Obesity gets a definition that isn’t BMI

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It's long been known that Body Mass Index is far from perfect when measuring overweight and obesity. People who are very muscular can have a high weight relative to their height but not carry much fat. BMI also doesn't account for where the fat might be, and fat in the abdomen, visceral fat, is the fat that's really toxic for health. That's why alternate measures such as waist circumference, waist to hip ratio and waist to height ratio have been developed. And then there's what the obesity might be doing to someone's health. Some people are surprisingly healthy despite a high BMI and need a different approach from their doctor than someone who's being made sick by their obesity.

So for all these reasons, an international group of experts has redefined obesity in relation to whether there are complications and signs of organ damage such as diabetes, heart disease, liver disease, some cancers, mental health issues or trouble with activities of daily living. They called this clinical obesity. People without these side effects of obesity should now be defined as having preclinical obesity, in other words they may be at future risk.

Some people are surprisingly healthy despite a high BMI and need a different approach from their doctor than someone who's being made sick by their obesity.

A new study has looked at what difference the new definition makes to assumptions about how much obesity is out there. Levels of clinical obesity in children are much lower than the obesity rate when measured by BMI alone. In mid-life clinical obesity is commoner in men than women, and in later life it is more common in women. The main point is that the harm done by obesity accumulates with age, and the younger it is tackled the better. In mid to late life it's important to know whether obesity is causing organ damage so it can be treated more comprehensively.

Healthy measures of body size and fat

• BMI between 18.5 and 24.9, with a lower upper cut-off for people of South Asian or First Nations ancestry
• Waist circumference of 94cm or less in men and 80cm or less in women, again lower in some ethnic groups
• Waist to height ratio below 0.5, the measure gaining ground because it applies regardless of gender or ethnicity

If waist to height ratio is above 0.5, experts argue the person should be investigated for metabolic dysfunction associated fatty liver disease (MAFLD). More on MAFLD from the RACGP.

Further viewing. Tonic's recent webinar Beyond Fatty Liver: Why MAFLD Matters Now covers MAFLD in depth with Prof Jacob George, Prof Simone Strasser and Dr Vijay Ramanathan. It is CPD accredited with RACGP and ACRRM, and you can register to receive the recording and podcast version.