Though it's smart to take steps to prevent skin cancer, people diagnosed with the non-melanoma types of the disease may have a reduced risk of developing Alzheimer's disease, a new study finds.
Study participants who had been diagnosed with either basal cell or squamous cell skin cancer were nearly 80 percent less likely to develop Alzheimer's, according to researchers at Albert Einstein College of Medicine in New York.
The researchers followed 1,102 people age 70 and older, for an average of 3.7 years, checking them annually for Alzheimer's and skin cancer. All participants were enrolled in an ongoing study of aging in New York City.
At the beginning of the study, none of the participants had signs of dementia or Alzheimer's, and 109 people reported being diagnosed with either basal-cell or squamous-cell skin cancer at some point previously.
During the study period, an additional 32 people developed one of the skin cancers. While 126 people were diagnosed with some form of dementia, only two people with non-melanoma skin cancer developed Alzheimer's disease.
The researchers did not explore a possible link between melanoma, the deadliest skin cancer, and Alzheimer's because not enough people in the study developed that cancer, said researcher Dr. Richard Lipton, vice chairman of neurology at Albert Einstein College of Medicine.
An estimated 2.8 million cases of basal-cell carcinoma, and 700,000 cases of squamous-cell skin cancer are diagnosed in the United States each year, according to the Skin Cancer Foundation. Basal-cell skin cancer develops in the deepest layer of the epidermis, the skin's outer layer. Although it can be disfiguring, the cancer usually doesn't spread if it's treated early. On the other hand, squamous-cell skin cancer — which affects the skin's topmost layers — can spread, and causes about 2,500 deaths each year.
The cell theory
What might link skin cancer with Alzheimer's risk still isn't clear. "Biologically, the most interesting idea has to do with cell division," Lipton said. "Alzheimer's is a disorder in which brain cells die and aren't replaced."
Conversely, cancer occurs when "cells divide out of control to form tumors," Lipton said, so one possible reason for the link is that the process of nonstop cell division raises the risk for basal- or squamous-cell skin cancer while lowering people's risk of Alzheimer's, he explained.
But other factors may also play a role. Exercise has been shown to lower the risk of Alzheimer's disease, Lipton said. "People who run, canoe, bike or swim outdoors may get more sun exposure," increasing their risk for skin cancer at the same time they are reducing their risk for Alzheimer's, Lipton said.
Another possibility is that exercising outdoors may boost levels of Vitamin D, the so-called sunshine vitamin. "There is some suggestion that Vitamin D has a protective effect against Alzheimer's," Lipton said.
The findings show an association, not a cause-and-effect link, between non-melanoma skin cancer and a reduced Alzheimer's risk. However, Lipton said the association held even after the researchers adjusted for diabetes, high blood pressure and coronary heart disease, which may contribute to Alzheimer's and other forms of dementia.
Despite what Lipton called "astonishingly strong" findings, he stressed that more research is needed to confirm the results and to tease out the mechanism that may be at work.
"It's hard to draw real specific conclusions from this study because a small number of people — 141 — had skin cancer," said Heather M. Snyder, director of medical and scientific operations for the Alzheimer's Association.
"This study underscores the need to understand the underlying biology of diseases like Alzheimer's and skin cancer, so we can develop new targets and potential therapies," Snyder added.
Alzheimer's disease — the most common form of dementia — affects memory, thinking and behavior. An estimated 5.1 million Americans have Alzheimer's disease, according to the Alzheimer's Association.
Because many people with a history of skin cancer develop Alzheimer's, sun protection remains crucial. "People should still wear sunblock and hats, and avoid sun exposure," Lipton stressed. "Nobody would say, 'Go get a sunburn so you won’t develop Alzheimer's.'"
The study appears today (May 15) in the journal Neurology.
Pass It On: Some skin cancers have been linked with a reduced risk of Alzheimer's disease.