#### Body mass index

Contents
1. The body mass index (BMI) or Quetelet index is a value derived from the mass (weight) and height of an individual.
2. It has traditionally been used to identify individuals who are the most likely to be overweight or obese. Generally, a high value indicates excessive body fat and consistently relates to increased health risks and mortality.
3. It is also used to identify chronic energy deficiency in adults.
1. BMI estimates are derived using the measurement of height and weight. The unit of the measurement (e.g. metric units or Imperial scale) should be clearly stated to avoid discrepancy in recording the measurements. Incorrect conversion may lead to errors.
2. If a study participant cannot stand straight or confined to a chair or bed, BMI can still be derived from special equations using arm span or knee height instead of height.
1. BMI is used in clinical settings, field surveys, and large scale population studies as a screening tool to indicate whether a person or a population is underweight, overweight, obese or a healthy weight for their height.
2. Typically used to track weight status in populations and as a screening tool to identify potential weight problems in individuals (underweight or overweight).
3. When applied to a population, BMI is used to identify independent and interactive risks of unfavourable health outcomes associated with body compositions, so as to inform policy, facilitate prevention programmes and assess the effect of interventions.

If using the metric system, BMI is calculated by dividing the weight (in kilograms) by the height (in metres) squared. As height is typically recorded in centimetres, height in centimetres will need to be divided by 100 to obtain the height in metres.

BMI = Weight (kg) / Height (m)2

If using pounds and inches, the formula needs to be altered slightly. The weight in pounds is multiplied by 703 and then divided by the height in inches, squared.

The Imperial BMI Formula is: BMI = (Weight (lbs.) * 703) / Height (inches)2

BMI categories and cut-offs are commonly used to guide patient management (Table 1).

Table 1 The cut-off points for defining malnutrition in adults by WHO.

Classification Body mass index (kg/m
Obese class III 40 +
Obese class II 35.0 - 39.9
Obese class I 30.0 - 34.9
Overweight 25.0 - 29.9
Normal 18.5 - 24.9
Mild underweight 17.0 - 18.4
Moderate underweight 16.0 - 16.9
Severe underweight < 16.0

Adapted from: Obesity, Preventing and managing the global epidemic, WHO technical report, NO. 894, WHO, Geneva 2000 [8]

Population specific cut offs are also available for assessing overweight and obesity in Asians as these populations show different associations between BMI, percentage body fat and health outcomes than their white counterparts. The proportion of Asians with a higher risk of diabetes and cardiovascular disease is significantly higher at BMIs lower than the recommended WHO cut offs for overweight. Thresholds of 23 kg/m2 or and 27.5 kg/m2 have been identified as increased risk and high risk respectively. The following categories have therefore been suggested:

1. <18.5 kg/m2 underweight
2. 5-23 kg/m2 increasing but acceptable risk
3. 23-27.5 kg/m2 increasing risk
4. >27.5 kg/m2 high risk

However, one clear BMI cut off point is not applicable to all Asian populations for overweight and obesity as the observed risk varies from 22 kg/m2 to 25 kg/m2 and for high risk from 26 kg/m2 to 31 kg/m2.

Increasing risk is a continuum with increasing BMI, and the cut off values are merely a convenience for public health and clinical use.

Strengths

1. Easy to measure.
2. Non-invasive.
3. Standardized cut off points for underweight, overweight and obesity available.
4. Strongly correlated with body fat levels measured by direct/objective methods such as DEXA or MRI.
5. An accurate predictor of morbidity and mortality associated with obesity.

Limitations

1. A measure of excess weight rather than excess fat.
2. It does not provide information on the proportions of fat and fat-free mass.
3. Poor indicator of fat distribution and central obesity.
4. Age, sex, ethnicity and lean mass influence the relationship between BMI and body fat and those factors are not accounted for in BMI:
1. On average, older individuals tend to have higher body fat than younger adults for an equivalent BMI.
2. On average, women have higher body fat than men with an equivalent BMI.
3. On average, Asians have higher body fat than whites with an equivalent BMI.
4. Athletes may have a high BMI due to increased lean mass.
5. In children height, sexual maturation and age influence the relationship between BMI and body fat. For this reason, for infants, children and adolescents, BMI has to be interpreted relative to a child's age and sex (refer to BMI for age and other growth indices).
6. The accuracy of this index in children and adolescents varies according to individualâ€™s degree of fatness:
1. Poor indicator in overweight children as a high BMI may be due to increased level of fat or fat free mass.
2. Poor Indicator in thin children as differences in BMI are often due to differences in fat free mass.
7. Unable to detect sarcopenic obesity (loss of muscle mass and function present in the context of obesity) and therefore its effect on morbidity and mortality may be underestimated.

Considerations relating to the use of body mass index in specific populations are described in Table 2.

Table 2 Application of body mass index in different populations.

Population Comment
Pregnancy Suitable. Women will also have a higher BMI during pregnancy because of increased weight associated with pregnancy, but not necessarily due to increased fat. Therefore, BMI will overestimate body fat in this population. Pre-pregnancy and post pregnancy BMI as well as weight gain during pregnancy are typically used to assess a pregnant womanâ€™s weight status.
Infancy and lactation Refer to BMI for age and growth indices as the relationship between body mass index and fatness in children varies substantially with age, height and sexual maturation. Age, sex, height and weight are taken into account when calculating these indices.
Toddlers and young children Refer to BMI for age and growth indices as the relationship between body mass index and fatness in children varies substantially with age, height and sexual maturation. Age, sex, height and weight are taken into account when calculating these indices.
Adolescents Refer to BMI for age and growth indices as the relationship between body mass index and fatness in children varies substantially with age, height and sexual maturation. Age, sex, height and weight are taken into account when calculating these indices.