Obesity: Obesity is an excess of body fat - frequently resulting in significant impairment of health - the excess fat accumulation is associated with increased fat cell size; and in individuals with extreme obesity, fat cell numbers are also increased. The underlying mechanisms of obesity are not fully understood, but the net effect leads to an imbalance between energy intake and expenditure. Genetic and environmental factors are likely to be involved in the pathogenesis of obesity. This includes excess calorie intake, decreased physical activity, metabolic and endocrine abnormalities.   Obesity and Disease: Obesity is directly linked to many chronic diseases such as heart disease, type II diabetes, hypertension, stroke, certain cancers, and osteoarthritis. The psychosocial effects of obesity can range from lowered self esteem to clinical depression. Obese individuals may suffer job discrimination and other forms of social stigmatization.