The body mass index (BMI) is one method used to estimate your total body fat and risk of developing weight-related diseases such as type 2 diabetes and cardiovascular disease. BMI uses weight and height to determine if an adult is within the healthy weight range, underweight, overweight or obese.It is calculated by dividing your weight (in kilograms) by the square of your height (in metres): BMI = Weight (kg)/Height (m)2

Differences in BMI between people of the same age and sex are usually due to body fat. However, there are exceptions to this rule, which means a BMI figure may not be accurate, which is why the use of it has led to much confusion and misinformation. The BMI can’t distinguish between fat and muscle (which tends to be heavier) and can place more muscular individuals into the overweight category, even if their fat levels are low.

Therefore, BMI is less accurate for assessing healthy weight in certain population groups, including:

  • some ethnic groups such as Pacific Islander populations (including Torres Strait Islander peoples and Maori), Indigenous Australians, South Asian, Chinese and Japanese
  • body builders or weight lifters
  • athletes
  • pregnant women
  • the elderly
  • people with a physical disability
  • people with eating disorders e.g. anorexia nervosa
  • people under 18 years
  • the morbidly obese

Another reason why the BMI has caused some controversy is because it does not take into consideration, where the body stores fat. Abdominal fat for example, known as visceral fat, is more harmful than subcutaneous fat that simply lies under the skin as it as it coats the internal organs and increase risk of chronic disease.

In this case, a person’s waist circumference is thought to be a better measurement and predictor of health risk than BMI.

In addition to the waist circumference, there are better ways to measure body fat that provide more useful readings on how likely a person’s weight will contribute to future chronic health issues. These include CT and DEXA scans, and MRIs, which can provide a clearer overview of your body composition by separating the fat mass from muscle, for example. However, these are expensive and more labourious for a health practitioner, which is why the BMI is still used. It’s a quick, cost effective and easy method to provide a “reasonable measure” of body fat for some people.

The use of the BMI in children, adolescents and teenagers is controversial because they are constantly growing until they reach adulthood. Therefore, it’s difficult to have set values for BMI cut-offs. It is very common for teens to gain weight quickly during puberty and as a result see their BMI go up. It is normal for children to have different amounts of body fat at different ages, and will differ between genders. Therefore, it’s important to take these factors into consideration when determining healthy weight categories for these age groups.

It’s also known that adolescents and teenagers experience growth spurts at different times and different ages. Therefore, BMI fluctuations will continuously occur during these stages of life, which can be detrimental to the mental health of a child and/or teenager if they are using this as a sole measure to compare body types with their classmates and friends.