Weight gain from smoking cessation attributed to changes in gut bacteria
In a study recently published in PLoS One, scientists have found that the weight gain attributed to smoking cessation is due to a change in gut bacteria composition. The study, which ran for 9 weeks, compared the composition of gut bacteria in 10 subjects who underwent a smoking cessation program to that of 5 subjects who were smokers and 5 subjects who were non-smokers. The results were shocking to say the least.
Not only did the composition of the gut bacteria change with smoking cessation when compared to smokers and non-smokers, but the change was not due to a change in the composition of the diet . In addition, the changes in composition of gut bacteria in the subjects who quit smoking resembled that of the changes seen between lean and obese people. Obese people tend to have a lower proportion of bacteroidetes and a higher proportion of firmicutes when compared to lean people. When the subjects went from smoker to non-smoker, their gut bacteria diversity basically switched from resembling that of a lean person to that of an obese person.
The ramifications for this are believed to be an increased ability to extract nutrients from the food one consumes. So the weigh gain, rather than being from an increased caloric consumption, appears to be from an increased ability to harvest nutrients from a calorically similar diet. Although it should be pointed out that diet was assessed by food questionnaire which is known to be error prone.
Since this is a small study, we need more research in this area to confirm it and identify causative factors. One fascinating thing to pull away from this study is that it points towards a 2 way road of communication between your body and your gut bacteria. Diet is known to have a significant impact on the composition of bacteria in your gut via providing nutrients for the proliferation of specific species. Friendly bacteria are known to impact health by providing nutrients, training the immune system, and positively affecting the hormonal status of the body. Unfriendly bacteria can introduce toxins and increase inflammation that will negatively impact health.
In this study, however, it would appear that changes in the blood are triggering changes in gut bacteria composition. Nothing in cigarette smoke should make it's way in to the digestive tract past the mouth as it is inhaled in to the lungs. Unless a change in the microbial communities in the mouth, pharynx, or trachea are able to influence gut bacteria in some way other than via the bloodstream, it would appear that something in the blood is influencing the composition of bacteria in the gut. Since smoking is known to cause a lot of inflammation, as is being obese, the attractive answer is that inflammation is the mode of communication.
Perhaps continued levels of inflammation signal a stressful state to the body in smokers. When the inflammation is removed, the body senses a need to store more calories for when the stressful state resumes and communicates this somehow to the microbiome. The bacteria in the microbiome then shift to benefit the host, aka you, by extracting more calories from the food you eat. Although this is possible, the fact that obese people are continually bombarded by inflammation makes it unlikely unless obese people are reinforcing the poor gut bacteria composition with poor dietary choices. This is where the autonomic nervous system could come in to play.
Obesity is known to lead to a constantly activated fight or flight response and going from smoking to non-smoking would be an obvious stressor. It is possible that the changes in gut bacteria could be mediated by the autonomic nervous system and your ability to move between sympathetic and parasympathetic states effectively(AKA, being able to move from fight or flight to rest and digest). Perhaps being in fight or flight for long periods of time induces a change in gut bacteria that allows you to extract more calories from your food once you enter rest and digest.
This could be a novel way that the body prepares you to deal with your environment. If stress is only minimal or intermittent, this change wouldn't be necessary. However, someone who is constantly bombarded by stress would benefit from an increased ability to extract nutrients from their food because if they are in fight or flight mode constantly, they are in rest and digest for smaller periods of time. There is no doubt that nicotine withdrawal would lead to a significant period of time of increased stress on the body.
For the vast majority of our time on the planet, stress has been related to our ability to find food. Therefore, constant stress would tend to be due to a lack of finding food and an enhanced ability to extract nutrients from a small amount of food would be beneficial. An enhanced ability to extract nutrients from a large amount of food would not. Quitting smoking is a serious stressor that would be novel to modern humans, and in the environment of a steady food supply it appears that changes in the composition of gut bacteria could be what's leading to those extra pounds and not an increase in food consumption. Whether or not this is mediated via the autonomic nervous system is only speculation, but interesting nonetheless.