In recent years, epidemiologists have found that waist circumference is a better measure of the burden of excess visceral fat tissue than body mass index (BMI). Progress towards making better use of this information has been slow, as is usually the case in the world of epidemiology. Visceral fat tissue generates chronic inflammation through a variety of mechanisms, from DNA debris activating the immune system to inappropriate signaling by fat cells to an accelerated pace of generation of senescent cells. Chronic inflammation disrupts function and accelerates the progression of all of the common age-related conditions. People who are overweight have a shorter life expectancy and higher lifetime medical costs as a result.
A 2015 large-scale retrospective cohort study of nearly 2 million people from the United Kingdom Clinical Practice Research Datalink showed that the incidence of dementia continued to fall for every increasing BMI category. Two Mendelian randomization studies showed no association between obesity and dementia. BMI is not a perfect measure of adiposity because it cannot discriminate between fat and lean body mass. Waist circumference is a more accurate indicator of abdominal visceral fat level than body mass index (BMI) in the elderly. Studies have been limited, however, and focused on the relationship between waist circumference and dementia in older persons. One study showed that central adiposity, represented by waist circumference, predicted an increased risk for cognitive decline during a 2-year follow-up period in older patients with diabetes. Another study reported that waist circumference was correlated with lower overall cognition and executive performance in older women with type 2 diabetes.
To help determine a healthy waist circumference, researchers compared relative risk of dementia associated with waist circumference and BMI categories using the Korea National Health Insurance Service program. The program is a mandatory social health insurance program that enrolls about 98 percent of Koreans who participate in biannual standardized health examinations. The study population comprised 872,082 participants aged 65 years and older who participated in the Korean national health screening examination between January 1, 2009 and December 31, 2009. The study population was observed from baseline until the date of development of dementia, death, or until December 31, 2015, whichever came first.
The results of the study showed participants with a waist circumference of greater than or equal to 90cm for men and 85cm for women had a significantly increased risk of dementia after adjusting for other factors such as age, BMI, blood pressure, cholesterol, liver function tests and various lifestyle factors. As for the association between BMI categories with dementia in older men and women who were underweight, they experienced a significant increased risk of dementia compared with normal weight individuals after factoring in comorbidities and various lifestyle factors. The relationship with BMI and dementia may be a result of the adverse effects of sarcopenia in the elderly.