Body Mass Index (BMI) Explained

Monday 15th March 2010 10:39am

bmi The Body Mass Index (BMI) is a statistical measure that calculates the ratio of a person’s weight and height.

Beware of statements like” Body Mass Index is used to estimate your total amount of fat”. As you can see below that is simply not true.

BMI is defined as the individual's body weight divided by the square of his or her height. The formulae universally used to produce a unit of measure of kilograms per height in metres squared. You can calculate it or read it from a number of readily available charts.

BMI = Mass (kg) / Height (m)²

The resulting BMI is then used to determine statistically where you are at:

Underweight - Less than 19
Acceptable - 20 to 25
Over weight - 26 to 30
Obese - 31+

Because of its simplicity the BMI has become a common tool to determine whether someone is underweight or overweight. Within this simplicity there are also some limitations:

BMI does not distinguish between weight due to muscle and weight due to fat, so it does not take into account differences in body composition.
BMI measures are not suitable for Aboriginal and Torres Strait Islander peoples, whose healthy BMI range may be lower than that for Australians of European descent.
For people of Asian descent the cut-off points for health risks appear to be lower than for other Australians.

BMI is however a good starting point for anyone that might consider themselves either underweight or overweight because it is estimating your total mass against your height and comparing it statistically to the overall population.

BMI is used as a screening tool to identify possible weight problems for adults. However, BMI is not a diagnostic tool. For example, a person may have a high BMI but in order to determine if excess weight is a health risk, further assessments would need to be performed. These might include skin fold thickness measurements, evaluations of diet, physical activity, family history etc.

It is also important to remember that BMI is only one factor related to risk for disease. Two other predictors are also important:

The individual's waist circumference - abdominal fat is a predictor of risk for obesity-related diseases.
Other risk factors the individual has for diseases and conditions associated with obesity eg high blood pressure or physical inactivity.

So what does it all mean for you?

Calculate your BMI and determine where you are at statistically.
If you are considered underweight or obese then consider consulting your healthcare practitioner for further assessment.
If you are considered overweight then re-consider your lifestyle and consider consulting you healthcare practitioner if you are physically inactive or suffering other health issues.

This information is not intended as a substitute for qualified medical advice. Please consult a properly qualified health care practitioner.