Although our own genes are generally fixed, each of us carries around the genes embedded inside 100 trillion microorganisms. Those microbes outnumber our own cells by a factor of 10 to one, making us "super-organisms," Evans said, composed of our own genes and the genes of our microbiome.
There are thousands of different kinds of bacteria that can make up the microbiome, and in recent years, studies have revealed that what we eat influences the kinds of microbes that thrive in our guts.
Two recent papers published in the journal Nature, for example, showed that high-fat, high-calorie diets were linked with simple microbiomes that were low in genetic diversity, and people with less diversity in their guts had more inflammation, more insulin-resistance and other signs of metabolic disease. When people switched to a low-fat healthy diet, the diversity in their guts increased.
That raises a chicken-and-egg question. Does becoming overweight change the microbiome? Or do changes in the microbiome drive weight gain?