A very large study of older women has found that although there was a small downward trend in cognitive function (as measured by the MMSE) with increasing obesity, this trend was almost entirely driven by those with a waist-hip ratio below 0.78 — that is, for women who carry excess weight around their hips, known as pear shapes (as opposed to carrying it around the waist, called apple shapes). The study of 8,745 post-menopausal women (aged 65-79) found a drop of around 2 points on the 100-point MMSE for those with a BMI over 40 compared to those who were of normal weight, after controlling for such variables as education, diabetes, heart disease and hypertension, all of which were also significantly associated with BMI and MMSE score. Because 86% of the participants were white, and women belonging to other ethnic groups were not equally distributed between BMI categories, only data from white women were used. Some 70% of the participants were overweight (36%) or obese (34%).
Fat around the middle is thought to make more estrogen, which protects cognitive function. However, although depositing fat around the waist may be better for the brain, it is said to increase the risk of cancer, diabetes and heart disease.
(2010). The Cross-Sectional Relationship Between Body Mass Index, Waist–Hip Ratio, and Cognitive Performance in Postmenopausal Women Enrolled in the Women's Health Initiative.
Journal of the American Geriatrics Society. 58(8), 1427 - 1432.