Geneva: Nervous nibbles alone do not explain the weight that people tend to gain when they give up smoking, Swiss researchers said today, turning the spotlight instead on a bacterial shift in the intestines.
Studies have shown that quitting smoking leads to an average weight gain of four to five kilogrammes (nine to 11 pounds) in the first year.
But according to researchers at Zurich University Hospital, former smokers who bulk up may not be eating more than before they kissed their cigarettes goodbye.
Noting that even people who cut back on calorie intake after quitting smoking tend to gain weight, Professor Gerhard Rogler said he and his colleagues had discovered another potential explanation: a change in the composition of the intestinal flora among smokers who kick the habit.
Their study, supported by the Swiss National Science Foundation and published in peer-reviewed scientific journal PLoS One, found that when a person stops smoking, the diversity of bacterial strains in their intestines shifts. It more resembles the gut flora found in people with obesity.
Both recent non-smokers and obese people tend to have more of two bacteria types, Proteobacteria and Bacteroidetes, Rogler told AFP.
These germs are believed to use energy more efficiently and break down otherwise indigestible fibres -- and as a result, more of what the person eats is transformed into fat rather than excreted as waste.
The researchers studied the genetic profile of intestinal bacteria found in faecal samples provided by 20 volunteers over nine weeks.
The participants comprised five non-smokers, five smokers and 10 people who had quit smoking one week after the study began.
Little difference was seen in the bacterial biodiversity among the persistent smokers and non-smokers.
But among those who had just given up smoking, there was a clear shift towards more Proteobacteria and Bacteroidetes, the study showed.
Over the study period, the people who had quit smoking also gained an average of 2.2 kilos (4.8 pounds), even though they insisted that their eating and drinking habits were unchanged.
"Under the same living conditions, they gained weight after the cessation of smoking, and they showed a change in the microbiota," Rogler said.