Obesity is defined as an excess accumulation of body fat. This excess accumulation is the result of a positive energy balance where caloric intake exceeds caloric expenditure. Recent evidence suggests that American adult men and women today consume, respectively, 116 and 112 more calories per day than in the mid 1970's. 
In a study of temporal trends in energy intake in the United States between 1970 and 1998, Harnack et al. concluded that consistent with trends in overweight, most ecologic data suggests that energy intake has increased and is a likely contributor to the observed increases in average bodyweight.  Since the caloric value of fat is known (~3,500 calories), this translates into a potential weight gain of one pound approximately every 31 days or 11–12 lbs per year. Obviously, the “fattening of America” hasn't occurred at this astounding rate, suggesting that energy outputs are also slightly greater due to larger body mass or an increase in physical activity and thus energy expenditure. Since physical activity rates over the past three decades are essentially unchanged or lower, it would seem that increased body mass has limited the weight gain due to increased energy output. 
Thus, overweight and obesity have increased dramatically, particularly since the 1980's, throughout the world. This trend prompted the World Health Organization (WHO) in 1998 to recognize a “global epidemic of obesity.” 
In 1998, the National Institutes of Health (NIH) published guidelines for the identification, evaluation, and treatment of overweight and obesity in adults.  Body mass index (BMI), expressed as a weight/height ratio ( kg/m2 ), is often used to distinguish between overweight (BMI is between 25 and 29.9) and obesity (BMI greater than or equal to 30). Although BMI is a widely-used estimate of obesity, unfortunately it underreports obesity (Rahman, 2010; Shah, 2012). Obesity is further subdivided into three classes based on increasing disease risk: 
|Obesity Class||BMI (kg / m)|
People with normal BMI but high body fat are at risk for cardiovascular disease.
It also is important to consider the distribution of body fat. Excess body fat in the abdomen is an independent predictor of the presence of risk factors such as diabetes mellitus (DM), high blood pressure, and abnormal blood lipids.  The NIH document recognized that, particularly in men and women with BMI's between 25–34.9, a waist circumference of > 40 and > 35 inches in men and women, respectively, further increases the risk of morbidity and mortality. Finally, studies support the importance of considering percentage of body fat or the ratio of fat to lean body mass, particularly in relationship to risk of obesity-related diseases.