How the gut digests

How does the gut work? Where do gastrointestinal (GI) symptoms come from? Some of the most common questions patients ask me revolve around making sure their gut is functioning normally. Let's briefly discuss what "normal" really is, knowing that there's obviously a lot we can't explore in a single post. Being able to localize your GI symptoms can be incredibly useful, and understanding the underlying physiology is an important first step.

Your GI tract extends from your mouth to your anus, and is a 30 ft long tube featuring 10 different organs that have a wide range of form and function. When patients have GI symptoms, localizing their symptoms to a part of the GI tract is always the first step. I think the most interesting way to learn about gut anatomy is through the perspective of food.


How the esophagus works

Before you even put food in your mouth, your GI tract is already hard at work. There are important digestive enzymes in your saliva that start to break down food even before you swallow. Chewing is obviously important to digesting your food, but the popular fad diet in the early 1900s of chewing every bite 100 times in an effort to lose weight has long been dispelled as mythical. Swallowing is incredibly coordinated and complex, involving over 30 muscles and nerves, transporting our food from the mouth into the esophagus. The "healthy" esophagus should really only play a brief role - moving our food into our stomach in less than 15 seconds typically. The sensation of food getting "stuck" on the way down is often indicative of a problem with how your esophagus is moving and should prompt a call to your gastroenterologist.


How the stomach works

Entering the stomach, our food is really going to take a beating both physically and chemically - stomach acid is actually more acidic than battery acid. The stomach is constantly churning and burning, breaking food down and storing it (up to 4 liters at a time) so that it can release it in smaller amounts to your small intestine. A common GI complaint is "heartburn" after eating. Part of a healthy stomach is allowing some of the acidic gastric juice to "reflux" into your esophagus, however, actually being able to feel that sensation can be due to a whole host of reasons that we will explore in a future post, and leads to heartburn in over 20% of adults. Differentiating heartburn from "indigestion" can be important in helping doctors understand your symptoms; although the two overlap, indigestion is often more related to abdominal pain than reflux.


What is the small bowel?

In the healthy stomach, food is only around for 2-4 hours. Where to next? The small bowel. While the small bowel (aka small intestine) has three distinct parts (duodenum, jejunum, ileum), for the purposes of this post we will oversimplify the physiology here to say that your small bowel is where most of your food gets digested and absorbed into the nutrients your body uses for energy. In order to perform this task, your small bowel has to have a huge amount of surface area, and even though it is only about 20 feet long, its millions of small folds ("microvilli") gives it the same square footage as a studio apartment!

To help facilitate this digestion, your pancreas, liver, and gallbladder make digestive juices (enzymes or bile are a few names you may hear doctors use) to break down carbs, protein and fat released into the small bowel. If you've ever heard of someone getting "gallstones" all that really means is that your bile is sitting around long enough to form little stones that then cause problems in their pursuit of reaching the small bowel. 


How does the colon work?

Food moves through the small bowel over the course of 90 minutes, but by the time it reaches the end of the small bowel, it's really not fair to call what's left "food". Your body has taken all the nutrients it can from what you ate, and can now begin the process of discarding the rest: fiber, water, and dead cells. That's essentially the function of the colon (aka the large intestine); moving water in and out of its tube to form your poop.

Even though your body has already accomplished the bulk of its absorption from what you ate, the process of it getting through your colon is a time consuming one - it takes an average of 30 hours . As your poop passes through the colon, water is drained out of it and reabsorbed into your body. Because all we are left with at this point is fiber, water, and dead cells, it's easy to imagine that having health fiber in your diet and drinking plenty of water are both crucial to a happy colon.

Both men and women pass gas on average 13 times per day, the majority of which occurs overnight. While choosing foods that ease your colon's job can help minimize your GI symptoms, thirty hours is a long time for these byproducts to cause problems. GI symptoms of constipation, diarrhea, bloating, and gas all stem from your colon. Most of your perception of these symptoms comes from your rectum, the last part of your colon. In coordination with muscles that make up the pelvic floor, your rectum stores your poop, and is connected to a ton of nerves that help you know when it's time to poop.

Obviously this is just a taste (too soon?) of how your gut functions. The GI system is a complex world inside your body, and the more you know about how it works, the healthier choices you can make on its behalf. Read more and learn to love your gut!

-The Gut Doc


Like our posts? Get our weekly newsletter to stay in the loop!