|Giulia Corrao, Dario Camperchioli|
Carnitine is a quaternary ammonium compound biosynthesized from the amino acids lysine and methionine . In living cells, it is required for the transport of fatty acids from the cytosol into the mitochondria during the breakdown of lipids for the generation of metabolic energy . Carnitine exists in two stereoisomers: its biologically active form is L-carnitine ,whereas its enantiomer, D-carnitine, is biologically inactive. In animals, the biosynthesis of carnitine occurs primarily in the liver and kidneys from the amino acids lysine (via trimethyllysine) and methionine. Vitamin C (ascorbic acid) is essential to the synthesis of carnitine.
Acyl carnitine translocase
Carnitine transports long-chain acyl groups from fatty acids into the mitochondrial matrix, so they can be broken down through β-oxidation to acetyl CoA to obtain usable energy via the citric acid cycle . The acyl group on CoA can now be transferred to carnitine and the resulting acylcarnitine transported into the mitochondrial matrix. This occurs via a series of similar steps:
1. Acyl CoA is conjugated to carnitine by carnitine acyltransferase I (palmitoyltransferase) located on the outer mitochondrial membrane
2. Acylcarnitine is shuttled inside by a carnitine-acylcarnitine translocase
3. Acylcarnitine is converted to acyl CoA by carnitine acyltransferase II (palmitoyltransferase) located on the inner mitochondrial membrane . The liberated carnitine returns to the cytosol.
Human genetic disorders, such as primary carnitine deficiency, carnitine palmitoyltransferase I deficiency, carnitine palmitoyltransferase II deficiency and carnitine-acylcarnitine translocase deficiency, affect different steps of this process .
It is widely available as a nutritional supplement:
|Chicken breast||100g||3.9 mg|
|American cheese||100g||3.7 mg|
|Whole milk||100ml||3.3 mg|
|Avocado||one medium||2 mg|
|Cottage cheese||100g||1.1 mg|
|Whole-wheat bread||100g||0.36 mg|
|White bread||100g||0.147 mg|
|Peanut butter||100g||0.083 mg|
|Rice (cooked)||100g||0.0449 mg|
|Orange juice||100 ml||0.0019 mg|
red meat, carnitine rich food
Role in atherosclerosis pathway
There may be a link between dietary consumption of carnitine and atherosclerosis. When certain species of intestinal bacteria were exposed to carnitine from food, they produced a waste product, trimethylamine N-oxide (TMAO), that is associated with atherosclerosis. The presence of large amounts of TMAO-producing bacteria was a consequence of a long-term diet rich in meat . Vegetarian and vegans who ate a single meal of meat had much lower levels of TMAO in their bloodstream than did regular meat-eaters, as vegetarian and vegans had lower levels of the intestinal bacteria that converts carnitine into TMAO . In addition the carnitines exert a substantial antioxidant action, thereby providing a protective effect against lipid peroxidation of phospholipid membranes and against oxidative stress induced at the myocardial and endothelial cell level.
In the study of Intestinal microbiota metabolism of L-carnitine, a nutrient in red meat, promotes atherosclerosis ,intestinal microbiota metabolism of choline and phosphatidylcholine produces trimethylamine (TMA), which is further metabolized to a proatherogenic species, trimethylamine-N-oxide (TMAO). The study demonstrates that metabolism by intestinal microbiota of dietary L-carnitine, a trimethylamine abundant in red meat, also produces TMAO and accelerates atherosclerosis in mice. Omnivorous human subjects produced more TMAO than did vegans or vegetarians following ingestion of L-carnitine through a microbiota-dependent mechanism. The presence of specific bacterial taxa in human feces was associated with both plasma TMAO concentration and dietary status. Plasma L-carnitine levels in subjects undergoing cardiac evaluation predicted increased risks for both prevalent cardiovascular disease (CVD) and incident major adverse cardiac events (myocardial infarction, stroke or death), but only among subjects with concurrently high TMAO levels . Chronic dietary L-carnitine supplementation in mice altered cecal microbial composition, markedly enhanced synthesis of TMA and TMAO, and increased atherosclerosis, but this did not occur if intestinal microbiota was concurrently suppressed . In mice with an intact intestinal microbiota, dietary supplementation with TMAO or either carnitine or choline reduced in vivo reverse cholesterol transport. Intestinal microbiota may thus contribute to the well-established link between high levels of red meat consumption and CVD risk .
To identify the mechanisms by which TMAO might promote atherosclerosis, the study first noted that TMAO and its trimethylamine nutrient precursors are all cationic quaternary amines that could potentially compete with arginine, thereby limiting its bioavailability and reducing nitric oxide synthesis . Recent studies showed that TMAO can promote macrophage cholesterol accumulation in a microbiota-dependent manner by increasing cell surface expression of two proatherogenic scavenger receptors, CD36 and scavenger receptor A . They envisioned three non-exclusive mechanisms through which cholesterol can accumulate within cells of the artery wall: enhancing the rate of influx (as noted above), enhancing synthesis or diminishing the rate of efflux. To test whether TMAO might alter the canonical regulation of cholesterol biosynthesis genes, it loaded macrophages with cholesterol in the presence or absence of physiologically relevant TMAO concentrations. However, TMAO failed to alter mRNA levels of the low-density lipoprotein (LDL) receptor or cholesterol synthesis genes.
They next examined whether TMAO might inhibit cholesterol removal from peripheral macrophages by testing whether dietary sources of TMAO (choline or L-carnitine) inhibit reverse cholesterol transfer (RCT) in vivo .
The dietary nutrient l-carnitine has been studied for over a century. Although eukaryotes can endogenously produce l-carnitine, only prokaryotic organisms can catabolize it. A role for intestinal microbiota in TMAO production from dietary l-carnitine was first suggested by studies in rats (L-Carnitine dissimilation in the gastrointestinal tract of the rat. 1984). Although TMAO production from alternative dietary trimethylamines has been suggested in humans, a role for the microbiota in the production of TMAO from dietary l-carnitine in humans has not previously been demonstrated. The present studies reveal an obligatory role of gut microbiota in the production of TMAO from ingested l-carnitine in humans . They also suggest a new nutritional pathway in CVD pathogenesis that involves dietary l-carnitine, the intestinal microbial community and production of the proatherosclerotic metabolite TMAO. Finally, these studies show that TMAO modulates cholesterol and sterol metabolism at multiple anatomic sites and processes in vivo, with a net effect of increasing atherosclerosis.
Consuming foods rich in l-carnitine (predominantly red meat) can increase fasting human l-carnitine concentrations in the plasma. Meats and full-fat dairy products are abundant components of the Western diet and are commonly implicated in CVD. Together, l-carnitine and choline-containing lipids can constitute up to 2% of a Western diet (Changes in l-carnitine content of fish and meat during domestic cooking. 2008.) (Concentrations of choline-containing compounds and betaine in common foods. 2003)
Numerous studies have suggested a decrease in atherosclerotic disease risk in vegan and vegetarian individuals compared to omnivores; reduced levels of dietary cholesterol and saturated fat have been suggested as the mechanism explaining this decreased risk (Vegetarian diets: what do we know of their effects on common chronic diseases? 2009)
Notably, a recent 4.8-year randomized dietary study showed a 30% reduction in cardiovascular events in subjects consuming a Mediterranean diet (with specific avoidance of red meat) compared to subjects consuming a control diet (Primary prevention of cardiovascular disease with a Mediterranean diet. 2013)
Analyses of microbial composition in human feces and mice cecal contents revealed specific taxa that segregate with both dietary status and plasma TMAO concentrations. Recent studies have shown that changes in enterotype are associated with long-term dietary patterns (Linking long-term dietary patterns with gut microbial enterotypes. 2011)
Recent reports have shown differences in microbial communities among vegetarians and vegans versus omnivores (Origins and evolution of the Western diet: health implications for the 21st century. 2005.).
Of note, we observed an increase in baseline plasma TMAO concentrations in what has historically been called enterotype 2 ( Prevotella ), a relatively rare enterotype that in one study was associated with low animal-fat and protein consumption (Origins and evolution of the Western diet: health implications for the 21st century. 2005.). In our study, three of the four individuals classified into enterotype 2 are self-identified omnivores, suggesting more complexity in the human gut microbiome than anticipated. Indeed, other studies have demonstrated variable results in associating human bacterial genera, including Bacteroides and Prevotella, to omnivorous and vegetarian eating habits (A vegan or vegetarian diet substantially alters the human colonic faecal microbiota. 2012). This complexity is no doubt in part attributable to the fact that there are many species within any genus, and distinct species within the same genus may have different capacities to use l-carnitine as a fuel and form TMA. Indeed, prior studies have suggested that multiple bacterial strains can metabolize l-carnitine in culture (Metabolism of L(-)-carnitine by Enterobacteriaceae under aerobic conditions. 1999) and species within the genus Clostridium differ in their ability to use choline as the sole source of carbon and nitrogen in culture. Our results suggest that multiple ‘proatherogenic’ (that is, TMA- and TMAO-producing) species probably exist. Consistent with this supposition, others have reported that several bacterial phylotypes are associated with a history of atherosclerosis and that human microbiota biodiversity may in part be influenced by carnivorous eating habits (A vegan or vegetarian diet substantially alters the human colonic faecal microbiota. 2011) (Origins and evolution of the Western diet: health implications for the 21st century. 2005.) (Diversity of the human intestinal microbial flora. 2005)
There are only a few reports of specific intestinal anaerobic and aerobic bacterial species that can use l-carnitine as a carbon nitrogen source (Carnitine metabolism and its regulation in microorganisms and mammals. 1998)
Some case reports of carnitine supplementation have reported beneficial effects in individuals with inherited primary and acquired secondary carnitine deficiency syndromes. Carnitine supplementation studies in chronic disease states have reported both positive and negative results (Critical update for the clinical use of L-carnitine analogs in cardiometabolic disorders. 2011.). Oral carnitine supplementation in subjects on hemodialysis raises plasma carnitine concentrations to normal levels but also substantially increases TMAO levels. A broader potential therapeutic scope for carnitine and two related metabolites, acetyl-l-carnitine and propionyl-l-carnitine, has also been explored for the treatment of acute ischemic events and cardiometabolic disorders .
Discovery of a link between carnitine ingestion, gut microbiota metabolism and CVD risk has broad health-related implications. The study reveals a new pathway potentially linking dietary red meat ingestion with atherosclerosis pathogenesis. The role of gut microbiota in this pathway suggests new potential therapeutic targets for preventing CVD. Furthermore, the study has public health relevance, as carnitine is a common over-the-counter dietary supplement. The results suggest that the safety of chronic carnitine supplementation should be examined, as high amounts of orally ingested carnitine may under some conditions foster growth of gut microbiota with an enhanced capacity to produce TMAO and potentially advance atherosclerosis .