Wednesday, June 26, 2013

Meat, Dairy, and Mortality: TMAO

Recently two articles in the New York Times shocked many of its readers. In review, the Times reported on the findings of two different studies conducted at the Cleveland Clinic and found a new and startling connection between diet and heart disease. 

Specifically, and unsurprisingly, the link was established between the consumption of animal products and the increased risk in mortality from heart disease. While the results of the work by researchers such as Dr. Caldwell Esselstyn has shown for years that those following a plant-based diet have a significantly reduced risk of heart disease (America’s number one killer), until these studies, it was not known exactly why that was. 

Basically it goes like this: within 24 hours after the consumption of carnitine, a chemical found naturally in meats, certain gut bacteria metabolize it into a more toxic chemical known as trimethylamine. Trimethylamine gets oxidized in our livers and converted into Trimethylamine-Oxide (TMAO) which circulates throughout our blood stream. 

The problem with TMAO is that it increases the buildup of cholesterol and plaque in our arteries. This, of course, greatly increases the risk of having a heart attack, stroke, aneurism, and a few other un-pleasantries.

Now, our bodies naturally produce carnitine, however that doesn’t get turned into TMAO. It is only when we ingest outside sources that the carnitine ends up in our guts and gets oxidized. Carnitine is found in very limited amounts in plants, but animal products are human's major source of the chemical. So obviously we should stay away from the greatest sources of canitine which is red meat- so think about avoiding all forms of cow.

But avoiding red meat is not enough. The problem is that a second chemical known as choline can also be oxidized into TMAO when ingested due to the similarity in structure between carnitine and choline. According to the research, “eggs, milk, liver, red meat, poultry, shell fish, and fish are all major dietary sources for choline, and hence TMAO production.” 

Now for the really interesting part: the researchers discovered the oxidation process doesn’t occur until these chemicals reached the microbial-gut bacteria. They discovered this by introducing a potent antibacterial to the gut, wiping out all of the microbial-gut bacteria and then measuring the production of TMAO after consuming animal products. The result was that no TMAO was produced when animal products were consumed. However, those same people were fed a steak a second time a few weeks later (after the gut was able to restore some of its habitat) and the production of TMAO skyrocketed after the meal was consumed. 

However, when the researchers got a “vegan” (or perhaps more accurately someone who avoids all animal products but apparently has no moral objection to consuming them in the name of science) to eat an 8oz sirloin steak, there was no increase in TMAO production- even without the antibiotics. This was explained by the differences in microbial communities between vegans and omnivores. Basically, when you eat certain foods, certain bacteria will grow in your gut. When you change your diet and start eating completely different foods, the make up of your floral gut will also change. Since the vegan hadn't been regularly consuming animal products, they did not have the bacteria that oxidizes into TMAO. This suggest, among other things, that those eating a plant-based diet will have a healthier floral gut.

Even if you were to eat a completely plant-based diet, you’re not 100% safe. Most energy drinks contain large amounts of carnitine in them. The weight-loss supplement lecithin contains choline and the regular consumption of these products could result in the development of certain bacteria in the gut that will oxidize TMAO, and thus, consuming these products regularly can increase the risk of heart disease.

As is so often the case with linking diet to disease and mortality, the consumption of these foods impacts more than just our hearts. The regular consumption of eggs was also found to increase the risk of prostate cancer in men. In a study looking at the diet of men who were already diagnosed with cancer, those who consumed more eggs had a rate of progression of prostate cancer by more than 2 to 1. According to researchers from Harvard University, the “plausible mechanism that may explain our observed association between eggs and prostate cancer progression is high dietary choline.” 

So not only did consuming eggs increase their risk, and potentially helped cause them to get prostate cancer in the first place, continuing to eat the eggs actually helped to spread the cancer from its isolated area in the prostate to the rest of the body. Once this happens, survival rates plummet down to 1 in 3. According to one study, those who ate 1 egg every few days had “an 81% increased risk of lethal prostate cancer compared to men who consumed less than .5 eggs per week.”

While the New York Times articles attempted to put their readers at ease by stating that a new drug is being worked on to target and wipe out the bacteria that oxides these chemicals, it seems to be far more simple, and reasonable to limit- or better yet- completely eliminate the consumption of animal foods.  

Further reading:

Kolata, Gina, “Culprit in Heart Disease Goes Beyond Meat’s Fat.” NYT April 7, 2013.

Z. Wang, E. Klipfell, B. J. Bennett, R. Koeth, B. S. Levison, B. Dugar, A. E. Feldstein, E. B. Britt, X. Fu, Y.-M. Chung, Y. Wu, P. Schauer, J. D. Smith, H. Allayee, W. H. W. Tang, J. A. DiDonato, A. J. Lusis, S. L. Hazen.Gut flora metabolism of phosphatidylcholine promotes cardiovascular disease”. Nature 2011 472(7341):57 – 63.

J. E. Lee, E. Giovannucci, C. S. Fuchs, W. C. Willett, S. H. Zeisel, E. Cho. “Choline and betaine intake and the risk of colorectal cancer in men. Cancer Epidemiol.” Biomarkers Prev. 2010 19(3):884 – 887.

Koeth RA, Wang Z, Levison BS, Buffa JA, Org E, Sheehy BT, et al.. “Intestinal microbiota metabolism of l-carnitine, a nutrient in red meat, promotes atherosclerosis.” Nat Med. 2013 Apr 7. 

E. L. Richman, S. A. Kenfield, M. J. Stampfer, E. L. Giovannucci, J. M. Chan. “Egg, red meat, and poultry intake and risk of lethal prostate cancer in the prostate-specific antigen-era: Incidence and survival.” Cancer Prev Res (Phila) 2011 4(12):2110 - 2121

Dr. Greger also has an excellent video summarizing much of this research and many other studies on this top.


  1. "it seems to be far more simple, and reasonable to limit- or better yet- completely eliminate the consumption of animal foods." Exactly!!! The mindset of just creating another pill to patch the problem is so frustrating, when the cause can be so easily removed by not eating animal products.

    Thanks for explaining the science! Great read!

  2. People don't want to change the way they eat; they want to continue to do what they've been doing and just take a pill. The question is: how do we change their minds?