People with Alzheimer's disease have a lower risk of cancer than other elderly adults, a new Italian study suggests.
Additionally, researchers found that seniors who were diagnosed with cancer were less likely to develop Alzheimer's.
Researchers said there are a number of genes that affect both neurology and cancer growth - and pathways by which the two are connected - that could explain the "unexpected" inverse link between the diseases.
"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 in Milan.
"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.
There are convincing data that Parkinson's disease is tied to a lower risk of cancer, said Dr. Jane Driver, who studies aging at Brigham and Women's Hospital in Boston.
More recently, the same pattern has been showing up for other neurological disorders, including schizophrenia and Alzheimer's, she noted.
But earlier studies haven't been able to rule out whether Alzheimer's disease might be keeping cancer symptoms from being noticed - or vice versa - or if people who die from one disease just have less time to be diagnosed with the other.
In their study, Musicco and his colleagues found people who ultimately were diagnosed with Alzheimer's had a lower risk of cancer both leading up to and after their diagnosis.
Likewise those with cancer were less likely to get Alzheimer's both before and after the cancer was caught.
"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," Driver told Reuters Health.
Musicco and his team tracked new cancer and Alzheimer's diagnoses among 204,000 people age 60 and older living in Northern Italy.
Between 2004 and 2009, just over 21,000 of them were diagnosed with cancer and close to 3,000 with Alzheimer's disease. There were 161 people diagnosed with both diseases.
The researchers calculated that 246 cases of Alzheimer's disease would be expected in members of the cancer group, based on their age and gender balance, and 281 cancers would be predicted among those with Alzheimer's.
The lower rates meant that people with cancer were 35 percent less likely to develop Alzheimer's disease than other adults, the researchers wrote in the journal Neurology. And those with Alzheimer's had a 43 percent lower risk of cancer.
The link held up when they looked at most cancers individually.
"These two diseases seem intrinsically related to human aging," Musicco told Reuters Health.
"Cancer may be conceptualized as a high tendency of cells to reproduce, which is so high that it is no (longer) controlled," he said. "Alzheimer's disease is exactly the reverse. It's a sort of incapacity of neuron cells to reproduce."
The study doesn't prove one disease is protective against the other. It also doesn't mean people with Alzheimer's or cancer never have to worry about getting the other condition, Driver said.
The researchers noted that they couldn't take into account people's lifestyle and whether some habits might affect the risk of cancer and Alzheimer's differently.
Driver said the inverse link has "prompted thinking outside the box" regarding ways to treat Alzheimer's disease - for which good treatment options are scarce.
"By further investigating this decreased risk, there's a good chance we'll be able to find completely new therapies," Driver said.
"Any new lead or any new avenue to go down, we really need to take it."