Is waist-to-height ratio better than BMI? – by Victoria Trowse
Can taking too large a size in trousers dramatically shorten your lifespan? According to experts in this field, “Your waist should be no more than half your height”.
Researchers claim that measuring the ratio of someone's waist to their height is a better way of predicting their life expectancy than body mass index (BMI), the method that has been widely used until now to judge overall health and risk of disease.
The BMI has been around since Belgian scientist Adolphe Quetelet invented it in the 1830s. It was designed to approximate whether people have a healthy weight. It is possible to arrive at an estimate of that individual's body fat by dividing a person's weight in kilograms by the square of their height in metres. However, while BMI is a useful indicator, it does not distinguish fat from muscle or different fat distributions. The latter is particularly important as evidence suggests that central fat or ‘android distribution’ is more strongly related to cardiovascular risk than peripheral fat or ‘gynoid distribution’. Measuring the waist circumference gives a better indication of fat distribution. However, it may over or under-evaluate the risk for tall and short individuals with similar waist circumferences.
It has, therefore, been suggested that correcting the waist circumference relative to height would be a better measure. So the waist-to-height ratio has been proposed as a useful tool for assessing abdominal obesity. In addition, as height has been found to be inversely associated with cardiovascular disease, it may be important to correct waist circumference for height. The importance of this point is further highlighted if we take into account the fact that height remains fairly unchanged during adulthood. The waist-to-height ratio will change only when there is a change in the waist measurement. Other indices, such as waist-to-hip ratio, are less sensitive to changes in body size, as both hip and waist could increase or decrease proportionately.
Research led by Dr Margaret Ashwell, former Science Director of the British Nutrition Foundation and now an independent consultant, found that measuring waist-to-height ratios was a better predictor for diabetes, high blood pressure, dyslipidemia and cardiovascular risk than BMI and waist circumference - in both sexes and in populations of various nationalities and ethnic groups.
Keeping your waist circumference to less than half of your height can help prevent the onset of conditions such as stroke, heart disease and diabetes and add years to your life. Researchers from Oxford Brookes University found that people with the highest waist-to-height ratio, whose waistlines measured 80% of their height, lived 17 years less than average.
There is also increasing evidence that this ratio works very well with children. As they grow, not only do they get taller but the size of their waist increases. Therefore, children as young as 5 years of age could be screened using the waist-to-height ratio to identify those at greatest risk of obesity and serious health conditions later in life.
Using the waist-to-height ratio could also help to end debates about the use of different BMI boundary values for assessing health risks in different populations. Communicating messages about health risk would be made much simpler if the same anthropometric index and public health message could be used throughout childhood, into adult life, and across the world. This simple message is: “Keep your waist circumference to less than half your height”.