Quitting Smoking Means Gaining Weight, But Why?

If you quit smoking, you often gain weight. Researchers have now investigated in twins how big the difference is and whether this is also due to genes. They come to an unequivocal conclusion.

Weight gain is one of the great fears of smokers who want to stop smoking. In fact, most ex-smokers gain three to four kilos in the first few days. Studies also show that smokers weigh, on average, less than former smokers. For some, this prospect is reason enough to continue smoking.


But is the combination of cigarettes and weight all about nicotine? Or is there another explanation? To find out, a group of researchers led by Maarit Piirtola from the University of Helsinki (1) now investigated twin siblings, including those who differed significantly in their smoking behavior. Her conclusion: genes and common environmental factors cannot explain it to a sufficient degree.

The connection between cigarettes and weight is not always clear. Smokers do not necessarily gain as much weight as when they smoked before. And not every smoker gains weight when he/she stops smoking; some even lose it. In addition, genes play a role in both weight and smoking. There is even a study in which the same genetic variable was associated with a low BMI in smokers, but with a higher BMI in non-smokers (2).

The researchers, therefore, believe that people who smoke might differ from non-smokers not only in their BMI and health behavior but also in their genotype, i.e. their genetic make-up and certain influences from their environment. In order to find evidence for this hypothesis, the researchers evaluated the international CODACTwins database. The database contains information on smoking behavior and the BMI of almost 157,000 twins from 1960 to 2012.

As expected, smokers had a lower BMI than non-smokers. Also, former smokers weighed more than smokers. These results were regardless of gender. The correlation also applied to both men and women. On the other hand, smokers do not have to worry too much. When comparing twin siblings, one of whom had smoked and quit and the other of whom had never smoked, there was hardly any difference in weight. The work of the scientists also coincides with the results of two other studies in which former smokers only gained as much weight as they were at the level of non-smoking twin siblings (3,4). According to the authors, this information could moderately invalidate the concerns of smokers who wish to quit.


And what do the researchers say about gene involvement? The researchers found no evidence for this. The common environment of the twin siblings is also said not to have played a role. So maybe it is the nicotine itself. Nicotine reduces the feeling of hunger in the brain. A reward system could also play a role. Without cigarettes, many former smokers look for something else, especially at first, that can give them the pleasant feeling of a reward. Not infrequently, these are sweets.

There is also discussion as to whether smokers’ bodies are more energy-wasting. Some small studies suggest that smokers consume about 200 kilocalories more per day than non-smokers. So far, however, the findings are rather contradictory.

The study of these researchers also has weaknesses and limitations. The data on smoking and BMI, for example, comes from the participants themselves. There was also no information on the health status of the participants, although diseases can have a significant influence on BMI and smoking behavior.

This is part 40 of a series covering twin health provided by Paul Enck from the Tübingen University Hospital and science writer Nicole Simon. Further studies in twin research can be found at the TwinHealth website. Translation was done with the assistance of DeepL translator.

Piirtola M, Jelenkovic A, Latvala A, et al. Association of current and former smoking with body mass index: A study of smoking discordant twin pairs from 21 twin cohorts. PLoS One. 2018 Jul 12;13(7):e0200140.

Taylor AE, Morris RW, Fluharty et al. Stratification by smoking status reveals an association of CHRNA5-A3-B4 genotype with body mass index in never smokers. PLoS Genet. 2014 Dec 4;10(12):e1004799.

Swan GE, Carmelli D. Characteristics associated with excessive weight gain after smoking cessation in men. Am J Public Health. 1995 Jan;85(1):73-7.

Carmelli D, Swan GE, Robinette D. Smoking cessation and severity of weight gain, Letter. New England Journal of Medicine. 1991;325(7):517–8.



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