Body Mass Index - Its Interpretation and Limitations

by Sharon Kirby - Date: 2006-12-15 - Word Count: 831 Share This!

What is Body Mass Index?

Body Mass Index (BMI) is a statistical measure of body fatness, involving a calculation based on weight and height. The range within which the BMI falls determines whether a person is of a suitable weight, which may otherwise lead to related health problems.

The BMI is a broad measure, and is not without its critics, but it is a straight forward, inexpensive method that can be used by women and men aged 20 and over to assess their weight in relation to the general population.

This is not suitable for pregnant and breastfeeding women. Also note that when interpreting BMI for children, age and gender need to be taken into account. In any case, BMI should not be used in isolation when assessing an individual's health status.

How to Calculate Body Mass Index

BMI can be calculated using metric units (kilograms and metres) or imperial units (pounds and inches). The metric system is more widely used.

Metric Calculation

Weight in kilograms is divided by height in metres squared.

Weight (kg) / height2 (m)

Here is an example:

Height = (1.58 m), Weight = 54 kg

Calculation: 54 (1.58)2 = 21.63

Imperial Calculation

Weight in pounds is divided by height in inches squared and multiplied by 703.

Weight (lb) / height2 (in) x 703

Here is an example:

Weight = 119 lbs, Height = 5'2" (62")

Calculation: (119 (62)2) x 703 = 21.76

Interpretation of Body Mass Index

The range within which a person's BMI falls will help determine whether they are of a healthy weight for their height. The ranges are as follows:

Starvation < 15
Underweight < 18.5
Normal weight 18.5 - 24.9
Overweight 25 - 29.9
Obesity 30 > 40
Morbidly obese 40 >

In the example given in the calculations above, the BMI value is between 21 and 22, which is normal for a person who weighs 54 kg at 1.58 m. This is an indicator of good health, and there is no need for a person who has a BMI within the normal range to lose or gain weight.

A BMI of 18.5 or under may indicate someone is deliberately restricting their food intake (or adopting other strategies) in order to achieve a desired level of thinness. This is unhealthy, and may lead to health problems such as malnutrition and osteoporosis, which are associated with the eating disorders anorexia and bulimia.

BMI is indeed one of the criteria stipulated by the World Health Organisation for a diagnosis of anorexia nervosa, whereby BMI must be 17.5 or less. In some cases the fashion industry is using BMI as an indicator of whether models are too thin for the catwalk.

A person with a BMI of 25 or over is probably overweight, which means they are at a greater risk of heart disease, osteoporosis, diabetes, high blood pressure, stroke, some cancers and other diseases. Steps should be taken to lose weight, including a healthier diet and regular exercise. There is the need to create a calorie deficit (burning more calories than are being consumed).

For someone with a BMI of 30 or over, advice from a health professional about how to lose weight may be beneficial.

Limitations of Body Mass Index

While BMI may be a useful tool in monitoring someone's health status, it has its limitations.

BMI measures total body weight, not the actual amount of fat a person is carrying. Some people are naturally stocky and have a BMI that is in the overweight category, when in fact their weight is due to muscle mass and a heavier bone structure rather than excess fat.

It is common for athletes such as rugby players and weight lifters to have a BMI indicating they are overweight when they are not. Their BMI is higher because of their extra muscle mass, not because of excess body fat (muscle weighs more than fat).

Conversely, some athletes will be underweight according to their BMI (such as long distance runners). This is due to low body fat and aerobic slow twitch muscle fibres, which develop naturally as a result of their particular sport.

Some people are thin-framed and their BMI will inaccurately determine them as being underweight. Those who have lost muscle mass (such as the elderly and infirm) may also appear to be underweight, when in fact it is normal to lose muscle mass and body fat in old age and as a result of poor health.

On certain occasions the way BMI is interpreted can be adjusted according to the person's lifestyle, natural build and health status.

If BMI falls within the normal range, this does not necessarily indicate someone is in perfect health, that they are getting enough exercise or eating a healthy range of foods. They may also be carrying more body fat than is good for them.

In addition to body weight, shape is also important. A person's BMI may be normal but their waist circumference may be too high. Being apple shaped carries more health risks (including a higher risk of heart disease) than being pear shaped -- regardless of a normal BMI.

It is important to be physically active and eat healthily, while limiting salt, sugar and alcohol, and not smoking, and BMI is not necessarily indicative of these things.

Related Tags: obesity, body, overweight, fat, anorexia, bulimia, weight, thin, index, height, mass, bmi, underweight, normal

If you would like to calculate your BMI easily and quickly, here is an online body mass index calculator.

Sharon Kirby is a freelance health writer who likes to write about exercise, fitness, nutrition and a multitude of other health issues. She also writes about eating disorders such as anorexia, bulimia and binge-eating disorder.

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