Gas in the Digestive Tract
Everyone has gas and eliminates it by burping or passing it
through the rectum. However, many people think they have too much
gas when they really have normal amounts. Most people produce about
1 to 4 pints a day and pass gas about 14 times a day.
Gas is made primarily of odorless vapors-carbon dioxide, oxygen,
nitrogen, hydrogen, and sometimes methane. The unpleasant odor of
flatulence comes from bacteria in the large intestine that release
small amounts of gases that contain sulfur.
Although having gas is common, it can be uncomfortable and
embarrassing. Understanding causes, ways to reduce symptoms, and
treatment will help most people find relief.
What causes gas?
Gas in the digestive tract (that is, the esophagus, stomach,
small intestine, and large intestine) comes from two sources:
- swallowed air
- normal breakdown of certain undigested foods by harmless
bacteria naturally present in the large intestine
Air swallowing (aerophagia) is a common cause of gas in the
stomach. Everyone swallows small amounts of air when eating and
drinking. However, eating or drinking rapidly, chewing gum, smoking,
or wearing loose dentures can cause some people to take in more
Burping, or belching, is the way most swallowed air-which
contains nitrogen, oxygen, and carbon dioxide-leaves the stomach.
The remaining gas moves into the small intestine, where it is
partially absorbed. A small amount travels into the large intestine
for release through the rectum. (The stomach also releases carbon
dioxide when stomach acid and bicarbonate mix, but most of this gas
is absorbed into the bloodstream and does not enter the large
Breakdown of Undigested Foods
The body does not digest and absorb some carbohydrates (the
sugar, starches, and fiber found in many foods) in the small
intestine because of a shortage or absence of certain enzymes.
This undigested food then passes from the small intestine into
the large intestine, where normal, harmless bacteria break down the
food, producing hydrogen, carbon dioxide, and, in about one-third of
all people, methane. Eventually these gases exit through the
People who make methane do not necessarily pass more gas or have
unique symptoms. A person who produces methane will have stools that
consistently float in water. Research has not shown why some people
produce methane and others do not.
Foods that produce gas in one person may not cause gas in
another. Some common bacteria in the large intestine can destroy the
hydrogen that other bacteria produce. The balance of the two types
of bacteria may explain why some people have more gas than others.
Which foods cause gas?
Most foods that contain carbohydrates can cause gas. By contrast,
fats and proteins cause little gas.
The sugars that cause gas are raffinose, lactose, fructose, and
Beans contain large amounts of this complex
sugar. Smaller amounts are found in cabbage, brussels sprouts,
broccoli, asparagus, other vegetables, and whole grains.
Lactose is the natural sugar in milk. It is
also found in milk products, such as cheese and ice cream, and
processed foods, such as bread, cereal, and salad dressing. Many
people, particularly those of African, Native American, or Asian
background, normally have low levels of the enzyme lactase needed to
digest lactose after childhood. Also, as people age, their enzyme
levels decrease. As a result, over time people may experience
increasing amounts of gas after eating food containing lactose.
Fructose is naturally present in onions,
artichokes, pears, and wheat. It is also used as a sweetener in some
soft drinks and fruit drinks.
Sorbitol is a sugar found naturally in fruits,
including apples, pears, peaches, and prunes. It is also used as an
artificial sweetener in many dietetic foods and sugarfree candies
Most starches, including potatoes, corn, noodles, and wheat,
produce gas as they are broken down in the large intestine. Rice is
the only starch that does not cause gas.
Many foods contain soluble and insoluble fiber. Soluble fiber
dissolves easily in water and takes on a soft, gel-like texture in
the intestines. Found in oat bran, beans, peas, and most fruits,
soluble fiber is not broken down until it reaches the large
intestine, where digestion causes gas.
Insoluble fiber, on the other hand, passes essentially unchanged
through the intestines and produces little gas. Wheat bran and some
vegetables contain this kind of fiber.
What are some symptoms and problems of gas?
The most common symptoms of gas are flatulence, abdominal
bloating, abdominal pain, and belching. However, not everyone
experiences these symptoms. The determining factors probably are how
much gas the body produces, how many fatty acids the body absorbs,
and a person's sensitivity to gas in the large intestine.
An occasional belch during or after meals is normal and releases
gas when the stomach is full of food. However, people who belch
frequently may be swallowing too much air and releasing it before
the air enters the stomach.
Sometimes a person with chronic belching may have an upper GI
disorder, such as peptic ulcer disease, gastroesophageal reflux
disease (GERD), or gastroparesis.
Occasionally, some people believe that swallowing air and
releasing it will relieve the discomfort of these disorders, and
this person may intentionally or unintentionally develop a habit of
belching to relieve discomfort.
Gas-bloat syndrome may occur after fundoplication surgery to
correct GERD. The surgery creates a one-way valve between the
esophagus and stomach that allows food and gas to enter the stomach
but often prevents normal belching and the ability to vomit. It
occurs in about 10 percent of people who have this surgery but may
improve with time.
Another common complaint is passage of too much gas through the
rectum (flatulence). However, most people do not realize that
passing gas 14 to 23 times a day is normal. Too much gas may be the
result of carbohydrate malabsorption.
Many people believe that too much gas causes abdominal bloating.
However, people who complain of bloating from gas often have normal
amounts and distribution of gas. They actually may be unusually
aware of gas in the digestive tract.
Doctors believe that bloating is usually the result of an
intestinal disorder, such as irritable bowel syndrome (IBS). The
cause of IBS is unknown, but may involve abnormal movements and
contractions of intestinal muscles and increased pain sensitivity in
the intestine. These disorders may give a sensation of bloating
because of increased sensitivity to gas.
Any disease that causes intestinal inflammation or obstruction,
such as Crohn's disease or colon cancer, may also cause abdominal
bloating. In addition, people who have had many operations,
adhesions (scar tissue), or internal hernias may experience bloating
or pain. Finally, eating a lot of fatty food can delay stomach
emptying and cause bloating and discomfort, but not necessarily too
Abdominal Pain and Discomfort
Some people have pain when gas is present in the intestine. When
pain is on the left side of the colon, it can be confused with heart
disease. When the pain is on the right side of the colon, it may
mimic gallstones or appendicitis.
What diagnostic tests are used?
Because gas symptoms may be caused by a serious disorder, those
causes should be ruled out. The doctor usually begins with a review
of dietary habits and symptoms. The doctor may ask the patient to
keep a diary of foods and beverages consumed for a specific time
If lactase deficiency is the suspected cause of gas, the doctor
may suggest avoiding milk products for a period of time. A blood or
breath test may be used to diagnose lactose intolerance.
In addition, to determine if someone produces too much gas in the
colon or is unusually sensitive to the passage of normal gas
volumes, the doctor may ask patients to count the number of times
they pass gas during the day and include this information in a
Careful review of diet and the amount of gas passed may help
relate specific foods to symptoms and determine the severity of the
Because the symptoms that people may have are so variable, the
physician may order other types of diagnostic tests in addition to a
physical exam, depending on the patient's symptoms and other
How is gas treated?
Experience has shown that the most common ways to reduce the
discomfort of gas are changing diet, taking medicines, and reducing
the amount of air swallowed.
Doctors may tell people to eat fewer foods that cause gas.
However, for some people this may mean cutting out healthy foods,
such as fruits and vegetables, whole grains, and milk products.
Doctors may also suggest limiting high-fat foods to reduce
bloating and discomfort. This helps the stomach empty faster,
allowing gases to move into the small intestine.
Unfortunately, the amount of gas caused by certain foods varies
from person to person. Effective dietary changes depend on learning
through trial and error how much of the offending foods one can
Many nonprescription, over-the-counter medicines are available to
help reduce symptoms, including antacids with simethicone. Digestive
enzymes, such as lactase supplements, actually help digest
carbohydrates and may allow people to eat foods that normally cause
Antacids, such as Mylanta II, Maalox II, and Di-Gel, contain
simethicone, a foaming agent that joins gas bubbles in the stomach
so that gas is more easily belched away. However, these medicines
have no effect on intestinal gas. Dosage varies depending on the
form of medication and the patient's age.
The enzyme lactase, which aids with lactose digestion, is
available in caplet and chewable tablet form without a prescription
(Lactaid and Lactrase). Chewing lactase tablets just before eating
helps digest foods that contain lactose. Also, lactose-reduced milk
and other products are available at many grocery stores (Lactaid and
Beano, an over-the-counter digestive aid, contains the
sugar-digesting enzyme that the body lacks to digest the sugar in
beans and many vegetables. The enzyme comes in liquid and tablet
form. Five drops are added per serving or 1 tablet is swallowed just
before eating to break down the gas-producing sugars. Beano has no
effect on gas caused by lactose or fiber.
Doctors may prescribe medicines to help reduce symptoms,
especially for people with a disorder such as IBS.
Reducing Swallowed Air
For those who have chronic belching, doctors may suggest ways to
reduce the amount of air swallowed. Recommendations are to avoid
chewing gum and to avoid eating hard candy. Eating at a slow pace
and checking with a dentist to make sure dentures fit properly
should also help.
Although gas may be uncomfortable and embarrassing, it is not
life-threatening. Understanding causes, ways to reduce symptoms, and
treatment will help most people find some relief.
Points to remember
- Everyone has gas in the digestive tract.
- People often believe normal passage of gas to be
- Gas comes from two main sources: swallowed air and normal
breakdown of certain foods by harmless bacteria naturally present
in the large intestine.
- Many foods with carbohydrates can cause gas. Fats and proteins
cause little gas.
- Foods that may cause gas include
- vegetables, such as broccoli, cabbage, brussels sprouts,
onions, artichokes, and asparagus
- fruits, such as pears, apples, and peaches
- whole grains, such as whole wheat and bran
- soft drinks and fruit drinks
- milk and milk products, such as cheese and ice cream, and
packaged foods prepared with lactose, such as bread, cereal, and
- foods containing sorbitol, such as dietetic foods and
sugarfree candies and gums
- The most common symptoms of gas are belching, flatulence,
bloating, and abdominal pain. However, some of these symptoms are
often caused by an intestinal disorder, such as irritable bowel
syndrome, rather than too much gas.
- The most common ways to reduce the discomfort of gas are
changing diet, taking nonprescription medicines, and reducing the
amount of air swallowed.
- Digestive enzymes, such as lactase supplements, actually help
digest carbohydrates and may allow people to eat foods that
normally cause gas.
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