What is obesity?


Definition of obesity

Obesity is an abnormal or excessive accumulation of fat that poses a risk to health. There are two main ways to determine whether you’re obese.

The first is your body mass index, calculated by multiplying your weight in kilograms by the square of your height in metres. This is interpreted as follows:

  • 8.5 to 24.9 means you’re a healthy weight
  • 25 to 29.9 indicates that you’re overweight
  • 30 to 39.9 means you’re obese
  • 40 or over is defined as severely obese

However, BMI is not a definitive indication of obesity, because it’s possible to be very muscular without being obese. A useful additional benchmark is your waist measurement. If you’re a man with a waist circumference of more than 94 cm (37 in), or a woman with a waist measuring more than 80 cm (31.5 in), you’re more likely to develop obesity-related problems.

Causes of obesity

Quite simply, it’s caused by consuming more energy than you’re using, and the excess is stored as fat.

Some medical conditions may make you more predisposed to obesity, such as an underactive thyroid and Cushing’s disease, and some people inherit a large appetite from their parents. And certain medications, such as some corticosteroids and antidepressants, can cause weight gain. But there’s no medical evidence to support the idea that obesity is caused by a slow metabolism. In the vast majority of cases, it’s preventable.

The obesity epidemic reflects radical changes in our lifestyles over the past few decades. These include:

  • Increased ownership and use of cars
  • Growing consumption of takeaway and restaurant meals rather than home cooking
  • Larger portion sizes
  • Ready availability of cheap, processed food with a high sugar content
  • Increasing mechanisation and computerisation at home and in the workplace, ranging from TV remote controls to industrial robots
  • The worldwide move away from rural areas to cities

Why is obesity a problem?

Being overweight or obese reduces life expectancy. It makes people more likely to get heart disease, stroke, type 2 diabetes, certain cancers, and other diseases. It’s often socially stigmatised, and it imposes huge additional costs on overstretched health services. The NHS spends some £5 billion a year on treating weight-related conditions – that’s £78 for every man, woman, and child in the country.

How common is it?

According to the World Health Organisation, 28.1 percent of UK adults were obese in 2014, and 62 percent were overweight. Britain has the highest rate of obesity in Europe, part of a major global epidemic that has grown fourfold in twenty-five years.

Increasing numbers of children are obese

Almost one in five children is obese when they start primary school. By the time they leave, this has risen to one in three. Obese children are often bullied, and tend to become obese adults, so they may face a lifetime of low self-esteem and ill health.

The growth in childhood obesity reflects changes in society. Where, a generation ago, children might have spent hours riding their bikes in the street, now they may spend that time snacking in front of the TV or computer.

Simple ways to lose weight

There are no quick fixes when it comes to weight loss. But some relatively easy lifestyle changes do have proven benefits.

  1. Exercise at least 30 minutes a day to maintain good cardiovascular fitness. Your weight depends on a number of factors, such as your food intake, but moderate intensity exercise of 200 minutes (over 3 hours) per week can help you notice some weight loss results on average.
  2. Use an app to track your food intake and exercise. This lets you monitor your progress over the long term rather than from day to day, and can be hugely motivating.
  3. Use a smaller plate. There’s ample research evidence that doing this makes you feel you’re eating more than you really are. The phenomenon even has a name – it’s known as the Delboeuf illusion.
  4. Plan and shop for your meals in advance, say a week at a time. This can help you to set and keep to calorie goals, and makes you more mindful of the food you eat. It also ensures you always have healthy food in the larder rather than subsisting on takeaways.
  5. Increase your fibre intake. Plant-based foods such as fruit, vegetables, grains, and wholemeal bread make you feel full and help you to stave off cravings.
  6. Get a good night’s sleep. Less than five or six hours a night is associated with an increased risk of obesity. This is because lack of sleep slows your metabolism, the process of converting calories to energy, and may therefore cause increased fat accumulation. It also reduces the production of the appetite-controlling hormones leptin and ghrelin, and increases the output of insulin and cortisol, which increase fat storage.
  7. Eat protein for breakfast. This helps to regulate hunger and satiety hormones for several hours. Good protein choices include eggs, oats, nuts, and porridge.


World Obesity Federation (2015). About obesity. Retrieved from

Diabetes Digital Media (2018). Diabetes and obesity. Retrieved from

Obesity Health Alliance. To tackle obesity we need urgent action in 10 areas. Retrieved from