Body Mass Index (BMI)

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Body Mass Index (BMI) is a statistical measure of the weight of a person scaled according to height. It tells whether a person’s weight is appropriate for their height or not.

Before 1980, doctors generally used weight-for-height tables -- one for men and one for women -- that included ranges of body weights for each inch of height. These tables were limited because they were based on weight alone, rather than body composition. BMI became an international standard for obesity measurement in the 1980s. It was introduced to general public in late 1990s, when the government launched an initiative to encourage healthy eating and exercise.

Contents

Why should I be aware of this?

  • A BMI which is not in the normal range indicates that our weight is likely to cause serious health problems in the future. Some of these problems could be cancer, heart disease, diabetes and depression.
  • BMI is also useful to know if we have a weight problem or not. For example, though our clothes might fit us fine and we feel like we are eating quite healthily – for we might not be hogging crisps, canned drinks and chocolate. Or we might be looking at all the other people out there who are bigger and heavier than us and feel that it is not a cause of concern for us.
  • If you think you might have a weight problem, BMI is a good place to start.
  • BMI is not the best measure of weight and health risk. A person’s waist circumference is a better predictor of health risk than BMI.

All about BMI

Body mass index (BMI) is used to estimate the total amount of body fat. It is calculated by dividing a person's weight in kilograms by their height in metres squared (m2).

Although BMI is a good indicator of obesity, there are instances when a BMI figure may not be accurate.

BMI calculations will overestimate the amount of body fat for:

  • Body builders
  • Some high performance athletes
  • Pregnant women.

BMI calculations will underestimate the amount of body fat for:

  • The elderly
  • People with a physical disability who are unable to walk and may have muscle wasting.
  • BMI is also not an accurate indicator for people with eating disorders like anorexia or people with extreme obesity.

Classification and health risk for adults of Asian descent

  • You are underweight if your BMI is less than 18.5
  • You are of normal weight if your BMI is in between 18.5-22.9
  • You are overweight if your BMI is greater than 23
  • You are obese and liable to health problems if your BMI is greater than 25 and less than 30
  • You are very obese and are prone to severe health problems if your BMI is greater than 30.

Classification and health risk for adults of European and American descent

  • You are underweight if your BMI is less than 19
  • You are of normal weight if your BMI is in between 19 and 25
  • You are overweight if your BMI is greater than 25
  • You are obese and liable to health problems if your BMI is greater than 30 and 39
  • You are very obese and are prone to severe health problems if your BMI is greater than 40.

BMI for children

The healthy weight range for adults of a BMI of 20 to 25 is not a suitable measure for children.

For adults who have stopped growing, an increase in BMI is usually caused by an increase in body fat. But as children grow, their amount of body fat changes and so will their BMI. For example, BMI usually decreases during the preschool years and then increases into adulthood.

For this reason a BMI calculation for a child or an adolescent must be compared against age and gender percentile charts


Reference

  • Body Mass Index (BMI)
  • Your weight and BMI
  • Waist-to-hip ratio, body mass index, and subsequent kidney disease and death
  • Body Mass Index
  • BMI Chart for Women