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What is obesity? A definition

Discover our definition of obesity and learn how body mass index (BMI) is defined. 

How is obesity defined?

The standards are based on the body mass index (BMI), which is defined as body mass in kilograms divided by the square of body height in metres.

Some countries vary the scale slightly. For example, Singapore lowers the cut-off points, recognizing that many of its citizens have higher proportions of body fat and are thus at increased risk for cardiovascular disease and diabetes mellitus.

Read our article The rise of globesity: a short introduction to a big problem.

What is BMI? A definition of body mass index

BMI as an indicator of body fat came into popular use in 1972, when researcher Ancel Keys published a paper in the Journal of Chronic Diseases. Dr Keys wanted BMI to be used in population studies, not for individual diagnoses, but the measure is now almost universally used as a quick way to assess whether a person is overweight or obese.

Today, experts point out several limitations to the measure’s effectiveness. Perhaps most significantly, it doesn’t account for the difference between fat and heavier muscle; since muscle weighs more than fat, a very physically active individual can fall into the overweight category even though he is carrying less fat than his sedentary, normal weight neighbour.

Some studies have shown that the waist-to-height or waist-to-hip ratio is a better indicator, but BMI remains as popular as ever. Taken as an indicator of population health, as originally intended, BMI trends since 1980 clearly show that the world is getting fatter than ever, faster than ever.

Longevity and obesity

Eleven minutes. That’s how much a single cigarette can shorten one’s life, according to rough calculations published in the British Medical Journal in 2000. Thus, a pack of 20 cigarettes means that the individual loses the time equivalent of two football matches or a Eurostar journey from London to Paris, while a carton of 200 cigarettes costs the time equivalent of a romantic night away.

It would be eye-opening to assign similar values to a doughnut, a sugar-sweetened beverage or a plate of fish and chips, but here the evidence is harder to come by.

The risk of premature death does not increase in lockstep with BMI. Instead, the survival curve is U-shaped: Those who are severely underweight or obese are most at risk, while those who are normal weight — or even slightly overweight — enjoy the best longevity.

Building a Healthier World

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We are committed to helping create a stronger, healthier global community by delivering comprehensive health care benefits and population health solutions worldwide.

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