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 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.
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 (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.
Which Foods Cause Gas?
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.
What Are Some Symptoms and Problems of Gas?
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.
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 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.
How Is Gas Treated?
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.
Conclusion
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.
Source: National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, May 1995
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