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Monday, April 8, 2013

Red meat and gut bacteria: partners in heart crime

We're used to thinking of nutrition the way we think about computers - input and output. For example, you eat carrots, you get lots of vitamin A. You eat strawberries, you get vitamin C. If you're deficient in a vitamin, take a supplement... or eat more foods containing that vitamin.

So far so good. Only, now science is teaching us it's more complicated.

For one thing, the exact nutrients you need may depend on your genes: see this article, "The nutrigenomics frontier". Which makes the ubiquitous "recommended daily intake" (RDI) nothing more than a ballpark estimate.

But there's another emerging facet of nutrition. There's increasing evidence that if we're going to make use of the nutrients we ingest through asparagus or chick peas or any other food, the body has to act on those nutrients. Sometimes the body's actions make things better for us, and sometimes they produce a bad result.

Hamburger (source: Wikimedia Commons,

Let's take red meat. Recently in Nature Medicine, a study found that omnivores who took a compound (found in red meat) called L-carnitine had higher markers of arterial plaque buildup than vegans or vegetarians who also took the compound. It also happened that the omnivores had very different gut bacteria, as measured through fecal samples, than the vegans/vegetarians. That would be correlation, not causation.

Enter the mouse part of the study. Here, the researchers found that arterial plaques increased in mice with normal gut bacteria when they were given L-carnitine. The red meat compound and the gut bacteria appeared to be partners in contributing to heart disease.

As a point of comparison, other L-carnitine-eating mice were given antibiotics that cleared their gut bacteria. Arterial plaques did not increase in these mice.

What are we left with? Certainly NOT that we should pop antibiotics alongside our red meat in order to prevent heart disease. (For reasons why antibiotics are not the answer, check out this article by Carl Zimmer.)

We're left with a caution against red meat, I guess. An explanation of why vegans and vegetarians might have a lower risk of heart disease. But the research is preliminary, so personally I'm not going to swear off burgers forever. 

Most importantly, I think we're left with a picture of nutrition that's about more than just input. It's a picture of an ecosystem, where each thing interacts with other things to produce an output - in this case, heart disease. Gut bacteria seems to be a big player in these actions. In this article, Stanley Hazen, head of cardiovascular medicine at the Cleveland Clinic, says: "Bacteria make a whole slew of molecules from food... and those molecules can have a huge effect on our metabolic processes."

One more thing: after reading this study, I wouldn't be too quick to take L-carnitine supplements. Certain gut flora might love them... but it's possible that could have negative results. Koeth, R., Wang, Z., Levison, B., Buffa, J., Org, E., Sheehy, B., Britt, E., Fu, X., Wu, Y., Li, L., Smith, J., DiDonato, J., Chen, J., Li, H., Wu, G., Lewis, J., Warrier, M., Brown, J., Krauss, R., Tang, W., Bushman, F., Lusis, A., & Hazen, S. (2013). Intestinal microbiota metabolism of l-carnitine, a nutrient in red meat, promotes atherosclerosis Nature Medicine DOI: 10.1038/nm.3145


  1. I'm a nutritionist turned novelist. I thought the link between gut bacteria and weight control was so interesting that I wrote a mystery MURDER: A NEW WAY TO LOSE WEIGHT. The diet doctors in the novel are altering the gut bacteria in obese subjects to see if they can lose weight faster and keep it off longer. You might learn a little science and laugh a lot as you follow the twists of this plot.
    JL Greger

  2. Hi Kristina, I just discovered your blog and I am fascinated. Gut flora is (are?) a whole new thing in my ongoing health education.

    But the mouse study confuses me. If L-carnitine produces arterial plaques in them, but not in vegans/vegetarians, then how is it implicated in plaque build up?

    Is there a study that compares vegans who dont' take L-carnitine with vegans who do?

  3. I really wish they'd control for more variables than vegetarian vs omnivore. Like... say people who eat equivalent amounts of veggies with or without meat. I tend to think high vegetable consumption is the distinguishing bit more than meat consumption, per se.

    These studies are ALWAYS this way though, they never make the distinctions I would make. Almost all of the "high fat" studies on mice use soy and corn oil, which makes them high omega 6 studies, that I'd like to see compared to high saturated fat and/or high omega 3 controls.