Vitamin toxicity
Definition
Vitamin toxicity is a condition in which a person develops symptoms
as side effects from taking massive doses of vitamins. Vitamins vary in the
amounts that are required to cause toxicity and in the specific symptoms that
result. Vitamin toxicity, which is also called hypervitaminosis or vitamin
poisoning, is becoming more common in developed countries because of the
popularity of vitamin supplements. Many people treat themselves for minor
illnesses with large doses (megadoses) of vitamins.
Description
Overview
Vitamins are organic molecules in food that are needed in small
amounts for growth, reproduction, and the maintenance of good health. Some
vitamins can be dissolved in oil or melted fat. These fat-soluble vitamins
include vitamin D, vitamin E, vitamin A (retinol), and vitamin K. Other vitamins
can be dissolved in water. These water-soluble vitamins include folate (folic
acid), vitamin B12, biotin, vitamin B6, niacin, thiamin,
riboflavin, pantothenic acid, and vitamin C (ascorbic acid). Taking too much of
any vitamin can produce a toxic effect. Vitamin A and vitamin D are the most
likely to produce hypervitaminosis in large doses, while riboflavin, pantothenic
acid, biotin, and vitamin C appear to be the least likely to cause problems.
Vitamins in
medical treatment
Vitamin supplements are used for the treatment of various diseases or
for reducing the risk of certain diseases. For example, moderate supplements of
folic acid appear to reduce the risk for certain birth defects (neural tube
defects), and possibly reduce the risk of cancer. Therapy for diseases brings
with it the risk for irreversible vitamin toxicity only in the case of vitamin
D. This vitamin is toxic at levels which are only moderately greater than the
recommended dietary allowance (RDA). Niacin is commonly used as a drug for the
treatment of heart disease. Niacin is far less toxic than vitamin D. Vitamin
toxicity is not a risk with medically supervised therapy using any of the other
vitamins.
Vitamin
megadoses
With the exception of folic acid supplements, the practice of taking
vitamin supplements by healthy individuals has little or no relation to good
health. Most adults in the
United States can
obtain enough vitamins by eating a well-balanced diet. It has, however, become
increasingly common for people to take vitamins at levels far greater than the
RDA. These high levels are sometimes called vitamin megadoses. Megadoses are
harmless for most vitamins. But in the cases of a few of the vitamins--
specifically vitamin D, vitamin A, and vitamin B6-- megadoses can be
harmful or fatal. Researchers have also started to look more closely at
megadoses of vitamin C and of vitamin E, since indirect evidence suggests that
these two vitamins may reduce the risks of cancer, heart disease, and aging. It
is not yet clear whether megadoses of either of these vitamins has any influence
on health. Some experts think that megadoses of vitamin C may protect people
from cancer. On the other hand, other researchers have gathered indirect
evidence that vitamin C megadoses may cause cancer.
Causes and
symptoms
Fat-soluble
vitamins
VITAMIN D
Vitamin D and vitamin A are the most toxic of the fat-soluble
vitamins. The symptoms of vitamin D toxicity are nausea, vomiting, pain in the
joints, and loss of appetite. The patient may experience constipation
alternating with diarrhea, or have tingling sensations in the mouth. The toxic
dose of vitamin D depends on its frequency. In infants, a single dose of 15 mg
or greater may be toxic, but it is also the case that daily doses of 1.0 mg over
a prolonged period may be toxic. In adults, a daily dose of 1.0-2.0 mg of
vitamin D is toxic when consumed for a prolonged period. A single dose of about
50 mg or greater is toxic for adults. The immediate effect of an overdose of
vitamin D is abdominal cramps, nausea and vomiting. Toxic doses of vitamin D
taken over a prolonged period of time result in irreversible deposits of calcium
crystals in the soft tissues of the body that may damage the heart, lungs, and
kidneys.
VITAMIN A
Vitamin A toxicity can occur with long-term consumption of 20 mg of
retinol or more per day. The symptoms of vitamin A overdosing include
accumulation of water in the brain (hydrocephalus), vomiting, tiredness,
constipation, bone pain, and severe headaches. The skin may acquire a rough and
dry appearance, with hair loss and brittle nails. Vitamin A toxicity is a
special issue during pregnancy. Expectant mothers who take 10 mg vitamin A or
more on a daily basis may have an infant with birth defects. These birth defects
include abnormalities of the face, nervous system, heart, and thymus gland. It
is possible to take in toxic levels of vitamin A by eating large quantities of
certain foods. For example, about 30 grams of beef liver, 500 grams of eggs, or
2,500 grams of mackerel would supply 10 mg of retinol. The livers of polar bears
and other arctic animals may contain especially high levels of vitamin A.
VITAMIN E
Megadoses of vitamin E may produce headaches, tiredness, double
vision, and diarrhea in humans. Studies with animals fed large doses of vitamin
E have revealed that this vitamin may interfere with the absorption of other
fat-soluble vitamins. The term absorption means the transfer of the vitamin from
the gut into the bloodstream. Thus, large doses of vitamin E consumed over many
weeks or months might result in deficiencies of vitamin D, vitamin A, and
vitamin K.
VITAMIN K
Prolonged consumption of megadoses of vitamin K (menadione) results
in anemia, which is a reduced level of red blood cells in the bloodstream. When
large doses of menadione are given to infants, they result in the deposit of
pigments in the brain, nerve damage, the destruction of red blood cells (hemolysis),
and death. A daily injection of 10 mg of menadione into an infant for three days
can kill the child. This tragic fact was discovered during the early days of
vitamin research, when newborn infants were injected with menadione to prevent a
disease known as hemorrhagic disease of the newborn. Today a different form of
vitamin K is used to protect infants against this disease.
Water-soluble
vitamins
FOLATE
Folate occurs in various forms in food. There are over a dozen
related forms of folate. The folate in oral vitamin supplements occurs in only
one form, however--folic acid. Large doses of folic acid (20 grams/day) can
result in eventual kidney damage. Folate is considered, however, to be
relatively nontoxic, except in cases where folate supplementation can lead to
pernicious anemia.
VITAMIN B12
Vitamin B12 is important in the treatment of pernicious
anemia. Pernicious anemia is more common among middle-aged and older adults; it
is usually detected in patients between the ages of 40 and 80. The disease
affects about 0.1% of all persons in the general population in the
United States, and
about 3% of the elderly population. Pernicious anemia is treated with large
doses of vitamin B12. Typically, 0.1 mg of the vitamin is injected
each week until the symptoms of pernicious anemia disappear. The patient then
takes oral doses of vitamin B12 for the rest of his or her life.
Although vitamin B12 toxicity is not an issue for patients being
treated for pernicious anemia, treatment of these patients with folic acid may
cause problems. Specifically, pernicious anemia is often first detected because
the patient feels weak or tired. If the anemia is not treated, the patient may
suffer irreversible nerve damage. The problem with folic acid supplements is
that the folic acid treatment prevents the anemia from developing, but allows
the eventual nerve damage to occur.
VITAMIN B6
Vitamin B6 is clearly toxic at doses about 1000 times the
RDA. Daily doses of 2-5 grams of one specific form of this vitamin can produce
difficulty in walking and tingling sensations in the legs and soles of the feet.
Continued megadoses of vitamin B6 result in further unsteadiness,
difficulty in handling small objects, and numbness in the hands. When the high
doses are stopped, recovery begins after two months. Complete recovery may take
two to three years.
VITAMIN C
The RDA for vitamin C in adults is 60 mg per day. Large doses of
vitamin C are considered to be toxic in persons with a family history of or
tendency to form kidney stones or gallbladder stones. Kidney and gallbladder
stones usually consist of calcium oxalate. Oxalate occurs in high concentrations
in foods such as cocoa, chocolate, rhubarb, and spinach. A fraction of the
vitamin C in the body is normally broken down in the body to produce oxalate. A
daily supplement of 3.0 grams of vitamin C has been found to double the level of
oxalate that passes through the kidneys and is excreted into the urine.
NIACIN
The RDA for niacin is 15-19 mg per day in adults. Niacin comes in two
forms, nicotinic acid and nicotinamide. Either form can satisfy the adult
requirement for this vitamin. Nicotinic acid, however, is toxic at levels of 100
times the RDA. It can cause flushing of the skin, nausea, diarrhea, and liver
damage. Flushing is an increase in blood passing through the veins
in the skin, due to the dilation of arteries passing through deeper parts of the
face or other parts of the body. In spite of the side effects, however, large
doses of nicotinic acid are often used to lower blood cholesterol in order to
prevent heart disease. Nicotinic acid results in a lowering of LDL-cholesterol
("bad cholesterol"), an increase in HDL-cholesterol ("good cholesterol"), and a
decrease in plasma triglycerides. Treatment involves daily doses of 1.5-4.0
grams of nicotinic acid per day.
Flushing of the skin
occurs as a side effect when nicotinic acid therapy is started, but may
disappear with continued therapy.
Diagnosis
The diagnosis of vitamin toxicity is usually made on the basis of the
patient's dietary or medical history. Questioning the patient about the use of
vitamin supplements may shed light on some of his or her physical symptoms. With
some vitamins, the doctor can confirm the diagnosis by ordering blood or urine
tests for specific vitamins. When large amounts of the water-soluble vitamins
are consumed, a large fraction of the vitamin is absorbed into the bloodstream
and promptly excreted into the urine. The fat-soluble vitamins are more likely
to be absorbed into the bloodstream and deposited in the fat and other tissues.
In the cases of both water-soluble and fat-soluble vitamins, any vitamin not
absorbed by the intestines is excreted in the feces. Megadoses of many of the
vitamins produce diarrhea, because the non-absorbed nutrient draws water out of
the body and into the gut, resulting in the loss of this water from the body.
Treatment
In all cases, treatment of vitamin toxicity requires discontinuing
vitamin supplements. Vitamin D toxicity needs additional action to reduce the
calcium levels in the bloodstream because it can cause abnormally high levels of
plasma calcium (hypercalcemia). Severe hypercalcemia is a medical emergency and
may be treated by infusing a solution of 0.9% sodium chloride into the patient's
bloodstream. The infusion consists of two to three liters of salt water given
over a period of one to two days.
Prognosis
The prognosis for reversing vitamin toxicity is excellent for most
patients. Side effects usually go away as soon as overdoses are stopped. The
exceptions are severe vitamin D toxicity, severe vitamin A toxicity, and severe
vitamin B6 toxicity. Too much vitamin D leads to deposits of calcium
salts in the soft tissue of the body, which cannot be reversed. Birth defects
due to vitamin A toxicity cannot be reversed. Damage to the nervous system
caused by megadoses of vitamin B6 can be reversed, but complete
reversal may require a recovery period of over a year.
Prevention
Vitamin toxicity can be prevented by minimizing the use of vitamin
supplements. If vitamin D supplements are being used on a doctor's orders,
vitamin toxicity can be prevented by monitoring the levels of plasma calcium.
The development of hypercalcemia with vitamin D treatment indicates that the
patient is at risk for vitamin D toxicity.
Key Terms
Absorption
The transfer of a vitamin from the
digestive tract to the bloodstream.
Ascorbic acid
Another name for vitamin C.
Hypercalcemia
Hypercalcemia is a condition marked by
abnormally high levels of calcium in the blood. It is an issue during vitamin D
toxicity.
Hypervitaminosis
Another name for vitamin toxicity.
Megadose
A very large dose of a vitamin, taken
by some people as a form of self-medication.
Menadione
A synthetic form of vitamin K. It is
sometimes called vitamin K.
Recommended Dietary Allowance (RDA)
The recommended dietary allowances
(RDAs) are the quantities of nutrients in the diet that are needed for good
health. RDAs are established by the Food and Nutrition Board of the National
Academy of Sciences and may be revised every few years.
Retinol
Another name for vitamin A.
