The ongoing epidemic of obesity and its associated complications such as diabetes, increased cancer risk, and cardiovascular disease is creating an unprecedented challenge for healthcare systems around the world and threatens to slow or even reverse the gains in life expectancy that have been achieved over the past 50 years.1 To tackle this growing problem it is essential to consider the fundamental causes of obesity and apply this knowledge to develop effective strategies to prevent and treat the condition and its consequences.

There is no question that regulation of body weight and fat content is under powerful biological control, and that much of this biology is rooted in genetics; identical twins are only rarely discordant for adiposity, and about 40-70% of body fatness is inherited.2 Given the importance of this genetic component, 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, …