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Gas
In The Digestive Tract
Why Do I Have
Gas?
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 3 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.
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 (colon).
Swallowed air
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 air.
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 intestine.)
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 harmless and normal bacteria break down the food, producing
hydrogen, carbon dioxide, and, in about one-third of all people,
methane. Eventually these gases exit through the rectum.
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.
Most foods that contain
carbohydrates can cause gas. By contrast, fats and proteins
cause little gas.
Sugars
The sugars that cause
gas are: raffinose, lactose, fructose, and sorbitol.
Raffinose
Beans contain large amounts of this complex sugar. Smaller amounts
are found in cabbage, brussels sprouts, broccoli, asparagus,
other vegetables, and whole grains.
Lactose
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, have
low levels of the enzyme lactase needed to digest lactose. 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
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
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 and gums.
Starches
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.
Fiber
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.
The most common symptoms
of gas are belching, flatulence, abdominal bloating, and abdominal
pain. 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. Chronic symptoms caused by too
much gas or by a serious disease are rare.
Belching
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 gastritis.
Believing that swallowing
air and releasing it will relieve the discomfort of these disorders,
this person may unintentionally develop a habitual cycle of
belching and discomfort. Frequently, the pain continues or worsens,
leading the person to believe he or she has a serious disorder.
Two rare chronic
gas syndromes are associated with belching: Meganblase syndrome
and gas-bloat syndrome. The Meganblase syndrome, which causes
chronic belching, is characterized by severe air swallowing
and an enlarged bubble of gas in the stomach following heavy
meals. The resulting fullness and shortness of breath may mimic
a heart attack.
Gas-bloat syndrome
may occur after 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.
Flatulence
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. Although rare, too much gas may be the result of severe
carbohydrate malabsorption or overactive bacteria in the colon.
Abdominal bloating
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 motility disorder,
such as IBS. Motility disorders are characterized by abnormal
movements and contractions of intestinal muscles. These disorders
may give a false sensation of bloating because of increased
sensitivity to gas.
Splenic-flexure syndrome
is a chronic disorder that seems to be caused by trapped gas
at bends (flexures) in the colon. Symptoms include bloating,
muscle spasms, and upper abdominal discomfort. Splenic-flexure
syndrome often accompanies IBS.
Any disease that
causes intestinal 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 much gas.
Abdominal pain and
discomfort
Some people have
pain when gas is present in the intestine. When gas collects
on the left side of the colon, the pain can be confused with
heart disease. When it collects on the right side of the colon,
the pain may feel like the pain associated with gallstones or
appendicitis.
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 period.
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 diary.
Careful review of
diet and the amount of gas passed may help relate specific foods
to symptoms and determine the severity of the problem.
If a patient complains
of bloating, the doctor may examine the abdomen for the sound
of fluid movement to rule out ascites (build up of fluid in
the abdomen) and for signs of inflammation to rule out diseases
of the colon.
The possibility of
colon cancer is usually considered in people 50 years of age
and older and in those with a family history of colorectal cancer,
particularly if they have never had a colon examination (sigmoidoscopy
or colonoscopy). These tests may also be appropriate for someone
with unexplained weight loss, diarrhea, or blood not visible
in the stool.
For those with chronic
belching, the doctor will look for signs or causes of excessive
air swallowing. If needed, an upper GI series (x-ray to view
the esophagus, stomach, and upper small intestine) may be performed
to rule out disease.
The most common ways
to reduce the discomfort of gas are changing diet, taking medicines,
and reducing the amount of air swallowed.
Diet
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 handle.
Nonprescription
medicines
Many nonprescription,
over-the-counter medicines are available to help reduce symptoms,
including antacids with simethicone and activated charcoal. Digestive
enzymes, such as lactase supplements, actually help digest carbohydrates
and may allow people to eat foods that normally cause gas.
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. The recommended dose is 2 to 4 tablespoons
of the simethicone preparation taken 1/2 to 2 hours after meals.
Activated charcoal
tablets (Charcocaps) may provide relief from gas in the colon.
Studies have shown that when taken before and after a meal,
intestinal gas is greatly reduced. The usual dose is 2 to 4
tablets taken just before eating and 1 hour after meals.
The enzyme lactase,
which aids with lactose digestion, is available in liquid and
tablet form without a prescription (Lactaid, Lactrase, and Dairy
Ease). Adding a few drops of liquid lactase to milk before drinking
it or 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 Dairy
Ease).
Beano, a newer 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 form. Three to 10 drops are added
per serving just before eating to break down the gas-producing
sugars. Beano has no effect on gas caused by lactose or fiber.
Prescription medicines
Doctors may prescribe
medicines to help reduce symptoms, especially for people with
a motility disorder, such as IBS. Promotility or prokinetic drugs,
such as metoclopramide (Reglan) and cisapride (Propulsid), may
move gas through the digestive tract quickly.
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 excessive.
- 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:
- Beans
- 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 salad dressing
- 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 motility 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 or prescription 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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National Digestive
Diseases Information Clearinghouse
2 Information
Way
Bethesda, MD 20892-3570
E-mail: mailto:nddic@info.niddk.nih.gov
NIH Publication
No. 97-883
May 1995
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