About Obesity

The epidemic of obesity is now recognized as one of the most important public health problems facing the world today. Tragically, adult obesity is more common globally than under-nutrition. According to the World Health Organisation (2014), there are around 2 billion adults overweight, of those 670 million are considered to be affected by obesity (BMI ≥30 kg/m²) and 98 million severely affected by obesity (BMI ≥35 kg/m²).  If current trends continues it is estimated that 2.7 billion adults will be overweight, over 1 billion affected by obesity and 177 million adults severely affected by obesity by 2025.

We estimate that around 224 million school-age children are overweight, making this generation the first predicted to have a shorter lifespan than their parents.

What is obesity?
Obesity is a medical condition described as excess body weight in the form of fat. When accumulated, this fat can lead to severe health impairments.

What causes obesity?
Obesity is caused by an energy imbalance; when intake of calories exceeds expenditure of calories, the surplus energy is stored as body weight. There are a multitude of ‘obesogenic’ factors contributing to the increased energy consumption and decreased energy expenditure that are responsible for obesity, including:

  • Declining levels of physical labour as populations move from rural to urban settings and abandon walking in favour of driving, labour-saving devices in the home, and the replacement of active sport and play by television and computer games. 
  • Higher levels of food consumption, or an increase in energy density (particularly fat content) of the food we eat.  
  • Social, economic, educational and cultural factors are important underlying causes of obesity, although how they inter-relate to promote or protect against the development of obesity is complex and varies considerably by country.


How is obesity measured?
The most widely-used method of measuring and identifying obesity is Body Mass Index (BMI).
BMI = weight in kg/height in m2

Overweight, or pre-obesity, is defined as a BMI of 25–29.9 kg/m2, while a BMI >30 kg/m2 defines obesity. These BMI thresholds were proposed by WHO expert reports and reflect the increasing health risk of excess weight as BMI increases above an optimal range of 21–23 kg/m2, the recommended median goal for adult Caucasian populations (WHO/NUT/NCD, 2000).

Table 1: The World Health Organisation International Classification of adult underweight, overweight and obesity according to BMI


BMI Kg/m²

Principal cut off points

Additional cut off points
Underweight <18.50 <18.50
Severe thinness <16.00 <16.00
Moderate thinness 16.00 - 16.99 16.00 - 16.99
Mild thinness 17.00 - 18.49   17.00 - 18.49
Normal range

18.50 - 24.99  

18.50 - 22.99

23.00 - 24.99

Overweight ≥25.00 ≥25.00
Pre-Obesity 25.00 - 29.99

25.00 - 27.49

27.50 - 29.99

Obesity   ≥30.00 ≥30.00
Obesity class I 30.00 - 34-99

30.00 - 32.49

32.50 - 34.99

Obesity class II 35.00 - 39.99

35.00 - 37.49

37.50 - 39.99

Obesity class III

≥40.00   ≥40.0

Source: WHO website (http://www.who.int/bmi).

While BMI is a simple measure that is very useful for populations, it should be considered a rough guide for predicting risk in individuals. The distribution and amount of body fat are also crucial determinants of some obesity-associated health risks. Visceral fat, particularly in the abdominal region, has a stronger association with type 2 diabetes and cardiovascular disease than BMI. Accordingly, measures of central obesity such as waist:hip ratio and waist circumference provide more robust indices of overall obesity-related health risk than BMI alone.

Health impact of obesity
Obesity is an important cause of morbidity, disability and premature death (WHO, 2004). Obesity increases the risk for a wide range of chronic diseases; BMI is thought to account for about 60% of the risk of developing type 2 diabetes, over 20% of that for hypertension and coronary-heart disease and between 10 and 30% for various cancers. Other co-morbidities include gall-bladder disease, fatty liver, sleep apnoea and osteoarthritis.

The disability attributable to obesity and its consequences in 2004 was calculated at over 36 million disability-adjusted life years (DALYs), due primarily to ischaemic heart disease and type 2 diabetes (WHO Global Health Risks Report, 2004).

Obesity shortens life expectancy. In 2004, increased BMI alone was estimated to account for 2.8 million deaths, while the combined total with physical inactivity was 6.0 million (WHO Global Health Risks Report, 2004) – surpassing the excess mortality associated with tobacco, and approaching that of high blood pressure, the top risk factor for death.Figure 1: Deaths attributed to 19 leading risk factors, 2004 (Source: WHO Global Health Risks Report, 2004)


Relationships between obesity and health risks vary between populations. Asians, for example, are more susceptible and thus BMI risk thresholds are lower than other populations, with an action point for overweight defined at 23 kg/m2.


Obesity in children
Childhood obesity is already common, especially in westernized countries. In 2004, according to IOTF criteria, it was estimated that ~10% of children worldwide aged 5–17 years were overweight and that 2–3% were obese (Lobstein et al., 2004). Prevalence rates vary considerably between different regions and countries, from <5% in Africa and parts of Asia to >20% in Europe and >30% in the Americas and some countries in the Middle East. Becoming obese earlier in life clearly amplifies certain health risks, particularly for type 2 diabetes.

The IOTF criteria for overweight and obesity in children have recently been updated.  Read more

Social impact of obesity
For individuals, psychological problems associated with obesity are common, wide-ranging and potentially serious. Growing worldwide awareness of obesity may have reinforced prejudice against the obese, who are often stigmatized. Depression and low self-esteem can affect an individual’s quality of life, mental health, educational achievement and employment prospects. Cultural and ethnic factors undoubtedly modulate the social impact of obesity, as well as its perception.  In some parts of the world – notably the Pacific Islands and parts of Africa – obesity may still carry historic and cultural connotations of power, beauty and affluence.

Costs of obesity
Obesity has substantial direct and indirect costs that put a strain on healthcare and social resources.

Direct medical costs include preventative, diagnostic and treatment services related to overweight and associated co-morbidities. European nations spend 2-8% of their health care budgets on obesity, equating to 0.6% of gross domestic product (GDP) for some (Müller-Riemenschneider, Reinhold, Berghöfer, and Willich, 2008). In the USA, estimates based on 2008 data indicated that overweight and obesity account for $147 billion in total medical expenditure (Finkelstein, Trogdon, Cohen and Dietz, 2009). Although indirect costs to society can be substantially higher, they are often neglected. They relate to income lost from decreased productivity, reduced opportunities and restricted activity, illness, absenteeism and premature death. In addition, there are high costs associated with the numerous infrastructure changes that societies must make to cope with obese people (i.e. reinforced beds, operating tables and wheel chairs; enlarged turnstiles and seats in sports-grounds and modifications to transport safety standards).

Obesity is now reaching pandemic proportions across much of the world and its consequences are set to impose unprecedented health, financial and social burdens on global society unless effective actions are taken to reverse the trend.

Page Updated: 10th October 2015