A simple brain scan determines how quickly we age.

A tool designed by scientists at Duke and Harvard Universities (USA) and the University of Otago (New Zealand) can determine the rate at which a person ages.

From a single brain MRI, the tool can estimate the risk of developing chronic diseases in middle age that often appear decades later. This information could help motivate lifestyle and dietary changes that improve health.

In older adults, the tool can predict whether a person will develop dementia or other age-related diseases years before symptoms appear, when they may have a better chance of slowing the course of the disease.

"What's really interesting about this is that we've captured how rapidly people are aging, using data collected in midlife," says Ahmad Hariri , a professor of psychology and neuroscience at Duke University. "And it's helping us predict dementia diagnoses among much older people."

The results are published in the journal ' Nature Aging '.

Although various algorithms have been developed to measure a person's aging, most of these " aging clocks " are based on data collected from people of different ages at a given point in time, rather than following the same people as they age.

"What appears to be more rapid aging may simply be due to differences in exposure" to factors such as leaded gasoline or tobacco smoke, which are specific to their generation, Hariri notes.

The challenge, he adds, is to find a way to measure the speed at which the process develops that is not affected by environmental or historical factors unrelated to aging.

To do this, the researchers relied on data collected from some 1,037 people who have been followed since birth as part of the Dunedin Study , named after the New Zealand city where they were born between 1972 and 1973.

Every few years, Dunedin Study researchers looked for changes in participants' blood pressure, body mass index, glucose and cholesterol levels, lung and kidney function, and other measures, including gum recession and tooth decay.

They used the overall pattern of change in these health markers over nearly 20 years to generate a score for how rapidly each person was aging.

The new tool, called DunedinPACNI, was trained to estimate this aging rate score using only information from a single brain MRI scan collected from 860 participants in the Dunedin Study when they were 45 years old.

The researchers then used it to analyze brain scans from other databases of people from the United Kingdom, the United States, Canada, and Latin America.

Researchers found that people who aged more rapidly on this measure performed worse on cognitive tests and showed faster shrinkage of the hippocampus, a brain region crucial for memory. They were also more likely to experience cognitive decline in later life.

In one analysis, researchers examined brain scans of 624 people between the ages of 52 and 89, who were part of a North American study of Alzheimer's disease risk.

People who, according to the tool, were aging more rapidly when they entered the study were 60% more likely to develop dementia in the following years . They also began to experience memory and thinking problems earlier than people who aged more slowly.

Researchers also found that people whose DunedinPACNI scores indicated they were aging more rapidly were more likely to experience overall declines in their health, not just in their brain function.

People with faster aging scores were more frail and likely to suffer from age-related health problems, such as heart attacks.