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Cigarette smoking remains the primary cause of lung cancer — yet recent studies report growing incidences of it among those who never smoked.

Lung cancer is the third most common cancer in the United States, according to the Centers for Disease Control and Prevention. And it’s too often deadly: 221,121 new cases were reported in 2017 (the most ­recent year for which data is available); in that same year, 145,849 people died from the cancer.

Lung-cancer-screening recommendations focus on cur­rent and former smokers, which in part ­accounts for the high rate of ­smoking-related causation, according to a 2020 analysis published in JAMA Oncology. Yet that study and two others from 2017 and 2018 show that an estimated 12.5 percent of U.S. lung-cancer patients never smoked — and the numbers seem to be rising.

“Since the early 2000s, we have seen what I think is truly an epidemiological shift in lung cancer,” surgeon Andrew Kaufman, MD, of New York City’s Mount Sinai Hospital told Stat.

For those who never smoked, the CDC notes, secondhand smoke is often the key factor in contracting the disease. Radon gas is next to blame, along with diesel exhaust and other air pollution.

A 2020 study published in the BMJ Journals’ Thorax suggests that another mechanism behind nonsmoker lung cancer could be specific types of bacteria in the mouth’s microbiome. American and Chinese researchers assessed the 136,421 participants of the Shanghai Men’s and Women’s Health Studies over 10 years. Hosting a smaller variety of bacterial species was associated with a greater risk of lung cancer.

In addition, an abundance of Bacteroidetes and Spirochaetes bacteria was linked with a reduced risk; high levels of Firmicutes — a lactic-acid bacteria usually found in decomposing plants and dairy products — was associated with a heightened risk.

“The oral microbiome . . . forms an ecosystem that helps to maintain health,” explains ­David Christiani, MD, professor of medicine at Harvard University’s T.H. Chan School of Public Health and a physician at Massachusetts General Hospital, in an accompanying ­editorial. “It remains unclear whether the oral microbiome as measured in this (and other) epidemiological studies represents a causative agent or only a marker of disease or immune activity. If it is the former, then it will be important to understand whether the oral microbiome actually seeds the lung microbiome and thus acts locally.”

The gut may also play a role. A 2019 analysis of 10 studies in JAMA Oncology finds that eating yogurt and ample fiber may lower lung-cancer risk because the probiotics may bolster the gut microbiome. Researchers found that people who consumed the highest amounts of fiber had a 17 percent lower risk of lung cancer, while those who ate 3 to 4 ounces of yogurt a day were 19 percent less likely to develop the disease than those who ate none. People with a high intake of fiber and yogurt had a 33 percent reduced risk compared with those who consumed the least.

This article originally appeared as “The Rise in Lung Cancer Among Nonsmokers ” in the July/August 2021 issue of Experience Life.

Michael Dregni

Michael Dregni is an Experience Life deputy editor.

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