Having cancer appears to protect against getting Alzheimer's disease-and vice versa.
Older people with Alzheimer's disease are less likely to also have cancer, and older people with cancer are less likely to also have Alzheimer's disease, researchers said.
"Since the number of cases of both Alzheimer's disease and cancer increase exponentially as people age, understanding the mechanisms behind this relationship may help us better develop new treatments for both diseases," said study author Massimo Musicco, MD, of the National Research Council of Italy in Milan.
The study involved 204,468 people age 60 and older in northern Italy during a six-year period. During that time, 21,451 people developed cancer and 2,832 people developed Alzheimer's disease.
A total of 161 people had both cancer and Alzheimer's disease, whereas that number would have been expected to be 281 for cancer and 246 for Alzheimer's disease when considering how often the diseases occur in the general population. Therefore the risk of cancer was cut in half for people with Alzheimer's disease, and the risk of Alzheimer's disease was reduced by 35 percent for people with cancer.
"While other studies have noted this relationship before, this is the largest study to date and it has several strengths over previous studies, such as looking for the presence of the second disease both before and after the first disease was diagnosed," Musicco said. "This controls for the possibility that the presence of one disease might obscure the diagnosis of other diseases because any new symptoms might be interpreted as a consequence of the already-diagnosed disease, or in the case of cancer, people might assume that memory problems were a side effect of chemotherapy."
The researchers found the same result in people who died during the course of the study as well as those still living, controlling for the possibility that the reduced life expectancy for the first disease would also reduce the likelihood of living to develop the second disease.
The study was recently published in the online issue of Neurology, the medical journal of the American Academy of Neurology.
