People with Alzheimer's disease have a lower risk of cancer, and people with cancer are less likely to get Alzheimer's.
Experts said genetics affected the pathway connecting cancer cell growth and Alzheimer's, which could explain the "unexpected" correlation, Reuters reported.
"Cancer and Alzheimer's have been viewed by researchers as completely separate," said Dr. Massimo Musicco, who led the study at the National Research Council of Italy's Institute of Biomedical Technologies.
"Some of the knowledge that we have on cancer can be used for a better understanding of what happens when a person has Alzheimer's disease, and vice versa," he said.
The study observed about 204,000 Italians over the age of 60 who were living with either cancer or Alzheimer's.
Participants with cancer were found to be 35 percent less likely to develop Alzheimer's than the average person in their age group. Patients suffering from Alzheimer's had a 43 percent lower chance of cancer.
"Cancer may be conceptualized as a high tendency of cells to reproduce, which is so high that it is no [longer] controlled. Alzheimer's disease is exactly the reverse. It's a sort of incapacity of neuron cells to reproduce," Musicco said.
Researchers have noticed the same patterns showing up in other neurological disorders such as schizophrenia.
Scientists have speculated the findings may relate to one disease masking symptoms of the other. Patients suffering from cancer or Alzheimer's often have shorter life spans, so they may not live long enough to have a second disease diagnosed.
The study found people with Alzheimer's had a lower risk of cancer both before and after they were diagnosed, and vice versa.
"I'm hoping this will then convince all the doubters that there is a true inverse association between Alzheimer's, Parkinson's and probably some other neurologic diseases and cancer," Dr. Jane Driver, of Brigham and Women's Hospital in Boston, told Reuters.
The correlation, while not set in stone, could prompt researchers to "think outside of the box" when looking for cures, according to Driver.
