Higher body mass index (BMI) and waist circumference are consistently associated with worse cognition among both sexes, according to study findings published in the International Journal of Obesity.Obesity is a known risk factor for cognitive decline and dementia, but it remains unclear how the timing of obesity across the lifespan, along with age- and sex-related differences in body fat distribution, influences this relationship.

In this cohort-based study, researchers aimed to assess the associations of BMI, waist circumference, abdominal overweight, and obesity with cognitive function and decline on the basis of age.

The study included 3873 participants aged 45 to 70 years at baseline (52% women) from the Doetinchem Cohort Study, an ongoing, population-based longitudinal study designed to examine lifestyle factors, biological risk determinants, and health across the lifespan between 1987 and 1991.

 

Our study underscores the importance of waist circumference as an independent risk factor for cognitive function and cognitive decline

Participants were classified as having abdominal obesity if their BMI was at least 30 or their waist circumference was at least 102 cm for men and at least 88 cm for women. Cognitive outcomes — global cognition, memory, flexibility, and processing speed — were derived by standardizing baseline test scores. Associations between time-varying BMI, waist circumference, and abdominal overweight or obesity with cognitive function and decline were analyzed using sex-stratified linear mixed models, adjusted for mental health, sociodemographic, and lifestyle factors.

The primary outcome of the study is cognitive function and cognitive decline, assessed across multiple domains (global cognition, memory, flexibility, and processing speed).

Higher BMI and waist circumference were consistently associated with poorer cognitive performance across all domains among both sexes, with the strongest effects seen for BMI and memory. Each unit increase in BMI affected flexibility the most among both sexes (β=-0.017) whereas each unit increase in BMI affected global cognition the least among men (β=-0.011; 95% CI, -0.017 to -0.003) and women (β=-0.012; 95% CI, -0.018 to -0.007).Among men, each 1 cm increase in waist circumference was linked to a small but significant accelerated decline in cognitive processing speed (β=-1.952×10–⁴; 95% CI, -3.859×10–⁴; -4.579×10–⁶). Across all ages, abdominal obesity and general obesity were significantly associated with worse memory (eg, abdominal obesity: β=-0.149; 95% CI, -0.222 to -0.077).

Among men aged no older than 55 years, abdominal obesity was also related with poorer global cognition (β=-0.099; 95% CI, -0.15 to -0.048) and flexibility (β=-0.102; 95% CI, -0.18 to -0.023). Among men aged older than 55 years, abdominal overweight compared with healthy waist circumference was associated with slightly better flexibility (β=0.166; 95% CI, 0.002-0.33).

Study limitations include potential selection bias and reduced generalizability due to a healthier cohort, attrition bias from selective dropout over time, and limited applicability to diverse populations given the reliance on a single, culturally homogeneous cohort.

The study authors concluded, “Our study underscores the importance of waist circumference as an independent risk factor for cognitive function and cognitive decline.”