Understanding Your BMI and Body Composition

What Is BMI?

Body Mass Index (BMI) is a simple calculation that uses your height and weight to screen for weight-related health risk. It is widely used in clinical practice because it is quick to calculate, reproducible, and correlates broadly with health outcomes at a population level.

BMI formula:

BMI = weight (kg) ÷ height² (m²)

Example: A person who is 170 cm tall and weighs 90 kg has a BMI of: 90 ÷ (1.70 × 1.70) = 31.1

BMI Categories (Caucasian Populations)

• Below 18.5 — Underweight
• 18.5 to 24.9 — Healthy weight
• 25.0 to 29.9 — Overweight
• 30.0 to 34.9 — Obesity Class I
• 35.0 to 39.9 — Obesity Class II
• 40.0 and above — Obesity Class III (severe obesity)

Note for Asian and Asian-Australian populations: Health risks occur at lower BMI values. The recommended thresholds are: overweight ≥ 23 kg/m², and obesity ≥ 27.5 kg/m². This reflects the higher metabolic risk associated with excess visceral fat at a given BMI in people of Asian descent.

The Limitations of BMI

BMI is a useful screening tool but has significant limitations that Dr Chong accounts for in individual assessments.

It does not distinguish between muscle and fat. A very muscular person may have a BMI in the overweight or obese range but have low body fat and excellent health. Conversely, a person with a "normal" BMI may carry excess body fat — particularly visceral (abdominal) fat — with significant metabolic risk. This is sometimes called "normal weight obesity."

It does not reflect where fat is carried. Where fat is distributed in the body is at least as important as how much there is. Abdominal fat (visceral fat, surrounding the internal organs) is metabolically active and directly drives insulin resistance, inflammation, and cardiovascular disease. Fat carried predominantly on the hips and thighs carries considerably less metabolic risk.

It does not account for age, sex, or ethnicity — all of which significantly affect the relationship between BMI and body composition.

Waist Circumference: A Better Predictor of Metabolic Risk

Waist circumference directly measures abdominal fat and is a stronger predictor of type 2 diabetes, cardiovascular disease, and metabolic syndrome than BMI alone.

High-risk thresholds (Caucasian populations):

• Men: ≥ 94 cm (increased risk), ≥ 102 cm (substantially increased risk)
• Women: ≥ 80 cm (increased risk), ≥ 88 cm (substantially increased risk)

How to measure correctly:

Stand relaxed. Place a tape measure at the level of your belly button (navel), parallel to the floor. Take the measurement at the end of a normal breath out — do not suck in or breathe out forcefully.

Body Composition Analysis

At Medi Weight Loss, Bayswater, Dr Chong offers medical-grade body composition scanning — an advanced bioelectrical impedance analysis (BIA) assessment that provides a comprehensive breakdown of body composition in approximately 60 seconds.

What the scan measures:

• Total body fat percentage
• Visceral fat level — fat around the internal organs (the most metabolically dangerous type)
• Skeletal muscle mass and distribution
• Bone mineral content
• Body water (intracellular and extracellular fluid)
• Basal metabolic rate — the number of calories your body burns at rest
• Segmental analysis — fat and muscle mass in each arm, leg, and the trunk separately

Why body composition data matters:

A patient may lose significant body fat while simultaneously gaining muscle mass through exercise. Their total body weight may change very little — or not at all — but their health is improving substantially. Without body composition data, this progress is invisible.

Similarly, identifying high visceral fat is critical because it is a key driver of type 2 diabetes, cardiovascular disease, fatty liver disease, and systemic inflammation — even in individuals with a normal BMI and body weight.

Healthy Body Fat Percentage Ranges

Body fat percentage varies with age and sex. The following are approximate healthy ranges:

Men:

• 20–39 years: 8–19%
• 40–59 years: 11–21%
• 60 years and over: 13–24%

Women:

• 20–39 years: 21–32%
• 40–59 years: 23–33%
• 60 years and over: 24–35%

Body fat consistently above these ranges — particularly when combined with elevated visceral fat — is associated with significantly increased risk of metabolic disease, regardless of BMI.