July 20, 2016
In recent years, terms such as gut health and microbiome seem to have just dropped into our vocabulary. Today, poor gut health is often blamed for being overweight, chronic diseases, illnesses, and even our intelligence. Our microbiome—that is, all the microorganisms, including the commensal and pathogenic bacteria in the human body—plays a complicated and important role in many aspects of our health. And the microbiome in the gut is particularly important.
How does my burger affect my gut?
Choosing the best foods to eat can be confusing. We hear a lot about taking probiotics, prebiotics, kefir and kimchi for better gut health, but food choices may actually be a little simpler. More common foods, such as meats and vegetables appear to play leading roles in the health of our gut as well as our heart. Interestingly enough, the environment in our gut heavily influences how some meat and other animal products are absorbed and digested—and whether we eat more meat or more vegetables influences the variety and number of bacteria we have in our gut and how we respond to the meat we eat.
It is fairly well known that artery-clogging saturated fat and cholesterol are bad for our heart, but the picture of our heart health is ever-expanding. Red meat, processed meat, and animal fats are primary sources of L-carnitine; egg yolks are primary sources of choline in our diet. In and of themselves, L-carnitine and choline have important roles in our health. But the trouble comes when these substances are metabolized in our intestines by gut bacteria to produce a chemical called TMA, which subsequently converts to TMAO. High levels of TMAO are associated with an increase risk of cardiovascular disease and cardiac events, such as heart attacks. [i]
This is where things get interesting. It appears that the type of bacteria we have in our gut determines whether we produce harmful TMAO from L-carnitine and choline. Long-term vegans and vegetarians do not convert L-carnitine and choline to TMAO at the same rate as omnivores. (In fact vegans produce a negligible amount of TMAO.)[ii] And heavy meat-eaters have a different mix of bacteria than those who eat little or no meat and more carbohydrates, particularly vegetables and fiber. The difference is most pronounced comparing omnivores with both vegans and vegetarians.
It turns out that the state of our gut bacteria may reflect our risk for heart disease, but the research is still young. We know that we have a lot of bacteria in our gut—in fact, our intestine houses >1014 microorganisms with important functions, and the microbial composition differs along the digestive tract[iii] . And we know that the bacteria in the gut—our microbiome—seem to gather together by type into different groupings or communities, called enterotypes.
An individual’s microbiome enterotype depends on many factors, but a major factor is long-term food intake. The Bacteriod enterotype is associated with diets that are high in animal protein and fats; the Prevotella enterotype is associated with carbohydrates as well as plant-based diets. It is more of a spectrum than a distinct category. It appears that people who have more heart disease, diabetes, and metabolic syndrome tend toward the Bacteroid enterotype. Differences in these microbial communities may also be responsible for digestive disorders, skin diseases, gum disease and even obesity.[iv]
An important question is what came first, the meat or the microbiome? Diet changes explain 57 percent of structural variation in gut microbiome, while DNA explains only 12 percent.[v] Our long-term eating habits, rather than what we ate yesterday, matter when it comes to the state of our microbiomes. Although the composition of bacteria in our gut will change quickly toward a healthier mix of bacteria even after one week on a plant-based diet, it will revert back as soon as the person returns to his or her usual diet. The enterotype reflects dietary habits over years —even decades—not months. [vi]
There are other factors at play as well when it comes to gut health and diet. A rich variety of bacterial genes is associated with health, whereas lower bacterial richness is associated with obesity, type 2 diabetes, inflammation and dyslipidemia. Increased consumption of fruits and vegetables in higher-fiber diets and calorie-restricted diets are both associated with increased bacterial gene richness.[vii] [viii] Omega-3 fatty acids may also have a roll in a healthier mix of bacteria and preventing these associated chronic conditions.[ix]
Diets rich in complex carbohydrates reveal fewer pathogenic species than diets higher in fat and protein. The altered microbiome of vegetarians could also reflect a higher fiber intake and the presence of more short-chained fatty acids (SCFAs), which are produced when intestinal bacteria metabolize indigestible carbohydrates (fiber). SCFAs regulate immune function, hormone production and formation of fats; they also decrease the pH in intestines, thus preventing the growth of potentially pathogenic bacteria such as E. coli.
In summary, while it may not inspire anyone to become a vegetarian, there are some compelling gut-related reasons to eat a lot more vegetables and fiber and a lot less meat:
- Vegans appear to have the lowest levels of TMAO even after L-carnitine; therefore, increased vegetable and fruit intake relative to red meat may be the simplest and safest recommendation for reducing TMAO levels.
- When you have a healthy microbiome, eating small amounts of red meat and animal fats may not have the same negative effects.
- Looking at several different aspects, a healthy gut reflects long-term diet choices toward more vegetables, fruits and whole grains and less red meat and animal fats.
So, for a happy gut, reach for the veggie burger!
[i] Mendelsohn A.R., Larrick J.W., Dietary Modification of the Microbiome Affects Risk for Cardiovascular Disease; Rejuvenation Research. June 2013, 16(3): 241-244. http://online.liebertpub.com/doi/full/10.1089/rej.2013.1447
[ii] Koeth R.A., Wang Z., Levison B.S., Buffa J.A., Org E., Sheehy B.T., Britt E.B., Fu X., Wu Y., Li L. Intestinal microbiota metabolism of l-carnitine, a nutrient in red meat, promotes atherosclerosis. Nat. Med. 2013;19:576–585.
[iii] Fouhy, F., Ross, R. P., Fitzgerald, G. F., Stanton, C. & Cotter, P. D. Composition of the early intestinal microbiota: knowledge, knowledge gaps and the use of high-throughput sequencing to address these gaps. Gut Microbes 3, 203–220 (2012).
[iv] American Society for Microbiology. (2008, June 5). Humans Have Ten Times More Bacteria Than Human Cells: How Do Microbial Communities Affect Human Health?; ScienceDaily. Retrieved April 14, 2016 from www.sciencedaily.com/releases/2008/06/080603085914.htm
[v] Brown, K., DeCoffe, D., Molcan, E., & Gibson, D. L. (2012). Diet-Induced Dysbiosis of the Intestinal Microbiota and the Effects on Immunity and Disease. Nutrients, 4(8), 1095–1119. http://doi.org/10.3390/nu4081095
[vi] Wu, G Chen J, et. al; Linking Long-Term Dietary Patterns with Gut Microbial Enterotypes; Science 07 Oct 2011; Vol. 334, Issue 6052, 105-108
[vii] Cotillard A., Kennedy S.P., Kong L.C., et al. Dietary intervention impact on gut microbial gene richness; Nature, 500 (2013), pp. 585–588
[viii] E. Le Chatelier, T. Nielsen, J. Qin, et al.; Richness of human gut microbiome correlates with metabolic markers; Nature, 500 (2013), pp. 541–546
[ix] Noriega, B. S., Sanchez-Gonzalez, M. A., Salyakina, D., & Coffman, J. (2016). Understanding the Impact of Omega-3 Rich Diet on the Gut Microbiota. Case Reports in Medicine, 2016, 3089303. http://doi.org/10.1155/2016/3089303