Obesity results from an imbalance between caloric consumption and caloric expenditure over a prolonged period. Weight gain occurs when there is a greater consumption of calories than expenditure. The expenditure of calories is complex and results from likely combinations of metabolic, genetic, and individual factors. Genetics and environment may predispose to weight gain, but it is only the consumption of calories in excess of utilization that can cause weight gain.
In the developed world, there is easy access to large quantities of low-cost, high-calorie, high-fat foods. Over the last 20–30 years portion size has also dramatically increased. The combination of increased availability to nutrients and sedentary lifestyle has resulted in decreased energy expenditure with increased caloric intake. This combination is likely the main reason for the dramatic increase in obesity occurring worldwide. Additionally, medical care has advanced so that the average life span is rising despite the age-associated increase in body fat and obesity-related disease.
Although energy imbalance is the underlying reason for the obesity epidemic, there are some medical conditions, genetic diseases, and medications that may cause the imbalance to occur or worsen. Problems such as hypothyroidism (inadequate production of thyroid hormone) and Cushing’s syndrome (an excess of endogenous steroid production) are examples of medical problems that can be associated with weight gain. Examples of genetic diseases associated with obesity (obesity genetic) include leptin deficiency (a hormone involved in the energy balance and discussed in detail later) and Prader-Willi syndrome (a complex disorder resulting in decreased muscle tone, short stature, mental retardation, behavioral problems, and excessive eating resulting in obesity).
Some examples of medications that are associated with weight gain include antipsychotic medications (especially newer generation medications) and steroids (chronic use), which may be given for a variety of inflammatory conditions. However, the aforementioned factors are rarely the primary cause of obesity and combined do not significantly contribute to the obesity epidemic.
