Despite some evidence tying the mineral selenium to a reduced risk of lung cancer, a new long-term study sees no connection between the two.
In fact, researchers found that among heavy smokers, those with the highest selenium levels had a greater risk of dying from lung cancer than those with the lowest levels.
The reason for that finding is not known, and it may have nothing to do with selenium itself.
But the researchers say they cannot rule out the possibility that heavy smoking and high selenium somehow act together to promote lung cancer.
The bottom line for smokers is that the best way to curb your lung cancer risk is still to kick the habit.
"The best advice regarding smoking is: Stop," lead researcher Poul Suadicani, of Bispebjerg University Hospital in Copenhagen, told Reuters Health in an email.
The study, reported in the European Respiratory Journal, included more than 3,300 older Danish men followed for 16 years. At the start, all had their blood selenium levels measured.
Over the next 16 years, five percent of the men died of lung cancer. And there was no difference in that rate among men with low selenium levels versus those with high.
Looking only at men who were heavy smokers at the study's start – measured by the amount of nicotine byproducts in their blood – the researchers found different results, however. In those men, high selenium was linked to an increased risk of lung cancer death.
Of the roughly one-third of heavy smokers with the highest selenium, 11 percent died of lung cancer. That compared with six percent of those with the lowest selenium levels, and nine percent of those in the mid-range.
When the researchers weighed certain other factors – like heavy smokers' alcohol, fat and salt intake – high selenium was linked to a doubling in the odds of lung cancer death.
Selenium is a mineral needed in trace amounts for good health. Its main job is to aid in the body's antioxidant defenses, which help limit the cell damage that can lead to diseases like cancer.
Some studies have found that people with relatively high selenium levels have a lower risk of certain cancers, including lung cancer. But others have failed to find a link.
What's more, clinical trials that put selenium supplements to the test against cancer have so far come up short.
A large U.S./Canadian trial was stopped when selenium, taken with or without vitamin E, showed no effect on men's risk of developing prostate cancer over 5.5 years. Vitamin E alone actually produced a slightly increased risk of the disease.
The current study is not the end of the story for selenium, however, according to Suadicani. More studies into selenium and a range of health outcomes are still needed, he said.
Researchers still are not sure how much selenium is needed for optimal health. And some say it's still possible that selenium affects cancer risk over a very long time period.
For now, the U.S. recommendation for adults is to get 55 micrograms of selenium per day (and 60 and 70 micrograms per day during pregnancy and breastfeeding, respectively).
Selenium is found in soil, so grains like whole wheat bread and rice contain the mineral. Other sources include beef, chicken and certain fish, like tuna. Brazil nuts contain an unusually high amount of selenium, often several hundred micrograms per ounce.
In the U.S., the "tolerable upper intake" limit for selenium is set at 400 micrograms per day for adults. Too much selenium can cause selenosis, with symptoms like gastrointestinal problems, hair loss, fatigue and mild nerve damage.