An abnormal protein linked to Alzheimer's disease can start accumulating inside the brains of people as young as 20, according to a new study.

In a study published Monday in the journal Brain, researchers from Northwestern University found that the protein amyloid can start accumulating in the neurons of young people. Scientists have already established that amyloid accumulates and forms clumps of plaque on the neurons of aging adults and in Alzheimer's patients.

Study lead investigator Changiz Geula said the discovery of amyloid accumulation in such young human brains is a first.

"Discovering that amyloid begins to accumulate so early in life is unprecedented," he said in a statement. "This is very significant. We know that amyloid, when present for long periods of time, is bad for you."

In the study, researchers examined the neurons from the brains of deceased people. Thirteen of the brains were from cognitively normal people between the ages of 20 to 66; 16 brains were from non-demented people between the ages of 70 to 99; and 21 brains were from Alzheimer's patients between the ages of 60 to 95.

Researchers found amyloid molecules had started accumulating inside the neurons of the young adults, forming small toxic clumps called amyloid oligomers. These clumps were found in individuals in their 20s and other normal individuals, and the size of the clumps grew larger in the older individuals and Alzheimer's patients.

Geula said the clumps of amyloid likely damage and eventually kill neurons.

"The lifelong accumulation of amyloid in these neurons likely contributes to the vulnerability of these cells to pathology in aging and loss in Alzheimer's," he said.

The amyloid clumps can also trigger an excess of calcium leaking into the cell, which can lead to their death.

The research group said future research will look into how amyloid damages the neurons.