No change in food intake after quitting smoking; a prospective study in Switzerland | BMC Nutrition
To our knowledge, this is the first study to assess weight gain and dietary changes in non-smokers to be conducted in Switzerland. Our results show that smoking cessation was associated with a mean weight change of 2.1 kg, corresponding to a 0.9 unit increase in BMI over a median follow-up of 5 years. Conversely, and contrary to our initial hypothesis, no change in the total reported calorie intake was found.
Changes in food intake after quitting
Weight gain increased by an average of 2.1 kg, a finding consistent with a meta-analysis and systematic review involving various populations around the world ; It should be noted that the authors did not find any studies carried out in Switzerland. Indeed, weight gain is a common phenomenon after quitting smoking and is one of the reasons why many smokers tend to relapse. . Yet, proper dietary management after quitting smoking has been shown to prevent weight gain. . In this study, only nine participants who quit reported dieting and did not lose weight compared to participants without dieting. Therefore, our results suggest that effective dietary support for weight control is not provided on a regular basis to people who quit smoking.
No change in total energy expenditure or the prevalence of inactivity was found. Our results mimic those of a previous study, where no change in physical activity was noted after stopping . Nevertheless, the data on physical activity were questioned via a questionnaire and reporting bias cannot be excluded; other studies evaluating physical activity by more precise methods (ie accelerometry) would be welcome.
Contrary to what was assumed, no increase in declared total energy intake was observed in ex-smokers. Our results do not reproduce those of studies conducted 30 years ago [2, 3] but are consistent with a recent Australian study, where weight gain associated with stopping smoking was not explained by worsening eating and physical activity behaviors . One possible explanation for the Australian study results is that the authors assessed food intake one year after quitting, and smokers have been shown to increase their energy intake soon after quitting. [2, 4]. Yet, in our study, no difference in energy intake was found between the different withdrawal periods. Therefore, our results suggest that smokers do not increase their energy intake in the first year after quitting. However, our sample size was small and it would be interesting to replicate the study in a larger sample.
The dietary intake of people who quit smoking changed little before and after quitting, and the results were reproduced after stratification by sex or BMI category. The absolute decrease in total and saturated fat (kcal) intake was small and clinically irrelevant, as it corresponded to 2 g of fat per day. Therefore, our results suggest that the changes in food intake seen in smokers do not contribute to weight gain.
Comparison between groups of smokers
Dropouts had higher weight gain than maintainers and non-smokers, suggesting the increase was not due to aging. No significant difference was found regarding changes in diet. Women who quit smoking showed a higher intake of total and animal protein compared to women who never smoked, while no difference was found between people who quit and those who did. quit smoking. Our results do not replicate those of a previous study where women who quit smoking had more energy and lower fat intake than women who continued to smoke. . Yet several studies indicate that the benefits of quitting smoking outweigh the benefits of weight gain. . Therefore, people who quit smoking should refrain from smoking, even at the cost of increased body weight. Ideally, people who want to quit smoking should receive lifestyle advice to prevent and control weight gain after quitting.
Association with time elapsed since shutdown
Few changes in anthropometric or dietary intake were found based on time since stopping. The exception was fruit, the consumption of which declined among participants who had quit for less than a year but increased thereafter. A previous study showed an inverse association between the consumption of fruits and vegetables and weight gain in people who quit smoking , while no such association was found in another . While we cannot rule out that this association may have occurred by chance, negative but statistically insignificant correlations were found between changes in fruit or vegetable consumption and weight (Supplementary Table 11). Nonetheless, our results suggest that food 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 changes in food intake could explain the weight gain that occurred in the majority of participants. It has been suggested that dietary changes occur within the first 6 months and return to baseline levels after a year. . This could explain the lack of differences in food intake, as most people quit smoking for more than a year. Another possible explanation would be changes in the gut microbiota after quitting  but more studies are needed to better identify the determinants of weight gain after quitting smoking.
Several studies have suggested that low socioeconomic status (SES) is associated with a low likelihood of quitting smoking , although the reverse trend (i.e. low-income people having a higher likelihood of attempting to quit) was reported in a German study . In a previous article, we found no clear association between education level and dropout, although a trend (p= 0.064) towards lower dropout rates with lower education levels was found . Therefore, it is possible that participants who quit smoking had a higher SES and therefore a healthier lifestyle, which did not change significantly after quitting.
Limitations of the study
This study has several limitations. First, the sample size was small, resulting in low statistical power. Therefore, it is likely that some changes in food intake have gone unnoticed; again, the sample size is comparable to another study (NOT= 124) . Second, the timing of smoking cessation was not available for almost half of the participants. Therefore, it was not possible to assess whether changes in food intake occurred during the first years of smoking cessation. Third, a large fraction of smokers were excluded, which may limit the generalizability of the results. However, this was necessary because many excluded participants had no dietary data or erroneous data. Fourth, the same food composition database was used at both times, and possible changes in the composition of some foods may have occurred. Further studies should attempt to assess this point. Finally, our study was conducted in a geographically limited population and the results may not be applicable in other settings.
We conclude that smoking cessation is associated with weight gain in most smokers and is not accompanied by significant changes in food intake. Systematic dietary support should be provided to all smokers wishing to quit smoking.