Table of contents


Background papers: Lifestyle and wider determinants :: Healthy weight :: Obesity :: Introduction

Overweight and obesity are terms used to describe increasing degrees of excess body fat. Excess weight is a significant risk factor for a number of diseases, including type II diabetes, cancer and heart disease. Overweight and obesity can also affect mental health and self-esteem. Obesity in adults is strongly correlated to obesity in children. The prevention and treatment of overweight and obesity is a central public health policy goal.

Excess weight is caused by an energy imbalance between 'energy in' (food consumption) and energy expenditure (energy used by the body during activity and metabolism).[1] If there is greater energy intake than is required, the excess energy will become excess fat. However, the underlying causes of this energy imbalance, which result in weight gain, are complex. Behavioural, psychological, social, cultural and environmental factors are thought to determine the increasing prevalence of obesity seen throughout the world.

Adult obesity classification

Overweight and obesity in adults is measured and classified using Body Mass Index (BMI) according to table 1.

  Body Mass Index
Healthy weight 18.5 – 24.9
Overweight 25.0 – 29.9
Obesity I 30.0 – 34.9
Obesity II 35.0 – 39.9
Obesity III 40.0 or more
Table 1: Classifying adults who are overweight and obesity using BMI (kg/m2) [2]

The BMI classifications may be less accurate in highly muscular people. For some ethnicities, risk factors for obesity may occur at a lower BMI. The Scottish guidance [3] recommends that until specific cut-offs are validated, South Asian, Chinese and Japanese individuals may be considered overweight at BMI >23 kg/m2 and obese at BMI >27.5 kg/m2. Waist measurements are also used to assess the health risks from overweight and obesity. Tables 2 and 3 detail the health risks associated with an increased BMI and waist circumference.

  Low High Very high
Male <94 94–102 >102
Female <80 80–88 >88
Table 2: Waist circumference classifications [2]
  Low High Very high
Overweight No increased risk Increased risk High risk
Obesity I Increased risk High risk Very high risk
Table 3: Health risks associated with being overweight or obese in adults on BMI and waist circumference [2]

The health problems associated with obesity are shown in table 4.

Greatly increased risk
(Relative risk much
greater than 3)
Moderately increased risk
(Relative risk 2-3)
Slightly increased risk
(Relative risk 1-2)
Type II diabetes Coronary heart disease Cancer
Insulin resistance Hypertension Polycystic overy syndrome
Gallbladder disease Stroke Impaired fertility
Dyslipidaemia Osteoarthritis Low back pain
Breathlessness Hyperuricaemia Anaesthetic risk
Sleep apnoea Psychological factors
Table 4: Relative risks of health problems associated with obesity (Relative risk — risk measured against that of non-obese person of same age and sex)[1]

Childhood obesity classification

The National Child Measurement Programme (NCMP) measures the height and weight of children in reception class (aged 4 to 5 years) and year 6 (aged 10 to 11 years) to assess overweight and obesity levels in children within primary schools. This data can be used at a national level to support local public health initiatives and inform the local planning and delivery of services for children.

Children's heights and weights are measured and used to calculate a Body Mass Index (BMI) centile. The measurement process is overseen by trained healthcare professionals in schools.

The method of assigning a BMI classification is different for children and adults. Defining children as overweight or obese is a complex process, given that their height and weight change at the same time. Instead of using fixed BMI thresholds to classify individuals (as used for adults), children's BMI is categorised using variable thresholds that take into account the child's age and sex. The National Obesity Observatory has produced a simple guide for classifying BMI in children.[4]


[1]   Swanton DK. Healthy Weight, Healthy Lives: A toolkit for developing local strategies 2008; National Heart Forum. .
[2]   The National Institute for Health and Care Excellence. Clinical guideline [CG189]. Obesity: identification, assessment and management. 2014;
[3]   Scottish Intercollegiate Guidelines Network. Management of Obesity, A national clinical guideline 2010; Scottish Intercollegiate Guidelines Network. .
[4]   National Obesity Observatory. A simple guide to classifying body mass index in children 2011;