“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.”
There has been a long-held theory that depression might affect cancer risk, and different studies have found evidence both for and against it. For example, one study in the 1990s suggested that individuals with a long history of depression had double the risk for cancer.
Yet those findings were never repeated, wrote Lemogne and his team in the American Journal of Epidemiology.
In fact, large-scale analyses that attempt to comb through all available studies on the subject have continued to produce conflicting results. Despite a lack of clear evidence, however, some cancer patients may blame themselves for somehow causing or worsening their condition 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,” said Coyne.
For the new study, researchers analyzed data taken from 14,203 individuals during a 15-year period. The men ranged in age between the 40s and 50s, and women between 35 and 50 years old. At the time of the study, all of the subjects were employees or retirees of France’s only gas and electricity company, GAZEL.
These participants had agreed to share their health information over time for a variety of research projects.
The researchers noted any depression-related absences from work that had been confirmed by company doctors during a four-year period. Then they averaged participants’ depression scores taken from a 20-item questionnaire given to the participants three times within ten years.
All participants 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 this time, about eight percent (1,119 participants) received a cancer diagnosis. The researchers grouped cancers into five categories: prostate, breast, smoking-related, colon or rectum, and “all others.”
The findings were adjusted for a wide range of factors, including age, job hierarchy, alcohol use, smoking habits, vegetable and fruit consumption, body mass index and exercise.
The researchers found no link between depression and any specific type of cancer. There was a very weak connection between the “all other cancers” category and seasonal depression — although statistically, it could have been due to chance. Also, men with depression had slightly fewer prostate cancer diagnoses, but that number was also statistically insignificant.
“It’s very difficult to close the book on this topic,” said Lemogne. The study could be repeated on different types of populations, and looking at different cancers, he added.
Human nature drives men and women to want to know the root cause of a disease, said Lemogne. “People continue to have a psychological need to believe an explanation about life and death,” he added.
Source: American Journal of Epidemiology