So say doctors from the Boston University School of Medicine and Maine University, Orono, who have presented their findings at an American Heart Association conference.

It had been shown previously that diabetes and high blood pressure - also both risk factors for heart disease - might impair the ability to learn and remember.

But it had not been shown that others could cause a similar decline in ability.

The research also found that the more risk factors present, the greater the decline in mental prowess.

Quality of life

Professor Merrill Elias, who led the research, said: "We want people to function at the highest possible level and for the longest possible time.

Obesity is a risk factor for heart disease

"Individuals are going to have a reduced quality of life if they cannot learn and remember."

Professor Elias's group tested 1,799 volunteers involved in the Framingham Heart Study, which has tracked the cardiovascular health of a group of residents in a community near Boston for about 50 years.

Between 1976 and 1978, the volunteers, aged 55 to 85, took a series of eight tests designed to assess their thinking and memory abilities.

Those scoring in the lowest 25% were considered to be "poor performers".

Researchers assigned each volunteer a number (0-4) representing how many risk factors for heart disease the individual had at the time of testing.

"The study showed that the more risk factors for heart disease a person has, the greater the risk of developing memory and learning impairments," Professor Elias said.

Test results

After analysing the scores on all eight tests, researchers found that people with one of four risk factors at the time of testing were, on average, 23% more likely to be poor performers than those with no risk factors.

With each additional risk factor, the volunteers' risk of decreased mental function increased another 23%.

The researchers then narrowed their analysis to only the scores on three tests that assessed learning and memory, excluding those that measured attention, concentration or word fluency.

Professor Elias said the findings in this analysis were even more alarming.

"Again, more risk factors for heart disease indicated a higher chance of mental decline.

"People with one risk factor averaged a 32% increased risk of mental decline over those with no risk factors and again this chance increased by about 32% with each additional risk factor," he said.

"This tells us that treating these risk factors provides not only a payoff in terms of decreased risk of stroke and heart disease, but there may be an additional benefit by helping to prevent or slow down the mental decline that comes with ageing."

He added that counselling from their doctor about the risk of cognitive decline might motivate people to control their blood pressure and diabetes, stop smoking, and lose weight.

Professor Elias recently published research confirming the link between high blood pressure and an increased rate of mental decline.

Writing in the US journal Health Psychology, Professor Elias's team found that in 140 participants studied over 20 years, all showed a decline in mental ability.