Body mass index (BMI) has traditionally been used to identify individuals who are the most likely to be overweight or obese. It is calculated by dividing the weight (in kilograms) by the height (in metres) squared. Generally, a high value indicates excessive body fat and consistently relates to increased health risks and mortality. Unusually large muscle mass, as in trained athletes, can increase BMI to 30, but rarely above 32. BMI categories and cut-offs are commonly used to guide patient management. BMI reference ranges assume health in other aspects—healthy weight may be lower with major muscle wasting.
Waist circumference was developed initially as a simpler measure—and a potentially better indicator of health risk than BMI—to use in health promotion. Waist circumference is at least as good an indicator of total body fat as BMI or skinfold thicknesses, and is also the best anthropometric predictor of visceral fat.
People with increased fat around the abdomen or wasting of large muscle groups, or both, tend to have a large waist circumference relative to that of the hips (high waist to hip ratio). Waist circumference alone, however, gives a better prediction of visceral and total fat and of disease risks than waist to hip ratio. Waist circumference is minimally related to height, so correction for height (as in waist to height ratio) does not improve its relation with intra-abdominal fat or ill health.
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Adolphe Quételet was a 19th century Belgian scientist who established the body mass index to classify people's ideal weight for their height
BMI is still a useful guide to obesity related health risks, but waist circumference is a simple alternative with additional value for predicting metabolic and vascular complications
People with a large waist are many times more at risk of ill health, including features of metabolic syndrome (such as diabetes, hypertension, and dyslipidaemia) as well as shortness of breath and poor quality of life. These increased risks also apply in people whose BMI is normal but who have a large waist. However, BMI and waist circumference are colinear, so combining the two measures adds relatively little to risk predicton.
Figure 3 Incidence of metabolic syndrome in people with different categories of body mass index and of waist circumference action levels (action level 1=94 cm in men and 80 cm in women, action level 2=102 cm in men and 88 cm in women). Adapted from Han TS et al. (more ...)
During weight loss, each kilogram of weight loss is equivalent to a reduction of 1 cm in waist circumference. However, there is greater measurement error for waist circumference, so body weight is the best measure for monitoring change.
Figure 4 Prevalence of diabetes (a); hypercholesterolaemia, low HDL (high density lipoprotein), or hypertension (b); shortness of breath (c); and poor quality of life (d) in people with large waist. Adapted from Lean ME et al.
1998;351: 853-6 [PubMed]
Waist to hip ratio was introduced—mainly as a result of Swedish research—on the assumption that it would predict fat distribution better than waist circumference alone. Subsequent research, however, showed that it did not.
Classification of overweight and obesity by body mass index, waist circumference, and associated disease risk*
(adapted from data from National institutes of Health)
Hip circumference does have a relation to health and disease, but in an inverse way, such that a relatively large hip circumference is associated with lower risks of diabetes and coronary heart disease. This is probably because hip circumference reflects muscle mass, which is reduced in type 2 diabetes and inactivity.
Waist to hip ratio and myocardial infarction
Weight gain leads to greater adverse metabolic changes in certain ethnic groups. As a result, Asians should be considered overweight if BMI ≥23 and obese if BMI ≥27.5. Waist levels associated with risk are also lower in Asian men (≥90 cm v ≥94 cm in Europoids)
Perceptions of anthropometry
The main difficulty with anthropometric measures is that doctors, scientists, and the public are not aware of the value of these measures. People often assume that technological devices—such as fat analysers—are better at measuring body fat, despite evidence to the contrary. This assumption often arises from better marketing of technology, yet no portable body fat analysers (including those that measure bioelectrical impedance, which is highly dependent of body hydration status) are better than waist circumference for measuring body fat in adults.
Figure 5 Accuracy in measuring waist circumference can be improved with use of a specially designed tape measure, although a change in body fat may not be detected by waist circumference in very fat people, when the abdominal fat mass is pendulous. During waist (more ...)
Cut-off levels of waist circumference relating to increased health risks have not been fully defined for different ethnic groups, although some African and Asian groups clearly have a greater risk of coronary heart disease than Europoids at the same cut-off levels. Two people of the same BMI may have very distinct body shapes, depending on the distribution of body fat and skeletal muscle. A change in single measures, such as the amount of weight loss or reduction in waist circumference, is easily understood by lay people, whereas a ratio (such as waist to hip ratio or BMI) is more difficult to conceptualise. BMI charts can help.
Figure 6 Variation in human body fat distribution in men and women. In each pair of men and pair of women (subjects A and B), the body mass indices are similar. However, the waist circumference and waist to hip ratios of subjects B are much higher, indicating (more ...)
Weight should be measured by digital scales or a beam balance to the nearest 100 g. Equipment should be calibrated regularly by standard weights (4×10 kg and 8×10 kg), and the results of test weighing recorded in a book. Patients should ideally be weighed in light clothing and bare feet, ideally fasting and with an empty bladder.
Images of different fat compartments by computed tomography. The inner elliptic ring shows intra-abdominal fat
Height is measured with a regularly calibrated stadiometer. Patients stand in bare feet that are kept together. The head is level with a horizontal Frankfort plane (an imaginary line from lower border of the eye orbit to the auditory meatus).
If a patient cannot stand—for example, is confined to a chair or bed—BMI can still be derived from special equations using arm span or lower leg length instead of height.
Waist circumference should be measured midway between the lower rib margin and iliac crest, with a horizontal tape at the end of gentle expiration. Waist circumference measurement at the umbilical level is not reliable because sagging of abdominal skin occurs in very obese subjects or those who have lost weight previously.