Carrying extra weight was a risk even if other factors, such as diabetes, were taken into account.

The research, from the Sahlgrenska University Hospital in Goteborg, Sweden, is published in the journal Neurology.

Nerve cell damage

Whether or not a person is of a healthy weight is determined by their BMI, or body mass index.

This can be calculated by dividing your weight in kilograms by the square of your height in metres.

A BMI of over 25 is classed as overweight, and over 30 as obese.

Just under 300 women aged between 46 and 60 were studied over 24 years.

Every six years, researchers carried out computer tomography (CT) scans to study their brain tissue and calculated their BMI.

The study found that the higher the women's BMI, the greater the chance they would experience brain tissue loss, known as cerebral atrophy.

Being overweight also raised a woman's chances of being affected, the study found.

Almost 50% of the women were found to have lost temporal lobe tissue. The average BMI in that group was higher than that in the unaffected group throughout the study.

The research team say the results are consistent with their previous findings showing that being overweight was a risk factor for Alzheimer's disease.

Ramifications

Writing in Neurology the researchers, led by Deborah Gustafson, said there were a number of potential mechanisms for overweight and obesity being linked to brain tissue loss.

They pointed to the recognised link between obesity and diseases such as heart disease and stroke.

Dr Gustafson said: "These conditions contribute to an unhealthy vascular system, and therefore to a higher dementia risk.

"Obesity may also increase the secretion of the hormone cortisol, which could lead to atrophy."

She added: "If overweight and obesity contribute not only to diseases of middle age, but also to degenerative diseases of later life, the health ramifications of excess body fat will stress healthcare systems for many years to come.

"Given the myriad of risk factors that preclude cerebrovascular events in the elderly, it appears that obesity is yet another factor that should be actively intervened upon to reduce diseases of advanced ageing, such as cerebral degeneration and dementia."

Dr David Haslam, a Hertfordshire GP and chairman of the National Obesity Forum, said: "Obesity is known as a risk factor for vascular diseases, whether that's cardiovascular or cerebrovascular.

"The areas of the brain which this study is talking about are very susceptible to microvascular damage.

"In addition, obesity affects every part of the body - no part escapes damage."

Dr Haslam added the study findings provided more evidence of the need to tackle obesity, to prevent people developing any of the raft of obesity-related disease.