Last edited 09/2022 and last reviewed 09/2022

An obese patient is 20% or more heavier than their ideal weight.

Excess body weight is an important public health problem because it is associated with:

  • type II diabetes mellitus
  • hypertension and stroke
  • hyperlipidaemia
  • some cancers

Obesity is graded according to the Body Mass Index (BMI):

  • Healthy weight 18.5-24.9

  • Overweight 25-29.9

  • Obesity I 30-34.9

  • Obesity II 35-39.9

  • Obesity III 40 or more

Measures of overweight, obesity and central adiposity in adults (3)

  • use BMI as a practical measure of overweight and obesity. Interpret it with caution because it is not a direct measure of central adiposity
  • in adults with BMI below 35 kg/m2, measure and use their waist-to-height ratio, as well as their BMI, as a practical estimate of central adiposity and use these measurements to help to assess and predict health risks (for example, type 2 diabetes, hypertension or cardiovascular disease)

People with a South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background are prone to central adiposity and their cardiometabolic risk occurs at lower BMI, so use lower BMI thresholds as a practical measure of overweight and obesity:

  • overweight: BMI 23 kg/m2 to 27.4 kg/m2
  • obesity: BMI 27.5 kg/m2 or above

For people in these groups, obesity classes 2 and 3 are usually identified by reducing the thresholds highlighted by 2.5 kg/m2

Interpret BMI with caution in adults with high muscle mass because it may be a less accurate measure of central adiposity in this group.

Interpret BMI with caution in people aged 65 and over, taking into account comorbidities, conditions that may affect functional capacity and the possible protective effect of having a slightly higher BMI when older.

BMI in children (3):

  • define the degree of overweight or obesity in children and young people using the following classifications:

    • overweight: BMI 91st centile + 1.34 standard deviations (SDs)

    • clinical obesity: BMI 98th centile + 2.05 SDs

    • severe obesity: BMI 99.6th centile + 2.68 SDs

      Use clinical judgement when interpreting BMI below the 91st centile, especially the healthy weight category in BMI charts because a child or young person in this category may nevertheless have central adiposity

The health benefits of modest (10%) weight loss was previously summarised by SIGN- although the precise benefits will vary in individuals depending on initial body weight, current health and degree of weight loss) (4)

  • mortality
    • 20-25% reduction in premature death
    • 30% reduction in the risk of dying from diabetes-related complications
    • 40% reduction in the risk of dying from cancer
  • Blood pressure
    • 10mmHg decrease in systolic blood pressure
    • 20mmHg decrease in diastolic blood pressure
  • Diabetes 50% fall in fasting blood glucose levels
  • Lipids
    • 10% fall in total cholesterol 15% fall in LDL cholesterol
    • 8% increase in HDL cholesterol

By 2030 an estimated 38% of the world’s adult population will be overweight and another 20% will be obese (5)