Weight Gain in Ex-Smokers Caused by Intestinal Bacteria

A new study claims that the weight gain experienced after quitting smoking is not due to an increased calorie intake, but to a change in the composition of the intestinal flora.

Researchers at Zurich University Hospital (Switzerland) investigated the role of smoking cessation on intestinal microbial composition in 10 healthy smoking subjects undergoing controlled smoking cessation. During the nine-week observational period, repetitive stool samples were collected. Bacterial composition was analyzed and compared to 10 control subjects (5 continuing smokers and 5 non-smokers) using terminal restriction fragment length polymorphism analysis and high-throughput sequencing of 16S rRNA genes.

The results showed profound shifts in microbial composition and increased microbial diversity after smoking cessation, with an increase of Firmicutes and Actinobacteria, and a lower proportion of Bacteroidetes and Proteobacteria on the phylum level. At the same time, the test subjects who had quit smoking gained an average of 2.2 kilos in weight, although their eating and drinking habits remained the same. The researchers concluded the changed microbial composition in the intestinal flora provides the body with more energy, resulting in new nonsmokers gaining weight. The study was published in the March 2013 issue of PLoS One.

“These results indicate that smoking is an environmental factor modulating the composition of human gut microbiota,” concluded corresponding author Prof. Gerhard Rogler, MD, PhD, and colleagues of the division of gastroenterology and hepatology. “The observed changes after smoking cessation revealed to be similar to the previously reported differences in obese compared to lean humans and mice respectively, suggesting a potential pathogenetic link between weight gain and smoking cessation.”

About 80% of smokers put on weight when they quit, with the average weight gain between five to six kilograms in the first year after stopping smoking. The two main causes of weight gain when quitting smoking are thought to changed eating habits due to increased hunger as a withdrawal symptom, and the effect of nicotine on the body, which speeds up the body’s metabolism.

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