A breakdown of the protective coating around nerve cells during middle age may explain some cases of Alzheimer's disease, say scientists.
Loss of the coating, called myelin, is also the cause of multiple sclerosis.
The theory, developed by an expert at the University of California Los Angeles, is published in the journal Neurobiology of Aging.
The loss of myelin is a normal part of growing older, he says, but it might be possible to slow it down.
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Medical advances, however, have expanded the lifespan well beyond the brain's natural capacity to operate in a healthy, efficient manner
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Professor George Bartzokis
Professor George Bartzokis compared the human brain to a high-speed internet connection.
He said: "The quality of the internet's connections is the key to its speed, fidelity and overall capability.
"Close analysis of brain tissue and MRI scans clearly shows that the brain's wiring develops until middle age and then begins to decline as the breakdown of myelin triggers a destructive domino affect. Our time at the peak is short indeed.
"The challenge for science and medicine is to figure out how to extend the brain's peak performance so that our minds function as long as our bodies."
Effective insulator
In essence, myelin, a fat which contains very high levels of cholesterol, acts as an insulator, speeding up connections between neighbouring nerve cells.
As the brain continues to develop in adulthood, myelin is produced in greater and greater quantities.
However, this increases cholesterol levels in the brain to such an extent that they stimulate the production of a toxic protein that attacks the brain.
The protein attacks myelin, disrupting the transmission of signals in the brain, and eventually triggering the development of tangles of protein called plaques that are a classic sign of Alzheimer's disease.
Professor Bartzokis' work suggests that the complex connections that take the longest to develop and allow humans to think at their highest level are among the first to deteriorate as the brain's myelin breaks down.
He said: "The body was designed to myelinate through the natural lifespan.
"Medical advances, however, have expanded the lifespan well beyond the brain's natural capacity to operate in a healthy, efficient manner.
"The process of adult brain development and becoming 'wiser' has this downside that evolution could not anticipate."
Professor Bartzokis said it might be possible to limit myelin breakdown during middle age by taking cholesterol-lowering drugs, anti-inflammatory medications, or by improving diet and exercise.
In addition, education or other activities designed to keep the mind active may stimulate the production of myelin.
However, he warned that by the time the effects of Alzheimer's disease become apparent it may be too late to reverse the damage.
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