You may have read that what causes cancer in some people may protect them from developing Alzheimer’s disease.

Extensive new research published in The Journals of Gerontology: Series B., however, suggests that those who have cancer are less likely to get Alzheimer’s simply because their lives are not long enough.

“Diagnosis of age-related diseases, such as Alzheimer’s disease, depends on someone surviving to an age when disease onset can occur,” said lead author Heidi Hanson, Ph.D., M.S., a research associate at the University of Utah’s Huntsman Cancer Institute and a research assistant professor of family and preventive medicine.

To illustrate the concept, Hanson and colleagues looked at data from people with pancreatic cancer, for which the average age of death is 73. That’s the same age at which Alzheimer’s disease typically is diagnosed.

Although the rate of Alzheimer’s diagnoses tripled as the cancer-free population aged from 75 to 89 years old (increasing from 25 to 75 per 1,000), it remained constant in patients with pancreatic cancer (20 per 1,000).

The researchers then examined data from 92,245 individuals with and without cancer from the Utah Population Database, a comprehensive set of demographic, medical and other records. Members of the group were aged 65 to 79 years in 1992 and had no record of dementia. Their data was followed for at least 18 additional years to determine, via Medicare claims data, how many later received Alzheimer’s disease diagnoses.

Contrary to previous studies, three different statistical methods showed that those with cancer did not have a decreased risk for Alzheimer’s disease. Each method factored in higher rates of death among cancer patients in a slightly different way.

The research shows that studies of age-related diseases must consider how they are affected by chronic diseases and conditions, Hanson said.