When we cut our knees, we bleed. When we feel sad, we cry. Every action has a reaction, and our gut microbiome is no exception. But how do our gut bacteria let us know when something is wrong? Well, these tiny critters have one heck of a way to get our attention — by altering our body size. The most obvious example of this in action may be what happens when someone quits smoking cigarettes.
Weight gain is a well-documented side effect of quitting cigarettes, but we still don’t know why it happens. One popular theory is that in quitting, people suddenly gain an appetite and eat more — after all, they suddenly have more time on their hands and no cigarettes occupying their mouths.
It’s a nice idea, but it is also not really borne out in science. Instead of worrying about overeating in the absence of cigarettes, immunologist Eran Elinav suggests a different reason why quitting can come with unwanted side effects: The microbiome.
Why does quitting smoking lead to weight gain?
Elinav researches microbiota at the Weizmann Institute of Science in Israel. In a recent study published in Nature, Elinay and his team show smoking cessation’s companion weight gain stems from gut bacteria. Specifically, our microbiomes — the gut bacteria and other microorganisms living within our GI tract — are trying to reverse all the metabolic changes induced by chemicals in cigarette smoke.
“If one component of our body changes for any reason, the other component tries to compensate. This is the nature of the interaction that is so fascinating to us,” Elinav says.
“The reasons leading to human weight gain upon smoking cessation are ... complex, but the microbiome is likely a key player.”
The study uses mice to show how this works: Mice were kept in an environment that basically mimicked the lives of humans who chronically smoke, inhaling a lot of cigarette smoke every day. After a while, the researchers tracked changes in the mice’s microbiomes both after exposure to smoke and after they stopped ‘smoking.’ Then, the researchers repeated the experiments but also gave the mice antibiotics to hamper their microbiome.
Curiously, the mice experienced weight loss or no weight gain when smoking. But when the researchers stopped the smoke, the mice experienced significant, rapid weight gain — unless they were treated with antibiotics.
“This was telling us that somehow the gut microbiome was probably contributing to this weight gain,” Elinav says.
How smoking changes the gut
Elinav and his team show how the inhaled chemicals in cigarette smoke are dispersed via the blood, and eventually find their way to the gut. In their experiments, these chemicals changed the composition and function of gut microbes.
Changing the gut microbes also alters the secretion of two little molecules that also help determine body weight:
- DMG, which promotes weight gain
- ACG, which promotes weight loss
“Once one of the two molecules is secreted by microbes in larger amounts, it causes a feedback loop that leads to the suppression secretion of the other molecule and vice versa,” Elinav says.
When mice smoked, their gut microbes secreted more DMG and less ACG — counterintuitively, they should have gained weight as a result. But other chemicals in cigarette smoke counter the weight gain by triggering weight reduction mechanisms separate from the microbiome.
If you stop smoking, the chemicals from cigarette smoke dissipate within hours. The absence of these chemicals — but still altered microbiomes — meant the microbes still secrete more of the weight-promoting DMG molecule, but there is no longer any counter to their effect.
Can the gut help people quit smoking?
Previous research suggests a link between cigarette smoking and an imbalanced microbiome. Toxic chemicals in cigarettes can reduce the diversity of the microbiome by changing the acidity of the intestinal environment. Different acidity levels can promote the growth of harmful bacteria while hindering the growth of beneficial ones.
Since gut bacteria play a hand in regulating the immune response, an altered microbiome also increases the risk of developing inflammatory bowel disease. Additionally, smoking can worsen the symptoms of Crohn’s disease and other bowel diseases.
In 2021, the World Health Organization reported that 1.3 billion people worldwide are cigarette smokers. Smoking is a leading cause of preventable death, accounting for more than 8 million deaths every year. Despite the health hazards, many people continue to smoke.
There are many reasons why people continue to smoke. The addictive nature of nicotine found in cigarettes changes your brain’s chemistry and increases the craving for cigarettes’ stress-relieving and mood-enhancing effects. Most smokers want to stop smoking — about 55.1 percent of smokers tried to quit in 2018 — but drawbacks such as weight gain makes it harder to stop.
“A reason people fail to or avoid quitting is that they’re concerned about the dramatic weight gain that often accompanies smoking cessation,” Elinav says.
But taking care of your gut bacteria before you quit may prevent smokers from causing further damage to their microbiome.
Cigarette smoke is only one of many environmental factors that can harm your microbiome — but it is also one within individuals’ control.
What’s next — Elinav’s work brings us closer to understanding the full role of the gut microbiome on health — including how it’s involved in smoking cessation. But Elinav says there needs to be more human-focused research to validate and expand on his team’s findings.
The human and mouse gut microbiome share many similarities that make mice a fair model to investigate our guts. But there are some behaviors that can’t be replicated — like the voluntary consumption of cigarettes.
“There is a distinction between human and mouse smoking [behavior] and the reasons leading to human weight gain upon smoking cessation are probably more complex, but the microbiome is likely a key player,” Elinav explains.
Even so, finding two molecules that modulate weight could help with future treatment interventions, he says. For instance, Elinav and others want to know if an intervention known as postbiotic therapy can bypass the microbe component, and instead, let researchers focus on the molecules themselves. Changing their function post-quitting may make the difference in whether people stay off cigarettes long term.