How does human digestion work?

The purpose of digestion is to break down large food particles into smaller molecules that can be absorbed (as nutrients) and used by the cells of the body as a source of energy for growth and reproduction. For example, fats are broken down into glycerol and fatty acids and proteins into amino acids. The digestive tract, also called the alimentary canal, is approximately 30 feet (9 meters) long from the mouth to the anus. It is lined mostly with smooth muscles (involuntary) that push the food through the tract in a process called peristalsis.

What are the major parts of the digestive system?

Humans have a special digestive system that allows them to break down large food particles into smaller molecules and absorb the nutrients within the food. The following lists the details of the major parts of the digestive system:

Mouth-The mouth, tongue, and teeth are responsible for breaking the food down with mechanical action. As omnivores, humans have teeth that include incisors for cutting, canines for tearing, and molars for grinding. As the food is in the mouth, salivary glands release saliva, which then chemically breaks down the starch in the food.

Esophagus-The esophagus is where the food goes after a person swallows. Food is directed into the esophagus by the epiglottis, or a flap of cartilage in the back of the pharynx (throat)-which essentially stops food from going into the windpipe (and which is why, in some circumstances, people choke on food).

Stomach-Food is churned mechanically in the stomach, an organ that secretes gastric juice, a mix of special enzymes and hydrochloric acid. A structure called a cardiac sphincter acts as a stop to prevent food in the stomach from backing up into the esophagus and thus burning it. A structure called the pyloric sphincter at the bottom of the stomach helps keep the food in the stomach long enough to be digested.

Small intestine-The upper part of the small intestine-about the first 12 inches (30 centimeters)-is called the duodenum. Bile from the liver (stored in the gallbladder) is released into the small intestine, breaking fats down and helping other digestive enzymes to work. The intestine is lined with millions of elongated projections called villi that absorb all the nutrients released from the digested foods.

Large intestine-The large intestine, also called the colon, has several functions. It removes undigested waste and excess water, and it harbors bacteria that create gas, digest some material, and help produce certain vitamins, such as biotin and vitamin K. All together, both intestines absorb 90 percent of the water that enters the mouth, with the small intestine absorbing most of that water. (But if too much water is removed, constipation results, or if not enough water is removed, diarrhea results.) The waste is finally released at the end of the digestive tract, or the anus.

What are the general steps in the digestive process?

There are five major steps in the process of digestion-all of which can be affected if a person has diabetes. In general, the five steps are as follows:

Ingestion-The eating of any food.

Peristalsis-The involuntary muscle contractions that move the ingested food through the digestive tract.

Digestion-The conversion of the food molecules into nutrients that can then be used by the body.

Absorption-The passage of the nutrients into the bloodstream and/or lymphatic system to be used by the body’s cells.

Defecation-The elimination of the undigested and unabsorbed ingested materials.

How does digestion produce glucose?

During digestion, the fats, carbohydrates, and proteins that are consumed are broken down so they are easily used by the body’s cells. One of the products from this process is glucose (sugar), one of the “fuels” that provide the energy needed to sustain all of the body’s living cells.

The human digestive system.

What is the total average time from eating to excreting?

It takes an average of 53 hours, although this number is often debated, because food moves through the digestive system at different speeds. In addition, foods are not eliminated in stool in the same order in which people ingest them. For example, vegetables are easier to digest than meat, so fibrous plants typically move through the large intestines much faster.

What is gastroparensis?

Gastroparensis is often a complication of diabetes in the digestive tract. It occurs when the nerves and muscles that help the stomach to empty do not work properly. Because there is no spontaneous movement of the muscles (motility) in the person’s stomach, gastroparensis delays the emptying of the stomach contents into the small intestine after a meal. Not only does the condition interfere with a person’s normal digestion, but it can also cause nausea and vomiting, along with poor nutrition and blood glucose-level problems, especially in people with diabetes.

How does the body normally metabolize glucose?

A person without diabetes metabolizes glucose in certain ways. First, the energy for the body’s cells is provided by food. When that person consumes food with carbohydrates-bread, pasta, cakes, fruits, vegetables, and legumes-the carbohydrates are converted to simple sugars (for example, glucose) in the small intestine. As the level of glucose in the blood (often referred to as blood sugar) begins to rise, the pancreas produces insulin. This insulin helps muscles and the liver absorb the glucose, preparing it to be used for energy. When a person has diabetes, the normal process of breaking down sugar goes awry, stopping the body’s cells from receiving the energy they need to function properly.

What is meant by the body’s metabolism?

Metabolism (from the Greek metabole, meaning “change”) refers to all the physical and chemical processes involved in the activities of the body. It includes changing nutrients into usable energy for the body, the synthesis of proteins, helping with the physical construction of the body’s many cells and cell parts, eliminating cellular wastes, and helping with the production of body heat (in other words, it regulates the body’s temperature).

In general, how does the body respond after eating carbohydrates in terms of blood glucose levels?

The body does several things when a person eats a food containing carbohydrates. In general, the process is as follows:

1. As the blood glucose level rises, the pancreas produces insulin. This hormone prompts the cells to absorb blood sugar for energy or storage.

2. As the cells absorb blood glucose, levels of sugar in the bloodstream begin to fall.

3. As the blood glucose levels fall, the pancreas starts making glucagon. This hormone signals the liver to start releasing stored sugar.

4. The glucose and glucagon travel throughout the body, ensuring that cells-especially brain cells-have a steady supply of blood sugar.


What happens when a person ingests foods containing fats?

When a person ingests fats, many processes take place. In particular, if a person eats food that contains fat (mostly triglycerides), the fats go through the stomach and intestines. When the fat reaches the small intestine, it goes through a series of changes to make the fats easier to be used by the body, mainly for energy. An excess of fat in the system, especially if a person becomes overweight or obese, can lead to many health problems, including cardiovascular disease and diabetes. (For more about fats, diabetes, and obesity, see the chapter “Diabetes and Obesity.”)

How does insulin help many of the body’s cells?

Insulin produced by the pancreas helps many of the body’s cells, especially those in the liver, muscles, and fat tissue. The main activities insulin “tells” the cells to do are: 1) to absorb glucose, fatty acids, and amino acids and stop breaking down glucose, fatty acids, and amino acids; 2) to turn glycogen into glucose, fats into fatty acids and glycerol, and proteins into amino acids; 3) to start building glycogen from glucose, fats (triglycerides) from glycerol and fatty acids, and proteins from amino acids.

How does the body store fat?

The body stores fat through the activity of many components. In particular, after insulin is secreted (see above), fatty acids are absorbed from the bloodstream into fat, liver, and muscle cells. The insulin acts on the cells, turning the fatty acids into fat molecules, which are stored as fat droplets. The fat cells can also take up glucose and amino acids, converting them to fat molecules. (For more about insulin and the pancreas, see below.)

What is the link between lipoprotein lipases and insulin in the body?

There is a definite link between lipoprotein lipases-the enzymes that break fats into fatty acids-and insulin in the body. In fact, the activity of the lipoprotein lipases greatly depends on the level of insulin in a person’s body. If insulin is high, then the lipoprotein lipases enzyme activity is also high; if the insulin is low, then so is the lipoprotein lipases enzyme activity.

Why does the body “prefer” to store fat rather than carbohydrates for energy?

The body’s fat cells grab fat from the system (after a person eats certain fats) more readily than carbohydrates. This is because of how much energy is expended to store fats versus carbohydrates. For example, if a person has around 100 calories in extra fat in the bloodstream, the fat cells only use up about 2.5 calories of energy to store that fat. If the person has 100 calories in extra glucose in his or her bloodstream, then it takes about 23 calories of energy to convert the glucose for the body’s fat stores.


What are accessory organs in the digestive system?

The pancreas, liver, and gallbladder are considered to be accessory organs in digestion. They are “accessory” in that none of these organs is a part of the digestive tract (beginning at the mouth and ending at the anus). But overall, they contribute important chemicals, enzymes, and lubricants necessary for the functioning of the digestive system.

What cells in the pancreas are important to digestion?

The pancreas has both endocrine and exocrine cells, supplying hormones and enzymes for digestion, respectively. The acinar cells (also called acini, from the Latin, meaning “grapes,” as their structure resembles clusters of grapes) are responsible for secreting digestive enzymes. (For more about how the pancreas acts as an endocrine gland, see the chapter “How Diabetes Affects the Endocrine System.”)

How are the enzymes in the pancreas involved in the small intestine?

The enzymes in the pancreas are responsible for a major part of chemical digestion in the small intestine. They reach the small intestine through a long tube that links the bile duct with the pancreatic duct. The secretions of digestive juices from the pancreas are highly alkaline (generally the opposite of acidic; a pH of 8) and are composed of many enzymes. These enzymes break down all types of food and neutralize what is called the chyme, or the acidic, soupy mixture of partially digested food that forms in the stomach (it’s the ingested food that sits in the stomach for one to three hours after eating). In most adults, nearly 1.6 quarts (1.5 liters)-sometimes more-of these digestive juices are secreted by the cells of the pancreas daily.

What is the function of the liver?

The liver is the second-largest organ in the body (the skin is the largest). It weighs 3 pounds (1.4 kilograms) in adults and represents about 2.5 percent of the total body weight. This large organ has more than 500 vital functions, too many to examine in this text. It is often called the “wastebasket” of the human body because it collects and processes so many substances essential for the body to function properly. For example, the liver is responsible for filtering the blood, making special proteins that protect the body from infection, removing alcohol and poisons from the blood, and producing proteins that help the blood to clot. In terms of the body and diabetes, the liver plays a crucial role in managing blood sugar levels.

The pancreas not only provides vital hormones to the blood, it also excretes digestive enzymes.

What is a unique feature of the liver?

The liver is unique in the human body, as it is the only organ that can regenerate itself. As much as 75 percent of the liver may be removed, and in most people (usually not people with hepatitis C; see sidebar), it will grow back to the same shape and form within a few weeks.

What are the digestive functions of the liver?

The liver’s major function in the digestive tract is to produce and secrete bile. Other liver functions that help digestion include separating and filtering waste products from food nutrients, storing glucose until it’s needed, and producing many chemical substances-such as cholesterol-that the body and cells need to survive.

What is hepatitis, and can a person with diabetes contract it?

The most common forms of hepatitis are A, B, and C (there are also hepatitis D and E)-and all have to do with the liver. According to many studies, there is often a connection between diabetes and hepatitis. The connection varies depending on the type of hepatitis. The following gives a general description and how it may be linked to diabetes:

Hepatitis A-This type of hepatitis is a virus that is transmitted from person to person mainly through feces, causing an inflammation of the liver. The most common transmission occurs when a person ingests food or water contaminated with the feces of an infected person. It can also spread from contaminated shellfish and from foods or drinks prepared by someone with the virus. It is divided by type into acute (with short-lived symptoms that usually begin suddenly) and chronic (in which the liver is inflamed for at least six months, producing mild symptoms). Acute symptoms can include stomach pain, jaundice, light-colored stools, and dark yellow urine. Hepatitis A can be transmitted to a person with diabetes, but there is thought to be no solid connection between the two conditions. Studies are being conducted to see whether a person with acute viral hepatitis also has underlying diabetes-and whether that can cause the hepatitis to become chronic.

Hepatitis B-This type of hepatitis is a virus usually spread when blood or other body fluids from an infected person enters another person’s body. For example, decades ago, when the blood supply was not as diligently screened, people could contract hepatitis B through routine blood transfusions. It is also spread through sexual contact and from an infected mother to her baby during childbirth. For people with diabetes, hepatitis B has been spread through the sharing of glucose meters, lancets, and other supplies such as syringes and insulin pens, which is why it is best for a person with diabetes not to share diabetic supplies.

Hepatitis C-This type of hepatitis is a virus and is the leading cause of liver damage in the United States. It often takes decades to manifest itself. Once it develops, the disease can lead to scarring of tissues that can eventually destroy the liver. At this stage, a liver transplant is often necessary, but even so, it will not eradicate the virus from the body. There is also a hepatitis C–diabetes connection: People with hepatitis C have a higher prevalence of type 2 diabetes, and people with diabetes are more likely than most people to have hepatitis C. In fact, some researchers suggest that type 2 diabetes is often a symptom of hepatitis C.

What does the liver produce if a person’s body runs low on glucose for energy?

If a person needs energy and is low on glucose, then the liver produces ketones. The ketones are made from fatty acids in the body in a process called ketogenesis. Ketones circulate in the body at any given time but their levels seem to be higher when a person is exercising for sustained periods or is fasting.

What is the connection between ketones and diabetes?

For a person with diabetes, the level of ketones increases when the body fails to produce enough insulin. Ketones are also produced when there is not enough insulin available to increase the glucose in the body. If glucose levels are not brought into balance, a condition called diabetic ketoacidosis can occur.

What is fatty liver disease?

Fatty liver disease is caused by an increase of fat buildup in the liver. It is usually broken down into two types: alcoholic and nonalcoholic fatty liver disease (NAFLD). As the name implies, alcoholic fatty liver disease is prevalent in alcoholics. Nonalcoholic fatty liver disease is most often found in overweight people, including those who have type 2 diabetes. In fact, it is estimated that of all overweight people, around 54 percent to as high as 74 percent have NAFLD. This disease can harm the liver and may add to a person’s risk for heart disease. In the United States alone, this disease affects around one in five American adults-some research suggests one in three-especially those who are overweight, obese, and/or who suffer from diabetes.

What conditions can cause a fatty liver?

At one time, it was associated with people who drank excessively, but now, between lack of exercise and poor diet, more people are experiencing the disease. In addition, research has shown that certain people may carry a special genetic variation called single nucleotide polymorphism (or SNP) that make them more susceptible to developing a fatty liver. Even the types of fat a person eats can play a part in developing a fatty liver. For example, some fats can cause inflammation that can increase the risk of liver disease, such as some polyunsaturated fats in red meat that contain omega-6 fatty acids. The best way to stave off nonalcoholic fatty liver disease is obvious: lose weight and eat more healthful foods.

Do only overweight or obese people with diabetes have fatty livers?

No, even people who have diabetes and are at a normal body weight can have a fatty liver, although overweight people are more likely to have the disease. Research has shown that the type of diabetes a person has can affect the health of the liver. For example, people who have type 1 diabetes with controlled hyperglycemia (high blood glucose levels) are much less likely to have a fatty liver. But there is a 70 percent correlation between liver disease and type 2 diabetes. In fact, some studies estimate that around 98 percent of people with any type of diabetes have some form of fatty liver disease.

When fat builds up in the liver, it can cause fatigue and abdominal pain. If left untreated, the next stage can be cirrhosis, liver failure, and cancer.

Too much cholesterol or bilirubin in the blood can lead to the formation of gallstones. The material builds up in the gallbladder, then calcifies to form the stones.

What is bile?

Bile is an alkaline liquid composed mostly of water, bile salts, bile pigments (bilirubin), fats, and cholesterol. It is essential for digestion of fats because it breaks down fats into fatty acids, which can then be absorbed by the digestive tract. Bile gets its color from bilirubin, a waste product from the breakdown of worn-out red blood cells.

What is the major purpose of the gallbladder?

The gallbladder (from the Latin galbinus, meaning “greenish yellow”), a pear-shaped, small sac, is mainly a storage vessel. It is connected by ducts to both the liver and small intestine. It stores bile until it is needed in the duodenum. Its name is derived from its usual color of green from the accumulation of bile. The gallbladder is a nonessential organ. It may be removed surgically-in a procedure called a cholecystectomy-if it is diseased or injured. Once the gallbladder is removed, bile flows from the liver directly to the small intestine, where it continues to aid digestion. The excess bile is then stored in the bile duct. This is why individuals who have had their gallbladder removed usually lead normal lives and enjoy a regular diet.

What are the two types of gallstones?

Gallstones are hardened masses (stones) of bile. Gallstones form when bile contains too much cholesterol, bile salts, or bilirubin. The two types of gallstones are cholesterol stones and pigment stones. Cholesterol stones are more common, accounting for nearly 80 percent of all instances of gallstones. They are usually yellow-green in color and are made primarily of hardened cholesterol. An insufficient amount of water may also contribute to the development of cholesterol gallstones. Pigment stones are small, dark stones made of bilirubin.

Is there a connection between gallstones and diabetes?

No one understands why, but in general, people with diabetes have more gallstone problems than the general population. One possible reason may be that people with diabetes-in particular, type 2-are often overweight or obese, both conditions being linked to the formation of gallstones. In addition, triglycerides are linked to type 2 diabetes and are thought to encourage gallstone formation.

How serious are gallstones?

Many individuals who have gallstones are asymptomatic, and treatment is not necessary. However, if the stones block a duct, bile may be prevented from entering the small intestine. Surgery is then often recommended to remove the gallbladder. Women between ages 20 and 60 are twice as likely to develop gallstones as men.