The ABC's of the RDA
This water-soluble vitamin is a coenzyme, or enzyme helper, aiding in the synthesis of proteins including hemoglobin and certain hormones. B6 deficiency can lead to a large variety of symptoms including fatigue, irritability, depression, tremor, and convulsions.
A manufacturing oversight helped clarify B6's role in the nervous system. In the early 1950s, Pennsylvania pediatrician David Coursin treated several infants who were experiencing seizures for no apparent reason. Coursin discovered that the infants were receiving milk formula whose B6 content had been completely destroyed through sterilization. When the infants were fed a different formula with adequate B6, they stopped having seizures.
National surveys suggest that certain segments of the U.S. population, particularly women of childbearing age, do not consume recommended amounts of B6, according to School of Public Health neurotoxicologist Tomás Guilarte, PhD '80. The RDA for B6 is 2.0 mg/day for men, and 1.6 mg/day for women. (Three ounces of chicken contains about 0.3 mg of B6.)
What's New at Hopkins:
Through extensive studies in cell cultures and animals, Guilarte discovered a biochemical clue that could explain how B6 deficiency causes nervous system impairments. Newborn rats who are B6-deficient, he found, had extremely high levels of a metabolite called 3-hydroxykynurenine (3HK) in their brain tissue. He also found that 3HK kills neurons in culture, possibly by triggering the production of free radicals.
In other studies, researchers have observed levels of 3HK that were several times higher than control amounts in the brains of patients with HIV dementia and in postmortem studies of people who had Huntington's disease. The studies do not show that B6 deficiency causes such disorders, stresses Guilarte. Elevated 3HK could occur through a different, unrelated mechanism.
Even slight vitamin B6 deficiency might impair the nervous system, causing subtle defects in learning and memory, suggests Guilarte. "But very little research has been done in this area." Researchers should explore the neurochemical and behavioral changes that result over the long term from diets containing marginal amounts of B6, he believes.
What We Can Do:
Pregnant women should be sure to get enough vitamin B6 so that the fetus's brain develops normally. The richest sources include high protein foods such as red meat, poultry, fish, wheat germ, soybeans, dried beans, and peanuts. Other sources include bananas and avocados. Caution: consuming amounts of B6 that are several times the RDA can cause numbness and more severe nervous system damage. --MH
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