genetics
How often do you hear overweight or obese individuals make statements like "it's not my fault it is in my genes". The likelihood of this is very small as the obesity epidemic has only really occurred in the past 30 years; this is too short a time for there to have been a change in the human gene pool. It is more likely to be environmental influences that have fuelled the obesity epidemic. It has been described as "having prehistoric genes in a toxic food environment. In plain English this means we have a predisposition to fat storage from our innate survival mechanisms and we have an abundance of highly calorific (energy dense) foods that have a relatively poor satiety value. When you combine these two factors it becomes evident why we have an obesity epidemic. It is true that obesity does tend to run in families, a child has a 70% risk of becoming obese if they have two obese parents whereas the risk drops to 20% with two lean parents. A lot of the associated risk could be due to factors such as cultural influences, lifestyle and diet since these will all be shared with the parents.
Studies with adopted children have shown weight gain/loss similar to the natural parents rather than the adopted parents thus demonstrating a degree of genetic disposition to weight fluctuation. In detailed studies of genetic transmission the influence of genetic factors has been placed from as low as 5% to as high as 50% (ASO fact sheet 2004).
Whether obesity is down to genetic factors may not be that crucial when addressing the underlying problem of how to reduce the weight. Genes can only influence a number of factors such as increasing energy intake or decreasing energy expenditure. For example someone may have a predisposition for fatty foods or a mainly sedentary lifestyle, which could have been inherited from their parents. Although in either of these scenarios it will be more of a struggle to reduce the overall body fat that is not to say it is not possible. The rules of energy balance still apply and by increasing energy output and decreasing energy intake a reduction in body fat can still be achieved.