Smoking can increase harmful belly fat

Science and Health

Among the excuses that many smokers give for refusing to quit is that they claim they will gain weight if they kick the habit. However, it seems that they are fooling themselves. 

Smoking prevention and cessation are critical for public health efforts to reduce the incidence of several chronic disorders, particularly respiratory and cardiovascular diseases. A new study from Denmark published in the journal Addiction under the title “Estimating causality between smoking and abdominal obesity by Mendelian randomization” found that both starting smoking and lifetime smoking can increase the amount of dangerous abdominal fat, especially visceral fat, which is the kind inside the abdomen that is linked to a higher risk of heart disease, diabetes, stroke, and dementia.

Smokers tend to have lower body weights than non-smokers, but they also have more abdominal fat and more visceral fat that is hard to see; individuals may have a flat stomach yet have unhealthy amounts of fat, raising risks of serious illness. This new study offers strong evidence that smoking can cause that type of fat to increase.

How the new study uncovered such findings 

Researchers at the Center for Basic Metabolic Research at the University of Copenhagen used a statistical method to determine whether smoking causes an increase in abdominal fat. It combines the results from different genetic studies to look for causal relationships between exposure (in this case, smoking) and outcome (increased abdominal fat). The two underlying European ancestry studies were extensive in scale: the smoking study looked at 1.2 million people who started smoking and over 450,000 lifetime smokers, and the body-fat distribution study included over 600,000 people.  

A woman blows cigarette smoke at another woman in the belief that this will help alleviate the effects of tear gas fired by police officers, during a protest to demand fair treatment of their loved ones, outside the Litoral prison in Guayaquil, Ecuador, January 22, 2024. (credit: REUTERS/VICENTE GAIBOR DEL PINO)

First, the team used previous genetic studies to identify which genes are linked to smoking habits and body-fat distribution. Second, they used this genetic information to determine whether people with genes linked with smoking tend to have different body fat distributions. Finally, they accounted for other influences, such as alcohol consumption or socioeconomic background, to ensure that any connections they found between smoking and body fat distribution were truly due to smoking itself and not to other factors.

Dr. Germán Carrasquilla explained: “We found that starting to smoke and smoking over a lifetime can cause an increase in belly fat, as seen by measurements of waist-to-hip ratio. In a further analysis, we also found that the type of fat that increases is more likely the visceral fat rather than the fat just under the skin. Because our study design used genetic variations, it did a better job of reducing or controlling for those variables.”