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The science of nutrition and healthy eating
The science of nutrition and healthy eating

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4.3 Measuring obesity

Overweight and obese are defined as the abnormal or excessive accumulation of fat that may impair health. The measure of whether someone is of normal weight, overweight or underweight is conventionally obtained by calculating their body mass index (BMI). This takes into account their height as well as their body mass (weight).

BMI postfix times equation left hand side open kg solidus m squared close equals right hand side weight in kg divided by height multiplication height in metres

After calculating the BMI, the result is compared with the values in Table 1.

Table 1 Body mass index classifications
Category BMI range (kg/m2)
Very severely underweight less than 15
Severely underweight 15.0 to 16.0
Underweight 16.0 to 18.5
Normal (healthy weight) 18.5 to 25
Overweight 25 to 30
Obese Class I (moderately obese) 30 to 35
Obese Class II (severely obese) 35 to 40
Obese Class III (very severely obese) over 40

However, there are problems with using a simple calculation like this. First, these values were only ever designed to be appropriate for adults. There are separate height–weight charts for children. These values are also not appropriate for all adults. For example, many rugby players and shot putters would be classed as obese although they probably carry very little body fat.

A BMI above the healthy range puts most people at risk of adverse health consequences. These include an increased risk of coronary heart disease (CHD), stroke and Type 2 diabetes.

It appears that some population groups are more susceptible to these health problems than others. In the UK, the National Institute for Health and Care Excellence (NICE) has issued additional BMI advice to people in South-Asian and Chinese ethnic groups, who appear to have a higher risk of developing Type 2 diabetes than white populations. They are advised to keep their BMI below the standard value of 25. The evidence is less clear-cut for people in black ethnic groups, but again, keeping their BMI below 25 is recommended.

You might like to calculate your own BMI and compare it with Table 1. (Don’t worry – you don’t have to share it with other people!)

In 2013, an alternative formula for calculating BMI was proposed by the mathematician Professor Nick Trefethen of the University of Oxford. It is based on mathematical analysis of the physics and mechanics of how bodies grow. His suggestion is that the equation should be:

alternative BMI equals weight left parenthesis in kg right parenthesis multiplication 1.3 divided by height postfix times open in m close super 2.5

This change means that some tall adults who were previously considered overweight would now be within the normal range. And some short adults, previously within the normal range, would now be considered overweight. You might like to try calculating your BMI using this alternative measure, although the maths is slightly more complicated.