By Iddo on September 27th, 2013As an ex-smoker I can attest to this: quitting is hard. It can also make you fat. I gained quite a few kilos when I quit, and those took a long time to lose. Happily, these days I am spending money on running marathons rather than on cigarettes. Weight gain after smoking cessation is fairly well known. In fact, fear of weight gain is cited as a primary reason many smokers are reluctant to quit. There's a large body of research on weight gain in those who quit. In fact, one thought is that smoking cessation, when accompanied by weight gain, can throw the quitter from Scylla to Charybdis: glucose tolerance becomes lower, and you are now in danger of developing metabolic disease if you do not watch your weight gain. Therefore, intervention against weight gain is common practice in smoking cessation programs. But why do smokers gain weight? Common answers are: nicotine is an appetite suppressant (and we even may know how); the metabolism of smokers is higher. Also, food is used as a replacement for smoking: quitters need that dopamine surge they are not getting anymore from nicotine. They are getting it from food, and especially the fatty, salty and sweet food that is so unhealthy, yet (like cigarettes) is designed to be addictive. But now, a group in Switzerland has found something new that may affect in post-cessation weight-gain: gut bacteria. Apparently, changes to the diversity and composition of gut bacteria are profound, and those changes seem to affect weight gain. Reminder: the bacterial composition of the gut flora of obese and lean people are markedly different. In a series of observations in humans, and experiments in mice, gut flora have been shown to directly affect weight gain. The type of bacteria that predominate in the gut of obese mice (and people) break down sugar more efficiently. Hence, a vicious cycle is created where an obese person would gain more calories from the same food than a lean person. Fortunately, the cycle can be broken through change of diet. And, as shown more recently, possibly by fecal transplant. (Personally, I'll stick with the change of diet, thankyouverymuchindeed.) But back to those who quite smoking (yay!) but started gaining weight as a result (grrrr...). The changes in the phyla of their gut bacteria resemble those of obese people, although with some important differences, which may or may not mean that the gut flora changes are responsible for weight gain. For instance, the species in the smokers' pre-quitting gut flora was fairly similar to that of a lean person, but much less diverse. That is an interesting finding by itself: somehow, smoking reduces gut flora diversity; but why that is, and what kind of effect this has on smokers' health, is the subject for other studies. The post-quitting flora, on the other hand, was mostly similar to that of an obese person, with the difference that the flora was more diverse than the pre-quitting flora. taken together, these findings are pretty incredible: smoking causes such profound changes to our body, that the number and type of bacteria that settle in our guts are different between smokers and non smokers. And if you are a smoker, less bacteria "like" you: smoking somehow selects for certain types of bacteria. To me, the main finding was the increase in microbial diversity after quitting, not the gain of Firmicutes which may or may not contribute to post-cessation weight gain. So: quit smoking. Even your bacteria will thank you. Eventually, a diverse gut is a healthy one. Biedermann et al. Smoking Cessation Induces Profound Changes in the Composition of the Intestinal Microbiota in Humans (2013) PLoS ONE 8(3): e59260. doi:10.1371/journal.pone.0059260
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