By Kathleen Raven
NEW YORK (Reuters Health) - Doctors can reassure patients that being depressed does not increase one's risk of cancer, French researchers say.
In a large group of middle aged French utility workers, the study looked for links between a history of depression and any type of cancer diagnosis.
"We have found nothing, and usually it is not very interesting when researchers have found nothing," said lead author Dr. Cédric Lemogne of Paris Descartes University in France.
"But in this case, it was the point."
The idea that depression might affect cancer risk has been around for decades, and researchers have found evidence for or against it.
At least one study in the 1990s suggested that people who repeatedly experienced depression might double their cancer risk. But that result was never repeated, Lemogne and his colleagues note in the American Journal of Epidemiology.
Large analyses that attempt to comb through all available studies on the subject have continued to produce conflicting results.
Still, researchers worry that despite a lack of clear evidence, some cancer patients may blame themselves for somehow causing or worsening their disease by being depressed.
"Many people are convinced when they develop cancer that they know exactly what caused it," said James Coyne, a health psychology professor at University Medical Center in Groningen, the Netherlands.
Coyne was not involved in the French study, but has investigated connections between depression and cancer.
"I get particularly concerned if patients are left with the idea that they can control the course of cancer through psychological training," Coyne told Reuters Health.
For their study, Lemogne and his team analyzed data collected from 14,203 men and women over a 15-year period. The men ranged in age between the 40s and 50s, and women between 35 and 50 years old. All the study participants were current employees or retirees of France's only gas and electricity company, GAZEL.
In 1989, these volunteers agreed to share their health information over time for various research projects.
The French researchers noted depression-related absences from work that had been verified by company physicians during a four-year period. Next they averaged participants' depression scores taken from a 20-item questionnaire given in 1993, 1996 and 1999.
All volunteers were tracked for cancer diagnoses between 1994 and 2009. These diagnoses were cross-checked against physician records and the country's cause-of-death registry.
During the period studied, about eight percent, or 1,119 men and women, received a cancer diagnosis. The researchers grouped cancers into five categories: prostate, breast, smoking-related, colon or rectum, and "all others."
They adjusted the analysis for a wide range of factors, including age, job hierarchy, alcohol use, smoking habits, vegetable and fruit consumption, body mass index and exercise.
They found no link between depression and any of the specific cancer types. They did find a very weak association between the "all other cancers" category and seasonal depression, though statistically, it could have been due to chance. They also found that men with depression had slightly fewer prostate cancer diagnoses, but that result was also statistically insignificant.
"It's very difficult to close the book on this topic," Lemogne told Reuters Health. The study could be repeated on different types of populations, and looking at different cancers, he said.
The researchers cautioned against generalizing results taken from mostly middle-aged men employed at a utility company in France.
"I would like to see an older sample followed," Mary Step of Case Western Reserve University's School of Medicine in Cleveland wrote in an email to Reuters Health. "Most people do not develop cancer until their mid 60s so I would be curious to see if more cancer developed," she said.
Though the current research is "well-grounded" and "very thorough," Step said that discussion and treatment of depression among cancer patients remains very important.
People may still want to believe a link exists between cancer and depression because human nature drives men and women to want to know the root cause of a disease, Lemogne said.
"People continue to have a psychological need to believe an explanation about life and death," he said.
SOURCE: bit.ly/HbTS2O American Journal of Epidemiology, online September 30, 2013.