A LOST study of the IQs of every 11-year-old in Scotland has been discovered after 60 years, shedding new light on the way that an unhealthy lifestyle can damage the brain as well as the body.

It has shown that smoking, obesity, lack of exercise and bad food can all contribute to mental decline, as well as to the physical diseases such as cancer and diabetes with which they are already closely associated.

The 1947 study, which covered the entire intake of 70,000 11-year-olds in schools that year, was found in the basement of a Scottish educational research centre.

Its findings also suggest that “dementing diseases” such as Alzheimer’s, which has 750,000 British sufferers, are partly self-inflicted and could be avoided or delayed by leading a healthy lifestyle. “This 60-year-old study gives us a unique database of people’s past mental abilities,” said Ian Deary, professor of differential psychology at Edinburgh University.

“By tracking down and retesting the people in that study we can see how their mental powers have changed over the decades and what impact their lifestyles have had on those changes.”

Deary was put on the trail of the original research after seeing a passing reference to it in a book. The Scottish Mental Survey had been the largest study of its kind ever undertaken and provided a unique record of an entire cross-section of society.

The information was filed away and forgotten as the policies that inspired it were replaced. However, Deary found that the body that sponsored the work, the Scottish Council for Research in Education (SCRE), still existed.

He found the ledgers recording the names and IQ scores of all the 70,000 people tested in 1947, in the basement of the SCRE’s latest home in Glasgow.

Since that discovery Deary, working with Lawrence Whalley, professor of mental health at the University of Aberdeen, has tracked down about 1,500 of the people who took part in 1947.

Each was persuaded to undergo a battery of tests, including resitting the same 1947 exam, answering questionnaires about their lifestyles and submitting to MRI scans of their brains to determine whether they had suffered any age-related deterioration.

Lesley Main, a retired violin teacher, took the original test as a pupil at Hilton Primary School in Aberdeen. Now 71, she associates her good health with eating well and staying active — including becoming a senior dive master in a local scuba club and travelling the world on diving trips.

“I never found out my scores, but now I know it’s important I’m delighted to help,” she said. “It also means I get a free brain scan and it’s reassuring to know everything’s fine.”

Now the first research papers to be published paint a picture of how lifestyle affects the brain. “What is emerging is that people who are brighter, more socially engaged, and who live healthy lives, retain their mental faculties as well as their physical health for much longer,” said Whalley.

The results have also caught the attention of Help the Aged. It believes that finding more survivors from the 1947 study could answer many important questions surrounding the afflictions of old age and is about to launch a £13.5m Disconnected Mind Appeal to expand the research.

“This will be one of the most exciting projects in British medical history,” said Michael Lake, director general of Help the Aged. “This project offers us a chance to determine the causes of cognitive decline and how to prevent them.”

The effect of people’s lifestyle on IQ levels will attract particular interest. “Between the ages of 11 and 70-plus, the way you live can raise or lower your cognitive skills by around 10%,” said John Starr, a consultant and researcher in geriatric medicine at the Royal Victoria hospital in Edinburgh.

Smoking is one of the clearest factors in IQ reduction. “If you smoke through life it typically reduces your IQ by about 2%,” said Starr, who works with Deary. He said that this might explain the memory lapses and slower thinking suffered by many people beyond the age of 60, and might even explain why some tip over into dementia.

Starr also found that psychotropic drugs prescribed against depression and other mental conditions had a similar effect.

By contrast, high physical fitness and a high-quality diet — particularly one rich in omega 3 fatty acids, found in fish oils — enhances mental power.

Other findings have been more surprising. Men of this generation tend to retain their faculties far better than women, showing an average IQ several points higher than their female peers. “We think this is because women then did not have so many opportunities to get an education or hold stimulating jobs,” said Whalley. “Intellectual stimulation provides protection against later decline, and men had more opportunities.”

One small group of men has been identified as the “elite old” because, Whalley says, they “defy ageing logic”. The group of about a dozen of the 500 men tested combine high levels of fitness with IQ scores that have risen throughout their lives and appear still to be increasing.

A separate finding, that will undermine the food supplements industry, now worth £362m a year, is that most vitamins and minerals have almost no effect on mental abilities. However, those containing folic acid and vitamin B12 do help to reduce levels of homecysteine, an amino acid that in high levels is associated with increased risk of vascular disease and dementia.

The key to how the brain ages may be to do with the “white matter” which provides an insulating coat for the billions of connections between its nerve cells. Damage to this white matter may cause mental decline by breaking the connections between different parts of the brain.

Such damage, Deary tentatively suggests, may result from a range of factors, some genetic but including smoking, diet and high blood pressure brought on, for example, by obesity.

By contrast, said Deary, people who seek out intellectual stimulation can actually enhance their brain, prompting it to build many extra connections that make it more resistant to ageing.

Another participant in the 1947 test is Margaret Lawson, of Edinburgh, a former university science technician. She said: “It’s so easy to relax into retirement and assume that you’re going to just go downhill.”

A review by Carol Brayne, professor of public health at Cambridge University, published last week in Nature, links mental activity with overall wellbeing. She found mental decline and dementia in old age closely related to lifestyle factors including levels of activity, socialising, obesity and high blood pressure.

She said: “The brain shows a marked increase in degenerative disorders with age but there is evidence that up to 50% of dementia might be preventable.”

Richard Wilson, the actor who played Victor Meldrew in the BBC comedy One Foot in the Grave, was one of the thousands of children tested in 1947 and is now helping Deary to publicise the project.

How diet may be harming our brains

1947 DIET (rationing still in force)

Low in fat — most oils and fats rationed

High in fruit and vegetables — when in season

High in fibre Low in red meat Low in sugar

Low obesity levels — rationing based on calories people needed

- Effect on brain

Less obesity brings lower blood pressure, slowing mental ageing

Fresh fruit and veg supplies more minerals to brain

More exercise delays ageing

2007 DIET

Low in fruit and vegetables — one third of people eat little or none

High in sugar — average diet contains 1lb/week

Low in fibre High in salt — 2 teaspoons/day

High in fat — comprises 38% of energy intake

- Effect on brain

High salt raises blood pressure which promotes ageing

Rising obesity also raises blood pressure

Surge in diabetes from obesity brings increased risk of dementia

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