Thursday March 27 2008
Is this an indicator of dementia in later life?
“A pot belly in middle age dramatically raises the risk of Alzheimer’s”, reports the Daily Mail. Men and women who have “large stomachs in their 40s are three times more likely to suffer serious mental decline when they reached their 70s”, the newspaper adds.
The story is based on the results of data gathered as part of a large American study. Researchers used waist diameter measurements taken 36 years ago and followed up the patients using medical records to see whether they developed dementia. The results add to a growing body of evidence that central obesity is harmful. It will be important to see these results repeated in studies that take into account the nutrition and physical activity levels of the participants; without these, there will be a lack of clarity about the degree of increased dementia risk.
Where did the story come from?
Dr Rachel Whitmer and colleagues from Kaiser Permanente Division of Research carried out this research. It is not clear how the study was funded though the authors report no conflicts of interest. It was published online in the peer-reviewed medical journal: Neurology.
What kind of scientific study was this?
The study was a retrospective cohort study of members of Kaiser Permanente of Northern California (a managed care organisation in America providing health plans for members). Participants had had the distance from the back to the upper abdomen while standing (called the sagittal abdominal diameter) measured between 1964 and 1973 when they were aged between 40 and 45 years. The researchers were interested in whether this measure of central obesity in midlife was a risk factor for developing dementia. There were 6,583 adults available for study and the researchers report that there was no significant difference between those who had measurements available and the 2,081 who didn’t.
The researchers accessed their participant’s medical records from 1994 onwards to see if they had developed any illnesses, including stroke, diabetes, hypertension and heart disease. Dementia status as noted in medical records between January 1994 and June 2006 was also recorded. At this point in their follow up, the participants would have been aged between 73 and 87 years.
The researchers analysed the data to see whether there was link between waist diameter (divided into quintiles from 10cm to 40cm) and thigh circumference (divided into quintiles from 7cm to 70cm) in midlife and development of dementia in later years. They took into account other factors that could affect the risk of dementia such as age, gender, education, diabetes, body mass index (BMI) and ethnicity. They were also particularly interested in whether the effect of waist diameter on the risk of dementia was constant across different BMIs.
What were the results of the study?
Between January 1994 and June 2006, 1,049 (16%) participants were diagnosed with dementia. The results were analysed by quintile, meaning that participants were divided into five groups depending on their waist diameter. They found that as the waist diameter increased there was an increase in the risk of dementia. Each quintile group was compared with the slimmest group. People in the second quintile were 1.2 times more likely to have dementia, those in the third quintile were 1.49 times more likely and those in the fourth quintile were 1.67 times more likely. Those in the fifth quintile (which had the largest range of waist diameters from about 23cm to 40cm) were 2.72 times more likely to have dementia than those with the smallest diameters.
When the researchers took into account the original BMI of participants, there was still an increasing risk of dementia associated with increasing waist diameter. By dividing participants according to their BMI (using three groups: normal weight, overweight and obese) the researchers found that those who were both obese and had a high waist diameter (25cm and above) were at 3.6 times greater risk of dementia (95% CI 2.85 to 4.55) compared with those with normal weight and a low waist diameter (less than 25cm). People who were overweight or obese but had a low waist diameter had a 1.8 times greater risk of dementia. Those with normal weight and high waist diameter were 1.9 times more likely to develop dementia, though this result was not statistically significant.
There was no association between thigh circumference and risk of dementia.
What interpretations did the researchers draw from these results?
The researchers conclude that central obesity is associated with an increased risk of dementia. This increase in risk is not affected by demographics, diabetes, cardiovascular comorbidities or BMI. Their study finds no link between peripheral obesity (as indicated by thigh circumference) and dementia risk.
What does the NHS Knowledge Service make of this study?
This cohort study provides evidence of a link between central obesity and the risk of dementia. The strength of the link increases as waist diameter increases across the five groups. However, there are some factors the authors did not include in their analysis that could be responsible for the results:
- Insulin resistance may have confounded the relationship between waist diameter and dementia, as the researchers say, “insulin resistance may be a consequence of central obesity and is associated with cognitive decline”, but as they took into account type 2 diabetes (one of the manifestations of insulin resistance), this might not completely explain the relationship.
- The researchers did not have measures of nutrition during midlife (which has been associated with dementia) or physical activity (the researchers say that, “physical activity in old age lowers the risk of dementia”). Both these additional factors might explain some of the link.
- There is no breakdown of the type of dementia people had. Although most are likely to have had Alzheimer’s disease, as this is the most common type of dementia, there are other types.
The researchers put forward several possible biological reasons for the link between central obesity and dementia, including that the fatty tissue might itself be toxic, leading to changes in the brain in obese middle-aged adults. They say that if their results are replicated, the findings imply that central obesity may contribute to a degree of cognitive ageing. However, there is no indication from this study how much one’s risk could be reduced by losing weight.
Overall, this observational study provides some evidence of a graded link between waist diameter and risk of dementia, and provides yet another reason to maintain a healthy weight.