Experts aren’t sure why, but people who survive cancer may have certain biological conditions that protect them from cognitive diseases.

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Experts say this latest research builds on previous studies that demonstrate a link between surviving cancer and a lower risk of dementia. Getty Images

The National Cancer Institute (NCI) confirms that cancer rates in the United States are steadily increasing.

And, according to the Centers for Disease Control and Prevention (CDC), as many as 5 million Americans were living with Alzheimer’s disease in 2014. That number is only expected to grow in the coming years.

However, a new study finds that middle-aged people who survive cancer may be at a reduced risk of developing Alzheimer’s and other forms of cognitive decline later on.

This study builds on previous research that also found an association between surviving cancer and reduced Alzheimer’s risk.

“We conducted this research because previous studies found a lower risk of Alzheimer’s disease in people with a previous diagnosis of cancer,” Dr. Monica Ospina-Romero, a master student in training in clinical research at the University of California San Francisco and lead study author, told Healthline.

“With our study, we wanted to evaluate if adults with new cancer had a more favorable memory performance before and after diagnosis when we compare them with adults who were never diagnosed with cancer,” she added.

This study included almost 15,000 people born before 1949 with no history of cancer from the Health and Retirement Study.

Participants had their memory tested twice yearly for up to 16 years — from 1998 to 2014.

During the study period, 2,250 received a cancer diagnosis versus 12,333 who didn’t.

The researchers discovered that, on average, those who received a cancer diagnosis performed better at memory tasks than those who were cancer-free.

Romero said the study findings were surprising.

“The inverse association between cancer and Alzheimer’s disease is very intriguing,” she said. “We were very excited when we found that this evidence was reproduced with a totally different approach and that cancer patients had better memory function even before the diagnosis.”

“This supports the hypothesis of a common causal factor between carcinogenesis and neurodegeneration,” she explained.

Romero said she and her colleagues are now trying to get more data on the types of cancer and the stages of cancer in the study group.

According to Romero, cancer survivors might be a special group of people with certain biological and social characteristics that protect them from Alzheimer’s disease.

“If this is the explanation, we would love to know what these characteristics are. Detection bias means that the diagnosis of Alzheimer’s disease could be different for someone with or without a history of cancer,” she said.

Dr. Diana Kerwin, a geriatrics specialist at Texas Health Presbyterian Hospital in Dallas, thinks changes to the immune system may hold the key.

“There could be an inverse relationship due to immune-related changes in the bodies of persons that develop cancer,” she told Healthline.

However, she noted the study’s design limited that sort of conjecture because it “didn’t have a way to determine what treatment the patients reporting cancer received.”

According to Romero, she considered chemotherapy one of the potential explanations for the reduced incidence of Alzheimer’s disease in the study group.

However, she also said the study showed that when chemotherapy is prescribed, people experienced a short decline in memory function.

“’Chemo brain’ is a very real entity, although how it happens is still unknown,” said Dr. Kevin Conner, a neurologist at Texas Health Arlington Memorial Hospital. “But, one explanation could be that chemotherapy disrupts the cell replication necessary for healthy cell function. That means all cells in the body are at risk for disruption, including those in the brain.”

“We don’t know if chemotherapy is the only cause of this decline in cognitive function,” he told Healthline, “but this phenomenon has been documented by other researchers previously.”

“Most cancer treatments, like chemo, radiation, and steroids… are associated with a decline in cognitive function that stabilizes after completing treatment,” Kerwin added.

Romero hopes her research will lead to new treatment options.

“That was definitely our motivation,” she said. “We’re so intrigued by the link between cancer and Alzheimer’s because it’s surprising.”

“Sometimes by looking more closely at a result that doesn’t seem to make sense at first, we can learn something really new,” she added. “That’s what we hope here. If we can identify the common biological mechanisms, we might be able to take advantage of that mechanism to reduce Alzheimer’s and dementia risk.”

The incidence of both Alzheimer’s and cancer is increasing as the U.S. population grows older.

New research, however, has concluded that people who have survived a cancer diagnosis have a reduced risk of developing different types of memory problems such as Alzheimer’s disease or mild cognitive impairment.

The findings of this study, and others like it, may help researchers develop new options to prevent or treat memory-related conditions.