A study by the International Agency for Research on Cancer (IARC) provides new evidence that increased weight and obesity may result in increased smoking.

The Cancer Research UK (CRUK) funded study, involving University of Bristol researchers and published in the British Medical Journal (BMJ), found that increased body mass index (BMI), body fat percentage, and waist circumference were associated both with a higher risk of being a smoker and with greater smoking intensity, measured by the number of cigarettes smoked per day. These results were consistent in both men and women.

The University of Bristol reports, in contrast to previous studies evaluating the relationship between body weight and smoking behaviour, this study was based on genetic markers of obesity using UK Biobank data with genetic information on nearly four hundred and fifty thousand participants.

Dr Paul Brennan, one of the authors of the study, said, based on genetic markers of obesity, the findings allow a better understanding the complex relationship between obesity and important smoking habits such as smoking initiation and intensity, as well as the impact of obesity on smoking cessation. The study also suggests that the link between BMI and tobacco exposure may originate in a common biological basis for addictive behaviours, such as nicotine addiction and higher energy intake.

It is well established that smokers have a lower body weight on average than non-smokers, possibly because of a reduced appetite in smokers, but that people tend to gain weight after quitting smoking. However, among smokers, those who smoke more intensively tend to weigh more.

This new analysis of genetic variants linked to body mass highlights the complex relationship between obesity and tobacco smoking.

IARC Director Dr Christopher Wild said prevention of smoking is key to reducing the global burden of cancer and other chronic diseases, such as cardiovascular disease and diabetes. Obesity is also among the most important preventable causes of the same diseases. These new results provide intriguing insights into the potential benefits of jointly addressing these risk factors with public health measures that combine weight control and tobacco control strategies.

Professor Richard Martin, Professor of Clinical Epidemiology at Bristol’s Medical School, said “In this comprehensive analysis of genetic data from 450,000 people, that was jointly led by the University of Bristol and the International Agency for Research on Cancer, we provide very powerful evidence that being overweight or obese increases the risk that people will take up smoking and that they will smoke more cigarettes per day. This provides new information about how these two common risk factors for heart disease and cancer are interlinked, that will help us better plan how to tackle them.”