A paper by Robert A. Koeth and colleagues was published in the Journal Nature Medicine on the 7th April 20131. The authors reported on a series of experimental studies (in both humans and mice) that looked at the effect of an amino acid called L-carnitine (found in red meat, dairy products and used in some dietary supplements) and heart disease risk. The abstract of the paper is available here.
The authors, who were from Cleveland Clinic in Ohio, USA, investigated whether naturally occurring bacteria in the gut converted L-carnitine into a waste product called TMAO (trimethylamine-N-oxide). It is thought that TMAO increases the formation of plaques in the arteries (known as atherosclerosis), a risk factor for heart disease.
The study does provide some evidence of a link (but not a causal association) between L-carnitine found in red meat and TMAO level (which is linked to heart disease risk).
Dietary habits (for example, being a vegan or vegetarian compared to an omnivore or carnivore) have been associated with significant alterations in intestinal microbiota composition. The researchers showed that the meat-eaters in their study had a different intestinal microbe composition compared to the vegetarians or vegans. The meat-eating volunteers produced more TMAO than vegans or vegetarians following the ingestion of L-carnitine. The authors reported that vegans and vegetarians produced less TMAO from L-carnitine ingestion. This is likely to be due to the bacterial strains present in the gut of meat-eaters having a greater capacity to synthesise TMAO from L-carnitine, compared to the gut microbes present in vegans and vegetarians.
It should be noted that volunteers in this study were fed a large amount of L-carnitine (430mg) in the form of an 8oz (227g) sirloin steak (corresponding to an estimated 180mg of L-carnitine) and a supplement containing 250mg L-carnitine. The UK Department of Health advises that adults who eat more than 90g of red and processed meat a day should reduce their intake to 70g per day (cooked weight) on average. Eating this amount of red meat would mean that your intake of L-carnitine will be minimal and therefore not at the level of risk seen by this research, which looked at much higher levels of L-carnitine consumption2.
L-carnitine is also available in supplements marketed as both a weight loss and body building tool as it is suggested that it can help convert fat into muscle. However, there is no strong scientific evidence to support these suggestions.
The researchers suggest that the safety of chronic L-carnitine supplementation should be examined, as high amounts of L-carnitine may, under some conditions, foster growth of gut microbiota with an enhanced capacity to produce TMAO and potentially advance the process of atherosclerosis. More research is needed to investigate these associations further.
1. Koeth et al. (2013) Intestinal microbiota metabolism of L-carnitine, a nutrient in red meat, promotes atherosclerosis Nature Medicine. Published online 7th April 2013.
2. NHS Choices. Red meat chemical link to heart disease. Available at: http://www.nhs.uk/news/2013/04April/Pages/Red-meat-chemical-link-to-heart-disease.aspx Accessed on: 15th April 2013.