Intended for healthcare professionals

Head To Head

Are the causes of obesity primarily environmental? No

BMJ 2012; 345 doi: (Published 11 September 2012) Cite this as: BMJ 2012;345:e5844
  1. Timothy M Frayling, professor of human genetics
  1. 1Genetics of Complex Traits, Peninsula College of Medicine and Dentistry, University of Exeter, Exeter EX1 2LU, UK
  1. tim.frayling{at}

John Wilding (doi:10.1136/bmj.e5843) believes that changes in our environment are responsible for increasing obesity but Timothy Frayling thinks that it is genetic factors that determine who gets fat

Genetic variation has not changed appreciably in the past 50 years and therefore cannot explain the secular increases in average body mass index observed over the past few decades. But changes in the environment (decreased need for physical activity and greater availability of cheap food) mean we are all at increased risk of obesity compared with our parents and grandparents. So why do many people remain slim, while others gain weight?

Genetic variation influences our appetites, metabolism, and tolerance of physical activity. This creates a strong genetic component to variation in body mass index in today’s environment. An analogy can be made with smoking—if everyone inhaled the same amount of cigarette smoke every day, the strongest risk factor for lung cancer would be genetic susceptibility to the adverse effects of cigarette smoke (G Davey Smith, personal communication).

Size of genetic effect

Twin and adoption studies show consistently that variation in body mass index has a strong genetic component. One study assessed the heritability of body mass index in over 20 000 young adult twin pairs from eight …

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