This stands for ‘Body Mass Index’ and is the most widely used form of measurement for obesity. This calculation is based upon a person’s height and weight in which one is divided by another. For example: body mass index = weight (kg) divided by height (metres)

This measurement is used to determine if someone is underweight, a healthy weight, overweight or obese.

BMI categories

A body mass index (BMI) measurement is assessed against the following categories:

  • Less than 16.5: this is described as severely underweight
  • 16.5 to 18.4: classed as underweight
  • 18.5 to 24.9: classed as a healthy weight
  • 25 to 29.9: classed as overweight
  • 30 to 34.9: classed as obese (class 1)
  • 35 to 39.9: classed as obese (class 2)
  • 40 and above: classed as obese (class 3)

(Source: adapted from the World Health Organisation BMI classification table)

In regard to the obese categories: a measurement of 30 upwards is often classed as severe obesity. A measurement of 40 and above is classed as morbidly obese.

A measurement of 50 and above (yes, there are BMI measurements for that) is classed as ‘super obese’.

To use the Alli weight loss programme you need to have a BMI of 28 or higher. BMI is used by many doctors and weight loss clinics due to its ease of use. However it is not perfect.

Problems with BMI

The main problem with BMI is that of distinguishing fat from muscle. This calculation does not take the amount of muscle mass a person has into account. So, you could have very low body fat levels and a high degree of muscle mass and be classed as overweight or obese. This is particularly problematic for athletes and bodybuilders who tend to have greater amounts of muscle mass and low body fat levels compared to other people. It is not the most accurate form of measurement for these people. But athletes and bodybuilders are not the only group who find the BMI calculation less than accurate. Pregnant women, those who are breastfeeding or the elderly are also likely to obtain an inaccurate measurement. You also have the situation in which someone is very thin (or skinny) but otherwise, is a healthy individual. According to the BMI calculator they are classed as underweight which means that they need to gain weight. But, if that person is fit and healthy with no obvious signs of discomfort then is this the right thing to do? So, the BMI calculation is a useful one but there are other ways of determining obesity which include:

  • Height-weight ratio
  • Waist circumference
  • Measuring body fat using callipers or other tools

Height-weight ratio This involves taking a measurement of a person’s skeleton (frame) size, e.g. small, medium or large and using this with height/weight charts to determine if someone is overweight or not.

Waist circumference The size of a person’s waist can indicate if they have too much visceral fat which increases their risk of heart disease, stroke or diabetes. For example, a waist measurement of 37 inches or more in men increases their risk of health problems such as heart attacks or strokes. A waist measurement of 32 inches or more in women increases their risk of the same health problems. The biggest risks occur in men with a waist of 40 inches or more and women with a waist of 35 inches or more. To obtain a waist measurement: take a tape measure and wrap it around your waist. Breathe out and note down your waist size.

Body fat measurement with callipers etc

Body fat can be measured using callipers which are specially designed for that purpose. These callipers are often used by fitness instructors and personal trainers and measure skin folds at certain points on the body.

These include the back of the arms (triceps), below the shoulder blades, above the hip bone and the front of the arms (bicep). These skin folds may differ between men and women. On average, men have between 15 to 18% body fat whereas women have between 18 to 22% body fat.

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