While the study tied apathy to reduced brain volume, it didn't prove a cause-and-effect relationship.
It's possible that apathy could serve as a "warning sign" of increased dementia risk or some other disorder affecting the brain, according to Lenore Launer, a researcher at the U.S. National Institute on Aging who worked on the study.
But she said more research is needed to understand what is going on.
A neurologist who was not involved in the study agreed. "It's important not to over-interpret the results," said Dr. Marc Gordon, chief of neurology at Zucker Hillside Hospital, in Glen Oaks, N.Y.
"This doesn't mean that if you change your behavior -- if you go out more, or somehow try to be 'less apathetic' -- you're going to increase your brain volume," Gordon said.
The findings are based on 4,354 adults from Iceland, mainly in their 70s, who underwent MRI scans to measure their brain volume. They also answered three questions aimed at gauging apathy: "Have you dropped many of your activities and interests?" "Do you prefer to stay at home, rather than going out and doing new things?" and "Do you feel full of energy?"
Almost half of the study participants had two or three signs of apathy. And on average, Launer's team found, they had slightly smaller brain volume than those with one or no signs of apathy.
That leaves open the chicken-and-egg question. "We can't say which comes first, apathy or changes in the brain consistent with symptoms of apathy," Launer said.
The researchers did try to tease out whether certain health conditions could explain the connection. Apathy symptoms are common in people with depression, for example, but Launer's team found that apathy was tied to lower brain volume even in the absence of depression. People with Alzheimer's or other forms of dementia often become apathetic as well, but none of the study participants had dementia.
The researchers also factored in diabetes, high blood pressure, smoking and study participants' exercise levels and walking speeds. Apathy was still linked to lower brain volume.
And what's the significance of that extra brain shrinkage? "Our brains do get smaller as we age. We know that," Zucker Hillside's Gordon said. But, he added, brain atrophy can be accelerated when people are on the path to dementia, or have another disorder affecting the brain.
However, it's impossible to tell why the study participants with apathy had a lower brain volume. "We don't know if this is related to neurodegeneration," Gordon said, referring to the death of brain cells that can lead to Alzheimer's disease -- as well as other brain disorders, like Parkinson's disease.
But the current study cannot show whether apathy is an early warning sign of dementia, or any other condition, Gordon stressed. To do that, he said, researchers would have to do a long-range study.
For now, study co-author Launer suggested that older adults who notice a loss of energy or motivation bring it up to their doctor. She said it could be a sign of some underlying problem that warrants a closer look.