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Body Mass Index (BMI)

 It is natural for your body to store energy as fat. However, too much or too little body fat can increase your risk of disease and illness, depending on where your body stores it.

The amount of fat stored in the body can be accurately measured with a dual energy absorptiometry (DXA) machine. This is known as having a DXA scan. A much cheaper alternative is to estimate your total body fat by calculating your body mass index (BMI).

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BMI is a useful indicator of health at the population level. However, when assessing disease risk, the distribution of fat in your body is more important than the amount. Because of this, your waist circumference is thought to be a better predictor of health risks than BMI. Increased abdominal obesity is associated with a higher risk of cardiovascular disease, type 2 diabetes, and cancer. Abdominal obesity is measured using waist circumference.

What is body Mass Index (BMI)?

Body mass index (BMI) is a rough measure of your ideal weight for health. It is calculated by dividing your weight in kilograms by the square of your height in meters (m2). BMI is intended for adults only, as children and adolescents are constantly growing. This makes it difficult to determine BMI cutoff values for young people. However, in stunted adults, increased BMI is usually due to increased body fat.

You can use the Body Mass Index (BMI) Calculator for Adults to calculate your BMI, as long as you know your:

  • Weight in kilograms (kg)
  • Height in meters (for example, if your height is 176 cm, it is equal to 1.76 meters)
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BMI classifies you as “underweight,” “healthy weight,” “overweight” or “obese” as defined by the World Health Organization. If your BMI is:

  • Less than 18.5 kg/m2: You are considered underweight and possibly malnourished
  • 5 to 24.9 kg/m2: You are in the healthy weight range for young and middle-aged adults.
  • 0 to 29.9 kg/m2: You are considered overweight
  • More than 30 kg/m2: You are considered obese.

BMI is not always the best measure of health

At the population level, BMI is used to indicate the level of risk of disease and mortality (death rate). Differences in BMI between adults of the same age and sex are usually due to body fat, however there are many exceptions to this rule, which is why the BMI figure may not always be accurate.

BMI calculations overestimate body fat for:

  • Body builders
  • Some high performance athletes
  • pregnant women


BMI calculations underestimate body fat for:

  • The elderly
  • People with physical disabilities, who are unable to walk and may suffer from muscle wasting.


BMI is also not an accurate indicator for people with:

  • Eating disorders such as anorexia nervosa
  • Severe obesity

Why is BMI not always the best measure of health?

In general, the more body fat you have, the greater your health risk. However, BMI cannot differentiate between body fat and muscle mass. This means that there are exceptions to the BMI guidelines:

  • Muscles: Bodybuilders and people with a lot of muscle mass have a high BMI, but are not overweight.
  • Physical disabilities: People who have physical disabilities and are unable to walk may experience muscle wasting. Their BMI may be slightly lower, but that does not necessarily mean they are underweight. In these cases, it is important to consult a nutritionist who can provide useful advice.
  • Height: BMI is not completely independent of height and tends to overestimate obesity in short people and underestimate it in taller people. Therefore, BMI should not be used as a guide for adults who are very short (less than 150 cm) or very tall (more than 190 cm).
  • People from different ethnic groups: for example, Asians and Indians have more body fat in every body mass index compared to people of European descent. Therefore, reductions in overweight and obesity may be smaller for these populations. This is because the increased risk of diabetes and cardiovascular disease begins with a BMI of less than 23 kg/m2 in Asian populations. Some populations have similar risks of higher BMI, such as people of Torres Strait Islander and Maori descent.
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What is a healthy BMI range for children?

BMI calculations used for adults are not a good measure of weight for children or adolescents. BMI calculations for children and adolescents are interpreted differently from adults and take into account the age and gender of the child or adolescent.

Current BMI charts for children are provided by the US Centers for Disease Control and Prevention. These charts are useful for assessing overweight and obesity in children over two years of age. To calculate a child’s BMI, you can use the BMI calculator for children and adolescents.

However, BMI charts should only be used as a guide to indicate when to make small lifestyle changes, and when to seek further guidance from a doctor or nutritionist.

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Risks of being overweight (high BMI) and physical inactivity

If you are overweight (with a BMI of more than 25 kg/m2) and physically inactive, you may have:

  • Cardiovascular disease (heart and blood circulation)
  • Gallbladder disease
  • High blood pressure
  • Type 2 diabetes
  • Arthritis
  • Certain types of cancer, such as colon and breast cancer
  • Depression and other mental health disorders

Risks of being underweight (low BMI)

Risks of being underweight (low BMI) If you are underweight (BMI less than 18.5 kg/m2), you may be malnourished and have:

  • Safety performance is compromised
  • Respiratory disease
  • Digestive diseases
  • Cancer
  • Osteoporosis

A person’s waist size is a better predictor of health risks than BMI. When identifying health risk in adults, it is recommended to combine your BMI classification with your waist circumference as a measure of disease risk. Having fat around your belly, regardless of your body size, means you’re more likely to develop certain obesity-related diseases.

Fat that is mainly deposited around the hips does not seem to carry the same health risk. Men in particular often deposit their weight in the waist area and thus increase the risk of obesity-related diseases.

Studies have shown that body fat distribution is associated with increased prevalence of diabetes, high blood pressure, high cholesterol, and cardiovascular disease. In general, the links between health risks and body fat distribution are:

  • Least risk: thin (uniform body fat)
  • Moderate risk: overweight without pot belly
  • Moderate to high risk: thin with a big belly
  • High risk: overweight with excess abdominal fat Waist circumference and health risks

Waist circumference can be used to indicate health risk for chronic diseases.

for men:

  • 94 cm or more: increased risk
  • 102 cm or more: significantly increased risk

For women:

  • 80 cm or more: increased risk
  • 88 cm or more: the risk is significantly increased

Physical activity, avoiding smoking, and eating unsaturated fats instead of saturated fats have been shown to reduce the risk of abdominal obesity.

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