1. How Your Digestive System Works

You’ve just finished a satisfying meal of stir-fried vegetables with chicken and brown rice. Your taste buds told you it was delicious, but you won’t be aware of what is happening after you swallow, as the food works its way down through your digestive system.

Whether the food you eat is basic or fancy, raw or cooked, it all serves an essential function: to provide the nutrients your body needs to survive. Under normal circumstances, the digestive process proceeds practically unnoticed. Each part of the digestive system—esophagus, stomach, liver, gallbladder, pancreas, small intestine, large intestine, rectum, and anus—plays a distinct role in the process (see Box 1-1, “The Digestive System”). Ultimately, the nutrients in your food are absorbed into the bloodstream, and the waste is eliminated through a bowel movement. But multiple medical conditions can affect these organs, disrupting the normal digestive process or causing other problems.

The Process of Digestion

To be useful, food must be broken down from large molecules of proteins, fats, and carbohydrates to very small molecules of amino acids, fatty acids, and sugars. This feat is accomplished by a combination of mechanical and chemical processes that occur during digestion.

The digestive process begins in the mouth, when food is chewed into more easily digestible pieces. Saliva produced by salivary glands in the mouth coats the food particles with enzymes that start to break down starches into smaller molecules. Saliva also makes the food wetter, softer, and easier to swallow.

The hollow, tube-like organs of the digestive system contain muscles that make the walls contract and relax in a rhythmic pattern called peristalsis. This action mechanically propels food contents through the system from one organ to the next. It begins with swallowing, which is voluntary.

Mouth to Esophagus to Stomach

Swallowed food enters the esophagus, where peristalsis pushes it downward toward the stomach. At the point where the esophagus and stomach meet, there is a ring-shaped valve (the lower esophageal sphincter) that usually remains closed. The presence of food in the esophagus causes the muscles to relax, which opens the valve and allows the contents to pass into the stomach.

The proper functioning of the lower esophageal sphincter is critical for sealing off the esophagus and preventing stomach acid from flowing backwards into it. This will be discussed in further detail in the section on gastroesophageal reflux disease (GERD).

Stomach to Small Intestine

In the stomach, food is stored, mixed with digestive enzymes, and slowly emptied into the small intestine. Mechanically, this means the muscles in the upper stomach must relax when large volumes of food need to be stored, and contract to mix up the contents.

How Food Is Digested

A layer of cells called the mucosa lines the inside of all hollow organs of digestion (mouth, esophagus, stomach, small intestine, large intestine). In the mouth, stomach, and small intestine, the mucosa secretes substances that help with digestion. In the stomach, the substances are mucus, hydrochloric acid, and the enzyme pepsin, which breaks down protein.

Hydrochloric acid plays a critical role in digestion, but it is a highly corrosive substance that damages any other body tissue with which it comes into contact. The mucus coating the lining of the stomach protects it from acid injury. Bacterial infections and certain medications can cause the lining of the stomach to wear away in places, causing ulcers.

Protein from foods such as meat, eggs, and beans must be broken down into smaller molecules called amino acids to be useful. This process begins in the stomach with pepsin. Hydrochloric acid provides the proper environment for pepsin to start breaking down proteins.

What Happens in the Small Intestine

Food passes from the stomach into the small intestine, where more digestion takes place, and the nutrients are absorbed into the body’s circulatory system. Once food is in the small intestine, the pancreas, liver, and gallbladder come into play. Digestive juices from these organs, along with enzymes secreted from glands in the wall of the intestine, contribute to the process.

The first section of the small intestine that receives food is called the duodenum. Enzymes secreted by the pancreas, plus bile from the gallbladder and liver, mix with food in the duodenum. The pancreas produces numerous enzymes that break down carbohydrates, fats, and proteins. Bile (which is produced in the liver and stored in the gallbladder) dissolves fat, so that enzymes from the pancreas and the intestinal lining can digest it. In addition, microorganisms called gut microbiota—which are present in the intestinal tract—help in the digestive process.

The parts of the small intestine below the duodenum are called the jejunum and the ileum. Most absorption of the digested molecules of food takes place in these sections. The small intestine is an exceptionally long organ that is coiled up inside the abdomen. This length is necessary to accomplish the task of absorbing all the nutrients. The lining of the small intestine contains many folds and small projections (villi and microvilli), which enhance contact with food, and provide more locations for absorption.

The now-small molecules of sugars, amino acids, and fatty acids are absorbed through the walls of the intestine and into blood vessels and lacteals (thin-walled vessels that transport fat and white blood cells away from the intestine), which carry them to other parts of the body, where they help build, repair, and maintain body tissues and provide energy.

What Happens in the Large Intestine

Food molecules that aren’t absorbed must be eliminated from the body. This is the job of the large intestine, which consists of the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anus. The liquid waste material enters the pouch-like cecum, and then moves along the colon. As it does, the colon absorbs the water and creates a solid stool. The waste material (feces) then passes into the rectum, where it is held until being expelled through the anus in a bowel movement.

The Role of Certain Foods in Digestion

Because the digestive system exists to extract nutrients from food, it makes sense that what we eat can impact our digestive health, and that certain digestive diseases and disorders can impact which foods we are able to tolerate.

In this section, we will discuss some of the most important aspects of foods as related to digestion: the role of dietary fiber; sugars that cause digestive problems for certain individuals, particularly those with irritable bowel syndrome; and food allergies.

Dietary Fiber

The importance of fiber in the diet cannot be overemphasized. Dietary fiber—what we used to call “roughage” or “bulk”—can help prevent or alleviate chronic constipation, hemorrhoids, diverticulosis, irritable bowel syndrome, and possibly colorectal cancer. There is new evidence a high-fiber diet may lessen or prevent food allergies (see Box 1-2, “Fiber May Dampen Food Allergies,” on page 10).

Dietary fiber also appears to have health benefits outside the gastrointestinal tract—for example, fiber may help lower cholesterol, and prevent obesity, heart disease, diabetes, and certain types of cancer.

Two Types of Fiber

Fiber is found in plant foods, such as grains, vegetables, and fruits. There are two types of fiber: insoluble and soluble. You don’t digest insoluble fiber: It simply passes through your digestive system. Sources of insoluble fiber include wheat bran, whole grains and whole-grain products, and some vegetables. Soluble fiber attracts water. It is found in beans, fruit, oats/oat bran, nuts, seeds, and vegetables, and can also be taken in capsule form as psyllium. A high-fiber diet cleans out your gut, while creating good stools that are easy to pass.

Introduce Fiber Slowly

If you’re planning to increase the amount of fiber in your diet, it’s best to do so slowly. Soluble fiber has the potential to produce gas, so you may notice more gas or bloating when you increase your fiber intake. If you add fiber slowly over time, your gastrointestinal tract will adjust. In addition, as you increase fiber, it’s important to drink more fluids (water, soup, broth, juices). Try to drink eight glasses of liquid a day.

While the average American takes in only 15 grams of fiber a day, the recommended intake is:

• 25 grams per day for women
• 38 grams per day for men
• If you consume less fiber than recommended, you’ll want to increase your daily fiber intake. You can do this by eating:
• Two to three cups of vegetables per day
• Two cups of fruit every day
• Three servings of beans, lentils or peas every week
• Whole-grain bread instead of white bread
• Whole-grain cereals and bran cereal
• Brown rice instead of white rice.

Foods that are high in fiber are listed in Box 1-3, “High-Fiber Foods,” and the fiber content (in grams) of some common foods is listed in Box 1-4, “Fiber Content of Some Common Foods.”

FODMAPs

FODMAPs—an acronym standing for fermentable oligosaccharides, disaccharides, monosaccharides, and polyols—are known for inducing diarrhea, bloating, abdominal pain, and flatulence. They are a particular problem for people with irritable bowel syndrome (see Chapter 9), but they can affect people with other digestive issues, as well.

FODMAPs are poorly absorbed short-chain carbohydrates (saccharides) that are rapidly fermented by bacteria in the digestive tract. The process produces gas, which causes the bowel to distend and results in an urgent need to have a bowel movement, which relieves symptoms. The FODMAP family includes:

• Oligosaccharides: fructans found in wheat, onions, and garlic, and galactins found in beans, lentils, and soybeans
• Disaccharides: lactose found in milk and other dairy products (see Box 1-5, “Lactose Intolerance a Common Problem”)



• Monosaccharides: fructose found in apples and honey
• Polyols: sorbitol and mannitol found in fruits and vegetables and artificial sweeteners.

Not everyone reacts to all FODMAPs, and some people can consume small amounts of a FODMAP without experiencing symptoms. A diet in which all FODMAPs are eliminated for two to four weeks, then slowly reintroduced one by one over six to eight weeks, can reveal which foods trigger the reaction. The good news is that avoiding the offending food improves symptoms and restores quality of life in up to 70 percent of patients with irritable bowel syndrome (see Box 1-6, “High-FODMAP Foods and Their Alternatives,” on page 13).

Food Allergies

Reactions to FODMAPS are food sensitivities, not food allergies. In allergies, the body’s immune system mistakes a harmless substance as an invader and attacks it. In the case of a food allergy, the body mistakenly perceives a particular food as being harmful, and creates antibodies against it. Whenever this food is eaten, the antibodies initiate an immune system attack. The immune system has a variety of mechanisms to fight off invaders—for example, mast cells in the skin airways (nasal passages and lungs) and digestive tract trigger the release of several chemicals, one of which is histamine. The release of substances like histamine causes symptoms that include:

• A tingling sensation in the mouth
• Hives
• Swelling of the tongue and throat
• Vomiting
• Difficulty breathing
• Abdominal cramps
• A drop in blood pressure
• Diarrhea
• Loss of consciousness
• Death

Symptoms typically appear within two hours after eating the food, and in the case of some allergies, may appear within seconds. There is no cure for a food allergy. The only way to avoid an allergic reaction is to avoid the offending food, although there is some suggestion that a high-fiber diet may help.

In some people, a food can cause a life-threatening allergic reaction called anaphylaxis. This is a medical emergency in which breathing becomes difficult, blood pressure drops, and swelling of the tongue or throat may cause suffocation.

A severe allergic reaction can be treated with the drug epinephrine. People who are prone to severe allergic reactions should obtain a prescription for a self-injectable device that contains epinephrine (EpiPen or TwinJect), which should be carried at all times. Antihistamines may be used to treat less-severe symptoms.

Just about any food can cause an allergic reaction, but the most common ones are milk, eggs, peanuts, tree nuts (such as almonds, cashews, and walnuts), wheat, soy, fish, and shellfish (see Box 1-7, “Common Food Allergens”).

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