Body mass index and cognition: Associations across mid- to late-life and gender differences.

BACKGROUND: Higher mid-life body mass index (BMI) is associated with lower late-life cognition. Associations between later-life BMI and cognition are less consistent; evidence suggests reverse causation may play a role. We aimed to characterize associations between BMI and cognition across a wide age range during mid- to late-life (55-85 years) and examine whether associations vary by gender.

METHODS: We used data from the Health and Retirement Survey (HRS) (N=39,153) to examine the association between BMI and three cognitive outcomes: cognitive level, cognitive decline, and cognitive impairment. We used a series of linear regression, mixed effects regression, and logistic regression models, adjusting for potential confounders.

RESULTS: Higher BMI before age 65 (mid-life) was associated with lower cognitive performance, faster rates of cognitive decline, and higher odds of cognitive impairment in late-life. Averaging across analyses assessing associations between BMI measured before age 60 and late-life cognition, a 5-unit higher level of BMI was associated with a 0.26 point lower cognitive score. Beyond age 65, associations flipped, and higher BMI was associated with better late-life cognitive outcomes. Associations in both directions were stronger in women. Excluding those with BMI loss attenuated findings among women in older ages, supporting the reverse causation hypothesis.

CONCLUSIONS: In this sample, age 65 represented a critical turning point between mid- and late-life for the association between BMI and cognition, which has important implications for recruitment strategies for studies focused on risk factors for late-life cognitive outcomes. Evidence of gender differences raises the need to further investigate plausible mechanisms.