Study reveals that smoking increases abdominal fat

The concern about potential weight gain is often cited by smokers as a reason to avoid quitting. However, a new study led by the University of Copenhagen has recently found that both initiating smoking and prolonged smoking habits may lead to an increase in abdominal fat, particularly the dangerous kind known as visceral fat. 

This type of fat, nestled deep within the abdominal cavity, is associated with heightened risks of several severe health conditions, including heart disease, diabetes, stroke, and dementia.

Smoking and visceral fat

Although smokers typically weigh less than non-smokers, they paradoxically tend to accumulate more fat in the abdominal region, including an increase in visceral fat. 

This type of fat is insidious because it can accumulate significantly without visible signs, such as a protruding belly, yet significantly elevate the risk of critical health issues. The study provides compelling evidence that smoking may be a direct contributor to the accumulation of visceral fat.

Focus of the study

Utilizing a statistical method known as Mendelian randomization (MR), researchers from the NNF Center for Basic Metabolic Research at the University of Copenhagen sought to establish a causal link between smoking and an increase in abdominal fat. 

Mendelian randomization analyzes genetic information from various studies to explore the causal relationships between certain exposures (like smoking) and outcomes (such as increased abdominal fat). 

Smoking behaviors and body fat

The research combined multiple genetic findings from studies focusing on European ancestries that looked at smoking behaviors and body fat distribution indicators, like the waist-to-hip ratio and measurements of waist and hip circumferences. 

The scientists incorporated data from two large-scale European ancestry studies: one examining the impact of starting smoking on 1.2 million individuals and another analyzing over 450,000 lifetime smokers. 

The research on body fat distribution involved more than 600,000 participants, providing a robust dataset for these significant findings.

Potential influencing factors

The experts first identified genetic markers linked to smoking habits and body fat distribution. They then leveraged this genetic data to assess whether individuals with genetic predispositions to smoking exhibited different body fat distributions. 

Importantly, the experts adjusted for other potential influencing factors, such as alcohol intake or socioeconomic status, to isolate the impact of smoking on body fat distribution from other variables.

Dangerous belly fat

“This study found that starting to smoke and smoking over a lifetime might cause an increase in belly fat, as seen by measurements of waist-to-hip ratio,” said lead author Germán D. Carrasquilla, an epidemiologist at the University of Copenhagen. “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.”

“Previous studies have been prone to confounding, which happens when an independent variable affects the results. Because our study design uses genetic variations, it does a better job of reducing or controlling for those variables,” he added. 

“The influence of smoking on belly fat seems to happen regardless of other factors such as socioeconomic status, alcohol use, ADHD, or how much of a risk-taker someone is.”

Excess abdominal fat in smokers

According to the experts, from a public health point of view, these findings highlight the importance of large-scale efforts to prevent and reduce smoking in the general population, in order to help reduce abdominal visceral fat and the various chronic diseases that are related to it. Thus, reducing a major health risk in the population will, indirectly, reduce another major health risk.

The discovery that the excess abdominal fat in smokers is mainly visceral emerged from examining the relationship between DNA variants associated with smoking behaviors and abdominal fat and how these relate to different body fat compartments. 

The analysis revealed that these genetic factors have a stronger connection to increased visceral adipose tissue – the deep fat surrounding abdominal organs – rather than to the subcutaneous fat located just below the skin.

The study is published in the journal Addiction.

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