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Wed 1 Oct 2014 04:08 PM

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Why BMI is not an accurate assessment of your health

The secret to a longer life: Keep your waist half your height

Why BMI is not an accurate assessment of your health

Researchers at Cass Business School discovered that using a waist-to-height ratio (WHtR) is a better predictor of health and mortality risks than using the body mass index (BMI) measurement, which has always had its critics.

Currently, BMI is used as the standard measurement in primary public health screening and measures. A healthy BMI score is between 20 and 25. A score below 20 indicates that you may be underweight; a value above 25 indicates that you may be overweight. It predicts body fat based on a height and weight ratio, but it fails to take the importance of waist circumference into account.

Academics are urging policymakers to adopt the simple message after finding WHtR is a more accurate predictor over BMI which could leave people at increased risk of serious health issues.

So how does it work? Simply, keep your waist trimmer than half your height and you could significantly boost your life expectancy.

Analysing more than 20 years of UK data for non-smoking men and women, they found a stronger link between WHtR and mortality rates than BMI and mortality rates. In the first study of its kind, the authors quantified the number of years of life lost to obesity as measured using WHtR.

They cite the example of movie stars Arnold Schwarzenegger and Danny De Vito.  Both actors have a BMI of 34. If BMI is used as the measure of obesity then a BMI of 34 would suggest that both actors stand to lose 3.6 years of life.

However, the researchers propose that WHtR should be considered instead. Using this measure of obesity, they estimate that Schwarzenegger, having around the optimal level of WHtR of 0.48, would not expect to lose any years of life; whereas, De Vito, with a WHtR of 0.71, could be expected to lose 5.8 years of life. This is because Schwarzenegger has a high BMI due to muscle whereas De Vito has the same level of BMI but due to fat.

Co-author of the study Dr Margret Ashwell said the average 30-year-old, 5 foot 10 inch tall man should have a waist of no more than 35 inches. This would put him in the healthy category. If his waist expanded to 42in or 60 per cent of his height, he risked losing 1.7 years of life and if it grew to 56in he could die 20.2 years earlier.

An average 30-year-old, 5ft 4in tall woman risked dying 1.4 years earlier if she let her waist increase from half her height, 32in, to 60 per cent of her height, 38.4in. If her waist increased to 51in, she could die 10.6 years earlier.

Dr Margaret Ashwell OBE, Director of Ashwell Associates, and a visiting academic at Oxford Brookes University, said:  “BMI measures fat and muscle and so it will be high in muscular people and cannot give information about fat distribution.  In contrast, WHtR is a better proxy for central fat, which has greater associated health risks than fat stored in other parts of the body.”

“The cut-off value of 0.5 has been proposed for people in all countries and ethnic groups. The cut off value even works for children too. This latest study on YLL supports the cut-off values of 0.5; hence the very simple global message: "Keep your waist circumference to less than half your height," she added.

In new unpublished research, the authors examine how further increases in BMI and WHtR would affect life expectancy.  They examine the impact of 5 percent, 10 percent and 15 percent increases in BMI and WHtR for average men and women aged 30, 50 and 70 years. The study shows that a 15 percent increase in WHtR for an average 50-year old man results in an additional 2.0 years of life being lost while for a 50-year old woman it is an additional1.1 years.

Professor Les Mayhew of Cass Business School said the latest findings highlighted the need for an urgent review of how obesity is measured.

Cass Business School’s Professor Ben Rickayzen added: “The use of WHtR in public health screening, with appropriate action, could help add years to life. If health professionals included this simple measurement in their screening procedures then many years of productive life could be saved.”

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chris janota 5 years ago

Many body builders have been labeled “obese” because of their high BMI, when they actually have a low percentage of body fat.