Dietary Supplement Fact Sheet: Vitamin B6
Vitamin B6: What is it?
Vitamin B6 is a water-soluble vitamin that exists in three major
chemical forms: pyridoxine, pyridoxal, and pyridoxamine [1,2]. It
performs a wide variety of functions in your body and is essential for your good
health. For example, vitamin B6 is needed for more than 100 enzymes involved
in protein metabolism. It is also essential for red blood cell metabolism. The nervous
and immune systems need vitamin B6 to function efficiently, [3-6]
and it is also needed for the conversion of tryptophan (an amino acid) to niacin
(a vitamin) [1,7].
Hemoglobin within red blood cells carries oxygen to tissues. Your body needs
vitamin B6 to make hemoglobin. Vitamin B6 also helps increase
the amount of oxygen carried by hemoglobin. A vitamin B6 deficiency can
result in a form of anemia [1] that is similar to iron deficiency anemia.
An immune response is a broad term that describes a variety of biochemical changes
that occur in an effort to fight off infections. Calories, protein, vitamins, and
minerals are important to your immune defenses because they promote the growth of
white blood cells that directly fight infections. Vitamin B6, through
its involvement in protein metabolism and cellular growth, is important to the immune
system. It helps maintain the health of lymphoid organs (thymus, spleen, and lymph
nodes) that make your white blood cells. Animal studies show that a vitamin B6
deficiency can decrease your antibody production and suppress your immune response
[1,5].
Vitamin B6 also helps maintain your blood glucose (sugar) within a
normal range. When caloric intake is low your body needs vitamin B6 to
help convert stored carbohydrate or other nutrients to glucose to maintain normal
blood sugar levels. While a shortage of vitamin B6 will limit these functions,
supplements of this vitamin do not enhance them in well-nourished individuals [1,8-10].
What foods provide vitamin B6?
Vitamin B6 is found in a wide variety of foods including fortified
cereals, beans, meat, poultry, fish, and some fruits and vegetables [1,11].
The table of selected food sources of vitamin B6 suggests many dietary
sources of B6.
When can a vitamin B6 deficiency occur?
Clinical signs of vitamin B6 deficiency are rarely seen in the United
States. Many older Americans, however, have low blood levels of vitamin B6,
which may suggest a marginal or sub-optimal vitamin B6 nutritional status.
Vitamin B6 deficiency can occur in individuals with poor quality diets
that are deficient in many nutrients. Symptoms occur during later stages of deficiency,
when intake has been very low for an extended time. Signs of vitamin B6
deficiency include dermatitis (skin inflammation), glossitis (a sore tongue), depression,
confusion, and convulsions [1,12]. Vitamin B6 deficiency
also can cause anemia [1,12,14]. Some of these symptoms can
also result from a variety of medical conditions other than vitamin B6
deficiency. It is important to have a physician evaluate these symptoms so that
appropriate medical care can be given.
Who may need extra vitamin B6 to prevent a deficiency?
Individuals with a poor quality diet or an inadequate B6 intake for
an extended period may benefit from taking a vitamin B6 supplement if
they are unable to increase their dietary intake of vitamin B6 [1,15].
Alcoholics and older adults are more likely to have inadequate vitamin B6
intakes than other segments of the population because they may have limited variety
in their diet. Alcohol also promotes the destruction and loss of vitamin B6
from the body.
Asthmatic children treated with the medicine theophylline may need to take a
vitamin B6 supplement [16]. Theophylline decreases body stores
of vitamin B6 [17], and theophylline-induced seizures have been
linked to low body stores of the vitamin. A physician should be consulted about
the need for a vitamin B6 supplement when theophylline is prescribed.
What are some current issues and controversies about vitamin B6?
Vitamin B6 and the nervous systemVitamin B6
is needed for the synthesis of neurotransmitters such as serotonin and dopamine
[1]. These neurotransmitters are required for normal nerve cell communication.
Researchers have been investigating the relationship between vitamin B6
status and a wide variety of neurologic conditions such as seizures, chronic pain,
depression, headache, and Parkinson's disease [18].
Lower levels of serotonin have been found in individuals suffering from depression
and migraine headaches. So far, however, vitamin B6 supplements have
not proved effective for relieving these symptoms. One study found that a sugar
pill was just as likely as vitamin B6 to relieve headaches and depression
associated with low dose oral contraceptives [19].
Alcohol abuse can result in neuropathy, abnormal nerve sensations in the arms
and legs [20]. A poor dietary intake contributes to this neuropathy and dietary
supplements that include vitamin B6 may prevent or decrease its incidence
[18].
Vitamin B6 and carpal tunnel syndrome
Vitamin B6 was first recommended for carpal tunnel syndrome almost
30 years ago [21]. Several popular books still recommend taking 100 to 200
milligrams (mg) of vitamin B6 daily to treat carpal tunnel syndrome,
even though scientific studies do not indicate it is effective. Anyone taking large
doses of vitamin B6 supplements for carpal tunnel syndrome needs to be
aware that the Institute of Medicine recently established an upper tolerable limit
of 100 mg per day for adults [12]. There are documented cases in the literature
of neuropathy caused by excessive vitamin B6 taken for treatment of carpal
tunnel syndrome [22].
Vitamin B6 and premenstrual syndrome
Vitamin B6 has become a popular remedy for treating the discomforts
associated with premenstrual syndrome (PMS). Unfortunately, clinical trials have
failed to support any significant benefit [23]. One recent study indicated
that a sugar pill was as likely to relieve symptoms of PMS as vitamin B6
[24]. In addition, vitamin B6 toxicity has been seen in increasing
numbers of women taking vitamin B6 supplements for PMS. One review indicated
that neuropathy was present in 23 of 58 women taking daily vitamin B6
supplements for PMS whose blood levels of B6 were above normal [25].
There is no convincing scientific evidence to support recommending vitamin B6
supplements for PMS.
Vitamin B6 and interactions with medications
There are many drugs that interfere with the metabolism of vitamin B6.
Isoniazid, which is used to treat tuberculosis, and L-DOPA, which is used to treat
a variety of neurologic problems such as Parkinson's disease, alter the activity
of vitamin B6. There is disagreement about the need for routine vitamin
B6 supplementation when taking isoniazid [26,27]. Acute
isoniazid toxicity can result in coma and seizures that are reversed by vitamin
B6, but in a group of children receiving isoniazid, no cases of neurological
or neuropsychiatric problems were observed regardless of whether or not they took
a vitamin B6 supplement. Some doctors recommend taking a supplement that
provides 100% of the RDA for B6 when isoniazid is prescribed, which is
usually enough to prevent symptoms of vitamin B6 deficiency. It is important
to consult with a physician about the need for a vitamin B6 supplement
when taking isoniazid.
What is the relationship between vitamin B6, homocysteine, and heart
disease?
A deficiency of vitamin B6, folic acid, or vitamin B12 may increase
your level of homocysteine, an amino acid normally found in your blood [28].
There is evidence that an elevated homocysteine level is an independent risk factor
for heart disease and stroke [29-37]. The evidence suggests that high levels
of homocysteine may damage coronary arteries or make it easier for blood clotting
cells called platelets to clump together and form a clot. However, there is currently
no evidence available to suggest that lowering homocysteine level with vitamins
will reduce your risk of heart disease. Clinical intervention trials are needed
to determine whether supplementation with vitamin B6, folic acid, or
vitamin B12 can help protect you against developing coronary heart disease.
What is the health risk of too much vitamin B6?
Too much vitamin B6 can result in nerve damage to the arms and legs.
This neuropathy is usually related to high intake of vitamin B6 from
supplements, [28] and is reversible when supplementation is stopped. According
to the Institute of Medicine, "Several reports show sensory neuropathy at doses
lower than 500 mg per day" [12]. As previously mentioned, the Food and Nutrition
Board of the Institute of Medicine has established an upper tolerable intake level
(UL) for vitamin B6 of 100 mg per day for all adults [12]. "As
intake increases above the UL, the risk of adverse effects increases [12]."
Selected Food Sources of vitamin B6
As the 2000 Dietary Guidelines for Americans states, "Different foods
contain different nutrients and other healthful substances. No single food can supply
all the nutrients in the amounts you need" [38]. As the following table indicates,
vitamin B6 is found in a wide variety of foods. Foods such as fortified
breakfast cereals, fish including salmon and tuna fish, meats such as pork and chicken,
bananas, beans and peanut butter, and many vegetables will contribute to your vitamin
B6 intake. If you want more information about building a healthful diet,
refer to the Dietary
Guidelines for Americans and the
Food Guide Pyramid.
* DV = Daily Value. DVs are reference numbers based on the Recommended Dietary
Allowance (RDA). They were developed to help consumers determine if a food contains
a lot or a little of a specific nutrient. The DV for vitamin B6 is 2.0 milligrams
(mg). The percent DV (%DV) listed on the nutrition facts panel of food labels tells
you what percentage of the DV is provided in one serving. Percent DVs are based
on a 2,000 calorie diet. Your Daily Values may be higher or lower depending on your
calorie needs. Foods that provide lower percentages of the DV also contribute to
a healthful diet.
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