BERKELEY – Research led by University of California, Berkeley, nutrition experts has found new evidence that vitamin C can significantly reduce levels of oxidative stress, which is associated with a variety of chronic diseases, for people exposed to environmental tobacco smoke.
The study, published Aug. 5 in the journal Nutrition and Cancer, provides hope for people who cannot escape secondhand smoke, said Gladys Block, professor of nutritional epidemiology at UC Berkeley's School of Public Health and principal investigator of the study.
"We know that nonsmokers who are exposed to secondhand smoke have an increased risk of lung cancer and heart disease," said Block. "The problem is that most people who are passively exposed to tobacco smoke can't do anything about it because they live with a smoker and are therefore exposed indoors at home. They may also live in a state that does not ban indoor smoking and so they end up working in a smoke-filled environment."
The researchers note that cigarette smoke contains large amounts of reactive free radicals, molecules that can cause oxidative damage to cells. Current research links oxidative stress to heart disease, cancer, atherosclerosis, and other chronic diseases.
"It has been well established in test tube studies that vitamin C quenches reactive free radicals from cigarette smoke," said Marion Dietrich, a UC Berkeley researcher in nutritional epidemiology and lead author of the study. "However, there are few studies that have looked at the effects of vitamin C on oxidative stress in secondhand smokers the way we have."
In the study, the researchers tested for levels of a reliable and sensitive biomarker, F2-Isoprostanes, which is generated through a form of oxidative stress called lipid peroxidation. Lipid peroxidation can lead to damage of the cell membrane.
The researchers compared 67 adult nonsmokers who had been randomly assigned to one of three treatment groups in the double-blind trial. One group took daily doses of 500 milligrams of vitamin C, a second group took a daily mixture of vitamin C with vitamin E and the antioxidant alpha-lipoic acid, and a third group took daily placebo capsules. Participants had been instructed to stop taking any vitamins at least five weeks prior to the start of their treatment.
The researchers recruited participants through the Kaiser Permanente Health Plan and advertised through local Bay Area media. They chose participants who were exposed to the smoke of at least one cigarette per day, five days a week, in an indoor setting. They also excluded people with a high intake of alcohol or who ate large amounts of fruits and vegetables because of the potential impact those factors have on the level of oxidative stress.
The researchers found that after two months of treatment, blood levels of F2-Isoprostanes dropped significantly - by 11.4 percent and 12.7 percent for those in the vitamin C and the mixture groups, respectively - compared with those taking the placebo. The difference between the vitamin C-only group and the mixture group was not statistically significant.
"The fact that there was no significant difference in the oxidative stress biomarker between people taking vitamin C and those taking the mixture suggests that the key antioxidant at work is the vitamin C," said Dietrich. "The other antioxidants in the mixture group did not seem to enhance the reduction of F2-Isoprostanes in our study."
They also found that by the end of the study, levels of ascorbic acid in participants' blood increased significantly by 32 percent in the vitamin C group and 41 percent in the mixture group.
"These results are very encouraging," said Dietrich. "They show that vitamin C may help protect nonsmokers from the oxidative damage caused by secondhand tobacco smoke. "
The researchers controlled for potentially confounding factors such as body mass index and gender, both of which have been associated in prior research with changes in oxidative stress.
"We cannot say from this study that vitamin C will keep you from getting heart disease or cancer because we did not measure those outcomes," said Block. "But, there is research that says oxidative stress is related to a number of chronic diseases, and that vitamin C reduces oxidative stress."
Despite the encouraging results, the researchers caution against misinterpreting the study's findings. "The message of the study is clearly not that taking vitamin C makes smoking or exposing others to smoke okay," said Block. "But, if you are in a situation where you cannot escape frequent exposure to secondhand smoke, it may be worthwhile to take vitamin C supplements as a precautionary measure. And, as always, eat a diet rich in fruits and vegetables."
Co-authors of the study are Dr. Neal Benowitz and Peyton Jacob III at UC San Francisco (UCSF) and San Francisco General Hospital Medical Center; Dr. Jason D. Morrow at Vanderbilt University Medical Center in Tennessee; Mark Hudes at UC Berkeley; Edward Norkus at Our Lady of Mercy Medical Center in New York; and Lester Packer, former professor at UC Berkeley and founder of a research laboratory at the University of Southern California.
The study was funded by the University of California Tobacco-Related Disease Research Program. The National Cancer Institute, part of the National Institutes of Health, also helped support the research.