The causes of Alzheimer's disease are unknown, but there are some apparent risk factors. First, there's age: The number of people with Alzheimer's increases dramatically after age 65. Genetic inheritance seems to play a role, particularly if an immediate blood relation had the disease. Women are more likely to have Alzheimer's because they live longer than men.
There are three genes -- known as amyloid precursor protein, presenilin 1 and presenilin 2 -- associated with early-onset Alzheimer's. For "normal-onset" Alzheimer's, there is the apolipoprotein gene (ApoE), which moves cholesterol through the bloodstream. A certain form of this gene, found in 15 percent of the population, boosts a person's chances of developing normal-onset Alzheimer's [source: NIH].
Genetic abnormalities certainly play a role in the development of the disease, but researchers have considered many other possible risk factors for Alzheimer's, including advanced age, high cholesterol, lack of folate, head injuries, lack of exercise and even exposure to toxic substances. However, many of these risk factors have produced conflicting evidence regarding their roles in the development of the disease. Age, gender, heredity and genetics are among the most commonly accepted risk factors.
The Alzheimer's Brain
A person's brain may start experiencing some of the changes associated with Alzheimer's up to two decades before an official diagnosis is made [source: Alzheimer's Association]. Chief among the changes in an Alzheimer's patient's brain is the development of clumps called amyloid plaques and neurofibrillary tangles, particularly in the brain's learning and memory centers. Plaques crowd the areas between neurons and are made up of clumps of a protein known as beta-amyloid. Tangles appear within neurons and even people without Alzheimer's disease can have them, though they're far more numerous in the brains of those with Alzheimer's.
There's a lot that remains unknown about plaques and tangles, but scientists do know that they play key roles in the development of Alzheimer's disease and the disruption of brain function. Both appear to inhibit the ability of neurons to communicate with each other. Tangles, which are knotted threads of tau proteins, likely contribute to cell death, a key effect of Alzheimer's. Normally, in the human brain, nutrients and cell parts are transported through straight, parallel channels. Tangles disrupt these transport avenues and prevent those vital nutrients and cell parts from getting where they need to go.
As Alzheimer's disease progresses, brain cells start to die and neurotransmitter levels decrease. (Neurotransmitters are the chemicals that transmit messages through the brain.) The brain also develops inflammation, though doctors don't know whether inflammation represents a positive or negative response to the plaques and tangles [source: Mayo Clinic]. Inflammation could be the result of the body's immune system attacking dying, tangle-filled cells.