Being obese can increase the risk of Alzheimer’s Disease by as much as 80 percent, according to a study in the May issue of Obesity Reviews. But it's not just weight gain that poses a risk. People who are underweight also have an elevated risk of dementia, unlike people who are normal weight or overweight.

US researchers carried out a detailed review of 10 international studies published since 1995, covering just over 37,000 people, including 2,534 with various forms of dementia. Subjects were aged between 40 and 80 years when the studies started, with follow-up periods ranging from three to 36 years.

The review, which included studies from the USA, France, Finland, Sweden and Japan, also included a sophisticated meta-analysis of seven of the studies, published between 2003 and 2007 with a follow-up period of at least five years.

All kinds of dementia were included, with specific reference to Alzheimer’s Disease and to vascular dementia – where areas of the brain stop functioning because the blood vessels that supply them are damaged by conditions such as high blood pressure or heart disease.

“Our meta-analysis showed that obesity increased the relative risk of dementia, for both sexes, by an average of 42 percent when compared with normal weight” says Dr Youfa Wang, Associate Professor of International Health and Epidemiology at Johns Hopkins Bloomberg School of Public Health, Baltimore. “And being underweight increased the risk by 36 per cent. But when we looked specifically at Alzheimer’s Disease, the increased risk posed by obesity was 80 per cent. The increased risk for people with vascular dementia was 73 per cent.

“The risks were greater in studies where sufferers developed Alzheimer’s Disease or vascular dementia before the age of 60 or in studies with follow-up periods of more than 10 years. We also found that obesity was more likely to be a risk factor for women when it came to developing Alzheimer’s Disease and for men when it came to vascular dementia.”

The authors estimate that 12 per cent of the dementia risk in the study population could be attributed to obesity, with this rising to just over 21 per cent in patients with Alzheimer’s Disease.

It’s estimated that up to 10 per cent of people aged 65 or more suffer from some form of dementia and two-thirds of those have Alzheimer’s Disease.

“There has been controversy about the links between obesity and dementia for a number of years, but previous findings have been mixed and inconclusive” says Dr Wang. “The advantage of carrying out a meta-analysis is that it provides researchers with access to a large number of study subjects and it is possible to iron out the inconsistencies and come to overarching conclusions.

“Our detailed analysis clearly shows a U-shaped relationship between weight and dementia, with people who are obese or underweight facing a greater risk. We believe that our results show that reducing the prevalence of obesity is a promising strategy for preventing the progression of normal ageing into Alzheimer’s Disease.”

Article: Obesity and central obesity as risk factors for incident dementia and its subtypes: a systematic review. Beydoun MA, Beydoun HA, Wang Y. Obesity Reviews. 9, pp 204-218. (May 2008).