As just indicated, obesity is linked closely to NASH. Therefore, to get a grasp on the frequency of NASH and the impact of obesity on NASH in the population, one has to understand how obesity is defined. A calculation of the body mass index (BMI) is a method that can be used to determine degrees of obesity.

The BMI is calculated by dividing a person's weight in kilograms by his or her height in meters squared (kg/m2). In the non-metric system, BMI = (lbs/inches2) x 703. In adults, normal weight is defined as a BMI between 20 and 25, overweight from 25 to 30, obesity from 30 to 35, significant obesity from 35 to 40, morbid obesity from 40 to 45, super obesity from 45 to 50, and super-morbid obesity greater than 50.

In addition to the BMI method, obesity can be described according to the distribution of body fat. Fat can be distributed predominantly either to the hips (gynoid or pear-shaped) or to the abdomen (central, android, or apple-shaped). An abdominal predominance of fat is the most commonly observed type of obesity in insulin resistance. Abdominal obesity is defined in men by a waist-to-hip circumference ratio greater than 1.0 or a waist circumference greater than 40 inches (102 centimeters). In women, abdominal obesity is defined by a waist-to-hip ratio greater than 0.8 or a waist circumference greater than 35 inches (88 centimeters). In fact, simple waist measurements alone seem to be the best predictor of the type of body fat distribution that is most closely associated with insulin resistance.

The normal limits that have been established for the BMI and waist circumference are not based on considerations of appearance. Rather, they are based on the significant risk of developing complications due to being overweight (BMI-related morbidity), such as coronary artery heart disease and diabetes. In children, however, normal ranges of the BMI vary according to age and gender. Furthermore, up until the age of 16, the upper limits for the normal BMI are lower than 25. Therefore, in children, the degree of overweight is more often expressed as a percentage above ideal body weight.