Function of B6 in the body

Metabolism
Like the other B complex vitamins, vitamin B6 is involved in the functioning of enzymes involved in the release of energy from food. The coenzyme forms of vitamin B6 are pyridoxal 5' phosphate and pyridoxamine 5' phosphate, and these are necessary for nearly 100 enzymatic reactions. These include the synthesis and breakdown of amino acids, the conversion of amino acids to carbohydrate or fat, and the conversion of one type of fat to another. Thus, it is involved in the manufacture of most protein-related compounds and plays a role in almost all bodily processes.

Cardiovascular system
Vitamin B6 is essential for the manufacture of fat-derived substances known as prostaglandins which are involved in processes such as blood pressure regulation and heart function. Vitamin B6 is also necessary for red blood cell formation.

Immune system
Vitamin B6 plays a vital role in maintaining a healthy immune system by affecting functions such as cell multiplication and antibody production.

Nervous system and brain
Adequate vitamin B6 is vital to the healthy development and function of the nervous system. It is involved in the manufacture of several neurotransmitters including serotonin, GABA, dopamine and noradrenaline, and plays an important role in regulating mental processes and mood. Concentrations of vitamin B6 are up to 25 to 50 times higher in the brain than in the blood.

Skin and hair
Vitamin B6 is important in maintaining healthy hair and skin.

Hormones
Vitamin B6 plays a role in modulating the effects of hormones, including male and female sex hormones and adrenal hormones.

Absorption and metabolism
Vitamin B6 is readily absorbed in the small intestine. Excess vitamin B6 is excreted in the urine so adequate daily intake is essential. The forms of vitamin B6 found in food are converted to active forms in the liver. Zinc and riboflavin are necessary for this process.

Deficiency
Adolescents, the elderly, people with heart disease, those on restricted diets and alcoholics are at risk of vitamin B6 deficiency. Others at risk include women on oral contraceptives, those under stress, those whose diets are high in sugar and fat and those taking certain medications. (See page 95) People who exercise heavily and athletes often have low vitamin B6 levels. Exercise causes vitamin B6 blood levels to increase during an exercise session possibly because of the release of vitamin B6-dependent enzymes from muscle storage or the transfer of the vitamin from the liver to the muscles. As vitamin B6 is involved in a wide range of body functions, the symptoms of deficiency are widespread.

Elderly people
Low vitamin B6 levels are common among elderly people and may lead to increased risk of several disorders including heart disease. In a study published in 1996 Dutch researchers studied the vitamin B6 intake and blood levels in 546 elderly Europeans, aged from 74 to 76, with no known vitamin B6 supplement use. They also examined links with other dietary and lifestyle factors, including indicators of physical health. The results showed that 27 per cent of the men and 42 per cent of the women had dietary vitamin B6 intakes below the mean minimum requirements. Twenty-two per cent of both men and women had low blood levels.

In a French study published in 1997, researchers assessed vitamin B6 levels in elderly patients with infections during hospitalization. During infection, vitamin B6 levels were much lower than in healthy patients.

Brain and nervous system
Vitamin B6 deficiency can cause the mental symptoms of irritability, weakness, drowsiness, depression and poor appetite. Even a marginal deficiency can affect enzymes involved in the metabolism of several brain neurotransmitters. Vitamin B6 deficiency causes convulsions in young children.

In a study published in 1996, US Department of Agriculture researchers investigated the effects of blood levels of the amino acid, homocysteine and vitamins B12 and B6 and folate, on performance in cognitive tests of 70 men, aged 54 to 81 years. The results showed that higher concentrations of vitamin B6 were related to better performance on memory tests.

Pregnancy
Low levels of vitamin B6 in pregnant women can affect the development of a baby's nervous system. Deficiency may also contribute to water retention, morning sickness, pre-eclampsia and birthing difficulties. It may also lead to diabetic and blood sugar problems in pregnancy.

Skin
Vitamin B6 deficiency can lead to greasy inflammation of the skin around the nose, eyebrows and hairline, and cracking of the lips and tongue.

Immune system
Immune response is adversely affected by vitamin B6 deficiency. Many different aspects of the immune system are affected, including the quality and quantity of antibodies and the number of infection-fighting white blood cells. Some immunosuppressive drugs affect the activity of a vitamin B6-dependent enzyme and vitamin B6 supplements may help to counter some of the side-effects of these drugs.

HIV/AIDS
Vitamin B6 deficiency is common in HIV-infected people. In a 1991 study, University of Miami researchers examined the relationship between deficiency and immune dysfunction. The results showed that while CD4+ and CD8+ cell numbers were not affected, other measures of immune system function were.

Kidney
Vitamin B6 deficiency may also play a role in the development of some kinds of kidney stones.

Cardiovascular disease
Vitamin B6 deficiency can raise the risk of developing cardiovascular disease. One theory of the development of atherosclerosis links high levels of a compound known as homocysteine to damage of the cells lining the arteries. A deficiency of vitamin B6 leads to an accumulation of homocysteine and may also lead to defects in artery wall formation. Several studies, including the Framingham Heart Study, have confirmed the link between low pyridoxine levels and high homocysteine levels Researchers analyzed blood samples from the study participants to assess levels of homocysteine and the relationship between B vitamins and carotid artery narrowing, which increases the risk of heart attack. The results showed that low intakes of folate and vitamin B6 were associated with high homocysteine levels. Those with the highest homocysteine levels were twice as likely to have carotid artery narrowing when compared with those in the lowest homocysteine group.

Another study, published in 1998 in the American Heart Association journal Circulation provides further evidence of the importance of vitamin B6 in preventing heart disease. Researchers involved in a study done in several centers in Europe compared 750 patients with vascular disease and 800 control subjects of the same ages and sex. They measured blood levels of homocysteine, folate, vitamin B12, and vitamin B6. The results showed that those with high blood homocysteine concentrations had a high risk of vascular disease. In addition, low concentrations of folate and vitamin B6 were also associated with increased risk. In this study, the relationship between vitamin B6 and atherosclerosis did not appear to be solely due to increased homocysteine levels, suggesting that vitamin B6 may have other important roles in heart disease prevention.

Intake of folate and vitamin B6 above the current recommended dietary allowance seems to be important in the prevention of coronary heart disease among women. Researchers from the Harvard School of Public Health investigated the links between intakes of folate and vitamin B6 and the incidence of heart attacks in 80 082 women taking part in the Nurses' Health Study. The women had no previous history of cardiovascular disease, cancer, high cholesterol levels or diabetes when they entered the study. During the 14 years of follow-up, there were 658 nonfatal heart attacks and 281 fatal ones. The results showed that those with the highest intakes of vitamin B6 had just over 30 per cent less risk of heart attack than those in the low intake group. Women in the group with the highest intakes of both folate and vitamin has just less than half the risk of women in the lowest intake group. Risk of coronary heart disease was reduced among women who regularly used multiple vitamins, the major source of folate and vitamin B6.

Low vitamin B6 levels are linked to an increased risk of heart attack. In a study published in 1996, researchers studied the links between homocysteine, vitamins B12, B6 and folate levels, and the risk of heart attack. The cases were 130 hospitalized heart attack patients and 118 healthy people. The results showed that average homocysteine levels were 11 per cent higher in cases compared with health people. Dietary and blood levels of vitamin B6 and folate were lower in cases than in controls.

Carpal tunnel syndrome
Several small studies have found low vitamin B6 levels in sufferers of the carpal tunnel syndrome, a neurological disorder of the wrists and hands which causes pain and stiffness.

Sources
The richest sources of vitamin B6 are chicken, fish, liver, kidney, pork, eggs, milk, wheatgerm and brewer's yeast. Other good sources include brown rice, soybeans, oats, whole wheat products, peanuts and walnuts.

Long-term storage, canning, roasting or stewing of meat and food processing techniques can destroy vitamin B6. Cooking reduces the vitamin B6 content of food because of losses into the water.
 
 

Recommended dietary allowances (RDA)

USA

Men 1.3 mg
(over 50) 1.7 mg

Women 1.3 mg
(over 50) 1.5 mg

Pregnancy 1.9 mg

Lactation 2.0 mg
 

The RDA for vitamin B6 is based on protein intake. Those with high protein intakes may need higher levels of vitamin B6. A 1998 study published in the American Journal of Clinical Nutrition suggests that the US RDA for women is not adequate. The researchers found that the amount of vitamin B6 necessary to normalize body levels of vitamin B6 metabolites after depletion was 1.94 mg.

Supplements
Vitamin B6 is available as pyridoxine hydrochloride and pyridoxal-5'-phosphate. The latter is the more active form and may be best for those with liver disease who cannot convert pyridoxine to pyridoxal-5-phosphate. Anyone at risk of deficiency or who is suffering from a condition possibly linked to vitamin B6 deficiency is likely to benefit from supplements.

Toxic effects of excess intake
Large doses of vitamin B6 (over 2000 mg) can cause nerve damage. Symptoms include tingling in the hands and feet, a stumbling gait and lack of muscle coordination. Daily doses of 300 mg taken for many months or years may also lead to damage which can be permanent even when high doses are discontinued. Night restlessness, vivid dreams, sun sensitivity and an acne-like rash may also occur with high doses.

Therapeutic uses of supplements
Vitamin B6 supplements are used to treat deficiency symptoms. They have also been widely used in the treatment of a number of other health conditions.

Asthma
Limited research suggests that some children with asthma have a partial defect in tryptophan metabolism. An increased intake of vitamin B6 may reduce the symptoms of asthma in these patients. A study done in the 1970s looked at the effect of five months of pyridoxine therapy (200 mg daily) in asthmatic children and found significant improvement in symptoms and decreased need for anti-asthma medications. Researchers involved in a 1985 study found a dramatic decrease in frequency and severity of wheezing or asthmatic attacks in those taking vitamin B6 supplements. However, not all studies have found beneficial effects.

Long-term therapy with theophylline, a drug often given to asthmatic patients, lowers vitamin B6 levels. Vitamin B6 supplements may be useful in preventing the side effects of the drug which include headaches, nausea, sleep disorders and convulsions.

Cardiovascular disease
Low vitamin B6 levels increase the risk of cardiovascular disease and there is some evidence that supplements may be beneficial in preventing this. The results of the 1998 Harvard Study mentioned on page 87 showed that risk of coronary heart disease was reduced among women who regularly used multiple vitamins, the major source of folate and vitamin B6.

In a study done in 1993, South African researchers measured vitamin B6, vitamin B12, and folic acid levels in a group of healthy men with moderately high homocysteine levels. They found these levels to be low. In a placebo-controlled follow-up study, they found that a daily vitamin supplement containing 10 mg vitamin B6, 1.0 mg folic acid and 0.4 mg of vitamin B12 normalized elevated plasma homocysteine concentrations within six weeks.

In a study published in 1998 Irish researchers screened a group of clinically healthy working men aged 30 to 49 years and selected 132 with mildly raised homocysteine concentrations. They then assessed the effects of eight weeks of supplementation with B group vitamins and antioxidant vitamins on homocysteine concentrations. The men were randomly assigned to one of four groups: supplementation with B group vitamins alone (1 mg folic acid, 7.2 mg pyridoxine, and 0.02 mg vitamin B12), antioxidant vitamins alone, B-group vitamins with antioxidant vitamins, or placebo. The results showed significant decreases in both groups receiving B group vitamins either with or without antioxidants. The effect of the B group vitamins alone was a reduction in homocysteine concentrations of almost 30 per cent.

Vitamin B6 may also exert beneficial effects on the cardiovascular system by protecting against the aggregation of blood platelets. This prolongs clotting time and helps to reduce atherosclerotic plaque build-up. Vitamin B6 has also been shown to lower blood pressure and blood cholesterol levels. In a Swedish study published in 1990, researchers assessed the effect of 120 mg a day of vitamin B6 on seventeen 88 year old men with low vitamin B6 levels. After supplementation for eight weeks, the average plasma total cholesterol and LDL cholesterol concentrations were decreased by 10 per cent and 17 per cent respectively.

Mood disorders
Vitamin B6 deficiency is often found in depressed people and some studies have shown that supplements can improve mood. Vitamin B6 is involved in the metabolism of serotonin, a neurotransmitter which is involved in the regulation of mood. Vitamin B6 is also used to treat stress conditions.

In a randomized placebo-controlled study published in 1992, researchers at Harvard Medical School assessed the effects of 10 mg each of vitamins B1, B2, and B6 in 14 geriatric inpatients with depression who were taking antidepressant drugs. The results showed that those patients taking the vitamins showed greater improvement in scores on ratings of depression and cognitive function when compared with placebo-treated patients.

Premenstrual syndrome and estrogen therapy
Vitamin B6 has also been used in the treatment of premenstrual syndrome symptoms including water retention, acne and mood changes. Many experts recommend doses of 50 mg to 150 mg per day started on day ten of the menstrual cycle and continued until day three of the next cycle.

Researchers in Oxford, UK conducted a double-blind trial to study the effects of 50 mg of vitamin B6 per day on premenstrual syndrome symptoms. The trial involved 63 women aged 18 to 49 years old who had noticed moderate to severe premenstrual symptoms during the previous year. Thirty-two women completed the full seven months of the study and the results showed a significant beneficial effect of vitamin B6 on emotional-type symptoms such as depression, irritability and tiredness.

As estrogen may suppress vitamin B6 metabolism, supplements may be beneficial for pregnant women, those on the contraceptive pill or hormone replacement therapy (HRT) who suffer from mood swings and depression. Vitamin B6 is sometimes known as the women's vitamin.

Nausea of pregnancy
Vitamin B6 has been shown to provide relief from nausea and vomiting during pregnancy. Researchers at the University of Iowa studied the effects of vitamin B6 in doses of 25 mg every eight hours in a randomized, double-blind placebo-controlled study. Thirty-one patients received vitamin B6 for 72 hours, and 28 patients received placebo. At the completion of therapy, only eight of 31 patients in the vitamin B6 group had any vomiting, compared with 15 of 28 patients in the placebo group.

Carpal tunnel syndrome
Vitamin B6 has been used to treat nerve compression including that seen in carpal tunnel syndrome. Some research suggests that treatment with supplements in doses of around 50 mg to 200 mg can improve symptoms. However not all studies have reported positive results. In a 1996 study of 125 randomly selected industrial plant workers, researchers did not find a link between vitamin B6 and carpal tunnel syndrome. Vitamin B6 has been shown to change pain thresholds in clinical and laboratory studies which may explain studies which have shown significant improvements in pain scores when nerve conduction test results showed only mild improvements. A 1998 study suggests that a high ratio of vitamin C to vitamin B6 may worsen the symptoms of the disorder.

Vitamin B6 supplements may be useful in the treatment of carpal tunnel syndrome but intake should not exceed 100 mg per day as large doses can cause nerve damage. Higher doses should only be used under supervision. Some experts believe that a therapeutic effect may take at least three months to become apparent.

Convulsions
Some babies are born with a metabolic defect which means that they are unable to metabolize pyridoxine efficiently. This results in convulsions which must be treated with large doses of pyridoxine in order to avoid mental retardation.

Immune system
Vitamin B6 supplementation may boost the immune system in older people, thus reducing the risk of infection and possibly even cancer. Recent research from Johns Hopkins University suggests that vitamin B supplements may help HIV-positive people live longer. In an eight-year study of 281 HIV-positive, people vitamin B6 supplementation at twice the RDA was shown to be particularly effective in prolonging survival time.
 

Interactions with other nutrients
Vitamin B6 requires riboflavin, zinc and magnesium for its normal function in the body. Vitamin B6 deficiency may result in low blood levels of vitamin C, increased excretion of calcium, zinc and magnesium, and reduced copper absorption.

Interactions with drugs
Alcohol increases breakdown of the biologically active form of vitamin B6 and long-term use may cause liver damage which interferes with the conversion of vitamin B6 to the active form. Amphetamines, levodopa, some antidepressants, isoniazid, penicillamine and hydralazine may alter vitamin B6 requirements. Oral contraceptives may also affect vitamin B6 metabolism and increase needs. Large doses of vitamin B6 may interfere with the action of anticonvulsant drugs.

Cautions
Vitamin B6 supplements should not be taken by those taking anticonvulsants or levodopa for Parkinson's disease as they may alter the effectiveness of these medications. Vitamin B6 may lower blood sugar and should be used with caution by diabetics.