First-year college students' weight change is influenced by their randomly assigned roommates' BMI

Background There is ongoing debate about whether friends' greater similarity in Body Mass Index (BMI) than non-friends is due to friend selection, shared environments, or peer influence. Methods First-year college students (n = 104) from a southwestern U.S. university were randomly assigned roommates during the university's housing process, effectively removing friend selection effects. Participant BMI was measured up to four times (T1-T4) across 2015- 2016. The influence of roommate baseline BMI (T1) on change in participant BMI over time (T2-T4) was analyzed using a linear mixed effects model adjusted for individual socio-demographics, linear time trends, baseline BMI, and physical clustering of students. A sensitivity analysis examining floormates was also conducted. Results Consistent with roommate influence, participants randomized to roommates with a higher BMI gained more weight between times T2 and T4 (β = 0.06; 95% CI = 0.02, 0.10). No shared environment effects (shared campus or floor) were found. Conclusions Randomly assigned roommates influenced each other's weight trajectories. This clarifies that BMI convergence can occur outside of friend selection or shared environments mechanisms.