Definition of Overweight and Obesity
- World Health Organisation (WHO) Classification
- Measurement of waist circumference (waist measurement)
- Other Measurements
World Health Organisation (WHO) Classification
The WHO has classified overweight and obesity for adults as below, using the calculation of Body Mass Index or (BMI - the weight in kilograms divided by the height in metres squared).
The person’s BMI may be calculated mathematically or using tables, which are readily available in clinics or the GP’s surgery and it is then compared to this classification.
The main limitation with using body mass index as a calculation is that it does not differentiate fat mass from lean mass; muscular athletes will have a larger BMI than is accurately reflected in their actual body fat levels, whereas individuals with a prominent “belly” but lean arms and legs may escape detection despite being at inflated risk.
So the BMI may also underestimate the level of obesity and people with the same BMI may be affected differently with relation to the actual level of obesity.
A person is considered to be overweight if they have a BMI of 25.0 or more and obese if the BMI is 30.0 or more.
Obesity is divided into three classes:
- Class 1 BMI 30 to 34.9 (waist circumference 102.0cm plus for males & 88.0 cm plus for females).
- Class 2 BMI 35 to 39.9.
- Class 3 BMI 40 and over. A person with a BMI of 40+ is classed as being morbidly obese.
Associated health risks increase with increasing BMI and range from mildly increased in the overweight to very severe in Class 3 obesity.
Abdominal obesity (“apple shape”) is a strong indicator for the development of coronary artery disease and is associated with insulin resistance and the development of Type 2 diabetes.
The risk is associated with the fat stores, which are not just under the skin but deep in the abdominal cavity.
Obesity definition = anyone with a BMI equal to or greater than 30.
Table 1. Classification of overweight and obesity in adults according to BMI.
| Classification | BMI (kg/m²) | Risk of co-morbidities |
|---|---|---|
|
Underweight |
< 18.5 |
Low (but risk of other clinical problems increased) |
|
Normal Range |
18.5 – 24.9 |
Average |
|
Overweight (pre-obese) |
25.0 – 29.9 |
Mildly increased |
|
Obese |
≥ 30.0 |
|
|
Class 1 |
30.0 – 34.9 |
Moderate |
|
Class 11 |
35.0 – 39.9 |
Severe |
|
Class 111 |
≥ 40.0 |
Very severe |
Table 2 - Obesity and likely effect on functional ability
| BMI (kg/m²) | Likely level of functional restriction |
|---|---|
|
BMI = 35 to 45 |
Unlikely to have such restricted walking ability or ability to self care that entitlement conditions are satisfied. |
|
BMI > 45 to < than 55 |
Unlikely to have such restricted walking ability or ability to self care that entitlement conditions are satisfied. |
|
BMI = 55 and over |
Likely to have significantly restricted walking ability and ability to self care. |
|
For further details about functional restriction, refer to the ‘Care & Mobility considerations’ page. |
|
Measurement of waist circumference (waist measurement)
Measurement of waist circumference as an indicator of excess visceral (or intra- abdominal) fat, which is also known as “central obesity”, is regarded as a more accurate indicator of risk than BMI in considering associated health risks. It is performed by locating the point halfway between the crest of the hip (iliac crest) and the lowest rib at the side, passing the tape measure around the waist parallel to the floor, whilst the person is in expiration (breathing out) with a relaxed abdomen.
A healthy waist measurement is below 37 inches (94 centimetres) for men and 32 inches (80 centimetres) for women. The greatest health complications occur with a waist measurement of greater than 40 inches (102 centimetres) for men and 35 inches (88 centimetres) for women, although risk increases with increasing waist circumference even at lower levels.
In South Asians and other ethnic groups, lower thresholds are appropriate.
A *study demonstrated that “British and Indian Asian patients had higher waist to hip ratios than control groups”.
Table 2. Classification of abdominal obesity by waist circumference.
Waist Circumference (cm)
| Classification | Males | Females |
|---|---|---|
|
Not overweight |
<94.0 |
<80.0 |
|
Overweight |
||
|
Pre-obese |
94.0 – 101.9 |
80.0 – 87.9 |
|
Obese |
≥102.0 |
≥88.0 |
An alternative is waist/ hip ratio (dividing the waist measurement by the hip measurement). If the waist/ hip ratio exceeds 1.0 in men or 0.9 in women, central obesity is diagnosed.
Other Measurements
- Measurement of Skin fold Thickness with callipers (assessing body fat at multiple sites) is not used as a measurement of obesity now.
- Fat Monitors measure bio-impedance and can be bought from some chemist shops. The principle is that lean body mass conducts electric current better than fat mass. Body fat is estimated by a combination of height/weight measurement, the measurement of the resistance to a weak electrical current and an equation.
Amended April 2011
