Change in physical activity after smoking cessation: the Coronary Artery Risk Development in Young Adults (CARDIA) study

Abstract

Aims: To estimate physical activity trajectories for people who quit smoking, and compare them to what would have been expected had smoking continued.

Design, setting and participants: A total of 5115 participants in the Coronary Artery Risk Development in Young Adults Study (CARDIA) study, a population-based study of African American and European American people recruited at age 18-30 years in 1985/6 and followed over 25 years.

Measurements: Physical activity was self-reported during clinical examinations at baseline (1985/6) and at years 2, 5, 7, 10, 15, 20 and 25 (2010/11); smoking status was reported each year (at examinations or by telephone, and imputed where missing). We used mixed linear models to estimate trajectories of physical activity under varying smoking conditions, with adjustment for participant characteristics and secular trends.

Findings: We found significant interactions by race/sex (P = 0.02 for the interaction with cumulative years of smoking), hence we investigated the subgroups separately. Increasing years of smoking were associated with a decline in physical activity in black and white women and black men [e.g. coefficient for 10 years of smoking: -0.14; 95% confidence interval (CI) = -0.20 to -0.07, P < 0.001 for white women]. An increase in physical activity was associated with years since smoking cessation in white men (coefficient 0.06; 95% CI = 0 to 0.13, P = 0.05). The physical activity trajectory for people who quit diverged progressively towards higher physical activity from the expected trajectory had smoking continued. For example, physical activity was 34% higher (95% CI = 18 to 52%; P < 0.001) for white women 10 years after stopping compared with continuing smoking for those 10 years (P = 0.21 for race/sex differences).

Conclusions: Smokers who quit have progressively higher levels of physical activity in the years after quitting compared with continuing smokers.

Keywords: behavior change; marginal structural model; middle age; mixed longitudinal model; physical activity; smoking cessation; trajectory analysis; young adults.

Figure 1. Unadjusted self-reported leisure-time physical activity score ^a over 25 years of follow-up according to smoking status ^b and stratified by sex and race in the Coronary Artery Risk Development in Young Adults (CARDIA) Study

a Self-reported leisure-time physical activity was assessed at each CARDIA examination (examination year 0, 2 5, 7, 10, 15, 20, 25) (see Appendix table 1 for description of the elements included in the score). b Participants reporting smoking 1 or more cigarette per day at the CARDIA examination were considered smokers. Ex-smokers were participants not smoking at baseline and reporting having smoked prior to baseline and participants smoking at baseline and not smoking at the considered CARDIA examination.

Figure 2. Difference in self-reported leisure-time physical activity over the years since smoking cessation compared with ongoing smokers

The main outcome variable, physical activity score, was natural log-transformed to address skewness, so that exponentiated model coefficients are interpretable as the percent change in physical activity. We used linear mixed models with correlated random subject-specific intercepts and slopes and random intercept for study center. Fully adjusted models included fixed effects for the current smoking status (never smoking, current smoking, quit smoking), cumulative exposure to years of smoking and years since smoking cessation, as well as examination year, physical activity level before high-school, age at smoking start and 4-knot restricted cubic splines for education. Estimates are from 4 models according to race and sex categories. We estimated the average increase in physical activity 5, 10, 15 and 20 years after cessation, relative to the expected level if smoking had continued, accounting for both immediate and cumulative effects of the change in smoking status (Appendix Figure 2 and Table 2) (P = 0.38 for interaction by race/sex 10 years after smoking cessation). CI do not cross zero for black women after 5 years; percent change 5 years after smoking cessation: 8% (95% CI: −4, 21%).