Obesity and type 2 diabetes mellitus are influenced both by genes and lifestyle. That is not news. However, the genes in the human microbiome also may play an important role, and that is news.

It has been known for decades that gut bacteria synthesize essential vitamins and amino acids and help degrade toxins. During the past decade, it has become clear that the influence of the microbiome on health may be even more profound.

Beginning at the moment of birth, each human increasingly coexists with microbes. By the time individuals reach adulthood, they are colonized by many more microbial cells than the roughly 13 trillion human cells. More important still, these microbial cells (the microbiota), collectively, have exponentially more genes (the microbiome) than do human cells, around 250 to 800 times more.