 We are walking communities comprised not only of a Homo sapiens host, but also of trillions of symbiotic commensal microorganisms within the gut and on every other surface of our bodies. There are more bacterial cells in our gut than there are human cells in our entire body. In fact, only about 10% of the DNA in our body is human. The rest is in our microbiome, the microbes that we share this walking community we call our body. What do they do? Our gut microbiota, our gut bacteria microbiome, serve as a filter for our largest environmental exposure, what we eat. Technically speaking, food is a foreign object that we take into our bodies by the pound every day, and the microbial community within each of us significantly influences how we experience those meals. Hence, our metabolism and absorption of food occurs through this filter of bacteria. But if we eat a lot of meat, poultry, fish, milk, cheese, eggs, we can foster the growth of bacteria that convert the choline and carnitine in these foods into TMA, trimethylamine, which can be oxidized into TMAO and wreak havoc on our arteries, increasing our risk of heart attack, stroke, and death. We've known about this troublesome transformation from choline into trimethylamine for over 40 years, but that was way before we learned about the heart disease connection. Why were they concerned back then? Because these methyl means might form nitrosamines, which have marked carcinogenic activity, cancer-causing activity. So where is choline found in our diet? Mostly from meat, eggs, dairy, and refined grains. The link between meat and cancer probably wouldn't surprise anyone. In fact, just due to the industrial pollutants alone, like PCBs, children probably shouldn't eat more than like five servings a month of meats like beef, pork, or chicken combined. But what about cancer and eggs? Studies going back to the 70s hinted at a correlation between eggs and colon cancer, but that was just based on so-called ecological data, showing that countries that ate more eggs tended to have higher cancer rates, but that could be due to a million things. It needed to be put to the test. This started in the 80s, and by the 1990s, 15 studies had been published, 10 suggesting a direct association between egg consumption and colorectal cancer, and five showing no association. By 2014, there were dozens more studies published confirming that eggs may indeed be playing a role in the development of colon cancer, though no relationship was discovered between egg consumption and the development of pre-cancerous polyps, which suggests that eggs might be involved more in the promotional stage of cancer growth accelerating cancer growth rather than initiating the cancer in the first place. Which brings us to 2015. Maybe it's the TMAO made from the choline and meat and eggs that's promoting cancer growth, and indeed in the Women's Health Initiative study, women with the highest TMAO levels in their blood had approximately three times greater risk of rectal cancer, suggesting TMAO levels may serve as a potential predictor of increased colorectal cancer risk. Though there may be more evidence for elevated breast cancer risk with egg consumption than prostate cancer risk, the only other study to date on TMAO and cancer looked at prostate cancer and did indeed find a higher risk. Diet has long been considered a primary factor in health. However, with the microbiome revolution of the past decade, we've begun to understand how diet can affect the back and forth between us and the rest of us inside. And the whole TMAO story is like a smoking gun in gut-bacteria disease interactions. Since choline and carnitine are the primary sources of TMAO production, the logical intervention strategy might be to reduce meat, dairy, and egg consumption. And if we eat plant-based for long enough, we can actually change our gut microbial communities such that they may not be able to produce TMAO even if we try. The theory of you are what you eat is finally supported by scientific evidence. We may not have to eat healthy for long, though, soon. We may yet be able to drug the microbiome as a way of promoting cardiovascular health.