Underweight seniors may face greater risks of developing Alzheimer’s disease than their portlier peers.

That’s the latest word from researchers at Brigham and Women's Hospital and Massachusetts General Hospital who have linked lower body weight with more extensive deposits of the Alzheimer's-associated protein beta-amyloid in the brains of older individuals.

The association, reported in the Journal of Alzheimer's Disease, was particularly true for individuals carrying the so-called APOE4 gene defect, which is known to increase the risk of Alzheimer's.

"Elevated cortical amyloid is believed to be the first stage of the preclinical form of Alzheimer's disease, so our findings suggest that individuals who are underweight late in life may be at greater risk for this disease," said researcher Dr. Gad Marshall, of the MGH and BWH Departments of Neurology.

"Finding this association with a strong marker of Alzheimer's disease risk reinforces the idea that being underweight as you get older may not be a good thing when it comes to your brain health."

The findings are part of the MGH-based Harvard Aging Brain Study (HABS), designed to identify markers that predict who is likely to develop Alzheimer's and how soon symptoms are likely to develop.

Investigators tracked body mass index (BMI) and beta amyloid levels in the brains of 280 participants in HABS, between the ages of 62 and 90.

The findings indicated those with a lower BMI tended to have more extensive amyloid deposits in the brain, particularly among those with a genetic predisposition to the memory-robbing disorder.

"A likely explanation for the association is that low BMI is an indicator for frailty — a syndrome involving reduced weight, slower movement and loss of strength that is known to be associated with Alzheimer's risk," said Marshall, an assistant professor of neurology at Harvard Medical School.

"One way to get closer to determining any cause and effect relationship will be following these individuals over time to see whether their baseline BMI does predict the development of symptoms, which we are doing in HABS, and eventually investigating whether maintaining or even increasing BMI in late life has an effect on outcomes. Right now, we're also studying whether BMI is associated with any other clinical and imaging markers of Alzheimer's disease."