Vegans, bacteria and heart disease: The generality of indirect genetic effects

I don’t spend much time reading Nature Medicine, and I imagine most ecologists are similar to me in that regard. However, I ran across an article from that journal that’s gotten a lot of play in the popular media, and it got me thinking about some concepts from ecology and evolutionary biology, and their relevance to disease treatment.

First, let’s take a step back. You have trillions of microbes living and reproducing inside of you, right now. This may not surprise you, but if it does, don’t worry – your life would be much less pleasant if they were all wiped out. In particular, your digestive system is teeming with microbes that help you digest and process food. These microbes, of course, have genes that determine their traits and how well they function as digestive assistants. They also have very short lifespans, so the microbial communities inside of you can evolve in response to your diet and other activities. With that out of the way, let’s move on to the study.

With the least possible offense to vegans and vegetarians, let’s assume you like a tasty steak every once in a while. However, you’ve probably heard about the links between red meat and heart disease. The Nature Genetics study that I mentioned above has provided a much clearer picture of why these links exist. Red meat contains a compound called carnitine that, when broken down by your gut microbes, forms trimethylamine-N-oxide (TMAO). In turn, TMAO causes plaque to build up and clog arteries. But here’s where it gets really interesting – the gut microbes of vegans and vegetarians didn’t break down carnitine into TMAO, or at least didn’t do so as fast as the microbes in meat-eaters. This suggests that gut microbial communities have evolved in response to their host’s diet.

Another implication of this research suggests that, in the absence of microbes that break down carnitine, you could eat red meat with less risk to your health. As a quick aside, a burgeoning field called “consumer microbiomics” may be able to help you alter your gut microbial community – and there are a lot of reasons that may be desirable that go beyond the red meat-heart disease link.

As a researcher who works at the intersection of evolution and ecology, this study made me think about indirect genetic effects (IGEs, which occur when your phenotype is affected by the expression of genes in an interacting species. There’s a big literature of IGEs that suggests they are responsible for, among other things, the promotion of genetic diversity in plant communities and the evolution of behavior in social animals. The results of the Nature Genetics study can also be interpreted as an IGE because the genetic composition of your gut microbial community affects part of your phenotype (specifically, how much plaque may be clogging up your arteries). I’d argue that, because they operate at so many different levels of organization, IGEs are a general evolutionary mechanism that deserves more consideration. So, what’s the take-home message? The days of assuming that we know an organism’s phenotype (how productive a tree is, or how susceptible to a disease a human is) just because we know its genotype should be a thing of the past, because the world is full of indirect genetic effects (whether we’re talking about a roadside field or a human’s digestive system) and as ecologists and evolutionary biologists we should be careful to remember this.

Read the paper by Robert Koezen and a host of others including senior author Stanley Hazen at: http://www.nature.com/nm/journal/vaop/ncurrent/full/nm.3145.html

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