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.
While researchers have yet to prove a clear connection between the two developments, he pointed out that a number of other studies have also showed a link between intestinal bacteria and weight gain.
On Wednesday, a study published in the journal Nature found that individuals with low bacterial "richness" in their intestines were more prone to obesity and associated diseases such as diabetes, heart and cholesterol problems.
Six bacterial species appear to play a key role in promoting this diversity.
Rogler said that more research was needed to answer the many questions that arise from such discoveries.
But he said it was clear we should pay more attention to how the environment influences gut functions.
"Nobody believed the people who stopped smoking and said they weren't eating more but still gained weight. Perhaps we simply should be more willing to believe what people tell us," he said.