Vitamin B6 (Pyridoxine)
Vitamin B6 has created a stir within the past few years. Most recently, the focus has been on some groups of Americans who may need more of the vitamin than the rest of us. Other hot issues have been the use of B6 in the treatment of premenstrual syndrome (PMS) and carpal tunnel syndrome. While these issues seem timely, this B vitamin has suffered relative obscurity since its discovery in 1934.
The scientist who first identified vitamin B6 reported that it was "that part of the vitamin B complex which cures a dermatitis developed by rats," and in fact dubbed it "the rat antidermatitis factor." How's that for humble beginnings? More than ten years later, researchers realized that there were several forms of the "rat antidermatitis factor" and curing itchy skin on a rat was the least of the new vitamin's abilities.
Three forms of the vitamin exist: pyridoxine (PN), pyridoxal (PL), and pyridoxamine (PM), which are relatives because of their chemical structure and some of their functions. PN is the major form of the vitamin in plant foods, while the other two forms are present in animal foods. All these forms are converted by the liver, red blood cells, and a few other tissues to the main active form, pyridoxal phosphate (PLP), which we'll call B6 One important roleof vitamin B6 is as a coenzyme in chemical reactions related to protein metabolism. The liver is able to swap the nitrogen-containing amine group on one amino acid to another, called the transaminase reaction. This reaction makes it possible for the liver to produce new amino acids that it needs, so that you don't have to ensure you eat foods containing all twentyone amino acids. Only nine are essential and must come from the diet. The liver utilizes the rest as the building blocks for new proteins. The body's cells are constantly making new proteins, protein for replacing old cells and tissues and for making enzymes.
One example of the kinds of reactions B6 helps with is the conversion of the amino acid tryptophan to the B vitamin niacin and the neurotransmitter serotonin. Serotonin is important in sleep and a general feeling of well-being. Because of its function in making serotonin and other neurotransmitters, researchers have learned that B6 is crucial to the development of the nervous system. Other interesting reactions that depend on B6 include the production of a part of hemoglobin, the protein that carries oxygen in the blood, and lecithin, which makes up cell membranes. The vitamin also helps out in producing new glucose to maintain blood glucose within a normal range, making this energy source available to all cells.
The last decade has witnessed a virtual explosion of B6 research, yielding exciting new roles for it. Studies have shown that the vitamin is involved in cognitive development, immune function, and the activation of certain hormones. A more controversial area has been a possible role for B6 in cholesterol metabolism, with early studies showing that the vitamin either lowered cholesterol or kept it from rising. Although it's not clear if the relationship is direct, higher blood levels of the vitamin are linked to higher HDL and lower LDL cholesterol, at least in monkeys. Vitamin B6 does appear to playa role in lowering another risk factor for heart disease, homocysteine, probably along with other B vitamins.
Legumes and some grains are fair sources, but dairy products and red meats are low in B6 Although you can find B6 in a variety of foods, several factors can increase requirements for the nutrient. Processing can destroy up to 70 percent of the vitamin during freezing, milling, and other common manufacturing practices. Many prescription drugs interfere with B6 metabolism, and oral contraceptives head the list. However, while contraceptives lower plasma levels of PLP, scientists aren't sure if this increases the risk for deficiency. Another drug that can pose problems is INH, which is used to treat tuberculosis. The drug has a structure similar to that of B6 and takes its place at sites where the vitamin exerts its effects, but it doesn't do the vitamin's job. This is a common source of B6deficiency. Alcohol is probably more of a concern because it actually destroys B6 when the body metabolizes alcohol, the first step forms acetaldehyde, a toxic compound. Acetaldehyde pries B6 from its place on enzymes, causing it to break down and be excreted.
For quite some time, researchers had believed that B6 deficiency was uncommon, probably occurring along with other B vitamin deficiencies in malnourished people. But some nutrition researchers aren't so sure. They cite the growing list of factors that can interfere with B6 or affect how the body uses the vitamin. The first question is, what happens to people who are deficient?
Because of B6's functions in protein metabolism, deficiency should cause severe effects in this area, and because of the vitamin's role in tryptophan conversion, you might expect niacin status to be in jeopardy. Certainly, as protein intake increases, the need for B6 increases, and only people whose diets are low in niacin would suffer any effects in status of that vitamin if B6 intake is off. More current thinking suggests that, instead of overt problems, the effects of suboptimal B6 status may be more subtle. The first deficiency symptoms could describe any number of modern people: irritability, insomnia, and weakness. Eventually, the situation gets worse with problems in nerve function, weakened immune defenses, and even convulsions.
One of the first group researchers pointed to as being at high risk for B6 deficiency were the elderly; studies showed many older people had low levels of the vitamin in various body tissues. Soon after, others countered that the depressed levels were merely reflecting natural changes of aging, not necessarily inadequate vitamin status. The consensus now suggests that the elderly probably do have a higher requirement for the vitamin as a result of age-related changes in B6 metabolism. Most nutritionists expected to see a higher B6 recommendation for the elderly with the new DRI, and technically the committee did this by creating a new age category. But since the previous RDA was higher, the net effect is actually a reduction in the vitamin recommendation.
Another vulnerable group appears to be Americans who smoke, based on several studies reporting that smoking increases the requirement for B6 One study compared levels of B6 compounds in smokers, ex-smokers, and nonsmokers and found that blood levels of the vitamin were significantly lower in smokers than in ex-smokers and nonsmokers. The only problem was that when researchers measured vitamin levels inside red blood cells, they found no diferences among the three groups. Since B6 does its coenzyme dance inside of cells, they questioned the significance of low blood levels. One important finding was that B6 blood levels may not be the best, and are certainly not the only indicator of B6 status.
When the RDA Committee last updated this vitamin's recommendation, it stated that acute toxicity of the vitamin is low. But a spate of studies proposing B6 as an effective treatment for PMS and carpal tunnel syndrome in which high doses were used seemed to suggest otherwise. Some women who took daily doses of 2,000 mg or more over a six-month period for PMS, which is 125 times the RDA, reportedly developed numbness and other nerve problems. However, they all recovered completely within six months after stopping the supplement. Other studies that purportedly showed the same nerve problems at lower doses, even as low as 100 mg, were not well controlled, and women took the vitamin for almost three years. A recent review of vitamin toxicity suggests that most of the concerns are exaggerated, but it's the length of time a supplement is used rather than the amount that predicts nerve problems.
Another concern was the possibility that B6 supplements could cause kidney stones, but studies showing this effect had used a special form of the vitamin, not the standard form. A subsequent study actually reported that men who took more than 40 mg a day had a lower risk for kidney stones than those consuming less than 3 mg. Besides PMS, people have used B6 to treat carpal tunnel syndrome. People with carpal tunnel syndrome suffer from numbness and tingling pain, which starts in the hand and wrist and radiates up the arm, sometimes causing swelling. There is some evidence to suggest that one possible cause in some people is B6 deficiency. This may be especially true in older adults, in whom clinicians have identified a B6-responsive carpal tunnel syndrome.
Pyridoxine is one of the B vitamins that has piqued the interest of researchers relative to a number of human conditions and diseases. While the results of some of their efforts remain to be proven, there is good evidence for several of these hypotheses. Is B6 an effective treatment for PMS and carpal tunnel syndrome? Look at the latest studies in the "Research Update" and decide for yourself.
Recommendation Too Low?
Most people in the United States eat too much protein, many consuming up to three times the RDA. As protein intake increases, the requirement for B6 also rises. Researchers at Washington State studied B6 requirements in young women eating a high-protein diet of .71 g/pound of body weight, which is almost double the RDA and varied the B6 content of the diet. The diet was lactoovovegetarian (no meats but including dairy products and eggs). Over the course of a month on the various diets, the researchers evaluated the women's B6 status every week. They found that the amount of dietary B6 the women needed to maintain the vitamin's functions was 1.94 mg, or 20 percent above the 1989 RDA, which was in effect when the researchers carried out the study. Interestingly, most studies show that men tend to eat even more protein than women, and diets containing meat products are more likely to be higher in protein than vegetarian diets such as the one used in this study. The authors concluded that since most Americans eat almost twice the amount of protein set by the RDA, the 1989 RDA for B6 wasn't adequate. The study was done before the new DRI, which lowered the recommendation for this vitamin.