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Why Ex-smokers Gain Weight: Blame the Bugs in Your Bowel

Why Ex-smokers Gain Weight: Blame the Bugs in Your Bowel

A shift in intestinal bacteria, not diet, could underlie weight gain after smoking cessation.

It may have been more than 2000 years ago, but Hippocrates was not far from the truth when he wrote “all disease begins in the gut”. Gut health is critical to overall health. Colonies of benevolent bacteria reside in your entire digestive tract, predominantly the large intestine. In fact, you are made up of more microbes than human cells. This live-in colony of microbes is your digestion powerhouse, breaking down food into its building blocks so it can absorb the nutrients.

An unhealthy gut can lead to inflammatory bowel diseases (IBDs) such as Crohn’s disease (CD) or ulcerative colitis (UC). These conditions are caused and exacerbated by interactions between genetic and environmental factors. Cigarette smoking is one of the main environmental factors thought to affect the inflammatory response by altering the intestinal microbiome. Surprisingly, although smoking worsens CD, it has a protective effect in UC which is lost after smoking cessation. Perhaps this is due to the aforementioned diseases affecting different areas along the digestive tract where there may also be differences in bacterial populations. In addition to IBD, the gut microbiota is also known to play a vital role in obesity and other metabolic disorders (read the recent UA article on gut microbiota and obesity).

80% of ex-smokers gain approximately 7-8 kilograms in weight after kicking the habit. This phenomenon is widely thought to be caused by increased calorie consumption through nervous nibbling, but could this be a common misconception? Should we be blaming the gut microbiota?

A recent study led by Professor Gerhard Rogler from Zurich University Hospital explored the hypothesis that smoking and smoking cessation may influence the composition of the intestinal microbiota, and that the weight change associated with smoking cessation may be due to this microbial shift. The study comprised 20 participants of which 10 became ex-smokers one week into the 9-week study, and 10 were controls (5 non-smokers and 5 continuing smokers). Throughout the study, repetitive stool samples were taken and body weight was measured. The participants who quit smoking gained an average of 5 pounds (2.2 kilograms) and, intriguingly, all but one participant maintained the same total average daily calorie intake. So how did they pile on the pounds?

After smoking cessation, there was a rapid and long-term change in microbial components of the stool samples: a significant increase in Firmicutes and Actinobacteria and a significant decrease in Proteobacteria. This change in composition is also observed in obese patients, suggesting it could promote more efficient gut function by extracting more nutrients from food and therefore increasing the body’s fat reserves.

Nevertheless, it will be necessary to conduct a long-term study as 8 weeks could be too soon for concrete conclusions to be made, especially since some ex-smokers continue to gain weight 4 years after cessation. However, for those of you thinking of ‘losing the butts’, do not fear as this weight gain is only temporary. Maybe it is your intestinal microbiota reverting back to its normal composition that is causing the extra weight to drop off. Whatever the case may be, love your gut and drop those ‘butts’.

Biedermann L, Zeitz J, Mwinyi J, Sutter-Minder E, Rehman A, Ott SJ, Steurer-Stey C, Frei A, Frei P, Scharl M, Loessner MJ, Vavricka SR, Fried M, Schreiber S, Schuppler M, & Rogler G (2013). Smoking cessation induces profound changes in the composition of the intestinal microbiota in humans. PloS one, 8 (3) PMID: 23516617

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