Since cancer tends to be a rare occurrence in Alzheimer’s patients, researchers have hypothesized that perhaps what causes cancer may somehow prevent Alzheimer’s disease. A new study, however, provides a much somber explanation — that many cancer patients don’t live long enough to develop Alzheimer’s disease.

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

First, the researchers looked at data from pancreatic cancer patients, whose average age of death is 73, the same age at which Alzheimer’s is typically diagnosed. While 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).

Concluding that pancreatic cancer protects against Alzheimer’s disease is similar to saying that gunshots prevent Alzheimer’s, says senior author and Huntsman Cancer Institute investigator Ken Smith, Ph.D., distinguished professor of family and consumer studies and population science.

“People who are shot rarely get Alzheimer’s because most of them die before they have the chance to. But no one would say that gunshot wounds protect against the disease,” says Smith.

He adds that analyses need to consider that as people age, they are more likely to be affected by any of a number of conditions. People dying of lethal diseases simply lack the time to develop another illness.

Next, the researchers conducted their own analysis, looking at data from 92,245 individuals (ages 65 to 79) with and without cancer from the Utah Population Database, a comprehensive set of demographic, medical, and other records. The participants had no record of dementia. Their records began in 1992 and continued for at least 18 additional years to determine how many were later diagnosed with Alzheimer’s disease, as indicated by Medicare claims data.

Contrary to previous research, 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.

To get a better understanding of this, the researchers tracked two groups of patients with prostate cancer. If cancer provides protection from Alzheimer’s disease, says Smith, groups of patients with the same type of cancer would be equally likely to develop Alzheimer’s.

Yet they found that patients with a shortened life expectancy due to metastasized prostate cancer trended toward a decreased risk for Alzheimer’s as compared to patients with early-stage prostate cancer. Once they adjusted for mortality, however, they determined that the difference was not statistically significant.

“These results call into question a protective association between cancer and Alzheimer’s,” says Hanson. “If we are going to understand aging-related diseases, we need to consider how other chronic diseases and conditions impact them.”

The research is published in The Journals of Gerontology: Series B.

Source: Huntsman Cancer Institute at the University of Utah