Energy balance is a fundamental principle of physics that regulate weight gain and loss. An organism will stock fat when absorbing more energy through feeding than what it consumed by physical activity and its basal metabolism.The opposite is also true: reducing calorie intake and increasing physical activity creates a negative balance that translates into weight loss.

Even when admitting that the main cause of obesity is an inadequacy between energy intake and expenditure, there remains a lot to be learned in order to properly manage the epidemic. Research is barely shedding light on the mysteries of this complex disorder. In Canada, between 1981 to 2004, daily calorie intake got up of 17%, that is 381 calories per day.

The theory may sound simple, but reaching and maintaining a healthy weight remains difficult in the present environment. Research on the influence of physiological, behavioural and environmental factors on energy balance is essential to the development and implementation of effective prevention and treatment programmes.

Despite significant recent advancements made towards unraveling the complexity of obesity, there still remain a large number of unanswered questions. The available data suggest that multiple systems regulate energy homeostasis. Familial clustering and the high concordance of body weight among monozygotic twins provide strong evidence for a genetic component to human obesity and a number of genes associated with human obesity have been identified. Research must advance knowledge within and across these domains in order to understand the interactions of known homeostatic mechanisms, recognize new ones, and place these into the context of the human-environment interface.

The obesity epidemic did not occur overnight. Obesity and overweight are chronic conditions. Overall there are a variety of factors that play a role in obesity. This makes it a complex health issue to address.

Overweight and obesity result from an energy imbalance. This involves eating too many calories and not getting enough physical activity. Body weight is the result of genes, metabolism, behavior, environment, culture, and socioeconomic status. Behavior and environment play a large role causing people to be overweight and obese. These are the greatest areas for prevention and treatment actions. Adapted from U.S. Surgeon General’s Call to Action to Prevent and Decrease Overweight and Obesity, 2001

Overweight and obesity are a result of energy imbalance over a long period of time. The cause of energy imbalance for each individual may be due to a combination of several factors. Individual behaviors, environmental factors, and genetics all contribute to the complexity of the obesity epidemic.

Details about energy balance: CDC

References...

Causes of obesity (WIN)
Understanding Obesity
Obesity: Nutritional disorder (Merck Manual)
Obesity: in the genes? (Dr. Claude Bouchard)
Weighing in On Obesity