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The latest BMJ Head to Head looks at what are the fundamental causes behind the rising rates of obesity: is it environmental or is it genetic?
John Wilding, professor of medicine, and arguing that environmental factors are the cause says that whilst “regulation of body weight and fat content is under powerful biological control...40-70% of body fatness is inherited.2” It may seem counterintuitive to suggest that the main causes lie in our environment. However, known single gene defects, such as leptin deficiency, are rare causes of obesity,3 and the only common gene polymorphism with well characterised effects on body weight, FTO, has a relatively small effect size (about 3 kg greater weight for those who are homozygous for the variant associated with obesity, and about 1.5 kg for heterozygotes).4
For someone to become obese, overall energy balance (the difference between energy consumed in the form of food and that expended in normal biological processes and physical activity) must be positive over time. Of course, this is more likely to occur in those genetically predisposed to gaining weight, especially if the environment is more conducive to increased consumption and decreased activity. The increase in the prevalence of obesity has mainly occurred over the past 30 years and has been seen in most parts of the world. Such rapid change cannot be due to genetic (or even epigenetic) changes. In contrast, the evidence that the environment has changed is overwhelming. The environmental changes are complex and include substantial shifts in the production and availability of food, occurring simultaneously with alterations in the physical environment that encourage sedentary behaviour.”
Timothy M Frayling, professor of human genetics, arguing on the genetics side, writes “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).”
In a recent systematic review of five adoption studies with several hundred parent-biological child and parent-adoptee comparisons, children’s body mass index was consistently more strongly correlated with that of their biological parents than of their adoptive parents.2
Genetic factors influence substantially where you are on the body mass index scale in a given population at a given time. Evidence is accumulating that these genetic factors may operate largely through appetite control. If true, plans based on changing our environment, such as banning the sale of supersized sugary drinks,12 will make genetic factors less important and be more successful than plans to increase awareness through education.”
What do you think? Are you genetically predisposed to be overweight or sedentary/environmental factors which make people overweight? Or could it be that some of us have genes that make us overweight (either through biology or behaviour) and the environment just accelerates this process? And as Prof Frayling says, is education about diet and weight just a waste of time?