The study of nearly 4,000 subjects, half with Alzheimer's, by Dr. Robert Green of Boston University School of Medicine noted that Alzheimer's has previously been linked to recent bouts of depression, past head trauma, and level of education.

The study, published in the Archives of Neurology, was also adjusted for people who have a genetic susceptibility to the disease.

The researchers concluded that 14 percent of Alzheimer's patients studied had problems with depression stretching back 25 years according to family members, compared to 7 percent of subjects who suffered prior depression but remained free of the disease.

The association between depression and Alzheimer's grew the closer one gets to the onset of dementia, the study found, but the root cause of Alzheimer's remains unknown.

"Does (Alzheimer's disease) begin much earlier in life than conventionally thought, perhaps as early as young or mid-adulthood?" Green wrote. "Is there something about depression that is potentially 'toxic' to the brain and predisposes to a later vulnerability to (Alzheimer's)?

"Or could a propensity for depression be a subtle proxy for lower neurologic 'reserve' that cushions and delays the onset of a dementing syndrome? The answer remains unclear ... (but) we are moving closer to developing accurate models for who will be at highest risk for (Alzheimer's)."