Heavy smoking in middle age more than doubles the risk of Alzheimer's disease and other types of dementia later in life, one of the first long-term studies on the issue claims.

Smoking has a clear effect on the heart and lungs, but whether it also damages the brain has been controversial.The study reported on in last week's Archives of Internal Medicine overcomes some of the obstacles that have made it difficult to assess such a link: earlier research suggesting smoking does not cause dementia examined mostly elderly people for only a short time.

For a complete look, researchers in Finland, Sweden and the Californian health care organisation Kaiser Permanente's Division of Research surveyed 21,123 middle-aged Kaiser members from 1978 to 1985 and then for an average of 23 years.

After allowing for other factors that can contribute to dementia - such as education level, race, age, diabetes, heart disease and substance abuse - the study found a significant link with heavy smoking during middle age.

Compared with non-smokers, people who smoked two packs a day or more had a 114 per cent increased risk of dementia; those who smoked one to two packs a day had a 44 per cent increased risk, while those who smoked a half-pack to one pack a day had a 37per cent increased risk.

Middle-aged people who had given up did not appear to have an increased risk of later dementia.

One way smoking might increase the risk of dementia would be via the narrowing of blood vessels in the brain, a process that leads to the well-established increased risk of stroke, says Rachel Whitmer, a research scientist at Kaiser Permanente and the principal investigator of the study. Yet even people who smoked heavily during mid-life and did not have any subsequent strokes had a higher risk of dementia, Whitmer says.

Of the 5,367 participants later diagnosed with dementia, only 416 were diagnosed with vascular dementia - a condition in which reduced blood flow to the brain triggers strokes that steadily erode memory. Most cases were diagnosed simply as dementia, with 1,136 cases diagnosed with Alzheimer's.

"Stroke is one of the pathways through which smoking causes dementia, but not the only one," says Whitmer; inflammation caused from smoking and oxidative stress may also harm the brain and lead to dementia.

Yet researchers do not know how many in the study kept smoking into old age, quit or cut back; Whitmer says the real rates of smoking-related dementia might be higher if people who quit were left out of the analysis.