Articles often refer to BMI or data about people being in a healthy weight, overweight or obese. I think it’s important to understand what defines these terms so that the meaning of what is being read can be interpreted and understood. So despite the perhaps mundane topic of conversation, I thought it appropriate and necessary to provide a few definitions:
Body Mass Index (BMI)
BMI is used to classify underweight, healthy weight, overweight and obesity in adults to be used as a predictor of health risk of developing chronic diseases e.g. type 2 diabetes. There is much debate about BMI but I think it is important to remember that it is designed as a tool to classify health risk. What weight range an individual should aim for depends on their circumstances for example, weight history (weight changes over the years) and most importantly what is realistic for them. Nevertheless, when you next read something that refers to underweight, healthy weight, overweight and obese, it is likely referring to the BMI classification and you how know what it means.
How is BMI calculated?
The weight (in kilograms) of an individual is divided by their height (in metres) squared. In otherwords:
BMI = kg/m2
To have your BMI calculated for you, you can visit the Heart Foundation.
What are the BMI classifications?
< 18.5 = underweight
18.5 to 24.9 = healthy weight
25 to 29.9 = overweight
≥ 30 = obese
When looking at your BMI, I think it is important to consider the context of who you are. For example, there are two men of the same height and weight which is equivalent to a BMI of 25 (overweight) however they are of different ages: 25 and 60 years. Pardon the use of stereotypes but let’s say that the 25 year old is an avid gym goer and the 60 year old goes for a short walk on the weekend. The 25 year old will have considerably more muscle mass than the 60 year old who will have considerably more fat mass. Their associated health risks will be quite different due to differences such as age and body composition.
Limitations (of which there are a few):
- The above BMI ranges are not suitable for people such as those from an Asian or Polynesian background
- A higher BMI range may be suitable for people over 70 years
- It is not suitable for pregnant and lactating women or those under 18 years
- It doesn’t reflect different genders or distribution of fat and muscle mass
Therefore it should be used with caution on individuals that fit into the above categories for example, muscular individuals, body builders and high performance athletes.
Waist circumference is also used as a measure of health risk as it indicates fat distribution. Fat distribution around the waist indicates a higher amount of visceral fat (fat that surrounds organs), which is associated with increased health risk. This is why is it is better for individuals to be ‘pear shaped’ (more fat stored around hips) opposed to ‘apple shaped’ (more fat stored around waist).
- Women: ≥80 cm = increased risk; ≥88 cm = high risk
- Men: ≥94 cm = increased risk; ≥102 cm = high risk
Similarly to BMI, the above cut offs may not be suitable for people from different ethnic backgrounds and should only be used for adults.
You can visit Measure Up for further information.
Source: Adapted from the Australian Dietary Guidelines