A population-based study of Alzheimer’s disease (AD) and cancer among older adults in Northern Italy has found that the two diseases have an inverse relationship, with AD dementia patients exhibiting a lower risk of cancer and vice versa, according to a study published July 10 online in Neurology.

Despite the fact that the occurrence of both diseases increases exponentially with age, the risk of cancer was halved in people with AD dementia and the risk of AD dementia was 35 percent lower among those with cancer, reported Massimo Musicco, MD, of the Institute of Biomedical Technologies in Segrate, Italy, and colleagues.

Findings were based on a cohort study of over one million residents in Northern Italy age 60 years or older. Cancer incidence was pulled from a local tumor registry and incidence of AD dementia was calculated from registries of drug prescription, hospitalizations and payment exemptions for the observation period of 2004 to 2009.

The authors took pains to avoid confounding factors that might mask the underlying interactions between the two diseases. Multiple sets of analyses were conducted that compared incidence of AD, for instance, with incidence of cancer both before and after diagnosis of AD, and vice versa. Incidence of one disease was compared with incidence of the other among the larger population, and various cancer types were examined individually in order to weed out confounding conditions of a specific type of cancer.

Results showed people with AD were less likely to develop cancer, and those with cancer were less likely to develop AD, though the lower than expected incidences of the diseases only became statistically significant in those aged 70 or older. Results were similar in survivors and non-survivors, which suggests the findings were not influenced by reduced life-expectancy from one disease curbing incidence of the other, according to Musicco and colleagues. The trend also held steady for different cancer types.

Since occurrence of both AD and cancer increases with age, the authors noted the results might appear counterintuitive. However, they explained the cause might stem from the differing biological mechanisms of the diseases. Broadly speaking, while AD results in cell death, cancer is the product of excessive cell growth.

In an accompanying editorial, Catherine M. Roe, PhD, of Washington University School of Medicine in St. Louis, and Maria I. Behrens, PhD, MD, of the University of Chile, Santiago, expanded on the interactions between AD and cancer.

“Alterations in the activity of key molecules involved in survival pathways related to the decision to repair encountered damage and survive, or to proceed towards death by apoptosis, may explain a tendency to either develop a tumor or a neurodegenerative disorder such as AD,” they wrote.