Leo Sher, M.D.

The ideal body weight is usually described as a weight that is assumed to be maximally healthful for an individual, based mostly on height but modified by factors such as gender, age, build, and degree of muscular development. Ideal body weight is a definition originally introduced by life-insurance companies to describe the weight statistically associated with maximum life expectancy. The concept of the ideal body weight is related to the concept of the body mass index (BMI), a measure of body fat based on height and weight (bodyweight [kg]/height² [m]). A formula for body mass was described in 1869 by Lambert Adolphe Jacques Quetelet (1796-1874), a Belgian scientist.

The body mass index is widely used to grade obesity. The body mass index is categorized as:
18.5–24.9 = Healthy weight,
25.0–29.9 = Overweight,
30.0–34.9 = Class I obesity,
35.0–39.9 = Class II obesity,
>40.0 = Class III obesity.
A patient with a BMI greater than 50 is considered to be super morbidly obese, and super-super-morbidly obese if greater than 60.

Obesity increases the risk of many disorders, including diabetes mellitus, hypertension, coronary heart disease, cholelithiasis, cancer of the endometrium, colon and kidney, stroke, infertility and joint conditions, e.g., osteoarthritis. It has been proposed that there are bidirectional associations of obesity and depression. Relative risk increases at a BMI of 25 for some diseases, and at a BMI of 30 for others. It has been suggested that in older people, an increased BMI is not associated with elevated mortality.

Adjusted body weight has been proposed as one method to improve the accuracy of predictive equations when calculating calorie expenditure of obese individuals. It is used to determine energy needs in hospitalized patients.

Severe underweight is defined as a BMI of <5th percentile. Disorders associated with severe underweight include lung conditions, including bronchial asthma, intestinal disease including coeliac and Crohn’s disease, and neurosis. Abnormal BMI, either too high or too low, remains a ‘disease of civilization.’