when gut microbes from easygoing, adventurous mice are transplanted into the guts of anxious and timid mice, they become more adventurous. The expression “thinking with your gut” may contain a larger kernel of truth than we thought.
So why haven’t we evolved our own systems to perform these most critical functions of life? Why have we outsourced all this work to a bunch of microbes? One theory is that, because microbes evolve so much faster than we do (in some cases a new generation every 20 minutes), they can respond to changes in the environment — to threats as well as opportunities — with much greater speed and agility than “we” can.
This plasticity serves to extend our comparatively rigid genome, giving us access to a tremendous bag of biochemical tricks we did not need to evolve ourselves. “The bacteria in your gut are continually reading the environment and responding,” says Joel Kimmons, a nutrition scientist and epidemiologist at the Centers for Disease Control and Prevention in Atlanta. “They’re a microbial mirror of the changing world. And because they can evolve so quickly, they help our bodies respond to changes in our environment.”
Unlike most tissues, which take their nourishment from the bloodstream, epithelial cells in the colon obtain much of theirs from the short-chain fatty acids that gut bacteria produce as a byproduct of their fermentation of plant fiber in the large intestine.
The less a food is processed, the more of it that gets safely through the gastrointestinal tract and into the eager clutches of the microbiota. Al dente pasta, for example, feeds the bugs better than soft pasta does; steel-cut oats better than rolled; raw or lightly cooked vegetables offer the bugs more to chomp on than overcooked, etc. This is at once a very old and a very new way of thinking about food: it suggests that all calories are not created equal and that the structure of a food and how it is prepared may matter as much as its nutrient composition.