Jay W. Marks, MD, is a board-certified internist and gastroenterologist. He graduated from Yale University School of Medicine and trained in internal medicine and gastroenterology at UCLA/Cedars-Sinai Medical Center in Los Angeles.
Lactose is a large sugar molecule that is made up of two smaller sugars, glucose and galactose. In order for lactose to be absorbed from the intestine and into the body, it must first be split into glucose and galactose. The glucose and galactose are then absorbed by the cells lining the small intestine. The enzyme that splits lactose into glucose and galactose is called lactase, and it is located on the surface of the cells lining the small intestine.
Lactose intolerance is caused by reduced or absent activity of lactase that prevents the splitting of lactose (lactase deficiency). Lactase deficiency may occur for one of three reasons, congenital, secondary or developmental.
Congenital causes of lactose
Lactase deficiency may occur because of a congenital absence (absent from birth) of
lactase due to a mutation in the gene that is responsible for producing lactase.
This is a very rare cause of lactase deficiency, and the symptoms of this type
of lactase deficiency begin shortly after birth.
Secondary causes of lactose intolerance
Another cause of lactase deficiency is secondary lactase deficiency. This
type of deficiency is due to diseases that destroy the lining of the small
intestine along with the lactase. An example of such a disease is
Developmental causes of lactose intolerance
The most common cause of lactase deficiency is a decrease in the amount of lactase that occurs after childhood and persists into adulthood, referred to as adult-type hypolactasia. This decrease in lactase is genetically programmed, and the prevalence of this type of lactase deficiency in different ethnic groups is highly variable. Thus, in Asian populations it is almost 100%, among American Indians it is 80%, and in blacks it is 70%; however, in American Caucasians the prevalence of lactase deficiency is only 20%. In addition to variability in the prevalence of lactase deficiency, there also is variability in the age at which symptoms of lactose intolerance appear. Thus, in Asian populations, the symptoms of lactase deficiency (intolerance) occur around the age of 5, among Blacks and Mexican-Americans by the age of 10, and in the Finnish by age 20.
It is important to emphasize that lactase deficiency is not the same as
lactose intolerance. Persons with milder deficiencies of lactase often have no
symptoms after the ingestion of milk. For unclear reasons, even persons with
moderate deficiencies of lactase may not have symptoms. A diagnosis of lactase
deficiency is made when the amount of lactase in the intestine is reduced, but a
diagnosis of lactose intolerance is made only when the reduced amount of lactase
Medical Author: Melissa Stoppler, M.D.
K. Hecht, Ph.D.
Sometimes people become sick from eating a particular
food, because they cannot properly process or digest the food, or because they
have a true allergic (immune) reaction to the food. Food allergies
and food intolerance are sometimes
confused with each other, but they are quite different in terms of their origin,
symptoms and treatment.
True allergic reactions to food involve the body?s immune system. When the
body identifies a food as harmful, it produces antibodies directed against that
food. The next time the food is consumed, the body mounts an immune response
with the release of histamine and other chemicals that trigger allergic symptoms. A common
example of a food allergy is to peanuts.
With a food allergy, symptoms may occur almost immediately or up to hours after consuming the
particular food. These symptoms may affect the respiratory
system, gastrointestinal tract, cardiovascular system, or the skin.
Food allergy symptoms can include:
skin rash or
swelling of the tongue and
breathing problems including
Severe allergic reactions may result in a
drop in blood pressure,
loss of consciousness, or even death.