Don’t Belittle this Vitamin

As vitamins go, B6 doesn’t fly high on the radar screen of most consumers. However, owing to its many benefits–which include protecting DNA–this unsung nutritional hero shouldn’t be neglected, argue a pair of scientists. Last week, they reported data showing that when people consume diets low in this vitamin, their blood has higher rates of DNA-strand breakage than when those same test volunteers get ample amounts of B6.

Cigarette smoking appears to rob the body of much of its vitamin B6, a nutrient important to protecting DNA.

“The good news is that following . . . diets containing ample amounts of the vitamin, the number of DNA-strand breaks fell back to normal,” according to study coauthor Terry D. Shultz of Washington State University. This was true even in smokers, who typically have low blood concentrations of vitamin B6. However, the nutritional biochemist points out that smokers who took part in this experiment lit up only an average of 16 cigarettes a day–less than a pack.

Why care about DNA-strand breaks? Because they’re “one thing that can lead to cancer,” notes Christine M. Hansen, now at Iowa State University, also a coauthor of the new study reported last week in San Diego at Experimental Biology 2003.

Indeed, support for the idea that B6 may have anticancer benefits also appears in a paper earlier this year in the Journal of the National Cancer Institute. In that study, researchers led by a team at the Harvard School of Public Health reported that among nearly 33,000 women, those with the highest blood concentrations of vitamin B6 face a 30 percent lower risk of breast cancer than do women with the lowest concentrations of the vitamin in their blood. The comparison was between the top fifth of study volunteers versus the bottom fifth, according to their B6 blood concentrations.

The way for DNA to B healthy

Vitamin B6 comes in six forms. All are coenzymes–partners for about 100 true enzymes that help build or break down various biologically active chemicals.

The rungs of DNA’s ladderlike structure are made of paired combinations of four chemicals, one of which is thymine. Vitamin B6 prods a reaction in the body that converts folate–another B vitamin–into thymine. So, when B6 concentrations are low, the availability of thymine also drops. DNA sometimes responds, Shultz notes, by inappropriately substituting uracil, a related chemical, in some of the rungs.

The body has various enzymes to cull heavily damaged DNA and others to repair what can be fixed. Ordinarily, when uracil gets into DNA, repair enzymes swoop in, snip it out, and insert a thymine. However, if too many uracil mistakes occur, “the repair-enzyme mechanisms can become overwhelmed,” Shultz told Science News Online. The result: A strand of DNA may get cut open at the uracil site but not stitched back together with its missing thymine.

In the new study . . .

To evaluate the special vulnerability of smokers, the Washington State team recruited 12 men and women, half of them smokers, for a feeding experiment. All volunteers initially exhibited about the same, low background concentration of DNA-strand breaks in a type of white blood cell, although the smokers had roughly 50 percent lower starting B6 blood concentrations, observes Shultz.

Over the next 5 months, the researchers provided all meals for these volunteers, ages 21 to 44. Month to month, the only nutritional change to their diet was the quantity of vitamin B6.

For the first month, the men and women received a diet low in B6. The recommended dietary allowance (RDA) for adults is 1.3 milligrams per day, but the researchers fed the study participants only 0.7 mg/day during this starting period.

By the end of the month, the volunteers had about 75 percent more DNA-strand breaks in their white blood cells than when they started.

The researchers then upped the recruits’ B6 intake to the RDA for 1 month–and counts of DNA breaks fell back to normal. In subsequent months, the scientists increased the B6 supplementation even more–eventually to 10 mg/day. Though the recruits’ DNA-break counts continued to fall, they never went far below their values at the beginning of the study.

The researchers are a bit perplexed why they didn’t see a bigger difference between smokers and nonsmokers, “but perhaps that’s because we didn’t look at subsets of [white blood cells],” says Shultz. Some classes of cells might have been more vulnerable to DNA-strand breaks than others, he says.

“The take-home message here is that there could be a high prevalence of inadequate vitamin B6 intake in the general population,” Shultz says. Indeed, the Institute of Medicine has found that much of the U.S. population doesn’t even consume the estimated average requirement, which at 1.1 mg/day is below the RDA. Some 10 to 25 percent of men over 50 don’t get 1.1 mg/day, and women do even worse. A quarter to a half of women over 50 and pregnant women of any age fail to consume the estimated average requirement of vitamin B6.

People who think they may be low in vitamin B6–and this probably should include all smokers–may want to increase their intake of foods rich in the nutrient, especially fortified cereals, bananas, avocados, beef, poultry, fish, and legumes.

Janet Raloff is the Editor, Digital of Science News Explores, a daily online magazine for middle school students. She started at Science News in 1977 as the environment and policy writer, specializing in toxicology. To her never-ending surprise, her daughter became a toxicologist.

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