As many as 25 percent of older people have symptoms of depression, which are in turn associated with brain changes that accelerate the mind's ageing process.
The new University of Miami study is one of many to document a link between depression and dementia, but cause-and-effect relationship difficult to discern because the two share many common symptoms.
Regardless, the study adds weight to the importance of mental health care to preventing dementia and its authors urge that treating depression early could play a key role in easing the burden of Alzheimer's on older Americans.
Rates of depression and Alzheimer's are both climbing in the US, as scientists continuously struggle to better understand the causes and prospective treatments for each.
So far, we know that harmful amyloid beta proteins build up in the brains of people developing Alzheimer's disease, impairing the ability of neurons to connect.
We know, too, that people with Alzheimer's disease suffer from depression as well, particularly during the disease's earlier stages when people are still fairly aware of what is happening to them.
There is also a great deal of overlap in the symptoms of the two conditions.
While many studies have linked depression and memory-loss, the science to explain the relationship is nascent.
Both depression and Alzheimer's are marked by tendencies to withdraw from people, lose interest in activities and others and difficulty concentrating or thinking.
A study from Brigham Young University also suggested that depression impairs the ability of people to distinguish between similar events, making each more difficult to recall.
But depression can occur for most people much earlier in life and its symptoms treatable with medication or talk therapy.
Catching and treating depression early could mean putting a swift end to the mood disorder's deleterious effects on the brain.
Letting it go undiagnosed or addressed, on the other hand, is associated with memory problems even before Alzheimer's sets in.
To try to clarify what if any relationship depression and memory loss have, the researchers from the University of Miami quizzed more than 1,000 people with an average age of 71 about their depression symptoms and their recall.
Worryingly, they found that the 22 percent of older adults in the group that were more depressed also had poorer episodic memory.
Episodic memories include simple recent information - like where you put your keys today - as well as the memories that make us, us - like what happened to us yesterday.
In addition to assessing their depression symptoms and memory performances, the researchers also looked at scans of the participants' brains.
The brains of those with depression were markedly different than those without such symptoms.
People who reported more feelings of depression had smaller brain volumes and were at a 55 percent greater risk of developing vascular lesions, or abnormal blood vessel formations in or on the brain.
If these blood vessels burst or become blocked, a stroke ensues.
Some previous studies have also suggested that brain lesions may predict or even cause Alzheimer's.
'Small vascular lesions in the brain are markers of small vessel disease, a condition in which the walls in the small blood vessels are damaged,' said study author Dr Zeki Al Hazzouri.
'Our research suggests that depression and brain ageing may occur simultaneously, and greater symptoms of depression may affect brain health through small vessel disease,' she added.
This is not the first time that scientists have linked depression and brain lesions, but it isn't clear which problem is the cause and which the effect.
But since depression could well lead to the lesions, and 'symptoms of depression can be treated, it may be possible that treatment may also reduce thinking and memory problems,' said Dr Al Hazzouri.
'With as many as 25 percent of older adults experiencing symptoms of depression, it's important to better understand the relationship between depression and memory problems.'
The Daily Mail
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