In 2002, a team of researchers from Portugal surveyed 54 people who either had Alzheimer's or were likely to, and 54 healthy people of the same age and gender. Comparing the results of the surveys, they found no differences between the groups in terms of smoking, alcohol consumption, education, family history of dementia, or occurrence of non-Alzheimer's diseases. However, there were differences in the amount of coffee consumption between the groups, and it appeared that increased coffee intake was associated with a decreased risk of developing Alzheimer's.
A study from 2010 investigated the effects of eicosanoyl-5-hydroxytryptamide (EHT), a component of coffee unrelated to caffeine, on Alzheimer's in rats. The rats' diets were supplemented with EHT for 6-12 months, and by the end of the study there were substantial reductions in cognitive impairment and the concentration of harmful beta-amyloid plaques and tau tangles.
A different 2010 study looked at whether caffeine could protect against or even reverse Alzheimer's symptoms in mice. Mice who were fed caffeine from a young age showed reduced beta-amyloid production over time compared to those who were not. Caffeine given to elderly mice lowered beta amyloid levels and restored memory function. Caffeine appeared to have multiple beneficial effects, including decreasing levels of enzymes involved in the formation of beta-amyloid plaques. Additional research published the following year reported that caffeinated coffee was more effective than caffeine alone or decaf, so it is likely that other components of coffee (such as EHT) boost caffeine's effectiveness.
It should be noted that studies on the subject of coffee and caffeine's effect on Alzheimer's have reported mixed results, so this treatment may not be effective for everyone.