Читать книгу Gluten Exposed: The Science Behind the Hype and How to Navigate to a Healthy, Symptom-free Life - Rory Jones, Dr. Green Peter - Страница 42

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 the GI tract—a street sign that is often ignored. The sphincters (ring-like muscles) connecting the esophagus to the stomach and the stomach to the small intestine are one-way valves. Occasionally, chyme refluxes or backs up into the esophageal area—a condition known as GERD, gastroesophageal reflux disease. The corrosive effect of gastric acid is well known to people who experience its effect on their less-well-protected esophagus. (See chapter 8, “The Gut in Disease.”)

When the chyme is sufficiently liquefied, muscle/peristaltic contractions gradually push it into the upper part of the small intestine, the duodenum. The stomach empties in a slow and controlled way so as not to overwhelm all the mechanisms of digestion in the small intestine. Everything is released according to particle size.

As the small intestine fills with chyme, it signals the stomach to decrease its activity and slow down the emptying process. This is one reason a large or fatty meal stays with you. It lingers in the stomach until the small intestine is ready to process it.

The arrival of chyme in the small intestine triggers the release of a cascade of secretions. The small intestine, pancreas, liver, and gallbladder all deliver digestive enzymes and fluids that break down the food into components small enough to be absorbed. Alkaline mucus with a high concentration of bicarbonate is secreted to neutralize the gastric acid in the chyme.

All of these actions are regulated by both the nervous system and gastrointestinal hormones and called into action only when needed by the digestive system.

Most of these activities occur without any conscious awareness or control. In this sense, it is truly a “second brain” in your gut.

The Pancreas

A carrot-shaped gland that has a dual function in digestion and metabolism, the pancreas is both an endocrine gland—producing insulin, which enables the digestion and absorption of carbohydrates—and an exocrine gland—producing enzymes such as trypsin, which breaks down proteins; amylase, which breaks down starches; and lipase, which breaks down fats. When the pancreas becomes inflamed or diseased (e.g., pancreatitis), these enzymes are not secreted, and as a result carbohydrate, protein, and fat digestion is impaired.

The Liver

The liver is the energy-processing center of the body. It is the first stop for nutrients absorbed from the intestine. Its many functions include metabolizing, storing, and transporting nutrients to the body, producing chemicals necessary for digestion, and breaking down drugs and alcohol.

The liver stores glucose, iron, and vitamins A, B12, and D, sending out the nutrients and substances digested from the food to the cells of the body as they are needed. It also secretes bile, a fluid that increases the solubility of fats, enabling them to pass through the intestinal wall into the bloodstream. Bile is made in the liver and stored in the gallbladder until needed, and delivered to the small intestine when fatty foods arrive and stimulate its release through contraction of the gallbladder.

The Small Intestine

The small intestine, which is actually the longest part of the GI tract, is designed to complete digestion and much of the absorption of nutrients. It is approximately 22 feet long in adults and consists of three parts:

 the duodenum (the first segment)

 jejunum (the second segment)

 ileum (the third segment, or distal small intestine)

All three segments have similar anatomy, but each has a specialized job, digesting and absorbing specific nutrients.

Slow waves of peristalsis push chyme from the stomach through the duodenum toward the jejunum. It actually takes several hours for an entire meal to travel the entire length of the small intestine to enable absorption to occur through the lining or mucosal wall of the intestine.

This lining has a unique structure that possesses a much larger surface area than the midsection of the body in which it is contained. (See Diagram 2.) This lining, the mucosa, consists of folds that increase its surface area. The folds are in turn covered with tiny fingerlike projections, or villi, that contain the cells that absorb nutrients and again expand the surface area. (See Diagram 3.)

The surface of each villus has a “brush” border consisting of microvilli or tiny hairs that increase the absorptive surface of the small intestine yet again. The brush border also secretes enzymes that are necessary for the digestion of specific food components.

If you were to flatten out the intestinal mucosa—all the villi, microvilli, and crypts that lie between the villi—the “small” intestine actually has a surface area about the size of a football field that is totally dedicated to absorbing food! This enormous capacity ensures that the intestine can sustain a fair amount of assault and/or damage and still feed the body.

Diagram 2

A CROSS SECTION OF THE INTESTINAL WALL

Diagram 3

Inflammatory cells normally inhabit the mucosa to protect the small intestine against toxins and bacteria. Since the food supply entering the GI tract is not sterile and may contain toxic substances, these white blood cells are another line of defense. This results in a state of constant, controlled inflammation in the mucosa.

The Villi

The villi are the workhorses of the intestine. They are the final intestinal link between your dinner plate and your bloodstream. This is where celiac disease does its primary damage and where other gluten and food-related disorders affect the body’s ability to properly absorb nutrients.

The villi play a crucial role by:

 dramatically increasing the surface area of the small intestine to allow the absorption of food

 releasing enzymes that continue and complete the breakdown/digestion of food

 absorbing the products of digestion and transporting them into the bloodstream for distribution throughout the body

 acting as a barrier that blocks bacteria, parasites, and toxins from entering the body

Each villus is an independent but intimately related part of the assembly line. It is important to understand that the final stages of digestion, absorption, and transport of nutrients occurs through—not between—these tiny, fingerlike projections. When there is inflammation, and a breakdown of the lining of the intestine, the bowel may become permeable, often termed “leaky.”

There are millions of microscopic villi in each section of the small intestine. Because of its enormous capacity to absorb, parts of it can be damaged with no obvious manifestations or symptoms. But when large sections of the lining are inflamed or destroyed, absorption, enzyme release, transport of nutrients to the body, and the defensive ability of the small intestine is compromised.

Absorption

Once the food components are sufficiently digested (broken down), they are absorbed by different parts of the small intestine.

This is why disease of or infection in one section of the small intestine is often revealed by the malabsorption of specific nutrients. Iron deficiency and metabolic bone disease (e.g., osteoporosis or osteopenia) occur when disease involves the proximal intestine, the first two segments of the small intestine. Fat and sugars are absorbed throughout the intestine. Therefore, when one part is diseased, another can compensate and absorb these vital nutrients.

Unless there is a disease process at work, absorption works efficiently and steadily until every usable nutrient in the chyme is absorbed. Nutrients that the body does not need for energy and efficient cellular function are stored for later use—primarily as body fat.

Transport

Once the intestinal wall absorbs the fully digested food, it is transported into the bloodstream and made available to cells throughout the body. Carbohydrates, protein, and fat are transported across the epithelial cell membranes by different mechanisms, with some foods requiring specialized chemical “porters” that literally bind to the components and carry them across the cells of the villi.

Carbohydrates

Carbohydrates supply the body with the fuel it requires for immediate and long-term muscle function and energy. Complex carbohydrates (starches) are usually broken into simple carbs (sugars) in the small intestine. The simple sugars are usually readily transported across the villi into the bloodstream. But some sugars such as lactose or fructose are not absorbed by some people and arrive in the large intestine (colon) mostly undigested. It is these carbohydrates that are thought to cause many of the symptoms of IBS, and their digestive results are often mistaken for gluten intolerance.

Proteins

Proteins are large and complex molecules that are an essential part of every cell, organ, and body system. They are made up of hundreds of amino acids bound together in chemical chains that must be broken down to be used by the body. This is done by enzymes secreted by the pancreas and from the brush border (microvilli), which split these chains into smaller and smaller molecules and amino acids that can then be absorbed.