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Body Mass Index (BMI) and its indication of health status

Hadyn Luke posted this on Friday 21st of September 2012 Hadyn Luke 21/09/2012

Tags: Nutrition weight management


Body Mass Index 

Body Mass Index  (BMI) is a formula based on height and weight that allows a personal trainer to calculate an individual’s body mass. It’s a good way of establishing whether a client is underweight, ideal weight, overweight or obese.

To calculate body mass index, the fitness instructor should first measure the client’s height in metres and weight in kilogrammes. The client’s weight is then divided by the client’s height squared.

For example, a client who is 1.8m tall and weighs 80kg:

  • 1.8 x 1.8  = 3.24
  • 80kg ÷ 3.24  = 24.7

This result should then be cross-referenced to a BMI table. 

  • Below 18.5  = underweight
  • Between 18.5 and 25  = ideal
  • Between 25 and 30  = overweight
  • From 30 to 35  = category 1 obese
  • From 35 to 40  = category 2 obese
  • 40+  = morbidly obese

The pros of using the BMI index

The benefits of a personal trainer using the BMI index to calculate body mass is that, for the general population  (excepting those who are overweight because of a specific illness), it’s a good way to calculate whether people are carrying excess body fat – and it’s both quick and accurate.

The cons of using the BMI index

The downside is that if somebody is carrying a lot of muscle mass but has low body fat, for example a professional rugby player, the BMI index will suggest that they are overweight or even obese, when this is not the case. So a BMI rate is not a true indicator of body fat, nor does it tell you where the body mass is being held. For example, an individual with android obesity has excessive body fat around the internal organs, which is linked to conditions such as diabetes, heart disease and some cancers.

 Waist circumference

To help counteract these false indications, a fitness instructor could also measure waist circumference. The benefit of doing this is that rather than only measuring overall body mass, the personal trainer can establish whether the client is carrying excess visceral fat around their major organs and is therefore at a higher risk from the various illnesses linked with obesity.

Factors to be aware of when measuring BMI

When measuring body weight, personal trainers should ask their clients to remove their shoes and any heavy clothing, such as jackets. They should also ensure they aren’t carrying anything heavy in their pockets.
For the height measurement, a client should again remove their shoes and the personal trainer should ensure they flatten down the hair to measure close to the head for the most accurate results.
The waist should be measured at the narrowest point, above the umbilicus  (belly button). For consistency, the client should be wearing a top of similar thickness each time their waist is measured.

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