Long-term use of beta carotene and some other carotenoid-containing dietary supplements may increase the risk of lung cancer, especially among smokers, according to a study by University of North Carolina at Chapel Hill researchers.

The study, which also examined use of retinol, vitamin A, lycopene and lutein, appears in the February issue of the American Journal of Epidemiology.

Researchers used questionnaires to assess use of dietary supplements, including multi-vitamins and individual nutrients, by more than 77,000 Americans over 10 years, and matched the results against data from the Surveillance, Epidemiology and End Results (SEER) cancer registry to track the rates of lung cancer among them.

They found that certain people – especially smokers – who took dietary supplements containing these nutrients, were at higher risk of developing lung cancer than the general population.

“In the 1980s, studies began showing the link between diet and cancers, and showing that eating fruits and vegetables could lower your risk of certain cancers, including lung cancer,” said Jessie Satia, Ph.D., associate professor of epidemiology and nutrition at the UNC Gillings School of Global Public Health and member of UNC’s Lineberger Comprehensive Cancer Center. “Scientists wondered, then, if you took the beneficial nutrients from fruits and vegetables and gave high doses of them to persons at high risk for lung cancer, such as smokers, if you could decrease the risk of lung cancer.”

However, subsequent clinical trials proved that high doses of beta carotene actually seemed to increase the risk of developing lung cancer, Satia said, and trials in the United States and Sweden were stopped when the increased risk was detected.

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“But that was in clinical trials, under controlled circumstances,” Satia said. “We wanted to see if the same results would be found if we looked at use of these supplements in the general population.”

Satia and colleagues from UNC and the University of Washington in Seattle gave questionnaires to men and women, aged 50 to 76, in western Washington state. Participants were asked about their use of multivitamins and individual nutrient supplements over the past 10 years (including frequency and dose), as well as about their health history and risk factors. Participants were followed for the next four years, at which time lung cancer rates were obtained.

The results show that smokers’ risk of developing lung cancer increased with the length of time they took dietary supplements containing beta carotene, retinol and lutein.

“The risk increased the longer the person had taken the supplements,” Satia said. “The amount of time the person took supplements seemed to have a greater effect than the dose. Even a modest dose, if taken for a long time, can increase the risks of lung cancer, especially among smokers.”

For example, the study found that use of retinol and lutein supplements for four years or longer was associated with increases in lung cancer risk of 53 percent and 102 percent, respectively.

The risk for nonsmokers could not be determined because lung cancer cases among nonsmokers was small, Satia said.

“We believe beta-carotene is an antioxidant, but it seems that if you take too much, at some point it can have pro-oxidant effects, which can result in elevated cancer risk.”

Co-authors of the study are: Alyson Littman, Ph.D., research assistant professor of epidemiology at the University of Washington in Seattle; Christopher G. Slatore, M.D., pulmonary and critical care medicine fellow, University of Washington at Seattle; Joseph A. Galanko, Ph.D., research assistant professor of medicine, UNC School of Medicine; Emily White, Ph.D., professor and associate dean of the School of Public Health at the University of Washington and member of the Fred Hutchinson Cancer Research Center.