Wednesday, April 10, 2013

Blame the Bacteria

For my previous blog entry, I wrote about the discovery of arterial plaque-buildup in the mummified remains of various ancient populations. With heart disease consistently being the leading cause of death in the United States year after year, it is clear that our relationship with our hardest-working organ has not much improved despite hundreds of years of technological and medical innovation and discovery.

But that’s not to say that progress is non-existent. Doctors and clinical researchers are working hard to uncover the roles of lifestyle, diet, and genetics in the development of heart disease. Still, there are some paradoxes that are hard to explain based on our current understanding of high-cholesterol and fatty diets promoting heart disease.
Take red meat, for example. A food that is high in protein and relatively low in fat and cholesterol. Theoretically these qualities are heart-healthy and a green light for incorporating as a staple into your diet. Why, then, is red meat the first thing to get kicked to the curb when trying to lower blood cholesterol levels? The connection to increased atherosclerosis was identified years ago, but not explained until quite recently, when researchers were able to put the blame on an unlikely source – the bacteria in our guts.
The gut microbiome is essential in the digestion of food and the proper function of our digestive system. It is a dynamic community of bacteria that changes in response to diet, disease, and antibiotic use, among many other factors. Dysfunctions of the microbiome may promote the development of Crohn’s disease or other gastrointestinal issues.
Researchers discovered that bacterial processing of L-carnitine, a molecule found in red meat, into trimethylamine-N-oxide (TMAO) caused increased cholesterol levels by slowing the removal of cholesterol from the blood stream. They confirmed their findings by observing that mice which were fed high levels of L-carnitine had double the risk of developing atherosclerosis. When the same mice were treated with antibiotics (diminishing their gut microbiome) they did not see the increased plaque deposits.
Furthermore, meat-eaters who took L-carnitine were found to have higher levels of TMAO than vegans who had the same treatment, suggesting the difference between their gut bacteria was the determining factor for TMAO levels. Meat-eaters are likely to have more L-carnitine converting bacteria simply because the consumption of meat promotes the growth of these bacteria. Vegans do not have enough of the bacteria to cause significant conversion of the L-carnitine into TMAO.
This research adds further insight towards the role of diet in the development of heart disease and opens up avenues for possible treatment or prevention plans that can hopefully put a dent in America's greatest health problem.

Referenced work:

Koeth RA, Wang Z, Levison BS, et al. Intestinal microbiota metabolism of l-carnitine, a nutrient in red meat, promotes atherosclerosis. Nat Med. 2013;

Credit for cc images:

Posted by Joseph Starrett(3)


  1. With this research, did you happen to come across red meat's connection with any other disorders/diseases. Has it shown any effects on other body parts, like the stomach or intestines?

    posted by Ashley Sterpka (1)

    1. The research I cited focused on the role of L-cartinine in atherosclerosis and therefore I couldn't commment on other health problems based on that paper alone.

      Red meat is a much-debated topic among nutritionists and doctors (partly why this study is so interesting).

      This review paper argues that the effects of meat consumption are inconsistent and may depend more on the preparation of the meat (i.e. processing) than anything else. I recommend reading it if you are interested in heart disease and/or nutrition.

      If you are interested in its effects on gastrointestinal disease and cancer, I would recommend doing a quick search on PubMed. Doing so gave me more nutritional correlational studies than I could list here.

      Joseph Starrett (3)

  2. so do you think that foods that are high in antioxidants would help reduce the effects of the chemicals in other foods such as red meats?
    (Tonya Sulham )

  3. I'm not sure about that. In fact, I don't know much about antioxidants. Here's a review that addresses previous observational findings that increased selenium (antioxidant) intake or levels were correlated with lower risk of cardiovascular disease. Clinical trials involving selenium did not provide strong evidence for any effect on CVD risk, however.
    To be honest, I'm not not sure how antioxidants would relate to red meat specifically.

    Joseph Starrett(3)

  4. Reading this entry made me realize why red meat is so correlated with heart disease. I guess I never understood why, since it is relatively low in fat and cholesterol. It was really informative to learn that cholesterol becomes elevated when L-carnitine is processed into TMAO. However, I wonder what percentage of the population is willing to reduce their intake of red meat to prevent heart disease, especially since so many obese people are unwilling to change their diets considering the health risks. It was interesting to know that vegans don't have significant levels of L-carnitine to cause this elevation in cholesterol.

    Lindsey Dugas (1)

  5. Do you know if anyone has already started to consider developing therapies to antagonize the L-carnitine pathway in these bacteria as a way of lowering cholestrol in red meat eaters? It seems to me that this finding stands to make some drug company a lot of money if they could develop such a drug. What impact do you predict this finding will have on the overall health and obesity crisis in this country? Do you thinbk it sends a somewhat counterproductive message, that medicine will eventually make healthy dieting unnecessary? and if so is that a bad thing?

    Posted by Sean McDougall (2)

    1. I'm not sure what the progress is on possible therapies coming out of this research. The antagonist you mentioned would be great but I imagine it will take time to develop and test something like that. It would be interesting to try and clear the gut flora out completely through broad-scale antibiotics, then do something like this:

      transplanting the "right" bacteria. The L-carnitine processing bacteria would probably become prominent again if the person starts consuming meat after the treatment, though. An antagonist to the conversion pathway would probably be way more practical and effective...if such a drug can be made and found to be safe.

      I think people who are mindful of their health would not see a drug like that as a "get out of jail free" card. Others might see it that way, but there's really not much anyone can do to change the way people treat their bodies. In the end that is a personal choice that people have every right to make. The benefits the drug presents to those who need it would outweigh a few people letting themselves go, in my opinion.

  6. This article has a valid point backed by scientific research but I really don't think eliminating red meat from someones diet is that important towards someone getting heart disease. I know many people that are active and reap many benefits from the nutritional benefits of red meat, yet they don't have high cholesterol. As a matter of fact I know many people who don't eat meat at all, yet have high cholesterol. Maybe a more simple solution to Americas health problem is due to inactivity and going overboard when it comes to portion size.

    Posted by Marshall Moini (2)

    1. If heart disease was simply caused by one factor (i.e. L-carnitine in red meat), I don't think it would be as huge a problem as it is. It seems like our biggest health issues are the ones that have many contributing risk factors and mechanisms of action. Still, we shouldn't impede progress by playing down the baby steps that, piece by piece, may lead to a greater understanding of a disease.

      Additionally, anecdotal evidence has a place in science but it does not offer a strong argument against experiments that try to show causality. Why not cite research that opposes this one's findings, or look for flaws in their methods and design that would refute a cause-effect relationship?

      I agree that claiming L-carnitine and red-meat consumption are the sole causes of heart disease and atherosclerosis is unfounded, but I don't think that was suggested anywhere. It seems like this is just small part within the greater issue of inactivity and poor nutrition, as you suggested.

      Joseph Starrett(3)

  7. It's not so much eating red meat, it's eating large portions of red meat and too small or non existent portions of vegetables along with it, and I don't mean salad with iceberg lettuce, I mean true leafy greens.

    It just means people need to have a well balanced diet, not necessarily excluding red meat all together, but finding a way to manage it.