The human body is a machine which winds its own springs.
—JULIEN OFFROY DE LA METTRIE, 18TH-CENTURY PHYSICIAN AND PHILOSOPHER
I don’t digest things with my mind.
—MARILYN MONROE
In many ways, the digestive system is an extension of the environment—a long tube, open at both ends, that is designed to supply the body with all the nutrients and fluids it needs to function. And everything we consume—food, liquids, drugs, supplements, Play-Doh—ultimately affects the entire body as it travels through the tract.
Digestion is an amazingly effective and efficient process. The concept is simple, the design and execution quite remarkable and, as scientists are discovering, intriguingly complex. There is a constant interaction of organs, muscles, nerves, hormones, enzymes, microbes, and blood vessels, every one doing tasks that monitor, regulate, and control the job of nourishing the body. As you will see, our gut goes well beyond simply processing the food we eat. It engages in a constant conversation with other parts of the body. And the volume is often deafening.
Problems in the digestive tract are one of the most common reasons people seek medical help—or self-medicate with restrictive diets, over-the-counter remedies, supplements, and/or pre-and probiotics. The list is long, and the discomfort and inconvenience of gastrointestinal distress a source of pain as well as embarrassment.
To understand what goes wrong, and the real basis of GI problems, it is important to understand first how digestion actually occurs. It is an essential foundation for comprehending how what we ingest affects both body and brain, and why going gluten-free or lactose-free or carbohydrate-free works for some and not for others. And why we were not meant to simply eliminate one category of nutrients without ample cause.
I spend a great deal of time on the toilet. In fact, I do some of my best work in the bathroom.
(ED, 44, CROHN’S DISEASE)
The Gastrointestinal Tract
Since nothing we eat can be used by the body in its ingested form, digestion is a combination of mechanical and chemical processes that tear food apart, grind it down, shake it vigorously, and create a soupy mix that is propelled through the entire length of the digestive tract (the gut), where it is ultimately absorbed and anything unusable is eliminated.
Food enters the mouth, where it is chewed into smaller pieces and enzymes begin digestion. It then travels through the pharynx, esophagus, stomach, small intestine (the duodenum, jejunum, and ileum), and large intestine (colon), and nondigestible products exit from the anus. Throughout the GI tract are organs and glands—salivary glands, pancreas, liver, and gallbladder—that secrete the enzymes and fluids needed to break down and digest food.
Diagram 1
THE DIGESTIVE TRACT
The digestive system is intimately joined to:
the circulatory system, which supplies nutrients to the organs and other tissues throughout the body
the enteric (intrinsic) and autonomic (automatic) nervous systems, which control enzyme release and contractions of the gut, and report back to the brain
the muscles of the digestive system, which provide coordinated motility (movement) to help digest and move/squeeze food through the long tract
the hormones that regulate movement as well as the secretions that stimulate and/or inhibit the activities of digestion
While there is enormous capacity and redundancy built into the digestive system, if one section malfunctions, it almost necessarily affects another, and there are numerous places for things to go wrong.
Digestion
Digestion is actually a three-part process:
1 Digestion—the breakdown of food products into smaller components that can be absorbed.
2 Absorption—the passage of food products that have been broken down into the intestinal wall.
3 Transport—the transfer of food from the intestinal wall to the rest of the body.
It consists of two basic activities:
the mechanical chewing and mixing of food in the mouth and stomach and propulsion by the intestinal muscle, called peristalsis. This muscular component is a crucial aspect of digestion since contractions of the smooth muscle both propel and mix the chyme (the liquid product of the broken-down food) as it travels down the digestive tract. Without peristalsis, there would be no digestion. When we discuss a lack of motility in various GI conditions, it means that peristalsis is affected.
the chemical breakdown of food by secretions and enzymes throughout the digestive tract. This starts with saliva in the mouth and is completed by microbes in the colon.
Digestion actually begins before the food even enters your mouth. When you see, think about, or smell food, the vagus nerve transmits a chemical message from your brain to release saliva in the mouth, increase stomach movement, and release gastric acid in the stomach. You begin to salivate, and the stomach “rumbles” at the very anticipation of food.
The Mouth
Digestion starts in the mouth as chewing tears, grinds, and crushes the food into smaller pieces. Saliva is secreted by glands under and around the tongue to lubricate and start to dissolve the food. Saliva contains enzymes that begin the digestion of fats and carbohydrates and acts as a glue to hold the food together as it travels toward the stomach.
The nervous system lends a hand to this process by inhibiting as well as stimulating the release of saliva. This is why we often get a “dry mouth” when fearful, and salivate at the thought or smell of food when hungry.
We swallow the ball or bolus of chewed food and saliva that is transported down our esophagus. While the skeletal muscles at work in the mouth and throat are voluntary—we consciously move our jaws and swallow—smooth muscles that function involuntarily then take over in the esophagus. This is where peristalsis begins and moves the food into the stomach, where the action really begins.
The Stomach
The stomach is a big muscular bag that holds the chewed food, mixes it with gastric juices, and starts many of the chemical processes of digestion. The muscle movements of the stomach act like a Cuisinart—chopping, blending, and mixing the ball of food into a soupy puree called chyme.
The main chemical ingredient in the stomach is hydrochloric (gastric) acid, a highly corrosive substance that both breaks down the food and converts the stomach into a disinfecting tank, killing bacteria and toxins in the food we have eaten. Gastric acid is an essential line of defense in the body’s monitoring of dangerous substances entering it from the outside. The stomach also releases pepsin, an enzyme that digests protein.
The walls of the stomach are composed of several layers of tissue—a structure found throughout the rest of the GI tract—that contain numerous mucous glands able to secrete mucus into the tract to lubricate the lining and protect it from friction and the acid bath of the chyme. The breakdown of this mucus coating is one of the causes of ulcers.
The communication system in the stomach also sends hormonal messages to the other digestive organs that food has arrived. This stimulates the secretion of pancreatic juices and bile from the liver that will further break down the chyme once it moves into the small intestine.
One-Way Street
Food is only meant to travel down