WEDNESDAY, Aug. 9, 2023 (HealthDay News) -- A large new study challenges the long-held idea that depression makes people more vulnerable to cancer, finding no association between the mental health condition and most types of cancer.

The study, of more than 300,000 adults, found that neither depression nor chronic anxiety were linked to increased odds of developing cancer in the coming years. And when researchers looked at specific types of cancer, the findings were largely the same.

The one exception was a slightly increased risk of cancers that are strongly linked to smoking, including lung cancer. And the analysis suggests that smoking — as well as alcohol and heavy body weight — are the real culprits, rather than depression or anxiety themselves.

Experts said the study, published online Aug. 7 in the journal Cancer, may offer reassurance to people who've blamed a cancer diagnosis on their mental health struggles.

"Our findings show that there is no evidence for this," said study leader Lonneke van Tuijl, a health psychology researcher at University Medical Center Groningen in the Netherlands.

Unfortunately, that kind of self-blame "comes up a lot," said Dr. William Breitbart, chief of psychiatry at Memorial Sloan Kettering Cancer Center in New York City.

"There are many patients who feel guilt, who say 'I gave this to myself,'" said Breitbart, who was not involved in the study.

The reality, he said, is that cancer is many different diseases, each caused by a complex interplay of genes and environment.

There has been a decades-old interest, though, in the question of whether daily stress, depression or even a less-than-sunny personality might help cause cancer, Breitbart noted.

Over the years, that has included studies suggesting that having a "fighting spirit" or "positive attitude" helps people survive cancer.

"We called it the tyranny of positive thinking," Breitbart said. "It was a real thing, and it still is."

There is a logic to the idea that depression or anxiety disorders could increase the risk of cancer. The conditions are linked to heightened odds of certain other physical health ailments, like heart disease. In that case, researchers believe the connection might partly be due to the ravages of stress hormones on the cardiovascular system.

Similarly, it's been proposed that stress hormones might dampen immune system activity enough to feed tumor development.

But when studies have delved into the question of whether depression and anxiety are linked to cancer risk, the findings have been "all over the place," Breitbart said.

The new findings come from a research consortium that was designed to dig into the relationship between psychological health and cancer risk. It involved 18 study groups in Europe and Canada that collectively recruited more than 319,000 adults who were mostly middle-aged at the outset.

Study participants were assessed for depression and anxiety symptoms, and followed for anywhere from eight to 26 years. During that time, just over 25,800 were newly diagnosed with cancer.

Overall, van Tuijl's team found, people with depression or an anxiety disorder were at no greater risk of developing cancer in general. Nor did they have higher risks of alcohol-related cancers or specific cancers, like breast, colon or prostate.

The mental health conditions were tied to a slightly elevated risk of smoking-related cancers. But that connection was largely diminished when the researchers accounted for known risk factors for the cancers — not only smoking, but drinking and heavy body weight as well.

"That suggests it's not so much the depression or the anxiety, but the associated health-risk behaviors," Breitbart said, noting that some people "self-medicate" with alcohol, nicotine or overeating.

People could, of course, feel guilt over those behaviors. But no one chooses to have clinical depression — which is a disease, not a "constitutional failing," Breitbart stressed.

He said the new findings offer "very useful" information that will hopefully help reassure people.

Van Tuijl noted that she and her colleagues presented the findings at a medical conference last year, and afterward an oncologist in the audience told them she was happy to hear the results.

"She often encounters patients who think that because they had suffered from depression or anxiety in the past, they were somehow to blame for their cancer diagnosis," van Tuijl said.

But these findings, she said, show no basis for that belief.

More information

The American Cancer Society has more on cancer risk and prevention.

SOURCES: Lonneke A. van Tuijl, PhD, postdoctoral researcher, health psychology, University Medical Center Groningen, Groningen, the Netherlands; William Breitbart, MD, chief, psychiatry service, chairman, psychiatry and behavioral sciences, Memorial Sloan Kettering Cancer Center, New York City; Cancer, Aug. 7, 2023, online