In a study published in April in Nature Medicine, US researchers show that the intestinal microflora can process L-carnitine, a nutrient abundant in red meat, to produce a compound linked to the risk of developing cardiovascular disease.
Nowadays, many people are aware that the high level of meat consumption in developed countries is linked to the risk of developing cardiovascular diseases (CVD). However, it is also generally assumed that this is due to the high content in saturated fats and cholesterol of red meat. But what about other factors associated with meat consumption? Is there more to it than saturated fat and cholesterol?
In a previous study1, the same research team had shown that choline (an essential nutrient found in for example eggs, milk or cauliflower) is metabolized by gut bacteria to produce an intermediate compound called trimethylamine (TMA). TMA is then oxidized by enzymes in the liver to form another compound, called trimethylamine-N-oxide (TMAO), which, the team showed, is associated with CVD risk and accelerates atherosclerosis in mice. Atherosclerosis is a chronic inflammatory disease characterized by the accumulation of plaques in arteries; these plaques may detach and induce the formation of blood clots, leading to for example myocardial infarction or stroke.
In their new study2, the researchers hypothesized that red meat may contain another dietary nutrient with a TMA structure that could be processed by gut bacteria to generate TMAO and thus contribute to the known link between high meat consumption and CVD risk.
- L-carnitine, gut bacteria and TMAO
The researchers turned their attention to L-carnitine, a nutrient that is abundant in red meat and possesses a TMA structure like that of choline. Although L-carnitine can be synthesized by mammals (and in fact by all eukaryotic cells), it can only be broken down by prokaryotes (for example the bacteria in our gut). L-carnitine is essential to normal cell function, being involved in the transport of long-chain fatty acids to the mitochondria (the cell’s energy factories).
In a first set of experiments, the researchers showed in five volunteers that ingestion of L-carnitine (by eating a steak and taking a pill) was followed by a peak in L-carnitine blood levels and an increase in TMAO levels some hours later, as the levels of L-carnitine were going down. However, this was not seen if the individuals had been taking antibiotics for a week before the test to suppress their intestinal microflora: although there was still an increase in L-carnitine blood levels, no TMAO was detected. The same phenomenon was observed in mice. These results showed that an intact gut microflora was necessary for TMAO to be produced after ingestion of L-carnitine (either by eating meat or taking a pill as a supplement).
- L-carnitine, TMAO and atherosclerosis
Given its abundance in red meat and the capacity of gut bacteria to metabolize it to produce TMAO, a pro-atherogenic compound, L-carnitine seemed like a good candidate in the search of what could contribute to the association between meat consumption and CVD risk. The researchers therefore looked at whether they could find a link between blood levels of L-carnitine and CVD risk in a cohort of 2595 individuals. They found that high levels of L-carnitine were statistically significantly associated with CVD risk, even after correcting for other known CVD risk factors such as age, sex, smoking or blood pressure. However, this association was dependent on blood levels of the compound TMAO, indicating that it was actually TMAO that underlied the association of L-carnitine with CVD.
Association studies can however only tell whether two factors are correlated or not (that is, whether they happen/change together or not), they do not inform on whether one factor actually affects/causes the other. Therefore, the researchers performed another set of experiments in mice to evaluate the potential involvement of L-carnitine in the development of CVD, more specifically of atherosclerosis. They used the Apoe-/- mouse, a strain that develops atherosclerosis, and fed the mice with either a regular diet, or a diet supplemented with L-carnitine. They observed almost a doubling of the number of atherosclerotic lesions in mice eating the L-carnitine-supplemented diet compared to the control mice. Of note, this increase in plaque formation was not seen when mice were also given antibiotics to suppress their gut microbiota, showing that gut bacteria were required for L-carnitine to have an effect on atherosclerosis development. This necessary contribution of the gut microflora is most likely through the production of TMAO, as treatment with antibiotics abolished the increase in blood TMAO levels that was otherwise seen in mice eating a diet supplemented with L-carnitine.
To further understand how L-carnitine-derived TMAO could promote atherosclerosis, researchers performed additional studies both in vitro and in vivo, and found that TMAO decreases a process called reverse cholesterol transport (which moves the cholesterol away from peripheral tissues and back to the liver) and affects the bile acid synthetic pathway (which is important for cholesterol elimination from the body).
Altogether, this study suggests that the nutrient L-carnitine contained in red meat may be another CVD risk factor associated with high consumption of meat, in addition to saturated fat and cholesterol. Importantly, the data presented by the researchers show that the host’s microbiota plays an essential role in the detrimental effects of dietary L-carnitine by processing it to produce TMAO, a compound that in turn affects cholesterol metabolism and promotes atherosclerosis.
- You are what you eat – or rather, your bacteria are (to some extent)
Another piece of data I found interesting in this study was the fact that long-term dietary habits have a clear visible effect on the composition of our gut microbiota and on how the foods we ingest are transformed. Indeed, when the researchers tested the levels of TMAO in the blood of a long-term vegan who had agreed to eat a steak and take a pill of L-carnitine for the sake of the study, they found virtually no increase in TMAO blood levels, by contrast to what was seen in an individual regularly eating red meat (nearly daily) and tested in the same way. When more vegetarian/vegan individuals were tested, the researchers observed the same reduced capacity to synthetize TMAO from ingested L-carnitine.
Since in their first set of experiments the researchers had shown that the presence of an intact gut microflora was required for TMAO to be produced after ingestion of L-carnitine, they hypothesized that differences in diet between vegans/vegetarians and omnivores over a long period of time could lead to changes in the gut microbiota and result in differences in the capacity to produce TMAO from L-carnitine. They therefore looked at the composition of the gut microflora in individuals who were either long-term omnivores or vegetarians, and found that there were indeed differences in the proportions of the different bacterial species present. The researchers also fed mice with a diet supplemented with L-carnitine for a while and observed changes in the composition of their microbiota compared to mice who had been kept on a normal diet.
Altogether, these data show that long-term dietary habits influence the composition of the gut microbiota, which in turn affects how the foods we ingest are transformed. In the particular case presented in this study, a high consumption of L-carnitine (frequently eating red meat) over a long period of time seems to foster the growth of gut bacteria that are particularly good at processing L-carnitine to produce TMAO. The authors of the study conclude that this process ultimately contributes to the known association between high meat consumption and increase in the risk of developing cardiovascular disease.
1. Gut flora metabolism of phosphatidylcholine promotes cardiovascular disease. Wang Z, Klipfell E, Bennett BJ, Koeth R, Levison BS, Dugar B, Feldstein AE, Britt EB, Fu X, Chung YM, Wu Y, Schauer P, Smith JD, Allayee H, Tang WH, DiDonato JA, Lusis AJ, Hazen SL. Nature. 2011 Apr 7;472(7341):57-63. doi: 10.1038/nature09922.
(Free access here)
2. Intestinal microbiota metabolism of l-carnitine, a nutrient in red meat, promotes atherosclerosis. Koeth RA, Wang Z, Levison BS, Buffa JA, Org E, Sheehy BT, Britt EB, Fu X, Wu Y, Li L, Smith JD, Didonato JA, Chen J, Li H, Wu GD, Lewis JD, Warrier M, Brown JM, Krauss RM, Tang WH, Bushman FD, Lusis AJ, Hazen SL. Nat Med. 2013 Apr 7. doi: 10.1038/nm.3145
Koeth RA, Wang Z, Levison BS, Buffa JA, Org E, Sheehy BT, Britt EB, Fu X, Wu Y, Li L, Smith JD, Didonato JA, Chen J, Li H, Wu GD, Lewis JD, Warrier M, Brown JM, Krauss RM, Tang WH, Bushman FD, Lusis AJ, & Hazen SL (2013). Intestinal microbiota metabolism of l-carnitine, a nutrient in red meat, promotes atherosclerosis. Nature medicine PMID: 23563705