To our knowledge, this is the first study assessing weight gain and dietary changes among smoking quitters conducted in Switzerland. Our results show that quitting smoking was associated with a mean weight change of 2.1 kg, corresponding to an increase of 0.9 BMI units over a median follow-up of 5 years. Conversely, and contrary to our initial hypothesis, no changes in reported total caloric intake were found.
Changes in dietary intake after quitting
Weight gain increased by an average of 2.1 kg, a finding in line with a meta-analysis and systematic review involving diverse populations around the world ; noteworthy, the authors failed to find studies conducted in Switzerland. Indeed, weight gain is a common occurrence after quitting smoking and is one of the reasons why many quitters tend to relapse . Still, adequate dietary management after quitting has been shown to prevent weight gain . In this study, only nine participants who quit reported being on a diet, and achieved no weight loss compared to the non-dieting participants. Hence, our results suggest that effective dietary support to control weight is not provided to quitters on a regular basis.
No changes in total energy expenditure or prevalence of sedentariness were found. Our results replicate those of a previous study, where no changes in physical activity were noted after quitting . Still, physical activity data was queried via questionnaire and reporting biases cannot be excluded; further studies assessing physical activity via more precise methods (i.e. accelerometry) would be welcomed.
Contrary to what was hypothesized, no increase in reported total energy intake was found among quitters. Our findings do not replicate those from studies conducted 30 years ago [2, 3] but are in line with a recent Australian study, where weight gain associated with smoking cessation was not explained by worsening dietary and physical activity behaviors . A possible explanation for the results of the Australian study is that the authors assessed dietary intake one year after quitting, and is has been shown that quitters increase their energy intake shortly after quitting [2, 4]. Still, in our study, no differences in energy intake were found between different quitting periods. Hence, our results suggest that quitters do not increase their energy intake in the first year after quitting. Still, our sample size was small and it would be of interest to replicate the study in a larger sample.
Dietary intake of quitters changed little before and after quitting, and findings were replicated after stratifying on gender or on BMI category. The absolute decrease in total and saturated fat intake (as kcal) was small and clinically irrelevant, as it corresponded to 2 g of fat per day. Hence, our results suggest that the changes in dietary intake observed among quitters do not contribute to the weight gain.
Comparison between smoking groups
Quitters had a higher weight gain than maintainers and never smokers, suggesting that the increase was not due to aging. No significant differences were found regarding changes in the diet. Women quitters showed a higher intake of total and animal protein compared to never smokers, while no differences were found between quitters and maintainers. Our results do not replicate those of a previous study where women who quit had higher energy and lower fat intake than did women who continued smoking . Still, several studies indicate that the benefits of quitting smoking outweigh those of weight gain . Hence, quitters should refrain from smoking, even at the expense of an increase in body weight. Optimally, people desiring to quit should be given lifestyle advice to prevent and control weight gain after smoking cessation.
Association with time since quitting
Few changes in anthropometric or dietary intake were found according to time since quitting. The exception was fruits, the consumption of which decreased participants who had quit for less than one year but increased afterwards. A previous study showed an inverse association between fruit and vegetable consumption and weight gain among people who quit smoking , while no such association was found in another . Although we cannot rule out that this association might have occurred by chance, negative albeit statistically nonsignificant correlations were found between changes in fruit or vegetable intake and weight (Supplementary Table 11). Still, our results suggest that dietary intake does not change after quitting smoking and is not associated with weight gain, a finding also reported elsewhere .
In this study, neither physical activity nor dietary intake changes could explain the weight gain that occurred in the majority of participants. It has been suggested that dietary changes occur during the first 6 months and return to baseline levels by one year . This could explain the lack of differences regarding dietary intake, as most quitters had quit for over one year. Another possible explanation would be changes in intestinal microbiota after quitting  but further studies are needed to better identify the determinants of weight gain after quitting.
Several studies have suggested that low socio-economic status (SES) is associated with a low likelihood of quitting smoking , although the opposite trend (i.e. lower income people having a higher likelihood of making a quit attempt) was reported in a German study . In a previous paper, we found no clear association between educational level and quitting, although a trend (p = 0.064) towards lower quitting rates with lower educational levels was found . Hence, it is possible that the participants who quit had a higher SES and thus a healthier lifestyle, which did not change significantly after quitting.
This study has several limitations. Firstly, the sample size was small, leading to a low statistical power. Hence, it is likely that some changes in dietary intake have gone unnoticed; still, the sample size is comparable to another study (N = 124) . Secondly, timing of smoking cessation was unavailable for almost half of the participants. Hence, it was not possible to assess if the changes in dietary intake occurred during the first years of smoking cessation. Thirdly, a sizable fraction of smokers was excluded, which might limit the generalizability of the findings. Still, this was necessary as many excluded participants had either no or misreported dietary data. Fourthly, the same food composition database was used at both time points, and possible changes in the composition of some foods could have occurred. Further studies should try to assess this point. Finally, our study was conducted in geographically limited population and results might not be applicable in other settings.
We conclude that quitting smoking is associated with weight gain in most quitters and is not accompanied by significant changes in dietary intake. Systematic dietary support should be provided to all smokers wishing to quit.