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Why Quitting Smoking Causes Weight Gain

Why Quitting Smoking Causes Weight GainMost smokers put on several pounds when they quit the habit. 

Contrary to popular belief, this is not due to a higher intake of calories, but to changes in the composition of the intestinal flora, according to a new study.

When smokers quit, 80 percent of them put on an average of 15 pounds. This weight gain occurs even when their calorie intake remains the same or even drops compared to what it was while they were still smoking.

For the study, researchers examined the genetic material of intestinal bacteria and studied stool samples which they had received from 20 different persons over a period of nine weeks — five non-smokers, five smokers and ten persons who had quit smoking one week after the start of the study.

They found that the bacterial strains that seem to prevail in the intestinal flora of obese people also become dominant in people who quit smoking.

While the bacterial diversity in the feces of smokers and non-smokers changed only a little bit over time, quitting smoking resulted in the largest shift in the composition of the intestinal bacteria.

For those who quit smoking, two types of bacteria, called Proteobacteria and Bacteroidetes, increased at the expense of two other types, Firmicutes and Actinobacteria phyla.

At the same time, the study participants who had quit smoking gained an average of 4.7 pounds although their eating and drinking habits stayed the same (with the exception that, towards the end of the study, they drank on average a little more alcohol than before quitting smoking).

The final results were similar to those found in previous mouse studies. In these studies, when scientists had transplanted the feces of obese mice into the intestines of normal-weight mice some years ago, they saw an increase in both Proteobacteria and Bacteroidetes in the gut flora as well as in the weight of the mice. The new gut flora apparently used the energy contained in the nutrition more efficiently.

The researchers hypothesize that the same effect is also occurring in their test subjects. The composition of the diverse bacteria in the intestinal flora, which changes after giving up smoking, probably provides the body with more energy, resulting in weight gain for new non-smokers.

Source:  PLoS One

Person’s intestines photo by shutterstock.

Why Quitting Smoking Causes Weight Gain

Traci Pedersen

Traci Pedersen is a professional writer with over a decade of experience. Her work consists of writing for both print and online publishers in a variety of genres including science chapter books, college and career articles, and elementary school curriculum.

APA Reference
Pedersen, T. (2018). Why Quitting Smoking Causes Weight Gain. Psych Central. Retrieved on November 29, 2020, from
Scientifically Reviewed
Last updated: 8 Aug 2018 (Originally: 31 Aug 2013)
Last reviewed: By a member of our scientific advisory board on 8 Aug 2018
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