The Irony of Smoking to Stay Thin: Smoking Increases Belly Fat

Posted on 21.03.2024

The worry of gaining weight is a common excuse for smokers not to quit. Dr. Germán D. Carrasquilla, who is a former Danish Diabetes Academy grant recipient, is lead author on a new study published today in the scientific journal Addiction. The study has found that both starting smoking and lifetime smoking may increase abdominal fat, especially visceral fat: the unhealthy fat deep inside the abdomen that is linked to a higher risk of heart disease, diabetes, stroke, and dementia.

Smokers Have Lower Weight, but More Abdominal Fat

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

Study Design Uses Genetic Variations

Researchers at the NNF Center for Basic Metabolic Research, University of Copenhagen used a form of statistical analysis called Mendelian randomization (MR) to determine whether smoking causes an increase in abdominal fat. MR combines the results from different genetic studies to look for causal relationships between an exposure (in this case, smoking) and outcome (increased abdominal fat). This new study combined multiple genetic results from European ancestry studies of smoking exposures and measures of body fat distribution (e.g., waist-hip ratio and waist and hip circumferences).

First, the researchers 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 associated 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 other factors.

Figure 1: Methodological considerations for examining causality between smoking and abdominal adiposity. Mendelian randomization can provide an unbiased estimation of causality (A), provided that three assumptions are valid (B). The first assumption is that the instrument is associated with the exposure. The second assumption is that the instrument is not associated with the outcome through a confounding pathway (uncorrelated horizontal pleiotropy). The third assumption is that the instrument does not directly influence the outcome but does so indirectly through the exposure (correlated horizontal pleiotropy). Depending on the direction of the causal estimate being tested, the analysis can be unidirectional or bidirectional (C). Abdominal adipose tissue consists of visceral adipose tissue and abdominal subcutaneous adipose tissue (D). Created with

Influence of Smoking on Belly Fat Happens Regardless of Other Factors

Lead author Dr. Germán D. Carrasquilla explains:

“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. 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. 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.”

Large-Scale Efforts to Prevent and Reduce Smoking are Important

“From a public health point of view, these findings reinforce the importance of large-scale efforts to prevent and reduce smoking in the general population, as this may also help to reduce abdominal visceral fat and all the chronic diseases that are related to it. Reducing one major health risk in the population will, indirectly, reduce another major health risk,” says Germán D. Carrasquilla.

The researchers determined that excess abdominal fat in smokers was predominantly visceral fat by studying how DNA variants linked to smoking habits and belly fat relate to fat compartments in different parts of the body. The key finding is that these genetic factors are more strongly linked to increased visceral adipose tissue—the deep fat that wraps around the abdominal organs—than to subcutaneous fat that is stored under the skin.

The two underlying European ancestry studies were large 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.  

Further Information

This paper is available online

To speak with lead author Dr. Germán D. Carrasquilla, please contact him at the University of Copenhagen by work telephone (+45 35 33 45 95), or email (

To speak with Dr. Tuomas O. Kilpeläinen, please contact him at the University of Copenhagen by work telephone (+45 35 33 22 47), or email (

Full citation for article: Carrasquilla GD, García-Ureña M, Romero-Lado MJ, and Kilpeläinen TO. Estimating causality between smoking and abdominal obesity by Mendelian randomization.  Addiction. 2024. DOI: 10.1111/add.16454

Primary funding: The Novo Nordisk Foundation, the European Union’s Horizon 2020 program, and the Danish Diabetes Academy (DDA).

Declaration of interests: None.

Further information on Mendelian randomisation

Addiction ( is a monthly international scientific journal publishing peer-reviewed research reports on alcohol, substances, tobacco, gambling, editorials, and other debate pieces. Owned by the Society for the Study of Addiction, it has been in continuous publication since 1884.

Copyright © 2023 Danish Diabetes and Endocrine Academy. All Rights Reserved • Privacy Policy