A big waistline in your 40s could almost triple the threat of dementia in old age, according to US research.
Obesity is a known risk factor for Alzheimer's, but scientists found even those of normal weight were more at risk if they had a large waist.
However, the study of 6,500 people, published in the journal Neurology, found obesity and bulging stomach was still the most dangerous combination.
An obesity expert said waist size was a good guide to future health problems.
Research linking obesity to dementia does not reveal precisely why being overweight can affect your ageing brain, but many specialists believe that associated problems such as high blood pressure, diabetes and high cholesterol levels may contribute.
Thickness of fat around the waist is thought to correspond closely with its presence around the major organs of the body.
The latest study suggests that while the standard measure of obesity - body mass index - can help predict those at risk, the lifestyles which produce large bellies may have a closer relationship with the long-term causes of dementia.
Researchers working for Kaiser Permanente, one of the biggest healthcare providers in the US, looked at 6,583 people aged between 40 and 45, measuring their abdominal fat levels with calipers.
They then followed all of these people into their 70s to see who became ill, and who managed to maintain relatively good health.
They found the 20% of people with the largest waistlines had a 270% greater risk of dementia than those with the smallest waists.
Even those reckoned to be normal weight using body mass index calculations had approximately an 90% increased risk of dementia if they had a large rather than a small waist.
People who were measured as overweight or obese using body mass index, but who didn't have a large belly, had an 80% increase in dementia risk but a combination of all these factors led to a bigger overall increase.
Being overweight and with a large waist raised the risk by 230%, but those who were large-waisted and were so overweight they could be officially classed as obese recorded a 360% rise in dementia risk compared to small-waisted people who met guidelines on normal weight.
Dr Rachel Whitmer, who led the research, said: "It is well known that being overweight in midlife and beyond increases risk factors for disease.
"However, where one carries the weight, especially in midlife, appears to be an important predictor for dementia risk."
She said that autopsies suggested that the changes in the brain characteristic of Alzheimer's disease started to appear decades before any symptoms became apparent.
Tam Fry, from the National Obesity Forum, said that waist size was potentially a far better way to predict future illness than body mass index.
"Many doctors are now coming around to the idea that this is a more reliable indicator.
"The problem is that waist measurements have to be carried out very precisely, in exactly the right spot, so this is something that is better done by your doctor or practice nurse.
"So while calculating your body mass index at home might give you a clue that something is wrong, this could be confirmed by your doctor measuring your waist size."
Professor Clive Ballard, of the Alzheimer's Society, said: "This new study highlights that having a large abdomen, regardless of weight, also significantly increases your risk.
"This is an important piece of research but the results are not that surprising as a large stomach is associated with high blood pressure, cholesterol and diabetes - all major risk factors for dementia. "
We are learning more and more that what is good for your heart is also good for your brain."
Rebecca Wood, of the Alzheimer's Research Trust, called for further studies to confirm the findings - and reveal how dementia develops.
Source: MJFM
