is the sugar making up around 2-8% of the solids in milk. The name comes from
the Latin for milk, plus the -ose ending used to name sugars. Lactose is a disaccharide
consisting of two subunits, a galactose and a glucose linked together. Its empirical
formula is C12H22O11 and its molecular weight
is 342.3. In the young of mammals, an enzyme called lactase is secreted by the
intestinal villi, and this enzyme cleaves the molecule into its two subunits for
as the young grow up, production of lactase gradually ceases, and they are then
unable to metabolise lactose. This is perhaps an evolutionary mechanism to enforce
weaning of the young. This loss of lactase on maturation is also the default pattern
in most of the human race. However, in some humans (mostly those with ancestry
in the approximate geographic region of Europe, the Middle East and India), the
enzyme is retained in adulthood, and dairy products form a substantial part of
the adult diet. It would appear that millennia of animal husbandry in those regions,
with the milking of sheep, cattle, goats and water buffalo, has caused an evolutionary
adaptation to an adult diet containing milk. (This process of retaining infant
characteristics into adulthood is one of the simplest routes of evolutionary adaptation,
and is known as neoteny) The fact that at least some humans have made adaptations
to lactose in the adult diet would, incidentally, appear to cast doubt on some
arguments by proponents of the so-called 'Stone-age diet', who argue that human
metabolic needs have not changed since the last ice age.
is lactose intolerance?
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.
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
are the symptoms of lactose intolerance?
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
is lactose intolerance diagnosed?
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
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.
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.
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.
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
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.
causes lactose intolerance?
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
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.
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.
is at risk for lactose intolerance?
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
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.
is lactose intolerance treated?
intolerance is easy to treat. No treatment can improve the body’s ability to produce
lactase, but symptoms can be controlled through diet.
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
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
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.
is nutrition balanced?
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.
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.
of calcium to consume daily, in milligrams (mg)|
addition, pregnant and nursing women need between 1,200 and 1,500 mg of calcium
meals, people with lactose intolerance should make sure that each day’s diet includes
enough calcium, even if dairy products are not included. Many non-dairy foods
are high in calcium, including dark green vegetables such as broccoli, or fish
with soft, edible bones, such as salmon and sardines. To help in planning a high-calcium,
low-lactose diet, the table that follows lists some common foods that are good
sources of dietary calcium and shows how much lactose they contain.
research shows that yogurt with active cultures may be a good source of calcium
for many people with lactose intolerance. Even though yogurt is fairly high in
lactose, the bacterial cultures used to make it produce some of the lactase enzyme
required for proper digestion.
many foods can provide the calcium and other nutrients the body needs, even when
intake of milk and dairy products is limited. However, factors other than calcium
and lactose content should be kept in mind when planning a diet. Some vegetables
that are high in calcium (Swiss chard, spinach, and rhubarb, for example) are
not listed in the chart because the body cannot use the calcium they contain because
these foods also contain substances called oxalates, which stop calcium absorption.
and Lactose in Common Foods
fortified, 1 cup||200–300
|Sardines, with edible
bones, 3 oz.||270
|Salmon, canned, with
edible bones, 3 oz.||205
|Broccoli, raw, 1 cup||90
|Orange, 1 medium||50
|Pinto beans, 1/2 cup||40
|Tuna, canned, 3 oz.||10
|Lettuce greens, 1/2
plain, low-fat, 1 cup||415
|Milk, reduced fat,
1 cup||295 mg||11
cheese, 1 oz.||270
|Ice cream, 1/2 cup||85
|Cottage cheese, 1/2
from Manual of Clinical Dietetics. 6th ed. American Dietetic Association,
2000; and Soy Dairy Alternatives. Available at: http://www.soyfoods.org/.
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.
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
is hidden lactose?
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
and other baked goods
potatoes, soups, and breakfast drinks
- lunch meats (other
and other snacks
for pancakes, biscuits, and cookies
products labeled non-dairy, such as powdered coffee creamer and whipped toppings,
may actually include ingredients that are derived from milk and therefore contain
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
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.
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.
Dietetic Association (ADA)
120 South Riverside Plaza
Foundation for Functional Gastrointestinal Disorders (IFFGD) Inc.
Milwaukee, WI 53217
Phone: 1–888–964–2001 or 414–964–1799
Source: National Digestive
Diseases Information Clearinghouse
Bethesda, MD 20892–3570
National Digestive Diseases Information Clearinghouse (NDDIC) is a service of
the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK).
The NIDDK is part of the National Institutes of Health of the U.S. Department
of Health and Human Services. Established in 1980, the Clearinghouse provides
information about digestive diseases to people with digestive disorders and to
their families, health care professionals, and the public. The NDDIC answers inquiries,
develops and distributes publications, and works closely with professional and
patient organizations and Government agencies to coordinate resources about digestive