To calculate your Body Mass Index (BMI), divide your weight (kg) by the square of your height (m). However as BMI is based on weight, muscular individuals may have BMI values equal to those who are overweight or obese. Therefore is BMI really a good indicator of your composition, and ultimately your health?

Different kinds of fat

BMI is a good basic indicator for total body fat, however it cannot tell between different types of fat distribution. Additional fat that is distributed deep in the abdomen makes a person’s appearanceseem ‘apple’ shaped. This is often linked with risk factors for heart disease, raised blood pressure and diabetes. The additional fat under the skin, around the buttocks, hips and thighs is described as a ‘pear’ shape. This is commonly accepted to be less risky to health.

’Keep your waist circumference to less than half your height’

The waist-to-height ratio (WHtR) offers the exciting possibility that it could be used to assess risk for adults in several ethnic groups. This is because the BMI boundary values were derived from Caucasian populations.

WHtR is also an easy way to assess health risk in children.

Waist circumference and height is simple to measure (tape measure in any units: inches, centimetres etc) whereas the BMI needs weighing scales and must be in metric units. The public health message is simple – “keep your waist circumference to less than half your height”.

Missing the risks

So what’sthe difference in assessing for central obesity using WHtR, instead of assessing for total obesity using BMI?

In 2009 BBC published a paper in which the WHtR was applied with a boundary value of 0.5 to nearly 2,000 British adults in the nationally representative National Diet and Nutrition Survey. The paper concluded that one in three ‘non-overweight’ men (judged by a BMI of 25 or less) and more than one in seven ‘non-overweight’ women had a WHtR value greater than 0.5. So due to central obesity these people may, unknowingly, be at higher risk of serious conditions such as heart disease and diabetes. These are known as ‘non-overweight apples’.

However, one in six women and one in twenty men who were overweight according to their BMI assessment, would actually have low cardiometabolic risk because their WHtR is 0.5 or less. These are known as ‘overweight pears’.

Most interesting of all, the ‘non-overweight apples’ had significantly higher levels of cardiometabolic risk factors (blood pressure and non-HDL cholesterol) than the ‘overweight’ pears’.

It is very concerning that patients using BMI alone for risks related to central obesity and might also be shocking those whose risk is not as great as it seems from their BMI.

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Posted by Mr. Trishul Vadi (Principal Osteopath in West Wickham, Beckenham, Bromley & South East London) on Friday, February 25th, 2011