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Definitions and prevalence of obesity

What measures do we use to define obesity? Watch Dr Angela Alexander explain measurements including, body mass index, as a tool for defining obesity.
So what is obesity? The World Health Organisation defines obesity as abnormal or excessive fat accumulation that may impair health. They use body mass index, BMI, as a simple index of the relationship between a person’s weight in relation to their height. It’s defined as a person’s weight in kilograms divided by the square of their height in metres. If you’re not familiar with the term ‘square’, it just means multiplying the height by itself. So, for example, if you’re 1.7 metres tall, the square of your height is 1.7 times 1.7, which is 2.89. And if your weight is 70 kilograms, then your BMI is 70 divided by 2.89, which is 24.2.
But you don’t need to worry too much about the calculation, as we have an app in the course for you to look at your own BMI in week two. We’ll also see later why it’s not necessarily the most appropriate measure in relation to health. But BMI is commonly used to classify overweight and obesity in adults. The World Health Organisation definition is a BMI of 24 or less is normal. A BMI greater than or equal to 25 is overweight. A BMI greater than or equal to 30 is obese. A BMI greater than or equal to 35 is severe obesity. And a BMI greater than or equal to 40 is classified as morbidly obese.
On the basis of these categories, the World Health Organisation has reported that worldwide obesity has nearly doubled since 1980, with 35% of adults age 20 or over being overweight in 2008 and 11% obese. 65% of the world’s population live in countries where overweight and obesity kills more people than underweight. And more than 40 million children under the age of five were overweight in 2011. You have read about the obesity epidemic and what it means to mankind, and generally, it seems to be a pretty gloomy view. But do you think the media overreacts when it reports about obesity? We’d really like to hear your thoughts about this, so share your comments in the section below.
In this video we explore some of the measures we use to define obesity, such as body mass index (BMI), and consider the implications of some obesity-related statistics provided by the World Health Organisation.
BMI is calculated by dividing your weight in kilograms by the square of your height in metres. We could represent this mathematically as BMI = weight ÷ (height × height).
So if I weigh 60kg and I am 1.6m tall I would calculate my BMI as follows:
First I multiply my height by itself to get the square of my height: 1.6 × 1.6 = 2.56. I can then divide my weight by this number to find my BMI: 60 ÷ 2.56 = 23.4 (rounded to one decimal place).
Let’s look at what the resulting numbers mean:
  • 18 – 24.9 is considered healthy.
  • greater than or equal to 25 is considered overweight.
  • greater than or equal to 30 is considered obese.
  • greater than or equal to 40 is considered morbidly obese.
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Remember you can also look at the glossary to help you with any terms in the course.
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Obesity: Causes and Consequences

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