To be healthy, humans need to think small. The tiny microbes in their digestive system provide a big boost to the health of their body, a scientist says. The actions of those microbes — known as the gut microbiome — are important to a person's overall health. Here's a short version of how they work. Grains such as wheat are a vital source of dietary fiber, but humans don't have the right enzymes to break that fiber down. Those enzymes belong to the microbes in their gut that break down the fiber. The resulting short-chain fatty acids are important fuel sources for intestinal cells, help lower blood pressure and improve heart health, and even travel to the brain and improve a person's mood.