The term ‘severe obesity’ or class III obesity by WHO classification refers to adults with a body mass index (BMI) 40kg/m2 or more. It is also sometimes used to refer to people with BMI 30 to 39.99 kg/m2 who have significant health problems (‘co-morbidities’) associated with obesity. In these web pages the definition of severe obesity refers to a BMI of 40kg/m2 or more, unless otherwise stated. There is no standard definition of severe obesity in children, although the 99.6th centile of the UK90 growth reference charts is often used to identify very obese children.
The prevalence of severe obesity in England increased  and is predicted to rise further over the next twenty to thirty years  . It is more common in women, although the rate of increase in recent years has been higher in men. Severe obesity is associated with an increased risk of co-morbidities including cardiovascular disease, diabetes and some cancers. Median survival is reduced by as much as eight to ten years compared with adults of normal weight .
The Health Survey for England data show that between 1993 and 2012, the prevalence of severe obesity was consistently higher among women (increasing from 1.4% in 1993 to 3.1% in 2012) than among men (increasing from 0.2% in 1993 to 1.7% in 2012) .
Prevalence of severe obesity among adults aged 16+ years
Health Survey for England 1993-2012
Adult (aged 16+) obesity: BMI ≥ 40kg/m2
Predicting future trends in morbid obesity has proved difficult. Two different models have estimated markedly different prevalence figures. The first predicts a prevalence of almost 3% in men and 6% in women by 2030 ; the second predicts a prevalence of 1% for men and 4% for women by 2050 . Straight-line extrapolation of the prevalence of adult men with BMI greater than 40 kg/m2 predicts a level of around 3% by 2050.
In most cases, severe obesity is caused by an imbalance between energy input and expenditure. However, some cases are due to identifiable and specific genetic defects (e.g. leptin deficiency, MC4 receptor mutations), damage to the hypothalamus (from trauma, tumours or surgery) or drugs (e.g. antipsychotics).
Severe obesity is associated with lower educational attainment, reduced employment prospects and lower socioeconomic status, although the directionality of this association is not known.
The most serious health consequences associated with overweight and obesity in adults include Type 2 diabetes, cardiovascular disease (including ischaemic heart disease, stroke and peripheral vascular disease), musculoskeletal disorders and some cancers. Some of these, such as heart disease and stroke, may represent the principal cause of death; others, such as diabetes reduce life expectancy. Other important health consequences of obesity which may impair quality of life and contribute to reduced life expectancy include obstructive sleep apnoea, obesity hypoventilation syndrome, musculoskeletal pain and osteoarthritis, gastro-oesophageal reflux, obstetric complications, polycystic ovarian syndrome, infertility, incontinence and mental health problems.
Both BMI and a measure of fat distribution (waist circumference or waist : hip ratio) are important in calculating the risk of obesity co-morbidities. However, amongst the severely obese, waist circumference adds little to the absolute measure of risk provided by BMI . Table 1 shows a simplistic relationship between BMI and the risk of co-morbidity, which can be affected by different factors including diet, ethnicity and physical activity level. The risks associated with increasing BMI begin at BMI above 25 kg/m2 and are continuous and exponentially graded.
Table 1: Classification of adult underweight, overweight and obesity according to BMI and risk of obesity-related co-morbidities (adapted from the International Classification  )
In 2009, the Prospective Studies Collaboration (PSC) published a pooled analysis of 57 prospective cohort studies of the association between obesity and causes of mortality . The analysis of almost 900,000 participants revealed strong associations between severe obesity and a number of causes of death. For example, people aged 35-59 years with BMI 40 to 50 were five times more likely to die from ischaemic heart disease than those with BMI 22.5 to 25. Their risk of dying from stroke was 6.5 times higher and their risk of dying from diabetes was 22.5 times higher. Associations with malignant disease were, in general, less strong but still important. For example, severely obese women were 6.2 times more likely to die from endometrial cancer than women of normal weight, and the corresponding figures for some other cancers were 2.8 times for kidney cancer, 1.7 times for ovarian cancer, and 2.4 times for colorectal cancer in men. The impact of severe obesity on health was so severe that median survival of affected people was reduced by about eight to ten years.
Click here to view a NOO briefing paper on obesity and life expectancy published August 2010
The same pooled analysis showed that vascular risk factors were strongly related to BMI. BMI had a positive association with systolic and diastolic blood pressure throughout the range 15 to 50 kg/m2. On average across all ages (15 to 89 years), each increase in BMI of 5 kg/m2 was associated with an increase of at least 5 mm Hg systolic blood pressure and around 4 mm Hg diastolic blood pressure. BMI was also strongly associated with the ratio of non-HDL to HDL cholesterol up to around 30 kg/m2 but above this level, and hence in the morbidly obese, there was only a weak association between BMI and either cholesterol fraction.
Obesity was strongly associated with diabetes, the sex-specific prevalence increasing more than five-fold over the BMI range 15 to 50 kg/m2 (from 2.8% to 11.4% for men; from 1.8% to 10.3% for women), most of this increase occurring above 25 kg/m2. Data from the Health Survey for England 2006  show a similar picture, although based on a small sample size and self-reported diagnosis of diabetes. Amongst adults for whom both BMI and diabetes status was reported, the prevalence of diabetes amongst those with normal weight (BMI 18.5 to 25 kg/m2) was around 1.5%, increasing to 15% in the severely obese.
It should be noted that the PSC analysis excluded around 300 individuals with BMI 50 kg/m2 or more so may have under-estimated the health impact of severe obesity.
For information on the management of severe obesity, please refer to the following documents:
- Health and Social Care Information Centre. The Health Survey for England - 2012 trend tables. London: Health and Social Care Information Centre, 2013 http://www.hscic.gov.uk/catalogue/PUB13219.
- Lobstein T, Jackson Leach R. International comparisons of obesity trends, determinants and responses. Evidence review. Foresight Tackling Obesities, 2007: Future Choices http://www.foresight.gov.uk.
- McPherson K, Marsh T, Brown M. Foresight tackling obesities: Future choices – modelling future trends in obesity and the impact on health. Foresight Tackling Obesities, 2007: Future Choices http://www.foresight.gov.uk.
- National Health & Medical Research Council (Australia). Clinical practice guidelines for the management of overweight and obesity in adults. NHMRC, 2003
- World Health Organization. Obesity: preventing and managing the global epidemic. Report of a WHO consultation. World Health Organ Tech Rep Ser 2000; 894: 1-253.
- World Health Organization. Physical status: the use and interpretation of anthropometry. Report of a WHO Expert Committee. World Health Organ Tech Rep Ser 1995; 854: 1-452.
- Prospective Studies Collaboration. Body-mass index and cause-specific mortality in 900 000 adults: collaborative analyses of 57 prospective studies. Lancet 2009; 373: 1083-96.
- Health and Social Care Information Centre. The health survey for England 2006. London: The Health and Social Care Information Centre, 2007.