Obesity class – Anthropometric weight classification

Obesity class or anthropometric weight classification (measurements of proportions or elements of the human body) is used to categorise variables of weight status relative to a pre-defined healthy or ‘normal’ range. Body mass index and waist circumference are two simple measures that are commonly used in the clinical and research settings.

Body Mass Index (BMI)

This is the most commonly used weight classification derived from height and weight measures. Body mass index is calculated by dividing a person’s weight (in kilograms) by their height (in meters) squared.

From this calculation, body mass index range classifications have been determined.

<img src="https://ugc.futurelearn.com/uploads/assets/11/8a/118a5baf-ec33-402b-b3f6-e2a8d9e6581e.JPG" alt="Underweight =40. Obese = Class I obese, Class II obese, Class III obese. Overweight = Overweight but not obese, Class I obese, Class II obese, Class III obese”>

_{Figure: Classifications of overweight and obesity based on body mass index from the Australian Institute of Health and Welfare.²}

However, the clinical limitations of body mass index should be understood. Body mass index is a surrogate measure of body fat because it is a measure of excess weight rather than excess body fat. Factors such as age, sex, ethnicity, and muscle mass can influence the relationship between body mass index and body fat.

For example, consider this scenario:

• A person who does no exercise, is 1.83 meters and weighs 92 kg would have a BMI of 27.
• An Olympic athlete, 1.83 meters and weighs 96 kilograms would have a BMI of 28.

According to body mass index, the athlete is more “overweight” than the person who does no exercise. However, this is clearly not true as muscle weighs about 18 percent more than fat.

Body mass index also does not provide an indication of the distribution of fat, which we now know is also important in influencing health risk. In particular, fat that is distributed mainly around the trunk and upper body is strongly associated with cardiometabolic risk factors.