What is the difference between overweight & obesity?

The terms ‘overweight’ and ‘obesity’ refer to the abnormal or excessive accumulation of fat which may impair health (2).

Here are a few common methods which may be used to establish if someone is overweight or obese:

1.Body Mass Index (BMI)

  • This is a simple way of estimating if someone is a healthy weight for their height.
  • It is calculated by using the equation: weight (kg) ÷ height 2 (metres) = BMI (kg/m2).
  • BMI often needs to be interpreted with caution as it does not measure adiposity (fat distribution) directly, and is therefore a less reliable indicator of health in certain population groups. For example,  elite athletes may have a higher BMI as they carry more weight due to muscle bulk (which weighs more) than fat mass (3).

2.Waist circumference

  • This measures abdominal fat including the level of fat deposits which surround the internal organs (4).
  • Carrying excess fat around the middles (‘apple shape’) is a stronger predictor of cardiovascular disease and Type 2 diabetes than the storage of more fat around the bottom and thighs (‘pear shape’) (4).

3. Bioelectrical impedance

  • This is an estimate of body composition, including body fat (4).
  • Various forms of equipment are available and you may find stand-on scales in your local gym.
  • Error in data entry, body position, hydration status and fluid balance, and variations in body shape and fat distribution may all affect the accuracy of measurements (4).
  • NICE does not recommended it as a substitute for BMI (3).

WHO (2016) and NICE (2014) classification of adult overweight and obesity and risk of associated co-morbidities (3,5&6).

Obesity Diagram 1


Some ethnic groups are at increased risk of health complications at lower BMI levels than stated in the table above, which has led to the recommendation of different BMI classifications.

Example: South Asian population – a lower BMI threshold of 23kg/m2 or less is recommended (7&8).

Men from South Asian, Chinese and Japanese descent are advised to maintain a smaller waist circumference of less than 90cm (8).

Equally it may not be appropriate to apply the BMI thresholds in the table above to  older populations, where health risks associated with a lower BMI may be of more concern and therefore a higher cut off is accepted (3).