Before you can understand what might be wrong with your digestion and how to fix it, you need to understand what happens when things are working well: how food makes the epic journey from your plate to the parts of your body that use it, and how you get rid of the foodstuffs you don’t need along the way.
The GI or gastrointestinal tract begins with your mouth and ends with your rectum. Whether you’re a Kourtney or Khloé height-wise, your GI tract clocks in at up to thirty feet long as you approach adult size—when it’s spread out from one end to the other, that is. This winding tube is lined with smooth muscle that helps any food you eat travel from your mouth to your stomach and intestines, and out the other side. We’ll get into the specifics of the role that each part plays soon—but for now, think about it as one long tube.
The craziest thing you may ever learn about your digestive tract: It’s technically located outside of your body. Sure, you’ve never glanced down in the middle of history class and seen it looking up at you. But any part of your body that has contact with the world around you is considered ever so technically by medical professionals to be outside the body. Your bones, for instance, protected by skin, muscle, and more, are inside. Your brain, protected by your skull and skin, is inside. Your lips mostly keep your mouth covered … but open that doorway to let some food or drink in and bam—your digestive tract is up close and personal with the outside world. And technically, anything that passes through your digestive tract remains outside of your body until it’s broken down and absorbed by the small intestine into the bloodstream (stay tuned for details on how that works).
So how does food make the epic journey from your mouth through this winding tube called the digestive tract, sending important nutrients out to your body and anything that’s left over out the other end? When I ask my clients what the first step of the digestion process is, they usually tell me that it’s when you swallow food or when it reaches your stomach. Most are surprised to hear that digestion begins before you’ve eaten a single thing.
Have you ever felt your mouth water when you’re waiting for a hot slice of pizza to come out of the oven? This process—technically called salivation—is part of your body readying itself for incoming food. (This is also why you might feel the urge to tackle your brother when he pops the last tater tot—which you were totally just about to eat—into his mouth.)
When you finally do take that first bite of food, the saliva you’ve already produced is at the ready to help soften what you’ve eaten so your teeth can break it down into an easy-to-swallow mush. In addition to turning food into a mass that can effortlessly slide down your throat, saliva contains digestive enzymes that work chemically to start turning nutrients into smaller components. One enzyme, called salivary amylase, chemically helps break down long chains of carbohydrates. If you put a piece of a carbohydrate-rich food like bread in your mouth and let it sit there, you can actually taste it becoming sweeter as the salivary amylase breaks the carbohydrate down into simple starches, the smaller building blocks that make up carbohydrates. While that happens, another enzyme, called salivary lipase, kicks off the process of digesting fats.
Next, you chew, turning your meal from, well, food, into what’s called a bolus—a word that’s Greek and Latin for “lump,” and in the case of digestion, refers specifically to a ball of food. Chewing is probably not something you’ve thought much about since you were an infant learning how to eat solid food and your parents were standing over you making exaggerated chompy faces to remind you not to swallow your banana whole. But the simple, mindless act of chewing your food deserves some attention even at your advanced age—and here’s why: Chewing does more than just ensure you don’t choke (though that is, admittedly, pretty important). When you chew, you increase the surface area of your food so that the enzymes produced by your saliva can reach more of what you’ve eaten and be more effective in breaking it down into smaller pieces. Chewing also signals your stomach to prepare for the food’s arrival by releasing acid that will help with digestion. What’s more, research shows that chewing for longer can help you feel more satisfied when you eat, and can even help your body absorb more nutrients from food.
As soon as you’re ready to swallow, your tongue propels the bolus of food back into your esophagus. The first door (called the upper esophageal sphincter, or UES) opens, welcoming the bolus into your esophagus, where a wave known as peristalsis carries it downward (with the help of gravity) and through a second door, called the lower esophageal sphincter (or LES). The LES separates your esophagus from your stomach.
Next, the bolus of food finally arrives at the stomach, which can hold as much as four cups (about a liter) of chewed food or liquid, expanding as you eat or drink to accommodate whatever you’ve taken in. The top part of your stomach serves as a waiting room, where the bolus rests as the lower stomach (called the antrum) begins to contract and mix the food with enzymes and acids. Hydrochloric acid creates conditions that are at just the right pH (or acidity level) for the digestion of protein as well as the killing off of bacteria, while the enzyme pepsin starts the process of breaking the protein down. The enzyme lipase continues to break down fats. As that happens, the lining of the stomach produces mucus, which will protect it from being harmed by all of that damaging acidity.
Once the bolus has been pummeled by the hydrochloric acid and pepsin, it’s now considered chyme—a liquefied mass of partially digested food, water, and more. Peristaltic waves continue to move the chyme downward toward the pyloric sphincter (also known as the pyloric valve), a ring of muscle that controls the flow of chyme from the lower stomach into the upper small intestine, called the duodenum. (It also prevents chyme from bouncing back up into the stomach.) You can think of the pyloric sphincter like the security at the entrance to the Apple Store the day a new iPhone comes out. If the duodenum is at capacity, it’s not letting any more food in. As food enters the duodenum, a little opening called the sphincter of Oddi funnels in digestive juices from the pancreas and bile from the gallbladder (the latter is produced in the liver). Bile emulsifies fat into smaller pieces, making it available for digestion and absorption, while the enzymes use chemicals to further break down protein, fat, and carbohydrates.
Up until this point, digestion has consisted of taking apart the nutrients in food and turning them into smaller and smaller pieces. We’ve now reached the stage of digestion in which things begin to get exciting. Exciting? you’re asking? Yes, exciting! Because, full disclosure, the small intestine is my absolute favorite digestive organ. It is. (Doesn’t everyone have one? No? Oh. Right. Well, I’m going to own this anyway.) The reason I love the small intestine? At its beginning, the process of breaking down food into smaller components is continuing, but by its end, you’re actually absorbing nutrients—real, live nutrients!—into your bloodstream, where they are then delivered to the various parts of your body to do the important things that nutrients do, like give you energy, repair your cells, make your bones strong, and so, so much more. The small intestine, in fact, is where most nutrients are absorbed into the bloodstream so that they can move on to do their jobs throughout your body. How cool is that?!
Where was I? Oh yes. So after the chyme passes through the duodenum, peristalsis continues, which helps it stay on its journey. The walls of the small intestine start out as smooth as a bedsheet, but farther along they develop folds called villi and microvilli, which make them resemble your grandparents’ 1970s-era shag carpet that’s sorely in need of a makeover. These microscopic hairlike projections increase the surface area of the intestinal lining, which allows for more and better absorption of nutrients. Villi and microvilli line the walls of the remaining two sections of the small intestine, the jejunum and ileum. The walls of these parts of the intestine absorb fats and other nutrients; then blood vessels transport those nutrients through the portal vein to the liver. By the time your food reaches the end of the small intestine, most of its nutrients, as well as all but about one liter of fluid, have been absorbed. Another sphincter, called the ileocecal valve, separates the small intestine from the large intestine and also keeps chyme from splashing back into the ileum.
The large intestine is also known as the colon or the large bowel. In healthy people, the chyme is mostly liquid when it enters the large intestine, and mostly solid when it exits the large intestine. So can you guess what the role of the large intestine is? You got it—removing all of that fluid through absorption, which occurs as what’s left over from the small intestine makes its journey through the various parts of the colon (from the cecum to the ascending colon to the transverse colon to the descending colon to the sigmoid colon and into the rectum). The large intestine also contains bacteria that, in addition to producing nutrients like vitamin K—the blood-clotting vitamin—further digest any remaining bits of carbohydrate and protein. The balance of bacteria throughout the digestive tract (known as the gut microbiome), and in the large intestine in particular, may hold the key to understanding and managing your symptoms; we’ll talk about this in great detail throughout the book. Stick with me, friends—before long, you, too, will have your very own fan-favorite digestive organ and tell tales of peristalses past.
The rectum and anus are where food ends its journey through the digestive tract. The rectum is typically empty, since your sigmoid colon holds on to poop until it’s ready to come out. Once the area above the sigmoid colon becomes full, you feel the urge to go—and under the best of healthy circumstances, you can control that urge until you’ve reached a bathroom. When you’re ready, the rectum pushes out the poop through the anus, the opening through which waste exits your body, controlled by a ring of muscle called the anal sphincter.
Processing and using food depends on much more than the parts of the body that are at the center of the action, however. Digestion is somewhat like the process of making a film. Have you ever watched the credits roll after a big blockbuster? There are thousands of people involved, many with jobs you never knew existed. Digestion, like making a movie, is an uber-collaborative effort that goes way beyond the mouth, stomach, and small and large intestines, which as you now know all play starring roles. The liver, pancreas, and gallbladder are supporting actors, supplying important enzymes and other compounds that help break down food. And behind the scenes, the nervous system is the producer/director, calling the shots and getting everyone to do his job—like stimulating production coordinators saliva and peristalsis, nudging the cleaning team to tidy up with the migrating motor complex (see “Why Grazing Isn’t Great” on p. 10). The whole time, bacteria work as the crew to keep things humming along; and like any group working together, some members do the heavy lifting while others kind of get in the way, which you’ll learn more about later because it’s at the crux of the low-FODMAP plan. Finally, the circulatory system gets those nutrients out to the parts of the body that will use them, much like the film distributor who puts the ready-to-watch movie in theaters for people to go and enjoy.
No doubt, digestion is a major motion picture that involves many players. This is why gut disorders are incredibly common; so many different parts need to align just right for healthy digestion to occur. Now that you have a good understanding of how this happens, we can take a look at what might be up with your body and prepare you for how a doctor might investigate.