Although smoking has long been associated with being thin, a recent genetic study suggests that a tendency to have excess body fat, especially around the waist, is also tied to a person's odds of being a smoker. The findings might indicate that extra body fat influences the likelihood of taking up smoking and how heavily a person smokes, or that the urges to overeat and to smoke may share some genetic origins, the authors note in The BMJ.
"These results highlight the role of obesity in influencing smoking initiation and cessation, which could have implications for public health interventions aiming to reduce the prevalence of these important risk factors," writes the study team, led by Robert Carreras-Torres at the International Agency for Research on Cancer in Lyon, France. The authors did not respond to a request for comments. The researchers analyzed data from the UK Biobank and the TAG Consortium on more than 450,000 people of European descent. These databases contain genetic, medical and lifestyle information for their volunteer participants.
Past studies, the authors note, have already linked genetic variations known as SNPs to both obesity and smoking, suggesting that particular SNPs increase a person's vulnerability to both forms of "addictive behavior" - overeating and smoking. It's not clear, however, if people who smoke stay thinner because smoking curbs appetite, or even if smokers really do stay thin.
To avoid the confusing influence of smoking's effect on appetite, the researchers didn't just look at participants' actual body mass and other body fat measurements. They also created a genetic profile of predicted body traits based on a person's SNPs. Using both real measurements and this genetic profile, the team then analyzed each person's smoking history.
For actual body mass index (BMI), a measure of weight relative to height, the researchers found that every additional 4.6 kilograms/meter squared was associated with a 5 percent lower risk of being a current smoker but also a 12 percent higher risk of ever having been a smoker, compared to never-smokers. The same real-BMI increment was also linked to a smoking-intensity increase of 1.75 cigarettes per day for current and former smokers combined.
When researchers looked at the genetic body fat profile, however, they found that each incremental increase in projected BMI based on SNPs was linked to 24 percent higher odds of being a current smoker and an 18 percent increase in odds of being a former smoker.
Projected increases in waist circumference and body fat percentage based on genetic profile were similarly linked with increases in the odds of ever having smoked and increased smoking intensity.
Genetic body type was not linked with odds of smoking cessation, though. This detail and others lead the authors to suggest that rather than genetic predisposition to addictive behavior, excess body fat itself might influence cravings for nicotine. Whatever the relationship between excess body fat and smoking, interventions to help people avoid these health risks need to take both into account, the authors conclude. People may be tempted to start smoking to help them lose weight, said Lucy Popova, a researcher at the Georgia State University School of Public Health in Atlanta who wasn't involved in the research. Smoking decreases appetite because nicotine, the primary addictive chemical in tobacco, activates various receptors in the brain, and some of these receptors are on the nerve cells that regulate appetite and eating behaviour, she said.
"Starting smoking in order to lose weight is a really bad idea. On one hand, you might weigh a couple of pounds less, but this weight reduction might come from lean muscles and not fat," Popova said.
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