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Brian Turner

Posted on June 28 2018

By Steve Blechman

 Thirty-five years ago, in 1983, I was the first to introduce and bring L-carnitine to America from Japan as a supplement during my 27 years as Head of Product Development at Twinlab (1974-2001). In 1987, I was also the first to introduce and bring coenzyme Q10 to America from Japan as a dietary supplement. At the time, there were a million Japanese people on coenzyme Q10 for cardiomyopathy, but not one bottle was sold in the United States. Today, L-carnitine and coenzyme Q10 are extremely popular supplements.

 In 1983, I introduced natural L-carnitine, not D,L-carnitine. D,L-carnitine is an inexpensive mixture containing equal amounts of the D- and L- forms. L-carnitine is found in nature, not D,L-carnitine. Only the L-isomer is biologically active. D-carnitine is completely inactive and can inhibit the body use of L-carnitine and potentially have negative effects on the body. At the time, the research showed that the D-isomer of D,L-carnitine competed with the L-carnitine in muscle and could cause muscle carnitine deficiency and potential health concerns. After the launch of Twinlabs’ L-carnitine, D,L-carnitine products started to appear in the marketplace. I was very concerned! At the time, D,L-carnitine was much lower in cost and therefore more competitive in the marketplace. Thankfully, several months later the FDA came in and banned D,L-carnitine products in the United States because of the health concerns and safety risks of D,L-carnitine.

 L-carnitine is an important chemical found in mitochondria – the cells’ energy centers – where it helps break down long-chain fatty acids and promotes oxygen consumption. A study published in the International Journal of Sports Nutrition and Exercise Metabolism showed that L-carnitine supplementation combined with aerobic training does not promote weight loss. A most recent study published in the Scandinavian Journal of Medicine & Science in Sports in 2018 found that increasing muscle carnitine during 24 weeks of HIIT (high-intensity interval training) did not enhance muscle metabolic adaptations or performance gains beyond those with HIIT alone. An Iranian meta-analysis (combination of many studies) showed that supplementing L-carnitine had no effect on physical performance.

 Nutrition experts recommend reducing intake of red meat in the diet because it is high in cholesterol and saturated fat, which promotes coronary artery disease. A Cleveland Clinic study led by Stanley Hazen found that L-carnitine, a popular athletic supplement and component of red meat, also triggers heart disease. Bacteria in the gut break down carnitine to trimethylamine N-oxide (TMAO), which promotes arterial disease by increasing cholesterol deposits in the arteries and inflammation. High TMAO in the blood was a good predictor of heart attack and stroke. A study by the same research group found that gut microbes also metabolized phosphatidylcholine, another athletic supplement to TMOA. Administration of antibiotics decreased TMOA levels. L-carnitine is a popular athletic supplement. However, a joint position statement from the American Dietetic Association and the Dietitians of Canada concluded that L-carnitine does not improve athletic performance. It might also promote heart disease. Further research is warranted.

 Also, too much carnitine might cause cancer. A study on rats, led by Christoph Weinert from the Federal Research Institute of Nutrition and Food in Karlsruhe, Germany showed that chronic administration of L-carnitine increased blood levels of trimethylamine-N oxide (TMAO), which has been linked to cancer and heart disease. Before you dump your L-carnitine, the study was on rats and not humans, and researchers did not find increased rates of cancer or heart attack – only increased levels of TMAO.

 Excessive consumption of red meat is linked to heart disease and cancer. Many cuts of red meat are high in cholesterol and saturated fats, which promotes coronary artery disease. Also, grilling meats produces dangerous chemicals that increase the risk of certain cancers.

 The research on L-carnitine supplementation has shown equivocal outcomes. Unfortunately, supplementation with L-carnitine has been shown to have no influence on fat loss when consumed alone. L-carnitine functions by transporting fatty acids into the mitochondrion, where they are burned for energy. Even though L-carnitine’s central role is enhancing fatty acid oxidation, L-carnitine has been shown to have no influence on fat burning or fat loss when consumed alone. In fact, a study in the American Journal of Clinical Nutrition, 2016, showed that when L-carnitine is consumed with whey protein, whey protein inhibited the uptake of L-carnitine. The combined use of whey protein to increase L-carnitine levels for fat loss is ineffective and therefore, not recommended. As recently as March 2018, it was reported in the International Journal of Sports Nutrition and Exercise Metabolism that, “it is likely that the lack of efficacy in many studies is due to its low bioavailability and failure to increase muscle L-carnitine stores.”

 Research has shown that L-carnitine absorption and retention can be increased when co-ingested with a large carbohydrate source, which enhances insulin-mediated uptake of L-carnitine. One study found that 3 grams a day combined with 94 grams of carbohydrate over 100 days increased muscle L-carnitine by only 10 percent! In a follow-up study, chronic supplementation twice daily with 2 grams L-carnitine plus 80 grams of carbohydrates for 28 weeks increased muscle L-carnitine by 28 percent. This is a huge amount of carbohydrates to take. “Regardless of these performance benefits, it is likely that these aforementioned ingestion protocols are impractical on a daily basis and it is also unclear as to the impact that such a dosage regimen may have on an individual’s health,” the 2018 report concluded.

 I agree, and that’s “The Carnitine Dilemma” for fat loss and health! It is very impractical to ingest all these carbohydrates when you’re dieting to increase L-carnitine levels, especially if you are on a low-calorie or low-carb diet! It just doesn’t make sense that you must take all these carbohydrates to increase muscle L-carnitine levels. High intake of carbohydrate can spike blood insulin levels, which can enhance fat storage and prevent fat burning. You want to lower blood and circulating insulin levels to enhance lipolysis (fat mobilization), fat oxidation and fat burning.



Evidence-Based Supplements for the Enhancement of Athletic Performance. P Peeling, MJ Binnie, et al. International Journal of Sport Nutrition and Exercise Metabolism, 28; 2. 3/2018 (28) 178-187.

 L-carnitine supplementation combined with aerobic training does not promote weight loss in moderately obese women. Villani RG, Gannon J, et al. Int. J Sport Nutr Exerc Metab, 2000; 10, 199-207.

Increasing skeletal muscle carnitine availability does not alter the adaptations to high-intensity interval training. Christopher E. Shannon, et al. Scand J Med Sci Sports. 2018;28: 107-115.

Chronic oral ingestion of L-carnitine and carbohydrate increases muscle carnitine content and alters muscle fuel metabolism during exercise in humans. Wall BT, Stephens FB, et al. J Physiol 2011; 589, 963-973.

Protein ingestion acutely inhibits insulin-stimulated muscle carnitine uptake in healthy young men. Shannon CE, Nixon AV, et al. Am J Clin Nutr 2016.

The Microbiome and Risk for Atherosclerosis. Komaroff AL. JAMA. 2018;319(23):2381-2382. doi:10.1001/jama.2018.5240

Plasma trimethylamine N-oxide is associated with vulnerable plaque characteristics in CAD patients as assessed by optical coherence tomography Liu Xinxin, et al. International Journal of Cardiology , Volume 265 , 18-23.

Intestinal microbiota metabolism of L-carnitine, a nutrient in red meat, promotes atherosclerosis. Koeth RA, et al. Nat Med 2013; May: 19(5) 576-85.

Gut flora metabolism of phosphatidylcholine promotes cardiovascular disease. Wang Z, et al. Nature 2011; Apr 7; 472(7341): 57-63.

Effect of L-carnitine supplementation on muscle and blood carnitine content and lactate accumulation during high-intensity sprint cycling. Barnett C, et al. International Journal of Sport Nutrition, 4(3), 280-288.

Effect of L-carnitine supplementation on the body carnitine pool, skeletal muscle energy metabolism and physical performance in male vegetarians. Novakova K, et al. European Journal of Nutrition, 55(1), 207-217.

Oral L-carnitine supplementation increases trimethylamine N-oxide but reduces markers of vascular injury in hemodialysis patients. Fukami K, et al. Journal of Cardiovascular Pharmacology. 65(3):289-295, Mar 2015.

Trimethylamine N-Oxide: The Good, the Bad and the Unknown. Manuel T Velasquez, et al. Toxins (Basel). 2016 Nov; 8(11): 326.

Intestinal Microbial Metabolism of Phosphatidylcholine and Cardiovascular Risk. WH Wilson Tang, et al. New England Journal of Medicine, 368: 1575-1584, 2013.

Molecular Nutrition, published online December 9, 2016.