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To examine the effect of body mass index (BMI) on weight change over 8 years in a cohort of continuing smokers and a cohort that quit and remained abstinent.
8 year prospective cohort study.
Participants smoking >15 cigarettes daily enrolled in a clinical trial of nicotine patch or placebo in Oxfordshire general practices and were reviewed 8 years later.
832 male and female participants. Abstainers were 85 participants who were biochemically proven abstinent at 3, 6, 12 months and 8 years. 613 people were smoking at each follow-up, 26 relapsed and 116 quit after 1 year.
Means, SDs, and 95% CIs were calculated for change in weight by smoking status. Linear regression analysis, using baseline BMI as an effect modifier, was used to investigate whether the effect of smoking status on weight change was dependent on baseline BMI in smokers and continuous abstainers. Modelling proceeded with separate regression equations for smokers and abstainers. Confounding variables were adjusted for.
Abstainers gained 8.79 kg (SD 6.36, 95% CI 7.42 to 10.17). Smokers gained 2.24 kg (6.65, 95% CI 1.7 to 2.77). Relapsers gained 3.28 kg (7.16, 95% CI 0.328 to 6.24). Later abstainers gained 8.33 kg (8.04, 95% CI 6.85 to 9.81). The difference in weight gain (6.56 kg, 95% CI 5.05 to 8.06, p<0.001) between abstainers and smokers was modified by baseline BMI. In abstainers a positive quadratic relationship of BMI fit best, resulting in a J-shaped curve. In persistent smokers there was a negative linear relationship of BMI (p<0.001). The model predicted that abstainers with a baseline BMI of 18 would gain 6 kg, with a BMI of 23 gain 5 kg, and with a BMI of 33 gain 14 kg more than would have been the case had they continued smoking for eight years.
Obese smokers who continue smoking are likely to not change or lose weight over eight years while obese people who quit are likely to gain the most weight. Weight gain is not as harmful as continuing smoking, but weight gain prevention interventions for obese people trying to stop smoking are needed.