have several important actions on 1 or
more body systems, and many are
involved in the activities of enzymes.
vitamin AA fat-soluble vitamin essen-
tial for normal growth, for the formation
of bones and teeth, for cell structure,
for night vision, and for protecting the
linings of the respiratory, digestive, and
urinary tracts against infection.
Vitamin A is absorbed by the body in
the form of retinol. This is found in liver,
fish-liver oils, egg yolk, dairy produce, and
is added to margarines. Carotene, which
the body converts into retinol, is found
in various vegetables and fruits.
Vitamin A deficiency is rare in deve-
loped countries. In most cases, it is due
to malabsorption. Vitamin A deficiency
may also result from long-term treat-
ment with certain lipid-lowering drugs.
Deficiency is common in some develop-
ing countries due to poor diet. The first
symptom of deficiency is night blindness,
followed by dryness and inflammation
of the eyes (see xerophthalmia), kera-
tomalacia, and eventually blindness.
Deficiency also causes reduced resis-
tance to infection, dry skin, and, in
children, stunted growth.
Prolonged excessive intake of vitamin
A can cause headache, nausea, loss of
appetite, skin peeling, hair loss, and
irregular menstruation. In severe cases,
the liver and spleen become enlarged.
Excessive intake during pregnancy may
cause birth defects. In infants, excessive
intake may cause skull deformities,
which disappear if the diet is corrected.
The drug tretinoin(a derivative of vita-
min A) is used to treat severe acne.
vitamin BSee vitamin B 12 ; vitamin B
complex.
vitamin B 12 A water-soluble vitamin
that plays a vital role in the activities of
several enzymesin the body. Vitamin
B 12 is important in the production of
the genetic material of cells (and thus
in growth and development), in the pro-
duction of red blood cells in bone
marrow, in the utilization of folic acid
and carbohydrates in the diet, and in
the functioning of the nervous system.
Foods rich in vitamin B 12 include liver,
kidney, chicken, beef, pork, fish, eggs,
and dairy products.
Deficiency is almost always due to
the inability of the intestine to absorb
the vitamin, usually as a result of
pernicious anaemia (see anaemia, meg-
aloblastic). Less commonly, deficiency
may result from gastrectomy, malab-
sorption, or veganism. The effects of
vitamin B 12 deficiency are megaloblas-
tic anaemia, a sore mouth and tongue,
and symptoms caused by damage to
the spinal cord, such as numbness and
tingling in the limbs. There may also be
depression and memory loss. A high
intake of vitamin B 12 has no known
harmful effects.
vitamin B complexA group of water-
soluble vitaminscomprising thiamine
(vitamin B 1 ), riboflavin (vitamin B 2 ),
niacin, pantothenic acid, pyridoxine (vit-
amin B 6 ), biotin (vitamin H), and folic
acid. Vitamin B 12 is discussed above.
Thiamine plays a role in the activities
of various enzymes involved in the uti-
lization of carbohydratesand thus in the
functioning of nerves, muscles, and the
heart. Sources include whole-grain cere-
als, wholemeal breads, brown rice, pasta,
liver, kidney, pork, fish, beans, nuts, and
eggs. Those susceptible to deficiency
include elderly people on a poor diet,
and people who have hyperthyroidism,
malabsorption, or severe alcoholdepen-
dence. Deficiency may alsooccur as a
result of severe illness, surgery, or
injury. Mild deficiency may cause tired-
ness, irritability, and loss of appetite.
Severe deficiency may cause abdominal
pain, constipation, depression, memory
impairment, and beriberi; in alcoholics,
it may cause Wernicke–Korsakoff syn-
drome. Excessive intake is not known to
cause harmful effects.
Riboflavin is necessary for the activi-
ties of various enzymes involved in the
breakdown and utilization of carbohy-
drates, fats, and proteins; the production
of energy in cells; the utilization of
other B vitamins; and hormone produc-
tion by the adrenal glands. Liver, whole
grains, milk, eggs, and brewer’s yeast
are good sources. People who are sus-
ceptible to riboflavin deficiency include
those taking phenothiazine antipsychotic
drugs, tricyclic antidepressant drugs, or
oestrogen-containing oral contraceptives,VITAMIN A VITAMIN B COMPLEX
V