Abstract
Obesity rates are rising in all parts of the world. It is generally believed that this is because of the rapid change in food availability and the removal of the obligatory need to do physical work. This chapter will discuss the powerful evidence that while these changes in lifestyle facilitate the development of obesity, on their own they cannot be the causes of obesity. They provide the necessary circumstances for the multiple genes selected over millions of years to improve the survival of the species, by causing excess deposition of stored energy as fat. There are single gene mutations that lead to severe obesity, such as a mutation in the leptin gene or the melanocortin 4 receptor gene or combinations of gene polymorphisms that interact to give different degrees of overweight. There are also multiple mechanisms described where environmental situations epigenetically imprint genes to lead to obesity. We also have two negative feedback systems to prevent obesity in genetically lean individuals. Finally, the body defends weight vigorously by increasing hunger and reducing energy expenditure when someone loses weight. This would not occur if obesity was only due to lifestyle changes.