Previous research has indicated that obesity in middle-age is linked to higher risk of cognitive decline and dementia in old age. Now a study of 32 middle-aged adults (40-60) has revealed that although obese, overweight and normal-weight participants all performed equally well on a difficult cognitive task (a working memory task called the 2-Back task), obese individuals displayed significantly lower activation in the right inferior parietal cortex. They also had lower insulin sensitivity than their normal weight and overweight peers (poor insulin sensitivity may ultimately lead to diabetes). Analysis pointed to the impaired insulin sensitivity mediating the relationship between task-related activation in that region and BMI.
This suggests that it is insulin sensitivity that is responsible for the higher risk of cognitive impairment later in life. The good news is that insulin sensitivity is able to be modified through exercise and diet.
A follow-up study to determine if a 12-week exercise intervention can reverse the differences is planned.
 . Insulin Sensitivity as a Mediator of the Relationship Between BMI and Working Memory-Related Brain Activation. Obesity [Internet]. 2010 ;18(11):2131 - 2137. Available from: http://dx.doi.org/10.1038/oby.2010.183