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Role of obesity in smoking behaviour: Mendelian randomisation study in UK Biobank

BMJ 2018; 361 doi: (Published 16 May 2018) Cite this as: BMJ 2018;361:k1767
  1. Robert Carreras-Torres, postdoctoral researcher1,
  2. Mattias Johansson, scientist1,
  3. Philip C Haycock, senior research associate2,
  4. Caroline L Relton, professor of epigenetic epidemiology2,
  5. George Davey Smith, professor of clinical epidemiology2,
  6. Paul Brennan, scientist1,
  7. Richard M Martin, professor of clinical epidemiology34
  1. 1Section of Genetics, International Agency for Research on Cancer, Lyon, France
  2. 2Medical Research Council Integrative Epidemiology Unit, School of Social and Community Medicine, University of Bristol, Bristol, UK
  3. 3Department of Population Health Sciences, Bristol Medical School, University of Bristol, Bristol, UK
  4. 4National Institute for Health Research Biomedical Research Centre at University Hospitals Bristol NHS Foundation Trust and the University of Bristol, Bristol, UK
  1. Correspondence to: P Brennan gep{at}
  • Accepted 5 April 2018


Objective To determine whether body mass index, body fat percentage, and waist circumference influence smoking status and intensity.

Design Mendelian randomisation study.

Setting UK Biobank, with replication of results from the Tobacco and Genetics (TAG) consortium.

Participants European descent participants from the UK Biobank cohort (n=372 791) and the TAG consortium (n=74 035).

Main outcome measures Risk of current and past smoking, number of cigarettes smoked per day, age of smoking initiation.

Results The Mendelian randomisation analysis indicated that each standard deviation increment in body mass index (4.6) increased the risk of being a smoker (odds ratio 1.18 (95% confidence interval 1.13 to 1.23), P<0.001). This association was replicated in the TAG consortium data (1.19 (1.06 to 1.33), P=0.003). Furthermore, each standard deviation increment in body mass index was estimated to increase smoking intensity by 0.88 cigarettes per day (95% confidence interval 0.50 to 1.26, P<0.001) in UK Biobank and 1.27 cigarettes per day in the TAG consortium (0.46 to 2.07, P=0.002). Similar results were also seen for body fat percentage and waist circumference in both UK Biobank and the TAG consortium data.

Conclusions These results strongly suggest that higher adiposity influences smoking behaviour and could have implications for the implementation of public health interventions aiming to reduce the prevalence of these important risk factors.


  • Contributors: RC-T, MJ, PB, and RMM initiated and designed this investigation, with important contributions from PCH, CLR, and GDS. RC-T conducted the statistical analysis for this investigation and wrote the first draft of the manuscript, with important contributions from RMM, MJ, PCH, CLR, GDS, and PB. All authors contributed to the interpretation of the study results and critical revision of the manuscript. RC-T and MJ are joint first authors. The corresponding author attests that all listed authors meet authorship criteria and that no others meeting the criteria have been omitted.

  • Funding: This work was supported by Cancer Research UK [the Integrative Cancer Epidemiology Programme, C18281/A19169] and the National Institute for Health Research (NIHR) Bristol Biomedical Research Centre based at University Hospitals Bristol NHS Foundation Trust and the University of Bristol. The Medical Research Council Integrative Epidemiology Unit at the University of Bristol is supported by the Medical Research Council and the University of Bristol (grant MC_UU_12013/2). The funders had no role in the analyses and interpretation of the results or the manuscript writing.

  • Competing interests: All authors have completed the ICMJE uniform disclosure form at and declare: support from Cancer Research UK and the NIHR Bristol Biomedical Research Centre for the submitted work; no financial relationships with any organisations that might have an interest in the submitted work in the previous three years; no other relationships or activities that could appear to have influenced the submitted work.

  • Ethical approval: Each participating study obtained informed consent from the study participants and approval from its institutional review board.

  • Data sharing: No additional data available.

  • The lead author affirms that the manuscript is an honest, accurate, and transparent account of the study being reported; that no important aspects of the study have been omitted; and that any discrepancies from the study as planned (and, if relevant, registered) have been explained.

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