But the researchers of the new study, from the University of California Los Angeles, suggest that iron accumulation is a third possible contributing factor.
Although iron is essential for cells to function properly, some researchers believe that it can become toxic in high quantities.
The highest iron concentrations in the brain are found in cells which produce myelin, the fatty tissue which coats nerve fibres and allows them to communicate with one another.
The destruction of myelin promotes the build-up of plaques which lead to further deterioration and cell death, the researchers explained.
They studied two brain regions in 31 Alzheimer's sufferers: the hippocampus, which plays an important role in memory and is usually damaged early in the disease, and the thalamus which is involved in sensory perception and motor skills, and generally remains healthy until a later stage.
Writing in the Journal of Alzheimer's Disease, they reported that unusually high levels of ferritin, a protein which stores iron, were found in the hippocampus of Alzheimers' sufferers, but not in the thalamus.
Prof George Bartzokis, who led the study, said: "The increase in iron is occurring together with the tissue damage. We found that the amount of iron is increased in the hippocampus and is associated with tissue damage in patients with Alzheimer's but not in the healthy older individuals, or in the thalamus. So the results suggest that iron accumulation may indeed contribute to the cause of Alzheimer's disease.
"The accumulation of iron in the brain may be influenced by modifying environmental factors, such as how much red meat and iron dietary supplements we consume and, in women, having hysterectomies before menopause."
Dr Marie Janson, of Alzheimer's Research UK, said: "This study suggests that during Alzheimer's, iron could accumulate in an area of the brain that is critical to memory, but it is not clear whether this build-up might be a cause or consequence of the disease.
"Research is under way to look at whether shifts in the balance of iron in the brain as we age could interfere with biological processes involved in Alzheimer's. However, iron is an essential part of our diet with many important roles in the body."
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