Lactose
intolerance is the inability to digest significant amounts of
lactose, the major sugar found in milk. Lactose intolerance is
caused by a shortage of the enzyme lactase, which is produced by
the cells that line the small intestine. Lactase breaks down
milk sugar into two simpler forms of sugar called glucose and
galactose, which are then absorbed into the bloodstream. Not all
people deficient in lactase have the symptoms commonly
associated with lactose intolerance, but those who do are said
to have lactose intolerance.
People
sometimes confuse lactose intolerance with cow’s milk
intolerance because the symptoms are often the same. However,
lactose intolerance and cow’s milk intolerance are not related.
Being intolerant to cow’s milk is an allergic reaction triggered
by the immune system. Lactose intolerance is a problem caused by
the digestive system.
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People who
do not have enough lactase to digest the amount of lactose they
consume may feel very uncomfortable when they digest milk
products. Common symptoms, which range from mild to severe,
include nausea, cramps, bloating, gas, and diarrhea. Symptoms
begin about 30 minutes to 2 hours after eating or drinking foods
containing lactose. The severity of symptoms depends on many
factors, including the amount of lactose a person can tolerate
and a person’s age, ethnicity, and digestion rate.
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Lactose
intolerance can be hard to diagnose based on symptoms alone.
People sometimes think they suffer from lactose intolerance
because they have the symptoms associated with the disorder, not
knowing other conditions such as irritable bowel syndrome can
cause similar symptoms. A doctor can use tests to diagnose
lactose intolerance but may first recommend eliminating cow’s
milk from the diet to see if the symptoms go away.
The most
common tests used to measure the absorption of lactose in the
digestive system are the lactose tolerance, hydrogen breath, and
stool acidity tests.
- The
Lactose Tolerance Test. This test requires fasting (not
eating) before the test and then drinking a liquid that
contains lactose. Several blood samples are then taken over a
2-hour period to measure the person’s blood glucose (blood
sugar) level. These measures indicate how well the body is
able to digest lactose.
Normally, when lactose reaches the digestive system, the
lactase enzyme breaks it down into glucose and galactose. The
liver then changes the galactose into glucose, which enters
the bloodstream and raises the person’s blood glucose level.
If, however, lactose is incompletely broken down, the blood
glucose level does not rise and a diagnosis of lactose
intolerance is confirmed.
- The
Hydrogen Breath Test. This test measures the amount of
hydrogen in a person’s breath. Very little hydrogen is
normally detectable. However, undigested lactose in the colon
is fermented by bacteria and produces various gases, including
hydrogen. The hydrogen is absorbed from the intestines,
carried through the bloodstream to the lungs, and exhaled. In
this test, the person drinks a lactose-loaded beverage and the
breath is analyzed at regular intervals. Raised levels of
hydrogen in the breath indicate improper digestion of lactose.
Certain foods, medications, and cigarettes can affect the
accuracy of the test and should be avoided before taking the
test. People should check with their doctor to make sure they
are not taking medications that may interfere with test
results.
The
lactose tolerance and hydrogen breath tests are not given to
infants younger than 6 months of age. A large lactose load can
be dangerous prior to this age, as infants are more likely to
become dehydrated from diarrhea that can be caused by lactose
intolerance.
- Stool
Acidity Test. This test may be used for infants and young
children to measure the amount of acid in their stool.
Undigested lactose fermented by bacteria in the colon creates
lactic acid and other fatty acids that can be detected in a
stool sample. Glucose may also be present in the sample as a
result of unabsorbed lactose in the colon.
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Some
causes of lactose intolerance are well known. Primary lactase
deficiency is a condition that develops over time. After about
age 2 the body begins to produce less lactase, though most
people will not notice symptoms until they are much older.
Secondary lactase deficiency occurs when injury to the small
intestine or certain digestive diseases reduce the amount of
lactase a person produces. These diseases include celiac
disease, inflammatory bowel disease, and Crohn’s disease.
Researchers have identified a genetic link for lactose
intolerance. Some people are born with a likelihood of
developing primary lactase deficiency because it has been passed
to them genetically (inherited from their parents). This
discovery may be useful in developing a diagnostic test to
identify people with the condition.
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Between
30 and 50 million Americans are lactose intolerant and certain
ethnic and racial populations are more affected than others. Up
to 80 percent of African Americans, 80 to 100 percent of
American Indians, and 90 to 100 percent of Asian Americans are
lactose intolerant. The condition is least common among people
of northern European descent.
Babies
that are born prematurely are also more likely to be lactose
intolerant, because lactase levels do not increase until the
third trimester of a woman’s pregnancy.
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Lactose
intolerance is easy to treat. No treatment can improve the
body’s ability to produce lactase, but symptoms can be
controlled through diet.
Young
children and infants with lactase deficiency should not consume
lactose-containing formulas or foods until they are able to
tolerate lactose digestion. Most older children and adults do
not have to avoid lactose completely, but people differ in the
amounts and types of foods they can handle. For example, one
person may have symptoms after drinking a small glass of milk,
while another can drink one glass but not two. Others may be
able to manage ice cream and aged cheeses, such as cheddar and
Swiss, but not other dairy products. People can also tolerate
more lactose by having smaller amounts of it at one time. The
level of dietary control needed with lactose intolerance depends
on how much lactose a person’s body can handle.
For
those who react to very small amounts of lactose or have trouble
limiting their intake of foods that contain it, the lactase
enzyme is available without a prescription to help people digest
foods that contain lactose. The tablets are taken with the first
bite of dairy food. Lactase enzyme is also available as a
liquid. Adding a few drops of the enzyme makes lactose more
digestible for people with lactose intolerance.
Lactose-reduced milk and other products are available at most
supermarkets. The milk contains all of the nutrients found in
regular milk and remains fresh for about the same length of
time, or longer if it is super-pasteurized.
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Milk and
other dairy products are a major source of nutrients in the
American diet. One of the most important of these nutrients is
calcium. Calcium is essential for the growth and repair of bones
throughout life. In the middle and later years, a shortage of
calcium may lead to thin, fragile bones that break easily, a
condition called osteoporosis. A concern for both children and
adults with lactose intolerance is getting enough calcium in a
diet that includes little or no dairy products.
The
Institute of Medicine released a report listing the requirements
for daily calcium intake. How much calcium a person needs to
maintain good health varies by age group. Recommendations from
the report are shown in the following table.
Calcium and Lactose in Common Foods
|
|
Vegetables |
Calcium Content |
Lactose Content |
|
Soymilk, fortified, 1 cup |
200–300 mg |
0 |
|
Sardines, with edible bones, 3 oz. |
270 mg |
0 |
|
Salmon, canned, with edible bones, 3 oz. |
205 mg |
0 |
|
Broccoli, raw, 1 cup |
90 mg |
0 |
|
Orange, 1 medium |
50 mg |
0 |
|
Pinto beans, 1/2 cup |
40 mg |
0 |
|
Tuna, canned, 3 oz. |
10 mg |
0 |
|
Lettuce greens, 1/2 cup |
10 mg |
0 |
|
|
|
Dairy Products |
|
Yogurt, plain, low-fat, 1 cup |
415 mg |
5 g |
|
Milk, reduced fat, 1 cup |
295 mg |
11 g |
|
Swiss cheese, 1 oz. |
270 mg |
1 g |
|
Ice cream, 1/2 cup |
85 mg |
6 g |
|
Cottage cheese, 1/2 cup |
75 mg |
2–3 g |
|
Adapted from Manual of Clinical Dietetics. 6th ed.
American Dietetic Association, 2000; and Soy Dairy
Alternatives. Available at:
www.soyfoods.org.
Calcium is absorbed and used only when there is enough
vitamin D in the body. A balanced diet should provide an
adequate supply of vitamin D from sources such as eggs and
liver. Sunlight also helps the body naturally absorb vitamin
D, and with enough exposure to the sun, food sources may not
be necessary.
Some
people with lactose intolerance may think they are not
getting enough calcium and vitamin D in their diet.
Consultation with a doctor or dietitian may be helpful in
deciding whether dietary supplements are needed. Taking
vitamins or minerals of the wrong kind or in the wrong
amounts can be harmful. A dietitian can help plan meals that
will provide the most nutrients with the least chance of
causing discomfort.
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Although milk and foods made from milk are the only natural
sources of lactose, it is often added to prepared foods.
People with very low tolerance for lactose should know about
the many food products that may contain even small amounts
of lactose, such as
-
bread and other baked goods
-
processed breakfast cereals
-
instant potatoes, soups, and breakfast drinks
-
margarine
-
lunch
meats (other than kosher)
-
salad
dressings
-
candies and other snacks
-
mixes
for pancakes, biscuits, and cookies
-
powdered meal-replacement supplements
Some
products labeled non-dairy, such as powdered coffee creamer
and whipped toppings, may actually include ingredients that
are derived from milk and therefore contain lactose.
Learn to read food labels with care, looking not only for
milk and lactose, but also for words such as whey, curds,
milk by-products, dry milk solids, and non-fat dry milk
powder. If any of these words are listed on a label, the
product contains lactose.
Lactose is also used in more than 20 percent of prescription
drugs and about 6 percent of over-the-counter medicines.
Many types of birth control pills contain lactose, as do
some tablets for stomach acid and gas. However, these
products typically affect only people with severe lactose
intolerance.
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Summary
Even
though lactose intolerance is common, it is not a threat to
good health. People who have trouble digesting lactose can
learn which dairy products and other foods they can eat
without discomfort and which ones they should avoid. Many
people can enjoy milk, ice cream, and other such products if
they eat them in small amounts or eat other food at the same
time. Others can use lactase liquid or tablets to help
digest the lactose. Even older women at risk for
osteoporosis and growing children who must avoid milk and
foods made with milk can meet most of their dietary needs by
eating greens, fish, and other calcium-rich foods that are
free of lactose. A carefully chosen diet, with calcium
supplements if the doctor or dietitian recommends them, is
the key to reducing symptoms.
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