The name carnitine comes from the Latin for flesh (carnus) because it was first isolated from meat. Carnitine is found in nearly all cells of the body, it is produced from the amino acids lysine and methionine. It is involved in fat metabolism, transporting fatty acids (fuel) into mitochondria (the cell’s powerhouse) to produce energy. This is why carnitine is sold as supplements and used in energy drinks.
We have no dietary requirement for carnitine; we can make all we need in the liver and kidneys, but so do animals – and that is where the problem lies. All meat, including chicken, contains carnitine. Meat is not the only dietary source of carnitine; cow’s milk, cheese, wholegrain products and asparagus also contain it, but in much smaller concentrations, and as stated, it is a common ingredient of energy drinks.
Many studies show that vegetarians and vegans have a lower risk of CVD compared to meat-eaters with the inferred mechanism being a lower intake of dietary cholesterol and saturated fat (Fraser, 2009; Key et al., 1999). However, a recent study suggested an additional explanation as to why meat intake may be related to mortality or early death (Koeth et al., 2013). This research suggests that people who eat meat have a different type of bacteria in their gut compared to vegetarians and vegans which breaks down carnitine to produce a substance linked to CVD.
It is known that certain gut bacteria use carnitine as an energy source, breaking it down and producing a waste product called trimethylamine. The liver converts this into another substance called trimethylamine-N-oxide (TMAO), which is excreted in urine. TMAO alters cholesterol metabolism in the gut, liver and artery walls increasing the build-up of cholesterol and decreasing the removal of cholesterol from artery walls – a double whammy! This causes a build-up of plaques on the artery walls that can lead to atherosclerosis and CVD.
The study by Koeth et al. found that meat-eaters produce higher levels of TMAO than vegans after they are fed carnitine, suggesting that they have more TMAO-producing bacteria in their gut. An unexpected finding was the huge difference in TMAO production between meat-eaters and vegans both before and after the ‘carnitine challenge tests’ (where carnitine was provided in two forms: a 250mg supplement and an eight ounce sirloin steak). One vegan participant who took the test showed a lower level of TMAO to start with – indicating a lower capacity to produce this harmful substance, then following ingestion of carnitine showed no increase either. On the other hand the meat-eaters responded with a spike in TMAO production. Results suggest that vegans don’t have TMAO-producing bacteria in their gut.
This makes sense as people who eat a lot of fibre appear to have more fibre-consuming bacteria. In other words – the diet we choose determines the type of gut bacteria we end up with. Reports have shown significant differences in microbial populations among vegans and meat-eaters (Cordain et al., 2005). More than 100 trillion microorganisms live in our gut, mouth, skin and other mucosal surfaces of our bodies. These microbes offer numerous beneficial functions and understanding our ‘microbiome’ is fast-becoming the next big thing in nutrition and health research.
The authors of the carnitine study suggest that the safety of carnitine supplementation should be examined, because consuming high amounts of carnitine may under some conditions prime our gut bacteria with enhanced capacity to produce TMAO and potentially promote atherosclerosis (Koeth et al., 2013).