That is, as the disease progresses in the brain, those who have a greater "reserve" do not show symptoms of Alzheimer's -- memory loss and impairment of day-to-day functions -- as quickly.

CNN: Less-stressed people may have lower dementia risk

Far from being harmful, amyloid is "actually a response to injury that the brain secretes to protect itself, like a scar, " argues Mark Smith, a neuroscientist at Case Western Reserve University.

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