JMIR Publications

ABSTRACT

Background:

The prevalence of obesity is rising. Most previous studies that examined the relations between body mass index (BMI) and physical activity (PA) measured BMI at a single time-point. The association between BMI trajectories and habitual PA remains unclear.

Objective:

We assessed the relations between BMI trajectories and habitual PA, among participants enrolled in the electronic Framingham Heart Study (eFHS).

Methods:

We used a semiparametric group-based modelling to identify BMI trajectories from eFHS participants who attended research examinations 1, 2, and 3 over a 14-year period. Daily steps were recorded from the smartwatch provided at examination 3. We excluded participants with <30 days or <5 hours of smartwatch wear data. We used generalized linear models to examine the association between BMI trajectories and daily step counts.

Results:

We identified three trajectory groups for the 837 eFHS participants (mean age: 53 years; 58% Female). Group 1 included 292 participants whose BMI was stable (slope: 0.005, P=.75); Group 2 included 468 participants whose BMI increased slightly (slope: 0.123, P<.001); and Group 3 included 77 participants whose BMI increased greatly (slope: 0.318, P<.001). The median follow-up period for step count was 357 days. Adjusting for age, sex, wear time, and cohort, participants in Group 2 and Group 3 took 422 steps (95% CI: -823, -21) and 1437 (95% CI: -2084, -790) fewer average daily steps, compared to participants in Group 1. After adjusting for metabolic and social risk factors, Group 2 took 382 (95% CI: -773, 10) and Group 3 took 1120 (95% CI: -1766, -475) fewer steps, compared to Group 1.

Conclusions:

In this community-based eFHS, participants whose BMI trajectory increased greatly over time took significantly fewer steps, compared to participants with stable BMI trajectories. Our findings suggest that greater weight gain may correlate with lower levels of physical activity.