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A form of vitamin E found in vegetable oils like corn and canola may worsen lung function, while another form typically found in olive oil may protect it, a new study suggests. The findings may help explain why studies of the health effects of the vitamin have had conflicting results.

Vitamin E comes in various forms called tocopherols, which are commonly found in fats and oils. Supplements of the vitamin may contain a single type of tocopherol, or a mix.

The new research, published in the journal Respiratory Research, found that a form of the vitamin called gamma tocopherol, the kind in corn, canola and soybean oils, was linked to poor lung function in adults. But another form of the vitamin more typically found in olive and sunflower oils, called alpha tocopherol, seemed to have a beneficial effect on the lungs.

“It’s mind-blowing that there’s this disparity,” said Dr. Clifford W. Bassett, an allergist and assistant clinical professor of medicine at the New York University School of Medicine. “What’s interesting is that most people taking vitamin E never stop to ask where it’s from. It’s not something you tend to be aware of.”

Dr. Bassett, who was not involved in the study, said the findings suggest that consumers who use the vitamin and doctors who recommend it might need to be “more acutely aware” of its source and formulation. That information is sometimes listed on the labels of supplements, but not always.

Vitamin E, considered a powerful antioxidant, is thought to play a role in cardiovascular and neurological health. There is also some evidence that the vitamin may help protect against asthma and other respiratory problems. But studies have also suggested that taking vitamin E has no effect on lung health, or even a potentially harmful one.

In research over the years at Northwestern University Feinberg School of Medicine, Joan Cook-Mills, an associate professor, has found that in addition being an antioxidant, vitamin E appears to play a role in inflammation. The vitamin influences a protein that allows white blood cells – an important part of the immune system – to exit the bloodstream and enter tissues, including cells in the lung, a critical step in the inflammatory response.

But different forms of the vitamin do not have the same effect. Gamma tocopherol increases the activity of the protein, and alpha tocopherol lowers it.

As a result, the two forms of vitamin E can have a drastically different impact on inflammation, Dr. Cook-Mills said. For example, gamma tocopherol can set off “hyper responsiveness” in the airways, a common feature of asthma, Dr. Cook-Mills said.

In the new research, Dr. Cook-Mills and her colleagues at Northwestern analyzed data from a nationwide study, sponsored by the National Institutes of Health, known as Cardia. The study has been tracking adults since 1985 as a way of discovering how genetics and habits affect heart disease, lung function and other aspects of health.

The research, involving 4,526 adults, assessed whether the levels of alpha and gamma tocopherol measured in their bloodstreams were linked to lung health. Ultimately, people with the highest levels of gamma tocopherol had a 10 percent to 17 percent reduction in lung function.

Dr. Cook-Mills and her colleagues found a higher incidence of asthma associated with higher blood levels of gamma tocopherol. But higher levels of alpha tocopherol – particularly in people with low levels of gamma tocopherol – were tied to better lung function.

Throughout the study, the subjects periodically underwent standard pulmonary tests that could identify lung capacity and airway obstructions.

Dr. Cook-Mills said the findings raised concern because gamma tocopherol is increasingly common in American food products. She pointed out that asthma rates in the United States have risen steadily in the past four decades, which corresponds with the rising use of canola, soybean and other vegetable oils rich in gamma tocopherol.

But she also cautioned that the research is observational. More rigorous studies are needed to see whether giving subjects alpha tocopherol in supplements or food can directly improve lung function, or if gamma tocopherol would have the opposite effect.

Dr. Bassett at New York University said there were other unanswered questions about the study, including the extent to which habits and diet may have played a role in the results. But the implications, if the findings are confirmed, are important, he said.

“It may explain why some of the studies on vitamin E have found it counterproductive,” he said.

Dr. Cook-Mills said that many studies have focused on vitamin E’s role as an antioxidant while neglecting the contrasting effects of alpha and gamma tocopherol on inflammation.

“You can see why in the medical literature it’s been totally confusing if researchers thought they were equal antioxidants but they got conflicting outcomes,” she said. “It’s because there are other functions of tocopherols that have opposing effects.”