Context:​ Gut microbiota-derived metabolite trimethylamine N-oxide (TMAO) may adversely affect bone by inducing oxidative stress. Whether this translates into increased fracture risk in older adults is uncertain.

Objective:​ Determine the associations of plasma TMAO with hip fracture and bone mineral density (BMD) in older adults.

Design and setting:​ Cox hazard models and linear regression stratified by sex examined the associations of TMAO with hip fracture and BMD in the longitudinal cohort of the Cardiovascular Health Study.

Participants:​ 5019 U.S. adults aged ≥65 years.

Exposure:​ Plasma TMAO.

Main Outcome Measures:​ Incident hip fractures;​ total hip BMD dual x-ray absorptiometry in a subset (n = 1400).

Results:​ Six hundred sixty-six incident hip fractures occurred during up to 26 years of follow-up (67,574 person-years). After multivariable adjustment, TMAO was not significantly associated with hip fracture (women:​ hazard ratio (HR) [95% confidence interval (CI)] of 1.00[0.92,1.09] per TMAO doubling;​ men:​ 1.12[0.95,1.33]). TMAO was also not associated with total hip BMD (women:​ BMD difference [95% CI] of 0.42 g/cm2*100 [−0.34,1.17] per TMAO doubling;​ men:​ 0.19[−1.04,1.42]). In exploratory analyses, we found an interaction between body mass index (BMI) and the association of TMAO with hip fracture (P < 0.01). Higher TMAO was significantly associated with risk of hip fracture in adults with overweight or obesity (BMI ≥ 25) (HR [95% CI]:​1.17[1.05,1.31]), but not normal or underweight.

Conclusions:​ Among older US men and women, TMAO was not significantly associated with risk of hip fracture or BMD overall. Exploratory analyses suggested a significant association between higher TMAO and hip fracture when BMI was elevated, which merits further study.