Is age an effect modifier of the association between body mass index and physical function in patients with obesity?

Amid hesitancy to pursue interventions, such as metabolic and bariatric surgery for obesity within some age groups, the impact of age on the relationship between body mass index (BMI) and perceived bodily function is fundamental to understand. Our objective was thus to determine whether this relationship was affected by older age such that it might alter how we consider interventions in different age groups.

Patient data (N = 1921) were obtained prospectively and assessed to determine if age decile influenced the association between BMI and BODY-Q Physical Function scores; BODY-Q PF is a validated instrument which assesses perceived physical function in patients who are being managed for weight loss. Normative BODY-Q PF scores were defined according to the previously published threshold. Stratified analyses, receiver operator characteristic curves, and logistic regression were utilized to assess for effect modification by age.

Among 1921 adults (80% female) seeking weight management interventions at an academic hospital consortium, the strongest association between normal BODY-Q PF and lower BMI was seen in patients 61-70 years of age, whether assessed through receiver operator characteristic curves, bivariable odds ratios, or multivariable logistic regression. Age was an effect modifier of the association between BODY-Q PF and BMI. When both age 61-70 years and BMI < 30 occurred concurrently, the odds of normal BODY-Q PF scores increased (OR 3.275, 95% CI 1.994-5.379). In contrast, age 61-70 years alone was associated with lower odds of a normal BODY-Q PF score (OR 0.434, 95% CI 0.298-0.603).

When BMI is improved in older patients, it is more closely associated with better physical function. Age is thus an effect modifier of the association between BMI and BODY-Q PF scores. Older patients may then experience at least as many physical function benefits from weight loss interventions as their younger counterparts.