Exploring an important feedback loop
The bacteria in our digestive tract interact with our diet in complex ways. What we eat affects gut bacteria, and gut bacteria affect how we metabolize what we eat. Some gut bacteria process carnitine, found in abundance in red meat, into a substance called trimethylamine-N-oxide (TMAO).
Researchers found that TMAO, which forms after carnitine intake, contributed to atherosclerosis in mice, prompting them to look at how carnitine is metabolized into TMAO in people.
The carnitine challenge
Researchers enrolled five people, including one long-term vegan, and four omnivores--people who reported eating meat (beef, venison, lamb, mutton, duck or pork) nearly daily--into several "carnitine challenges." For the challenge, study participants (including the vegan) ate an 8-ounce steak providing 180 mg of carnitine, and took a 250 mg carnitine capsule.
Compared with blood and urine carnitine and TMAO levels before the challenge, levels after the challenge:
increased significantly in the omnivores
did not change measurably in the vegan participant
did not change after the omnivores had taken antibiotics for one week to decrease the bacteria in their guts, and
again increased significantly in the omnivores after several weeks off of antibiotics, after the gut bacteria had time to recover.
These results suggest that if certain types of gut bacteria are absent from the body, carnitine may not be processed into TMAO, and that differences in gut bacteria between the vegan and the omnivores affected TMAO formation.
The researchers also evaluated blood carnitine and TMAO levels in 2,595 people who had undergone cardiac evaluation. Although high TMAO blood levels were associated with more advanced cardiovascular disease, and a higher likelihood of having experienced a heart attack, stroke, or death, a closer look at the details suggests that TMAO, rather than carnitine, was the primary reason for the association between carnitine levels and cardiovascular risk.
Complex analyses yield complex answers
Red meat has long been considered a contributor to atherosclerosis due to its saturated fat content, so whether or not carnitine is an additional factor in developing atherosclerosis, it is prudent to follow the standard advice of health experts and eat meat in moderation--no more than a couple of 3-ounce servings per week--for best heart health.
Carnitine's role in all this is less straightforward. While this research suggests that carnitine may promote atherosclerosis, Alan Gaby, MD, Chief Medical Editor of Aisle7, submits that more information is needed to interpret these results, pointing out, "Carnitine supplements have been successfully used therapeutically in people with angina, intermittent claudication, and congestive heart failure, and have been shown to improve the odds of survival when given after a heart attack." Interpreting these latest results, he notes that "carnitine might conceivably be harmful for a subset of people with a certain type of gut bacteria, but this study is very preliminary, and it doesn't provide enough evidence to justify dropping carnitine supplements for people who can benefit from them."
While the debate continues, in addition to limiting your meat intake, consider the following sensible advice to create your heart healthy, self-care plan:
- Track your ticker. Talk with your doctor about your blood pressure and cholesterol numbers, and if needed, come up with ways to ensure they stay in a healthy range.
- Dig deeper. Consider getting a high-sensitivity, C-reactive protein (hsCRP) blood test, which can add to a more complete picture of your heart disease risk.
- Take smart steps. Take sensible measures to protect heart health: exercise regularly, eat plenty of vegetables, fruit, legumes, and whole grains, and keep your weight in a healthy range.
- Ask the expert. If you are interested in, or already taking carnitine supplements, talk to your doctor about any concerns you have about heart disease risk.
(Nat Med 2013; doi: 10.1038/nm.3145)