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Sharon Curtain

Sharon Curtain

Accredited Practising Dietitian
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What is Body Mass Index?

Guides — Posted 26/08/15

BMI is a simple tool that gives you a snapshot of how healthy your weight is in relation to your height.

Two adults who are exactly the same height are unlikely to weigh the same amount. This is because body weight depends on factors such as bone structure (small, medium or larger frame), and the proportion of body fat compared to denser muscle.

Body Mass Index (BMI) assesses your weight in relation to your height and can be a useful indicator of whether your weight is above, below or within a healthy range.

What do the results mean? 

The World Health Organization classifies BMI results as follows:

• Below 18.5 – Underweight

• 18.5 – 24.9 – Healthy weight

• 25.0 – 29.9 – Overweight

• 30.0 and above – Obese

If your BMI falls between 18.5 and 24.9, it indicates you are within the healthy weight range. If your BMI is greater than 25, then you fall into the overweight category. As weight increases above a healthy range, the increased body fat levels are associated with a higher risk of developing health problems such as high blood pressure, high LDL cholesterol levels, type 2 diabetes, coronary heart disease, stroke and some cancers. A BMI below 18.5 is classified as underweight and if the low body weight is a result of restricted food intake, an inadequate nutrient intake is probable.

Is it always accurate?

It’s important to understand that BMI is only a guide for healthy weight ranges and health risk in adults – it doesn’t always tell the whole story. People who have higher proportions of muscle, such as athletes and body builders, may be incorrectly classified as overweight. Similarly, people with small frames and lower amounts of muscle mass, including the elderly and some ethnic groups, may be classified as being a healthy weight, despite carrying excess weight for their body type.

BMI can be a much better indicator of health status when paired with the Waist Hip Ratio, which considers where the body fat is distributed.


References and more information

World Health Organization BMI Classification

Dietitians Association of Australia

Centers for Disease Control and Prevention

Extra   Learn more about the Australian Dietary Guidelines and get weight management tips here.
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