August 14, 2014

Learn how the human digestive system functions and how healthy and diverse gut flora play a part.

Humans, and most animals, have a tube-like digestive system. Food goes in our mouth and then travels along a twisting 30-foot-long chute. While the mechanics of this chute play a big role in breaking our food into nutrients, it's mostly the work of tiny microbes.

Gut flora is a growing area of study for scientists like Jeff Leach, profiled in our July/Aug issue. [See "The Wild World Within"]

We heavily rely on the trillions of living bacteria cells every step along the way. In fact, the human body is only about 25% human cells-most of our cells are bacterial.

To understand how digestion works, we thought it would help to see how it all comes together in the human digestive system.

-Gretel H. Schueller

Location: Your Mouth

In the first phase of digestion, your teeth tear up food into smaller bits. Saliva and enzymes help break it down into a slurry.

Your Mouth's Bacteria: It's not just food in there. Emerging science shows the diverse community of creatures in your mouth may help you maintain oral health by preventing cavities and gum disease.

Location: Your Stomach

The stomach is like a powerful washing machine that churns food around and bathes it in enzymes and powerful gastric acid. This low pH is necessary to activate pepsin, an enzyme that breaks down protein. The acid bath helps kill off most-but not all-things that may be living in your food.

Some bacteria-both good and bad-can survive in your stomach, depending on their structure, how much you've eaten or if your stomach is not producing enough acid. Because stomach acid is so corrosive, your stomach lining secretes a thick mucus layer to protect itself.

Location: Small Intestine and Enzymes

Coiled just below your belly button, the 15- to 20-foot-long small intestine is where nutrient absorption in your digestive tract begins. Enzymes do the heavy lifting here. Bile (made in the liver), pancreatic enzymes and other digestive enzymes produced by the inner wall of the small intestine help break down your food.

Most of those enzymes come from bacteria. For example, just one bacterial species in our gut, Bacteroides thetaiotaomicron, possesses more than 260 carb-chomping enzymes. In contrast, our bodies contain less than 20 carbohydrate-digesting enzymes.

Location: Intestinal Lining

The lining of the small intestine is covered with millions of microscopic, fingerlike projections called villi, which increase surface area to better absorb nutrients, which are then sent off into your body. If you were to stretch out all the folds of your small intestine it could cover a tennis court!

Large Intestine and Hungry Bacteria

When food reaches the large intestine, a.k.a. the colon, this is where the microbial magic really happens. At up to 100 billion to 1 trillion bacterial cells per millimeter, the colon is home to the microbial density in our body-and the highest known on Earth!

Those tiny bacteria critters help digest the remaining food-feasting on fibers, fermenting them and creating crucial byproducts, such as fatty acids that your body needs. If the bacteria don't get enough to eat-namely fiber that survives the small intestine-they eventually begin to chomp away at the cells lining the colon. And that can lead to inflammation.

The Very End

Depending on what you've eaten and how much water you've been drinking, the journey typically reaches the end of the line 12 to 16 hours after entering your mouth.

More on the ways to add diverse bacteria and how it plays a vital role in your health: