Older people with cancer may have a lower risk of Alzheimer's disease and vice versa, a large study suggests.
Both diseases increase as people age and can be viewed as opposite faces of aging, researchers say in Wednesday's issue of the journal Neurology.
The study involved 204,468 people age 60 and older in northern Italy over six years. During that time, 21,451 people developed cancer and 2,832 people developed Alzheimer's disease.
The risk of cancer in those with Alzheimer's decreased by 43 per cent compared with other adults with the same age and sex, and risk of Alzheimer's was reduced 35 per cent, Dr. Massimo Musicco of the National Research Council of Italy in Milan and his co-authors found.
During the study period, 161 people were diagnosed with both diseases.
But based on age and gender, the researchers expected 246 cases of Alzheimer's disease among those with cancer and 281 cancers in those with Alzheimer's.
Musicco's team found the association also held up for most cancers individually.
"Alzheimer disease dementia and cancer can be viewed as opposite faces of senescence," the researchers concluded.
The finding might appear counterintuitive, they acknowledged. But cancer and Alzheimer's disease might have opposite biological mechanisms.
DNA damage sparks normal cells to become cancerous and control mechanism can either prevent or repair the damage or eliminate cancerous cells, the researchers said.
Altering the activity of key molecules in survival pathways to repair damage and survive could explain a tendency to develop a tumour, while neurons die off in neurodegenerative disorders such as Alzheimer's disease, Catherine Roe of Washington University School of Medicine in St. Louis, Mo., said in a journal editorial published with the study.
Earlier studies couldn't rule out whether Alzheimer's disease kept cancer symptoms from being noticed or vice versa. It's also possible that people who die from one disease just have less time to be diagnosed with the other.
The editorial authors said the researchers used a clever study design to check. The results were similar both before and after the diagnosis of with the index disease, which suggests that the lower risk of Alzheimer's in patients with cancer is not due to underdiagnosis of new diseases once Alzheimer's or cancer is diagnosed.
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.
Since administrative data was used to identify those with Alzheimer's disease, some milder cases may have been overlooked. The researchers also lacked information on lifestyle-related risk factors like smoking. Another limitation is that most skin cancers weren't included.
The study was supported by the National Research Council of Italy and the Santa Lucia Foundation in Rome. Several authors had financial ties to pharmaceutical companies.