Body Mass Index (BMI) is a simple index of weight-for-height that is commonly used to classify underweight, overweight and obesity in adults.1 It is defined as weight in kilograms divided by height in metres squared (kg/m2).
BMI does have its limitations. Factors such as muscle mass, ethnic origin and age can affect how well BMI reflects body fatness. However, we use it because, for most people, it correlates reasonably well with their level of body fat. It is also relatively easy to calculate, cheap and non-invasive.
Although BMI can give a fairly good indication of overall body fatness, it isn’t as good at telling us about someone’s fat distribution. For example, two people could have the same BMI but very different levels of visceral and subcutaneous fat.
Subcutaneous fat is the fat that we store under our skin. The fat we may be able to feel on our arms and legs is subcutaneous fat. This type of fat is not as harmful to our health as visceral fat.
Visceral fat is stored within the abdominal cavity around the internal organs such as the liver, pancreas and intestines.
Research has shown that visceral fat has a distinctive and potentially dangerous affect on our hormones. Storing higher amounts of visceral fat is associated with increased risks of a number of health problems including Type 2 diabetes.2
The best way to tell if someone is storing excess visceral fat is using an MRI scan (Magnetic Resonance Imaging scan). However, MRI scans are not cheap and therefore not used for this purpose in clinical practice.
A relatively good indicator of visceral fat is waist measurement. Taking someone’s waist measurement along with their BMI will give you a good indication of body composition. If someone has a healthy BMI but a large waist, they may benefit from making lifestyle changes.