Clinical Public Health Summit: Physician Assistants: Overview

Obesity refers to an excess of fat. However, the methods used to directly measure body fat are not available in daily practice. For this reason, obesity usually is assessed by the relationship between weight and height (ie, anthropometrics), which provides an estimate of body fat that is sufficiently accurate for clinical purposes (Perreault, Mar 2020; Kilsh & Skelton, Oct 2020).

Body mass index (BMI) is the accepted standard measure of overweight and obesity for adults and children two years of age and older. BMI provides a better estimate of total body fat compared with body weight alone. Other indices of childhood obesity, including weight-for-height (which is particularly useful for the child younger than two years), measures of regional fat distribution (eg, waist circumference and waist-to-hip ratio), and the growth standards developed by the World Health Organization (WHO), are discussed separately (Perreault, Mar 2020; Kilsh & Skelton, Oct 2020).

Calculation — The BMI is the most practical first step for evaluating the degree of excess weight. It is calculated from the weight and square of the height as follows:

 BMI  =  body weight (in kg)  ÷  height (in meters) squared

Adult BMI-based classifications — Body mass index (BMI) classifications are based upon risk of cardiovascular disease (CVD) [21]. The recommended classifications for BMI adopted by the National Institutes of Health (NIH) for Caucasian, Hispanic, and black individuals are:

• Underweight – <18.5 kg/m²
• Normal weight – ≥18.5 to 24.9 kg/m²
• Overweight – ≥25.0 to 29.9 kg/m²
• Obesity – ≥30 kg/m²
  • Class I – 30.0 to 34.9 kg/m²
  • Class II – 35.0 to 39.9 kg/m²
  • Class III – ≥40 kg/m² (also referred to as severe, extreme, or massive obesity)

Note: Because these cutoffs underestimate risk in the Asian and South Asian population, the WHO and NIH guidelines for Asians define overweight as a BMI between 23 and 24.9 kg/m² and obesity as a BMI >25 kg/m².

For children between 2 and 20 years of age, the following weight categories are used

• Overweight – BMI between the 85^th and 95^th percentile for age and sex.
• Obesity – BMI ≥95th percentile for age and sex.
• Severe obesity – BMI ≥120 percent of the 95^th percentile values or a BMI ≥35. This corresponds to approximately the 99^th percentile or BMI Z-score ≥2.33.

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