People with a normal Body Mass Index (BMI) who carry their weight around the middle are at the highest risk of death from any cause and cardiovascular causes compared to these who are obese according to BMI but carry their weight elsewhere, a new study co-led by University of Sydney researchers shows.

Published today in the Annals of Internal Medicine, the study shows that normal weight people who carry fat around the middle of their body are 22 per cent more likely to die from any cause and a 25 per cent higher risk for death from cardiovascular causes compared to those who are classified as normal weight without carrying fat centrally.

The University of Sydney’s Associate Professor Emmanuel Stamatakis co-led the research and says the study shows that diagnosis of obesity cannot solely rely on a person’s BMI.

“The research suggests that treatment decisions cannot be based solely on BMI because it may lead to inaccurate diagnosis of obesity,” he says.

Associate Professor Stamatakis says there are a range of reasons as to why fat carried centrally can lead to an increased risk of death.

“We cannot be absolutely certain about the exact mechanisms but we do know that excessive fat around the waist is associated with insulin resistance, abnormal lipids profile and increased systemic inflammation,” he says.

The Body Mass Index has commonly been used as a way to measure obesity in people, dividing a person’s weight by their squared height. However, the BMI has been questioned given it doesn’t take into account waist circumference or hip-to-waist ratio – two factors Associate Professor Stamatakis says are crucial to measuring obesity.

“BMI alone is limited; a patient’s waist-to-hip ratio should also be obtained and if those measurements are above the acceptable limit (0.85 for women and 0.90 for men), it may be a good idea to address it.

“If I had to choose between making sure my BMI or my waist-to-hip ratio are within “normal” range I would go for the latter. BMI is confounded by many things, including muscularity, and a well-trained individual can easily be classified as overweight, even though their body fat content is in the very healthy range,” says Associate Professor Stamatakis.

“On the other hand, a high waist-to-hip ratio most likely means high amounts of abdominal fat and we know this comes with quite serious health risks.”

Associate Professor Stamatakis says those with larger waistlines are best advised to see their doctor to understand how to best lose excess weight.

“Those with larger waistlines may want to have a discussion with their doctor and/or start thinking how to alter their lifestyle to lose or reduce that “paunch”.

“Addressing the root causes of abdominal obesity through increased physical activity, better diet and perhaps lower alcohol consumption has a myriad other physical health and psychological benefits beyond fat loss.”