Food Is the Body's Fuel Source

What's the first step in digesting food? Believe it or not, the digestive process starts even before you put food in your mouth. It begins when you smell something irresistible or when you see a favorite food you know will taste good. Just by smelling that homemade apple pie or thinking about how delicious that ice cream sundae is going to taste, you begin to salivate — and the digestive process kicks in, preparing for that first scrumptious bite.

If it's been a while since your last meal or if you even think about something tasty, you feel hungry. You eat until you're satisfied and then go about your business. But for the next 20 hours or so, your digestive system is doing its job as the food you ate travels through your body.

Food is the body's fuel source. The nutrients in food give the body's cells the energy and other substances they need to operate. But before food can do any of these things, it has to be digested into small pieces the body can absorb and use.

Almost all animals have a tube-type digestive system in which food enters the mouth, passes through a long tube, and exits as feces (poop) through the anus. The smooth muscle in the walls of the tube-shaped digestive organs rhythmically and efficiently moves the food through the system, where it is broken down into tiny absorbable nutrients.

During the process of absorption, nutrients that come from the food (including carbohydrates, proteins, fats, vitamins, and minerals) pass through channels in the intestinal wall and into the bloodstream. The blood works to distribute these nutrients to the rest of the body. The waste parts of food that the body can't use are passed out of the body as feces.

About the Digestive System

Every morsel of food we eat has to be broken down into nutrients that can be absorbed by the body, which is why it takes hours to fully digest food. In humans, protein must be broken down into amino acids, starches into simple sugars, and fats into fatty acids and glycerol. The water in our food and drink is also absorbed into the bloodstream to provide the body with the fluid it needs.

The digestive system is made up of the alimentary canal and the other abdominal organs that play a part in digestion, such as the liver and pancreas. The alimentary canal (also called the digestive tract) is the long tube of organs — including the esophagus, the stomach, and the intestines — that runs from the mouth to the anus. An adult's digestive tract is about 30 feet long.

How Digestion Works

Digestion Begins in the Mouth

The process of digestion starts well before food reaches the stomach. When we see, smell, taste, or even imagine a tasty snack, our salivary glands, which are located under the tongue and near the lower jaw, begin producing saliva. This flow of saliva is set in motion by a brain reflex that's triggered when we sense food or even think about eating. In response to this sensory stimulation, the brain sends impulses through the nerves that control the salivary glands, telling them to prepare for a meal.

As the teeth tear and chop the food, saliva moistens it for easy swallowing. A digestive enzyme called amylase (pronounced: AH-meh-lace), which is found in saliva, starts to break down some of the carbohydrates (starches and sugars) in the food even before it leaves the mouth.

Swallowing, which is accomplished by muscle movements in the tongue and mouth, moves the food into the throat, or pharynx. The pharynx (pronounced: FAIR-inks), a passageway for food and air, is about 5 inches long. A flexible flap of tissue called the epiglottis (pronounced: ep-ih-GLAH-tus) reflexively closes over the windpipe when we swallow to prevent choking.

From the throat, food travels down a muscular tube in the chest called the esophagus (pronounced: ih-SAH-fuh-gus). Waves of muscle contractions called peristalsis (pronounced: per-uh-STALL-sus) force food down through the esophagus to the stomach. A person normally isn't aware of the movements of the esophagus, stomach, and intestine that take place as food passes through the digestive tract.

The Stomach

At the end of the esophagus, a muscular ring called a sphincter (pronounced: SFINK-ter) allows food to enter the stomach and then squeezes shut to keep food or fluid from flowing back up into the esophagus. The stomach muscles churn and mix the food with acids and enzymes, breaking it into much smaller, more digestible pieces. An acidic environment is needed for the digestion that takes place in the stomach. Glands in the stomach lining produce about 3 quarts of these digestive juices each day.

Most substances in the food we eat need further digestion and must travel into the intestine before being absorbed. When it's empty, an adult's stomach has a volume of one fifth of a cup, but it can expand to hold more than 8 cups of food after a large meal.

By the time food is ready to leave the stomach, it has been processed into a thick liquid called chyme (pronounced: kime). A walnut-sized muscular tube at the outlet of the stomach called the pylorus (pronounced: py-LOR-us) keeps chyme in the stomach until it reaches the right consistency to pass into the small intestine. Chyme is then squirted down into the small intestine, where digestion of food continues so the body can absorb the nutrients into the bloodstream.

The Small Intestine

The small intestine is made up of three parts:

1. the duodenum (pronounced: due-uh-DEE-num), the C-shaped first part
2. the jejunum (pronounced: jih-JU-num), the coiled midsection
3. the ileum (pronounced: IH-lee-um), the final section that leads into the large intestine

The inner wall of the small intestine is covered with millions of microscopic, finger-like projections called villi (pronounced: VIH-lie). The villi are the vehicles through which nutrients can be absorbed into the body.

The Liver

The liver (located under the ribcage in the right upper part of the abdomen), the gallbladder (hidden just below the liver), and the pancreas (beneath the stomach) are not part of the alimentary canal, but these organs are still important for healthy digestion.

The pancreas produces enzymes that help digest proteins, fats, and carbohydrates. It also makes a substance that neutralizes stomach acid. The liver produces bile, which helps the body absorb fat. Bile is stored in the gallbladder until it is needed. These enzymes and bile travel through special channels (called ducts) directly into the small intestine, where they help to break down food.

The liver also plays a major role in the handling and processing of nutrients. These nutrients are carried to the liver in the blood from the small intestine.

The Large Intestine

From the small intestine, food that has not been digested (and some water) travels to the large intestine through a valve that prevents food from returning to the small intestine. By the time food reaches the large intestine, the work of absorbing nutrients is nearly finished. The large intestine's main function is to remove water from the undigested matter and form solid waste that can be excreted. The large intestine is made up of three parts:

1. The cecum (pronounced: SEE-kum) is a pouch at the beginning of the large intestine that joins the small intestine to the large intestine. This transition area allows food to travel from the small intestine to the large intestine. The appendix, a small, hollow, finger-like pouch, hangs off the cecum. Doctors believe the appendix is left over from a previous time in human evolution. It no longer appears to be useful to the digestive process.
2. The colon extends from the cecum up the right side of the abdomen, across the upper abdomen, and then down the left side of the abdomen, finally connecting to the rectum. The colon has three parts: the ascending colon and transverse colon, which absorb water and salts, and the descending colon, which holds the resulting waste. Bacteria in the colon help to digest the remaining food products.
3. The rectum is where feces are stored until they leave the digestive system through the anus as a bowel movement.

Things That Can Go Wrong

Nearly everyone has a digestive problem at one time or another. Some conditions, such as indigestion or mild diarrhea, are common; they result in mild discomfort and get better on their own or are easy to treat. Others, such as inflammatory bowel disease (IBD), can be long lasting or troublesome. GI specialists or gastroenterologists (doctors who specialize in the digestive system) can be helpful when dealing with these conditions.

Conditions Affecting the Esophagus

Conditions affecting the esophagus may be congenital (meaning people are born with them) or noncongenital (meaning people can develop them after birth).

These include:

• Tracheoesophageal fistula (pronounced: tray-KEE-oh-ih-saf-uh-jee-ul FISH-chuh-luh) and esophageal atresia (pronounced: ih-saf-uh-JEE-ul uh-TREE-zhuh) are both examples of congenital conditions. Tracheoesophageal fistula is where there is a connection between the esophagus and the trachea (windpipe) where there shouldn't be one. In babies with esophageal atresia, the esophagus comes to a dead end instead of connecting to the stomach. Both conditions are usually detected soon after a baby is born — sometimes even beforehand. They require surgery to repair.
• Esophagitis (pronounced: ih-saf-uh-JEYE-tus) or inflammation of the esophagus, is an example of a noncongenital condition. Esophagitis is usually caused by gastroesophageal reflux disease (GERD), a condition in which the esophageal sphincter (the tube of muscle that connects the esophagus with the stomach) allows the acidic contents of the stomach to move backward up into the esophagus. GERD can sometimes be corrected through lifestyle changes, such as adjusting the types of things a person eats. Sometimes, though, it requires treatment with medication. Occasionally, esophagitis can be caused by infection or certain medications.

Conditions Affecting the Stomach and Intestines

Almost everyone has experienced diarrhea or constipation at some point in their lives. With diarrhea, muscle contractions move the contents of the intestines along too quickly and there isn't enough time for water to be absorbed before the feces are pushed out of the body. Constipation is the opposite: The contents of the large intestines do not move along fast enough and waste materials stay in the large intestine so long that too much water is removed and the feces become hard.

Other common stomach and intestinal disorders include:

• Celiac disease is a digestive disorder caused by the abnormal response of the immune system to a protein called gluten, which is found in certain foods. People with celiac disease have difficulty digesting the nutrients from their food because eating things with gluten damages the lining of the intestines over time. Some of the symptoms are diarrhea, abdominal pain, and bloating. The disease can be managed by following a gluten-free diet.
• Irritable bowel syndrome (IBS) is a common intestinal disorder that affects the colon. When the muscles in the colon don't work smoothly, a person can feel the abdominal cramps, bloating, constipation, and diarrhea that may be signs of IBS. There's no cure for IBS, but it can be managed by making some dietary and lifestyle changes. Occasionally, medications may be used as well.
• Gastritis and peptic ulcers. Under normal conditions, the stomach and duodenum are extremely resistant to irritation by the strong acids produced in the stomach. Sometimes, though, a bacterium called Helicobacter pylori or the chronic use of certain medications weakens the protective mucous coating of the stomach and duodenum, allowing acid to get through to the sensitive lining beneath. This can irritate and inflame the lining of the stomach (a condition known as gastritis) or cause peptic ulcers, which are sores or holes that form in the lining of the stomach or the duodenum and cause pain or bleeding. Medications are usually successful in treating these conditions.
• Inflammatory bowel disease (IBD) is chronic inflammation of the intestines that affects older kids, teens, and adults. There are two major types: ulcerative colitis, which usually affects just the rectum and the large intestine, and Crohn's disease, which can affect the whole gastrointestinal tract from the mouth to the anus as well as other parts of the body. They are treated with medications, but in some cases, surgery may be necessary to remove inflamed or damaged areas of the intestine.

Disorders of the Pancreas, Liver, and Gallbladder

Conditions affecting the pancreas, liver, and gallbladder often affect the ability of these organs to produce enzymes and other substances that aid in digestion.

These include:

• Cystic fibrosis is a chronic, inherited illness that not only affects the lungs, but also causes the production of abnormally thick mucus. This mucus blocks the ducts or passageways in the pancreas and prevents its digestive juices from entering the intestines, making it difficult for a person to properly digest proteins and fats. This causes important nutrients to pass out of the body unused. To help manage their digestive problems, people with cystic fibrosis can take digestive enzymes and nutritional supplements.
• Hepatitis is a viral infection in which the liver becomes inflamed and can lose its ability to function. Some forms of viral hepatitis are highly contagious. Mild cases of hepatitis A can be treated at home; however, serious cases involving liver damage may require hospitalization.
• The gallbladder can develop gallstones and become inflamed — a condition called cholecystitis (pronounced: ko-lee-sis-TYE-tus). Although gallbladder conditions are uncommon in teens, they can occur when a teen has sickle cell anemia or is being treated with certain long-term medications.

The kinds and amounts of food a person eats and how the digestive system processes that food play key roles in maintaining good health. Eating a healthy diet is the best way to prevent common digestive problems.

Reviewed by: Yamini Durani, MD
Date reviewed: October 2012