WHEN MILK MAKES YOU SICK
presented for NABT Nov. 4, 1998
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INFORMATION FOR THE TEACHER
The milk produced by human mammary glands contains about the
same percentage of fat and less protein than cow's milk, but
it has more carbohydrate than that of other mammals. This carbohydrate
is in the form of lactose, a disaccharide. Mammalian infants
depend on their mother's milk for nourishrnent and immunological
protection. As a fresh, temperature-controlled, uncontaminated
food source, the milk from mother's breast is the best at meeting
the needs of the infant.
Except for certain populations of Western humans, milk consumption
stops ( or is greatly reduced) with weaning. It also happens
that in animals and most humans there is a decline in the level
of production of lactase with aging. Lactase is the enzyme that
digests milk sugar. (Curtis).
Since the sugar lactose is known to occur only in milk, the
advantages offered during infancy by this nutrient have been
studied. A component of the disaccharide, called galactose, is
not found free in nature, and in the synthesis of lactose by
the body a molecule of glucose is converted to galactose then
joined with another glucose molecule. A special advantage of
galactose in infant nutrition may be as a building material in
brain formation (glycolipids usually contain galactose). During
digestion lactose is digested and absorbed more slowly than other
sugars and may improve calcium retention and absorption for increasing
the mineral content of bone. In addition, growth factors, for
Lactobacillus bifudus present in human milk may establish
and maintain intestinal flora that are favorable. (Kleiner and
SYNTHESIS OF LACTOSE ( Keeton and Gould)
Galactose and glucose are combined in b-linkage to
yield the sugar lactose. The enzyme lactase, secreted by the
brush border cells of the intestinal epithelium, is capable of
splitting the molecule by hydrolysis. Without lactase the lactose
would pass into the colon undigested. (Keeton and Gould)
HYDROLYSIS OF LACTOSE (Keeton and Gould)
Since in most persons the body does not continue to secrete
large amounts of lactase enzyme after weaning, milk is indigestible
by a large majority of the world's population. The term "primary
adult lactase deficiency" refers to the NORMAL condition
for most people in whom lactase production has practically ceased.
The decline begins in youth and continues to decline, but the
onset of symptoms varies as to age. There is also a condition
known as "secondary lactase deficiency" which is temporary
and the result of a nutritional deficiency or digestive tract
Primary adult lactase deficiency leads to lactose malabsorption.
Lactose intolerance is a condition that results from the
malabsorption and produces symptoms that may cause the person
to avoid milk and other dairy products (some authors would prefer
to change the terrn to lactose malabsorption). (Hein) This is
not an allergy to milk. Allergy is associated with hypersensitivity
to the protein component of milk. (Houts)
GENETICS AND EVOLUTION:
Studies concerning the development of lactose deficiency centered
on an induction hypothesis have not been supported by the evidence.
Because the age at which the lactase activity declines varies
(from 3 years of age to more than 20 years), it was postulated
that the presence of lactose in the diet induced the intestinal
cells to continue lactase production. However, long-term feeding
of lactose to people who are malabsorbers did not result in an
increase in lactase activity. Also, depriving persons who are
absorbers of lactose for months at time, did not produce a loss
of lactase activity.
Although in the entire human population the trait may actually
be a genetic polymorphism, for the purpose of the study of a
family, it may be considered as autosomal recessive. (Allen and
At this time it appears that the loss or persistence of lactase
is a developmental change that is part of a genetic program.
The genetic difference seems to be in a regulatory gene and not
a structural gene. The most favored model is that of an autosomal
recessive condition. Pedigree information from families provide
support for this explanation. (May)
The following three sample pedigrees of American Indian families
illustrates this concept. Open symbols = lactose absorbers. Closed
symbols = lactose malabsorbers (from Newcomber et al). (May)
Evolution in pastoral ancestors that kept dairy animals, in
populations of Europeans and some Africans, is thought to have
produced the modern people who are lactose absorbers throughout
adult life. There may have been a selective advantage in that
group during times when milk and dairy products were relied on
almost exclusively for nutrition during shortages of other foods
(Houts). About 10,000 years have passed since the milking of
domestic animals was begun. Nigeria is a case that demonstrates
the significance of this lifestyle on human evolution. In the
southern region, where conditions are not favorable for cattle,
milk is not part of the diet. People there develop lactose intolerance.
In contrast, a nomadic tribe that has been raising milk cattle
for thousands of years remain lactose tolerant. Most African
Americans are descendants of the nonpastoral tribes from Western
Africa and do not tolerate lactose well. (Keeton and Gould)
In Caucasians only about 15% develop lactose intolerance while
80-90% of the African American and Asian populations are affected.
This has strong implications in relief efforts to aid those suffering
from starvation in various parts of the world. Powdered milk
has been a favorite food for shipping to needed areas but it
frequently causes illness in those people the aid is meant to
help. The sickness can lead to further malnutrition due to the
symptoms and resulting malabsorption (Martini).
Calcium deficiency can be a dietary concern for seven out
of ten African Americans who are lactose intolerant and who should
avoid eating large quantities of dairy products. Foods rich in
calcium that could be substituted in the diet include canned
fish with bones, dark green vegetables and beans. (Meister)
The presence of lactose in the colon is a boon for organisms
inhabiting that part of the digestive tract. However, the lactose
intolerant patient experiences acute gastrointestinal distress
as a result. The lactose acts as an osmotic laxative producing
diarrhea. (Greifru) This is usually accompanied by gas pains,
cramping, and bloating, Even small amounts of dairy products
or a single glass of milk can cause extreme discomfort . The
onset of symptoms normally occurs within 15 minutes to 3 hours
after consuming the offending food. (AAP)
Studies have been done to learn whether the malabsorption
of lactose and accompanying symptoms may be associated with reduced
absorption of other nutrients as well. Results at this time are
not conclusive. (Houts)
One can submit to clinical tests to determine his/her tolerance
for lactose. The tests are of two types: one measures hydrogen
breath and the other is a lactose-loading test following a period
of fasting. Blood glucose levels depend on the absorption of
the lactose break down products. (May) When the lactase enzyme
is present in sufficient quantity, lactose in the digestive system
is broken down into glucose and galactose. In the liver the galactose
is changed to glucose and the level of blood glucose is elevated.
If the lactose is not digested this rise is not detected. (NIH)
Another option is to test yourself in a simple experiment.
For two days do not eat any foods containing lactose. Drink two
glasses of skim milk on an empty stomach. If the symptoms develop
soon afterward, this indicates lactose intolerance. (McCall's)
Knowledge of the causes and carefully monitoring one's intake
of lactose can reduce discomfort. Some people can tolerate small
amounts of lactose without too much of a problem by eating it
with a meal. Whole milk is tolerated better than skim milk. Plain
unflavored yogurt with active bacterial cultures may be a good
substitute, however, frozen yogurt usually has been pasteurized
and the bacteria eliminated. (Hunter and McCarthy) Lactose is
an additive in some medications, vitamins, and processed foods.
Read labels and be aware of its presence.
Several dairy products have been developed to address the
problem of lactose intolerance. It is possible to buy lactose-reduced
products that have had lactase enzyme added. Most of these remove
up to 70% of the lactose in the milk. Persons who still have
symptoms after using such products can buy a tablet or liquid
to add to milk and rernove more of the lactose. The source of
the lactase enzyme is a cultured bacteria. (AAP, McCarthy, and
In summary, persons who are lactose intolerant have three
options regarding dairy product consumption. The first is to
consume less than their threshold amount of lactose; a second
is to limit usage to fermented forms such as yogurt and hard
cheeses; and the third is to add lactase to fluid milk prior
to use. (Houts) If an adequate amount of calcium in the diet
is a concern, broccoli, soybean products, and canned salmon with
the bones are optional natural sources of this mineral.
Allen, John S. and Cheer, Susan M.(1996) The Non-Thrifty Genotype.Current
Anthropology. Vol 37, No. 5, p. 831-842. Dec. 1996.
American Family Physician. (1984). Lactose Intolerance.
Vol. 30 p. 272.
Cerrato, Paul L. (1992). The Patient's Eating --Why is He
Losing Weight? RN Vol.55 p. 77-80 April 1992.
Curtis, Helena. (1993). Biology. Worth Publishers,
Inc.New York, New York. p. 678.
Gorman, Christine. (1993). Milking a Fad. Time. Vol
41 p. 53. May 17,1993.
Greifzu, Sherry. (1989). An inability to digest milk sugar.
RN Vol 52 p.93-94, Sept. 1989.
Hein, Pam and Weinzierl, Kathy. Taking Another Look at Lactose
Intolerance. The Pantagraph, Bloomington, Il. April 22,1996.
Houts, Sandra S. (1988). Lactose Intolerance. FoodTechnology.
Vol 42 p.110-113, March 1988.
Hunter, Beatrice Trum. (1986). Lactose Intolerance. Consumer's
Research Magazine Vol.69 p. 8-9, March 1986.
Keeton, William T. and Gould, James L. Biological Science.
W.W. Norton and Company, New York. 1994.
Kieiner, Israel S. and Orten, James M. Biochemistry.
The C.V. Mosby Company, St. Louis, 1966.
Martini, Frederic. FundamentalsofAnatomyandPhysiology.
Prentice Hall, New Jersey, 1992.
May, Daniel S. An Interdisciplinary Look at Lactose Malabsorption.
The American Biology Teacher. Vol. 47 p. 154-9 March 1985.
McCall's, When Milk Makes You Sick. Vol. 113 p. 62-63.
McCarthy, Paul. Many are Cold, Few are Frozen. American
Health. Vol. p. 138-9. July/August 1988.
Meister, Kathleen. Lactose Intolerance. Essence. Vol.16
Morholt, Evelyn and Brandwein, Paul. A Sourcebook for the
Biological Sciences. Saunder Publishing, New York.1986.p.177.
Tamarin, Robert. Principles of Genetics. Wm. C Brown
Therrien, Lois, Making the Undrinkable Thinkable. Business
Week.p. 40 May 14, 1990
Research done and lab developed by Therese Passerini July,1995.
NOTE: The chemical test for glucose may be altered depending
on materials available. Use the same type of milk throughout
the test: whole, or nonfat even for the lactose-reduced milk.
If you prefer, use Lactaid drops and reduce the milk as part
af the lab procedure.
Instead of using the Benedict's solution method, glucose test
strips may be used.
KEY to QUESTIONS & ANALYSIS
QUESTIONS TO ANSWER
1. Lactose is a disaccharide type of sugar. It is made up
of two subunits of monosaccharide sugars - galactose and glucose
2. The results of the test indicated the breakdown of lactose
by the enzyme lactase.
3. Hydrolysis is the chemical reaction in which a bond is
broken by the addition of a water molecule. One part of the molecule
takes the OH and another part takes the H atom. The result is
two separate molecules.
4. the hydrolysis of lactose.
5. galactose and glucose
6. the lactase enzyme acts on the beta linkage bond and causes
the disaccharide lactase to break by hydrolysis
7.From the pedigree charts it appears that lactose intolerance
is an autosomal recessive trait in a family. Children who inherit
two recessive genes will be lactose intolerant.
8. The data from the world map suggests that lactose tolerance
may have had its beginning in certain tribes in Africa. It is
also evident in Northern Europe.
9. The early American settlers were mainly from European countries.
10. The founder effect can be important in establishing the
gene pool for an area being colonized. The early settlers may
have been lactose tolerant and their descendants exhibit this
trait. Later migrants to the U.S. were from other populations
and were mostly lactose intolerant.
1. The symptoms of lactose intolerance include gas, bloating,
and diarrhea. Since bacteria in the intestine receive an undigested
quantity of lactose in the lactose intolerant person, the bacterial
activity increases with this food supply. There may be a temporary
increase in the production of gas by the bacteria.
2. The parents may have been heterozygous (Ll) in their genotype.
Each may pass on a recessive gene to a child.(ll) 25% chance
if they are heterozygous.
3.The process of evolution involves natural selection. A factor
in the environment is responsible for the survival and reproductive
succes of individuals with adaptive characteristics. Members
of the population with an adapative advantage would be the more
fit in terms of leaving more offspring. During periods of drought,
cold weather, disease or other conditions affecting the success
of crops, individuals who were lactose tolerant may have had
an adaptive advantage. In those isolated popualtions they may
have left more offspring with the lactose tolerant trait.
4. The earliest settlers in the United States were mostly
descended from European countries, where cattle and dairy products
were used a a main source of nutrition. Natural selection may
have produced populations of lactose tolerant individuals there
and when they migrated the founder effect established a gene
pool of lactose tolerance.
WHEN MILK MAKES YOU SICK
BACKGROUND INFORMATION: What is the topic of this ad? Who
would use such a product? Why would they need it?
According to statistics approximately one-third of all Americans
feel ill after consuming milk and other dairy products. We are
taught that we should "drink our milk" by our parents.
"It is good for you, will help you have strong bones and
teeth, and grow healthy and tall." The nutritional value
of milk is a fact. However, most animals stop drinking milk after
they are weaned and their body chemistry changes so that they
can no longer digest the sugar in milk. Worldwide this is also
true for the human population. That is what is normal. It is
actually unusual for adults to be able to digest milk easily.
In this lab activity you will learn more about lactose intolerance
You will understand the chemical structure of lactose, why a
person may have difficulty digesting milk, how lactose tolerance
may have started, why so many Americans can digest it, and how
lactose-reduced products can help people.
MATERIALS: 3 test tubes and rack Benedict's solution
distilled water boiling water bath
milk 3 10-ml pipettes
100% lactose-reduced milk
PROCEDURE: Part A. Test for lactose
1. Place 10 ml of distilled water in test tube A
Place 10 ml of milk in test tube B
Place 10 ml of 100% lactose-reduced milk in test tube C
WAIT 10 MINUTES
2. Add 5 ml of Benedict's solution to each test tube
Heat the tubes for 3 minutes in the boiling water bath
3. Compare the tubes
4. Record the results in the table:
Test Tube: A B C
PROCEDURE: Part B. Lactose Intolerance in Families
1. Fill in the pedigree charts for three families. Parents
Joe and Lucy Anderson are both lactose intolerant. The four children:
Alicia, Eric, Ben, and Rodney are all lactose intolerant.
* Remember that we shade in persons with the phenotype being
2. Parent Mary Wallace is lactose intolerant and her husband,
John is lactose tolerant. They had five children. Ann, David,
and Dan are lactose intolerant. Nancy and Scott are lactose tolerant.
3. Parents Mike and Donna Miller are both lactose tolerant.
Their children Fred, Niles, and Linda are lactose tolerant. The
other child, Jane i s lactose intolerant.
PROCEDURE: Part C. Study the map of the world. The numbers
indicate the percentage of the population in each area that is
USE THE MAP DATA TO MAKE A GRAPH (draw vertical and horizontal
axes below, using the labels for the axes as given)
Percentage of lactose intolerance
QUESTIONS TO ANSWER:
1. Which type of sugar is lactose?
2. What did the results of the Benedict's test for sugar mean?
3. Define hydrolysis.
4. What reaction is catalyzed by the enzyme lactase?
5. What are the products of lactose digestion?
6. What is the effect of adding lactase enzyme to milk?
7. Does it appear that lactose intolerance (or tolerance)
is an inherited characteristic? Explain your answer.
8. Where did lactose tolerance probably first originate?
9. Which parts of the world were the original home of the
early American settlers?
10. How can migration and gene flow affect a population?
1. When the body does not secrete lactase enzyme in the intestine,
the lactose sugar is not digested. Bacteria that are a normal
part of the colon use the lactose for food and produce gas. How
does this contribute to the symptoms of lactose intolerance?
2. How can two parents who both tolerate lactose (digest it)
produce children who do not?
What is the most probable mode of this inheritance?
Show your reasoning with a Punnett square or pedigree chart.
3. During the past 10,000 years agriculture has been important
to human populations. In sorne isolated areas crops did not perform
as well or the climate did not permit relying on them year round.
In these places animals and their milk were the main food supply.
Use your knowledge of evolution and natural selection to explain
how some populations may have become lactose tolerant.
4. What would explain the statistics that approximately 30%
of all Americans are lactose intolerant compared to other parts
of the worid where that number is more than 80%?
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