosterone. Athletes are specifically vulnerable to zinc deficiency since strenuous physical exercise causes significant loss of zinc in the urine. Among many functions, zinc is involved in normal production of testosterone. Small deficiencies in zinc can lead to reductions in testosterone and supplementing with zinc may bring levels back to normal.
Vitamin D deficiency is also widespread among the general population including athletes. Animals that are made to have a defect in the way they respond to vitamin D show signs of testosterone insufficiency. Low circulating vitamin D levels are associated with low testosterone. One recent study showed that supplementation with vitamin D for one year increased circulating testosterone 20% (22).
The Carnitine/T-Booster Connection
Combining carnitine, vitamin D, and zinc with other testosterone-boosters such as those found in ProSource's AndroTest Extreme may elicit even more impressive mass building effects. AndroTest Extreme contains of a powerful blend of Tongkat Ali and Tribulus terrestris extract with extraordinarily high levels of saponins (40-48% protodioscin, 80% total). A recent 4 week randomized, placebo-controlled, double blind clinical trial tested the effects of AndroTest. Healthy adult men aged 35 to 55 years showed a 29% increase in free testosterone and a 42% increase in total testosterone.
Androtest Extreme also contains another clinically validated testosterone booster Testofen containing Fenugreek extract standardized for 50% fenusides.
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[Editor's Note: AndroFury, a new product from BioQuest that includes a significant dosage of superior-quality Tribulus terrestris (along with a potent performance-enhancing matrix of pre-workout co-factors) would be another interesting option for use in tandem with L-carnitine.]
In addition to potentially enhancing fat burning at rest and during exercise, carnitine offers additional benefits related to faster recovery, increased muscle anabolism, and testosterone enhancement. Carnitine's unique effects make it ideal to combine with other nutrients to enhance its effects. For testosterone augmentation, combining AndroCept with its effective form and dose of carnitine with AndroTest Extreme would appear to be a worthy supplemental strategy.
1. Wutzke KD, Lorenz H. The effect of l-carnitine on fat oxidation, protein turnover, and body composition in slightly overweight subjects. Metabolism. 2004 Aug;53(8):1002-6.
2. Müller DM, Seim H, Kiess W, Löster H, Richter T. Effects of oral L-carnitine supplementation on in vivo long-chain fatty acid oxidation in healthy adults. Metabolism. 2002 Nov;51(11):1389-91.
3. Wall BT, Stephens FB, Constantin-Teodosiu D, Marimuthu K, Macdonald IA, Greenhaff PL. Chronic oral ingestion of L-carnitine and carbohydrate increases muscle carnitine content and alters muscle fuel metabolism during exercise in humans: the dual role of muscle carnitine in exercise metabolism. J Physiol. 2011 Jan 4.
4. Spiering BA, Kraemer WJ, Hatfield DL, Vingren JL, Fragala MS, Ho JY, Thomas GA, Häkkinen K, Volek JS. Effects of L-carnitine L-tartrate supplementation on muscle oxygenation responses to resistance exercise. J Strength Cond Res. 2008 Jul;22(4):1130-5.
5. Kraemer WJ, Volek JS, French DN, Rubin MR, Sharman MJ, Gómez AL, Ratamess NA, Newton RU, Jemiolo B, Craig BW, Häkkinen K. 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.
6. Rubin MR, Volek JS, Gómez AL, Ratamess NA, French DN, Sharman MJ, Kraemer WJ. Safety measures of L-carnitine L-tartrate supplementation in healthy men. J Strength Cond Res. 2001 Nov;15(4):486-90.
7. Volek, J. S., Kraemer, W. J., Rubin, M. R., Gomez, A. L., Ratamess, N. A. & Gaynor, P. L-Carnitine L-tartrate supplementation favorably affects markers of recovery from exercise stress. Am J Physiol Endocrinol Metab. (2002); 282:E474-482.
8. Spiering, B. A., Kraemer, W. J., Vingren, J. L., Hatfield, D. L., Fragala, M. S., Ho, J. Y., Maresh, C. M., Anderson, J. M. & Volek, J. S. Responses of criterion variables to different supplemental doses of L-carnitine L-tartrate. J Strength Cond Res. (2007); 21:259-264.
9. Ho JY, Kraemer WJ, Volek JS, Fragala MS, Thomas GA, Dunn-Lewis C, Coday M, Häkkinen K, Maresh CM. l-Carnitine l-tartrate supplementation favorably affects biochemical markers of recovery from physical exertion in middle-aged men and women. Metabolism. 2010 Aug;59(8):1190-9.
10. Kraemer WJ, Spiering BA, Volek JS, Ratamess NA, Sharman MJ, Rubin MR, French DN, Silvestre R, Hatfield DL, Van Heest JL, Vingren JL, Judelson DA, Deschenes MR, Maresh CM. Androgenic responses to resistance exercise: effects of feeding and L-carnitine. Med Sci Sports Exerc. 2006 Jul;38(7):1288-96. Erratum in: Med Sci Sports Exerc. 2006 Oct;38(10):1861.
11. Volek JS, Judelson DA, Silvestre R, Yamamoto LM, Spiering BA, Hatfield DL, Vingren JL, Quann EE, Anderson JM, Maresh CM, Kraemer WJ. Effects of carnitine supplementation on flow-mediated dilation and vascular inflammatory responses to a high-fat meal in healthy young adults. Am J Cardiol. 2008 Nov 15;102(10):1413-7.
12. Ringseis R, Keller J, Eder K. Mechanisms underlying the anti-wasting effect of L-carnitine supplementation under pathologic conditions: evidence from experimental and clinical studies. Eur J Nutr. 2013 Mar 19. [Epub ahead of print]
13. Busquets S, Serpe R, Toledo M, Betancourt A, Marmonti E, Orpí M, Pin F, Capdevila E, Madeddu C, López-Soriano FJ, Mantovani G, Macciò A, Argilés JM. L-Carnitine: an adequate supplement for a multi-targeted anti-wasting therapy in cancer. Clin Nutr. 2012 Dec;31(6):889-95.
14. Keller J, Ringseis R, Koc A, Lukas I, Kluge H, Eder K. Supplementation with l-carnitine downregulates genes of the ubiquitin proteasome system in the skeletal muscle and liver of piglets. Animal. 2012 Jan;6(1):70-8.
15. Keller J, Couturier A, Haferkamp M, Most E, Eder K. Supplementation of carnitine leads to an activation of the IGF-1/PI3K/Akt signalling pathway and down regulates the E3 ligase MuRF1 in skeletal muscle of rats. Nutr Metab (Lond). 2013 Mar 15;10(1):28.
16. Bøhmer T, Hoel P, Purvis K, Hansson V. Carnitine levels in human accessory sex organs. Arch Androl. 1978;1(1):53-9.
17. Palmero S, Leone M, Prati M, Costa M, Messeni Leone M, Fugassa E, De Cecco L. The effect of L-acetylcarnitine on some reproductive functions in the oligoasthenospermic rat. Horm Metab Res. 1990 Dec;22(12):622-6.
18. Bidzinska B, Petraglia F, Angioni S, Genazzani AD, Criscuolo M, Ficarra G, Gallinelli A, Trentini GP, Genazzani AR. Effect of different chronic intermittent stressors and acetyl-l-carnitine on hypothalamic beta-endorphin and GnRH and on plasma testosterone levels in male rats. Neuroendocrinology. 1993 Jun;57(6):985-90.
19. Gentile V, Vicini P, Prigiotti G, Koverech A, Di Silverio F. Preliminary observations on the use of propionyl-L-carnitine in combination with sildenafil in patients with erectile dysfunction and diabetes. Curr Med Res Opin. 2004 Sep;20(9):1377-84.
20. Cavallini G, Caracciolo S, Vitali G, Modenini F, Biagiotti G. 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.
21. Sakai K, Fukami K, Yamagishi SI, Kaida Y, Adachi T, Ando R, Manabe R, Otsuka A, Sugi K, Ueda S, Okuda S. Evidence for a positive association between serum carnitine and free testosterone levels in uremic men with hemodialysis. Rejuvenation Res. 2013 Mar 14. [Epub ahead of print]
22. Pilz S, Frisch S, Koertke H, Kuhn J, Dreier J, Obermayer-Pietsch B, Wehr E, Zittermann A. Effect of vitamin D supplementation on testosterone levels in men. Horm Metab Res. 2011 Mar;43(3):223-5.