Why is there an inverse association between cancer and Alzheimer's?
New research investigates the possibility that older people who experience memory and thinking problems may have a lower risk of cancer than older people who do not have these problems.
In 2013, Medical News Today reported on a study by Italian researchers, who found that Alzheimer's patients were half as likely to develop cancer and that cancer patients were 35% less likely to develop Alzheimer's.
Another study from 2009 looked at 3,020 people aged 65 and older and reported that Alzheimer's and cancer are much less likely to occur simultaneously.
"In white older adults, prevalent Alzheimer's disease (AD) was longitudinally associated with a reduced risk of cancer, and a history of cancer was associated with a reduced risk of AD," the authors of that study concluded, adding:
"Together with other work showing associations between cancer and Parkinson's disease, these findings suggest the possibility that cancer is linked to neurodegeneration."
Other neurodegenerative diseases also seem to have similar associations with reduced cancer risk. One study found that people with multiple sclerosis were not only much less likely to develop cancer, but that their risk for colorectal cancer in particular was much lower.
But why do cancer and Alzheimer's appear to have these inverse associations? Are the properties of one illness protecting against the other? Or is it that the symptoms of one disease will mask the other, leading to only one condition being diagnosed in patients who have both Alzheimer's and cancer?
"Studies have shown that people with Alzheimer's disease are less likely to develop cancer, but we don't know the reason for that link," says Dr. Julián Benito-León, author of the new study from University Hospital 12 of October in Madrid, Spain.
"One possibility is that cancer is underdiagnosed in people with dementia, possibly because they are less likely to mention their symptoms or caregivers and doctors are focused on the problems caused by dementia. The current study helps us discount that theory."
What did the new study find?
The group that had the fastest decline in memory and thinking abilities were 30% less likely to die from cancer.
In Dr. Benito-León's study, which is published in the journal Neurology, 2,627 people aged 65 and older took tests to assess their memory and thinking skills. The participants repeated the tests 3 years later and were followed by the researchers for an average of 13 years.
The study participants were divided into three groups by the researchers:
- Those whose scores on the thinking tests were declining the fastest
- Those whose scores improved on the tests
- Those in the middle.
Across the 13-year study period, 1,003 participants died. Of these deaths, 34% were in the "fastest decline in thinking skills" group and 66% were in the other two groups.
Of the deaths in the fastest decline group, 21% were due to cancer, compared with 29% of the deaths in the other groups.
Dr. Benito-León's team adjusted their results for factors such as smoking, diabetes and heart disease, and found that patients in the fastest decline group were still 30% less likely to die of cancer.
Although this new study adds to the growing body of research on the peculiar relationship between cancer and neurodegenerative disease, again, the findings as to why this inverse association occurs are not conclusive.
"We need to understand better the relationship between a disease that causes abnormal cell death and one that causes abnormal cell growth," Dr. Benito-León says. "With the increasing number of people with both dementia and cancer, understanding this association could help us better understand and treat both diseases."
Written by David McNamee
Copyright: Medical News Today
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