• What is assessed?
• How is the measurement conducted?
• When is this method used?
• How are estimates of body composition derived?
• Strengths and limitations
• Populations
• Further considerations
• References

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.

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 individual’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.

Refer to section: practical considerations for objective anthropometry

1. Chung, S. (2015). Body mass index and body composition scaling to height in children and adolescent. Ann Pediatr Endocrinol Metab, 20(3), 125-129.
2. Cole, T. J., Faith, M. S., Pietrobelli, A., & Heo, M. (2005). What is the best measure of adiposity change in growing children: BMI, BMI %, BMI z-score or BMI centile? Eur J Clin Nutr, 59(3), 419-425.
3. Mei, Z., Grummer-Strawn, L. M., Pietrobelli, A., Goulding, A., Goran, M. I., & Dietz, W. H. (2002). Validity of body mass index compared with other body-composition screening indexes for the assessment of body fatness in children and adolescents. Am J Clin Nutr, 75(6), 978-985.
4. Shetty PS & James WPT (1994) Body mass index: a measure of chronic energy deficiency in adults. FAO Food and Nutrition Paper no. 56, pp. 1–57. Rome: FAO.
5. Tyrrell, V. J., Richards, G. E., Hofman, P., Gillies, G. F., Robinson, E., & Cutfield, W. S. (2001). Obesity in Auckland school children: a comparison of the body mass index and percentage body fat as the diagnostic criterion. Int J Obes Relat Metab Disord, 25(2), 164-169.
6. Wells, J. C. (2001). A critique of the expression of paediatric body composition data. Arch Dis Child, 85(1), 67-72.
7. Wells, J. C. (2014). Toward body composition reference data for infants, children, and adolescents. Adv Nutr, 5(3), 320s-329s.
8. World Health Organization. Obesity: Preventing and Managing the Global Epidemic: Report on a WHO Consultation (WHO Technical Report Series 894). Geneva, Switzerland: World Health Organization; 2000.
9. World Health Organization Expert Consultation. Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies [published correction appears in Lancet. 2004;363:902]. Lancet. 2004;363: 157–163