Differential Diagnosis
Body mass index, or BMI, which is calculated by dividing weight in kilograms by height in meters squared, is the standard for determining obesity. A BMI under 25 is considered normal, with no weight-associated health risk. A BMI of 25-30 is considered overweight, with low to moderate health risk. Obesity is diagnosed by a BMI over 30, with high associated health risk. A BMI over 35 has a very high risk of weight-related health problems, and morbidly obese individuals (a BMI greater than 40) are at extremely high risk, with mortality rates up to 90% greater than those for normal-weight individuals.

In assessing obesity, the clinician must rule out medical illnesses such as hypothyroidism.

Prognosis
Although the prognosis for short-term weight loss has improved with the advent of new dieting and exercise strategies and the development of behavior modification programs, the long-term prognosis for losing excess weight and keeping it off remains poor, with few patients losing more than 40 pounds and most regaining the weight they lose. It is estimated that if an obese child does not achieve nearly normal weight by the end of adolescence, the odds against doing so later are 28:1. Morbidity and mortality rates for obese individuals increase in direct proportion to increases in the BMI.