Body Mass Index is an equation used by specialists to determine whether or not a person lies within a ‘healthy’ weight range.

It is a very common method of evaluating individual people to see if they are under or over weight. The method involves comparing a subject’s weight to their height. By dividing the weight measurement (expressed in kilograms or lbs) by the square of the height (expressed in meters or inches) results in the BMI. A BMI of below 18.5 is considered underweight, between 18.5 and 25 is an indication of a healthy weight, 25 to 30 is considered overweight, a BMI of over 30 is referred to as obese, over 35 is known as morbid obesity, and over 40 indicates extreme obesity.

The BMI test does however have its limitations, so is only used as a general guideline for assessing healthy weight.

There are many cases where BMI would not be suitable, such as the following:

• Pregnant women.
• Amputees
• Athletes
• Babies
• The elderly

The BMI equation is:
Weight divided by (height x height)

Specialists who deal with these particular groups of people will rarely use the BMI test to assess the condition of their clients for the following reasons…

• A heavily pregnant woman would be heavier than usual due to the weight of her unborn baby and the fluid sac that surrounds the baby for protection.

• An amputee would often be much lighter than usual, due to the weight of the missing limb being subtracted from their body weight. This would produce figures that would suggest the patient was severely underweight.

• Athletes tend to be very muscular, and muscle is a heavier tissue than fat. For this reason, an athlete can be very heavy compared to their height, but in peak fitness. If this was not taken into account, many athletes would appear obese on the BMI scale.

• Babies carry a lot of fat, but this fat is naturally present in babies for warmth and nourishment and is not a cause for health concerns.

• Elderly people often suffer from muscle and bone wastage, which can make them very light. If this is misinterpreted it could lead the practitioner to believe the person was suffering from ill health, when in reality nature has simply taken its course.

So how are these groups measured if the BMI scale is not accurate?

There are various alternative means for assessing the physical condition of these groups, such as body fat testing, waist to hip ratio, circumference measurement of the body and measurement of bone thickness.

This is a specialist subject in its own right, and is known as Kinanthropometry, which is the science of measuring the body.

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