Smoking may suppress appetite and increase metabolism in some people, making them appear thin. But this can be misleading. There seems to be a complex relationship between smoking and body fat, especially visceral fat (the fat that develops around internal organs). This fat is linked to a higher risk of heart disease than subcutaneous fat, which is found under the skin.
Smoking and Body Fat
Whilst some observational studies have indicated that smokers may have a lower body mass index (BMI) than non-smokers, these studies also suggest that smokers tend to have more abdominal fat than non-smokers (1). Looking for a better understanding of the relationship between smoking and body fat distribution, researchers conducted a new study, hoping to learn three things:
- Whether smoking directly causes or changes body fat distribution,
- What other factors might influence both smoking and body fat distribution, and
- Whether people with a certain body fat distribution are more likely to become smokers, or vice-versa.
Building on previous studies
This study has built on previous studies that used Mendelian randomization (MR) to understand the connection between smoking and body fat. The previous studies focused on genetic variants (variations in DNA sequences) that have been linked to smoking and body fat through large-scale studies called Genome-Wide Association Studies (GWAS).
The researchers noted that past MR studies relied on just one genetic variation linked to smoking. They refer to three studies that relied on a specific gene variant linked to smoking heaviness, which refers to how much a person smokes in a day. Two of the studies found no evidence that heavy smoking directly caused an accumulation of abdominal fat.
The third and largest study did, however, suggest a causal link between smoking more cigarettes per day and higher waist-to-Hip ratio (WHR). While BMI estimates overall body fat content, WHR is a more accurate indicator of how fat is distributed throughout the body. A higher WHR suggests a higher proportion of visceral fat, which is linked to an increased risk of conditions such as heart disease, type 2 diabetes, and some cancers.
The authors suggest that relying on just one genetic variant could lead to inaccurate assumptions about cause and effect, as genetic variants can have more than one effect that may influence the outcome. The researchers therefore decided to look at several genetic variations associated with smoking. This was made possible by two recent developments. First, the results of recent GWAS studies have uncovered over 400 new gene locations linked to smoking behavior. Secondly, new and improved Mendelian randomization methods have paved the way for more precise and reliable studies regarding the causal effects of smoking on fat distribution.
Eliminating other confounding factors
Studies have found a genetic link between smoking and a number of other factors, including socioeconomic status, alcohol consumption, attention-deficit/ hyperactivity disorder (ADHD), and risk-taking behavior. The current study employed statistical techniques to control these factors to get a more accurate determination of whether smoking itself causes the increase in fat.
The new study
The data gathered from Genome-wide association studies (GWAS) was limited to people of European ancestry. This is important because genetic variations can differ between populations. They focused on three aspects of smoking:
- Smoking initiation (whether someone has ever started smoking),
- Lifetime smoking (how long someone has smoked throughout their life), and
- Smoking heaviness (how much someone smokes, measured by the number of cigarettes per day).
They also looked at three measures of adiposity (abdominal fat) as the “outcome traits”:
- Waist circumference (WC): This is the measurement around the waist.
- Hip circumference (HC): This is the measurement around the hips.
- Waist-to-Hip Ratio (WHR): This is the ratio of waist circumference to hip circumference. WHR is a common indicator of fat distribution.
They analyzed the data both with and without adjustment for BMI.
The results
Key findings suggest that smoking initiation and lifetime smoking are likely to cause an increase in abdominal fat as measured by WHR, whether adjusted for BMI or not. Researchers found no evidence of a causal link between smoking heaviness and abdominal fat.
The increase in fat seems to be driven more by growth in visceral fat (found around the organs) than to abdominal subcutaneous fat (found under the skin). This is important because visceral fat is linked to a higher risk of heart disease and other chronic health conditions.
Interestingly, the study also found that higher abdominal fat might cause people to smoke more cigarettes. And smoke for a longer period, suggesting a potential two-way relationship.
Limitations of the study
The researchers noted several strengths and limitations of their study.
Some of the limitations they listed relate to the data. For example, the sample size for body fat distribution in current smokers was smaller than the sample sizes used for smoking initiation and lifetime smoking. They suggested that this could limit the statistical power of their smoking heaviness analysis.
Due to the fact that no data was available for analysis. They were also unable to determine whether fat distribution changes when a person stops smoking.
They further noted that not all cigarettes are alike. One important difference is their nicotine content, which may affect any results related to smoking intensity.
And lastly, as mentioned previously, the data was based exclusively on people of European ancestry. Their findings may not apply to other populations.
The hidden danger of smoking?
The link between smoking initiation and lifetime smoking on visceral fat is concerning. Visceral fat tends to surround vital organs like the liver, pancreas, and intestines; it can negatively impact their function. Chronic conditions such as cardiovascular disease, type 2 diabetes, and metabolic syndrome have been linked to the accumulation of visceral fat. This study and others like it reveal another potential danger of smoking. This should give smokers everywhere another good reason to break the habit.
References
- Carrasquilla GD, García-Ureña M, Romero-Lado MJ, Kilpeläinen TO. Estimating causality between smoking and abdominal obesity by Mendelian randomization. Addiction. 2024. https://doi.org/10.1111/add.16454
