Australian & New Zealand Journal of Psychiatry, 52(8) https://doi.org/10.1177/0004867418791282 Australian & New Zealand Journal of Psychiatry 2018, Vol. 52(8) 810–812 DOI: 10.1177/0004867418791282 © The Royal Australian and New Zealand College of Psychiatrists 2018 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav journals.sagepub.com/home/anp Measure what can be measured and make measurable what cannot be measured Galileo (1564–1642) The term BMI, which denotes body mass index, seems ubiquitous both in clinical practice and day-to-day ver- nacular. It is based on a ratio of weight to height, and as clinicians, we are increasingly using this index to gauge the physical health of our patients (see Figure 1). But is it a reliable and accurate measure of body habitus? There are several immediate prob- lems. First, it does not account for dif- ferential density of tissues – for example, muscle and bone are gener- ally heavier than other tissues and simply measuring the overall weight of an individual provides little informa- tion regarding tissue composition. Second, the BMI does not accommo- date gender and age differences, which can be substantial given the natural variance in the distribution of fat within the body in men and women of all ages. Third, even within tissues, there are important differences. For instance, fat within the body can be visceral or subcutaneous, with impor- tant implications for overall health – and again the BMI is unable to capture such detail. So why is the BMI used at all? To consider this, we need to review its origins. Lambert Adolphe Jacque Quetelet (1796–1874), an aptly named Belgian polymath who contributed to fields as diverse as astronomy, mathematics and sociology, but was perhaps best recognised as the patriarch of statisti- cians, devised the BMI for the purpose of defining the ‘average man’. Interestingly, this was possibly one of the very first direct applications of mathematics to human characteris- tics. Hence, initially, when first devised in 1832, the BMI was referred to as the Quetelet Index; it was almost a cen- tury and a half later, in 1972, that the American scientist Ancel Keys coined the term BMI and introduced it into our lexicon. A normal BMI is considered to lie between 18.5 and 25.0, overweight between 25.0–30.0 and obesity beyond 30.0. The latter is further sub- divided into Class 1 I obesity a BMI of 30.0–34.9; Class II obesity a BMI of 35–39.9 and Class III denotes obesity that extends beyond 40.0. Of note, the higher the BMI, the greater the risk of type 2 diabetes, cancer, sleep apnoea and cardiovascular illnesses. Interestingly, prior to 1998, Americans in the United States had a BMI cut off that was 2 BMI points above the World Health Organization guidelines for normal and overweight categories – but this has since been brought into line. Critics of the BMI suggest that sim- ply squaring height (m 2 ) does not accommodate sufficiently for the nat- ural variations in habitus observed at the extremes of height. For example, in short people, the BMI creates a dis- proportionately large denominator, whereas in tall people it is relatively modest. Consequently, tall people are prone to have their weight overesti- mated as compared to short people who tend to have it underestimated. This occurs simply because people are three-dimensional, not two, and healthy bodies grow at different rates – ultimately achieving diverse shapes and sizes. To address this, Nick Trefethen (2013), a University of Oxford Professor of mathematics, who questioned the usefulness of the BMI formula, described it as a ‘bizarre measure’, and proposed an alterna- tive: the ‘new BMI’ (see Figure 1) Despite having devised a new measure – purported to be more accurate – Trefethen maintained that any calculation that assigns one num- ber to a person is doomed to be imperfect, because the shape and composition of humans is fundamen- tally too complex to be described by a single figure. Nevertheless, his new calculation of human shape and size was thought to better approximate reality than the traditional BMI. In addition to the technicalities of the BMI, recent studies have cast doubt over the assumption that a higher BMI automatically denotes a health risk and suggest that especially from middle age onwards, a slightly higher BMI may actually protect Changing body mass index: The need for a more measured approach? Cornelia Kaufmann 1,2 , Neelya Agalawatta 1 , Tim Outhred 1,3,4 and Gin S Malhi 1,3,4 1 Sydney Medical School Northern, The University of Sydney, Sydney, NSW, Australia 2 Ramsay Health Care and Northside Group St Leonards Clinic, St Leonards, NSW, Australia 3 Academic Department of Psychiatry, Northern Sydney Local Health District, St Leonards, NSW, Australia 4 CADE Clinic, Royal North Shore Hospital, Northern Sydney Local Health District, St Leonards, NSW, Australia Corresponding author: Gin S Malhi, CADE Clinic, Royal North Shore Hospital, Northern Sydney Local Health District, Level 3, Main Hospital Building, St Leonards, NSW 2065, Australia. Emails: gin.malhi@sydney.edu.au 791282ANP ANZJP ReflectionsANZJP Reflections Reflections