Rapid Responses to:
|
|
Rapid Responses: Rapid Responses published:
-
![[Read Rapid Response]](/icons/misc/sectionDOWN.gif)
Thinness,BMI and Body Size
- C Kevin Connolly (1 August 2007)
-
New criteria for thinness are very close to Dutch criteria for (severe) underweight
- Stef van Buuren (6 August 2007)
-
Re: Thinness,BMI and Body Size
- Tim J Cole (6 October 2007)
-
Clarifying the grades
- Tim J Cole (14 April 2008)
|
|
||||||||
|
C Kevin Connolly, Retired Physician Aldbrough House,Aldbrough St John, Richmond ,North Yorkshire, DL11 7TP
Send response to journal:
|
Dr Cole and his colleagues are to be congratulated on producing tables that accurately reflect risk of malnutrition as assessed by BMI, but they might have questioned why their results appear to suggest that the hazard of thinness is least in infants. In his editorial Dr Cameron points out that BMI is not a measure of obesity. He suggests it does have some relationship to body composition, but justifies it by using correlation coefficient, the incorrect measure of an anticipated association [2]. One might go further and say that the discussion in the paper itself is fundamentally flawed in suggesting BMI is a measure of thinness or indeed obesity. The recognized index which is closest to being a surrogate for these dimensionless concepts is Ponderal Index (Weight/Height3). This is a true index in being itself effectively dimensionless, because with objects of similar density, mass, and therefore weight, has dimension equivalent to length cubed. Body Mass Index is a misnomer, as it is not dimensionless, being proportional to height (Height3/Height2). In simple terms the taller individuals of similar build, the greater is their BMI. For example a very tall person (1.9 m) of the same composition as a short person (1.55m) of ideal proportions (BMI 25) will be assessed as obese (BMI 30.6), or a tall modern female fashion model (1.85m) of the same degree of thinness as an undernourished more traditional one (1.65m, BMI 17) will appear to be adequately nourished (BMI 19.1). BMI is conventionally used as the measure of risk from obesity and thinness because it has been shown to perform better than ponderal index in the populations studied. This implies that obesity is less dangerous or conversely that thinness is a greater risk in those of short stature within those populations, but because BMI has a dimension cut –off values cannot be transferred to other groups. As might be deduced from the first paragraph, the curves in Dr Cole’s paper are similar to normal growth charts, but the gradients are strikingly attenuated. As heat loss per unit mass is inversely proportional to body size, and so such factors as skin thickness and calorific reserve become more critical in infants. This explains the attenuation of the gradient in the graphs in the paper. Had ponderal index rather than BMI been used the gradients would have been reversed, with a greater desirable ponderal index in the young rather than older children, reflecting the greater danger of thinness in infants. Aside from the potential and recognized errors arising from differences in the proportion of fat and lean body mass which may be less than expected [3], there is a further complication. The relationship between desirable body mass and both BMI and ponderal index is confounded because both only consider length along the longest axis. Potential for heat loss is related to shape, being least when it approximates to spherical. Therefore probably the best measure to use to determine optimal mass is the dimensionless index weight/height x waist2. 1. Cameron N (2007) Body mass index cut offs to define thinnessin children and adolescents. BMJ, 335, 166-167. 2.Bland JM, Altman DG. (1986). Statistical methods for assessing agreement between two methods of clinical measurement. Lancet, i, 307-310 3.Freedman DS, Wang J, Maynard LM, Thornton JC, Mei Z, Pierson RN, et al. Relation of BMI to fat and fat-free mass among children and adolescents. Int J Obes 2005;29:1-8. Competing interests: None declared |
||||||||
|
|
||||||||
|
Stef van Buuren, Professor TNO Quality of Life, P.O. Box 2215, 2301 CE Leiden, The Netherlands
Send response to journal:
|
Stef van Buuren (stef.vanbuuren@tno.nl) 1 TNO Quality of Life, Leiden
Cole et al. (2007) present new cut off values for three classes of thinness. In their motivation, the authors state that “There are no valid BMI cut offs for assessing underweight or wasting in adolescents or children over 5 years” and “There are no suitable thinness cut offs for this age group”. I like to point out that some years ago I published criteria for underweight and severe underweight for children 2-18 years (van Buuren, 2004). This work appears to be somewhat unknown, presumably because it appeared in Dutch. I used a similar approach as in Cole et al (2007), and derived cut off values that were anchored at BMI-values of 17 and 18.5. However, having no access to the references of all six studies, I restricted the analysis to the data from the Third Dutch Growth study only (Roede & Van Wieringen, 1985), for which Cole and Roede (1999) had published BMI references. It is of interest to study the differences between the criteria. Figure 1 plots the cut offs as published by Van Buuren (2004) and Cole et al (2007). Despite the differences in methodology, the values are very similar across the entire age range. The new cut offs for thinness are virtually always located within 0.2 BMI points of the “Dutch cut offs” for (severe) underweight. In clinical work, such differences easily fall within the bounds of measurement error. Differences in prevalence estimates are likely to be minor, and will nearly always be swamped by sampling variation. The differences between both sets of cut offs are considerably smaller than those observed between the centile curves of the six international studies as portrayed in figures 1 and 2 of Cole et al (2007). All in all, it appears that the cut off values proposed by Van Buuren (2004) and Cole et al (2007) are remarkably similar. This begs the question what cut offs to use. Evidently, the large exposure of BMJ combined with Cole’s authority will be a major factor in the worldwide adoption of the cut offs, which in itself would be a good thing. On the other hand, we should not be blind to the shortcomings of the new thinness cut offs. In the process of combining data from different studies, Cole et al. average centile curves. There is no statistically defensible rationale for this averaging operation, and it results in an intractable statistical distribution. Consequently, as noted by the authors, it is not possible to calculate Z-scores related to the new thinness cut offs. The criteria derived by Van Buuren (2004) do not have this problem. The cut offs correspond to a set of fixed centiles from a well defined reference distribution. The availability of such a distribution allows for the calculation of Z-scores. Thus, in addition to establishing cut offs, it is possible to assess BMI corrected for age and sex in a continuous way. The possibility to calculate Z-scores may well give the edge to the “Dutch” criteria for underweight and severe underweight.
References Cole TJ, Flegal KM, Nicholls D, Jackson AA. Body mass index cut offs to define thinness in children and adolescents: international survey. BMJ 2007;335;194. Cole TJ, Roede MJ. Centiles of body mass index for Dutch children aged 020 years in 1980—a baseline to assess recent trends in obesity. Ann Hum Biol 1999;26:3038. Roede MJ, van Wieringen JC. Growth diagrams 1980: Netherlands third nation-wide survey. Tijdschr Soc Gezondheidsz 1985;63(suppl):1-34. pdf Van Buuren S. Afkapwaarden van de 'body-mass index' (BMI) voor ondergewicht van Nederlandse kinderen. [Body-mass index cut-off values for underweight in Dutch children] Ned Tijdschr Geneeskd. 2004;148(40):1967-72. pdf
Competing interests: None declared |
||||||||
|
|
||||||||
|
Tim J Cole, Professor of medical statistics UCL Institute of Child Health, London WC1N 1EH, UK
Send response to journal:
|
Dr Connolly suggests that our paper [1] is fundamentally flawed in viewing body mass index (BMI) as a measure of adiposity. Our earlier related paper [2] made clear that BMI is only a proxy for adiposity, which to be identified accurately requires a direct measure of body fat. However BMI is popular as it is moderately correlated with adiposity and has the benefit of being widely applicable, since weight and height measurements are ubiquitous. Dr Connolly goes on to state that ponderal index (PI) is better than BMI, but it all depends on what is meant by "better". PI like BMI is an index of weight and height, and so lacks information about body composition. Thus both indices are poor proxies for adiposity, be it thin or fat. His dimensional argument supporting PI over BMI (i.e. dividing weight by height3 rather than height2) is valid when applied across species (e.g. from mouse to elephant) where the correlation between weight and height is close to 1. But within the species of homo sapiens the weight-height correlation is only about 0.7, so the best predictor of weight (based on log-log regression) is 3 x 0.7 ~ 2. This imperfect correlation, which is biologically inevitable, attenuates the height power and ensures that BMI rather than PI is uncorrelated with height.[3] In practice the optimal height power to predict weight varies with age in children, due to heterogeneity in the rate of maturation; it increases from 2 near birth to 3 in puberty and back to 2 or less in adults.[4] Thus PI is briefly better than BMI in puberty. But this is not the whole story. Obesity itself is positively related to height (fatter children tend to be taller), so the optimal index should be correlated with height. Through most of childhood BMI is positively correlated with height while PI is negatively correlated. Thus BMI is consistently better than PI.[5] Dr Connolly argues that the gradients of the BMI cut- offs are strikingly attenuated compared to the centiles of normal growth charts. This is also quite wrong as the cut-offs are centiles, averaged across surveys, and are very similar in shape to the centiles of the constituent national BMI charts. His argument involves heat exchange in infancy, but the cut-offs only start at age 2 years, well past infancy. Dr Connolly's final point is that an index incorporating weight, height and waist should be better than one using just weight and height. This is almost certainly true, but not for the reasons he gives. Waist circumference is a fairly direct measure of central fat, and so provides the information on adiposity that weight-height indices like BMI and PI lack. 1. Cole TJ, Flegal KM, Nicholls D, Jackson AA. Body mass index cut-offs to define thinness in children and adolescents: international survey. Brit Med J 2007;335:194-7. 2. Cole TJ, Bellizzi MC, Flegal KM, Dietz WH. Establishing a standard definition for child overweight and obesity: international survey. Brit Med J 2000;320:1240-3. 3. Cole TJ. Weight-stature indices to measure underweight, overweight and obesity. In: Himes JH, editor. Anthropometric assessment of nutritional status. New York: Wiley-Liss; 1991. p. 83-111. 4. Cole TJ. Weight/heightp compared to weight/height2 for assessing adiposity in childhood: influence of age and bone age on p during puberty. Ann Hum Biol 1986;13:433-51. 5. Freedman DS, Khan LK, Serdula MK, Dietz WH, Srinivasan SR, Berenson GS. Inter-relationships among childhood BMI, childhood height, and adult obesity: the Bogalusa Heart Study. Int J Obes Relat Metab Disord 2004;28:10-6. Competing interests: None declared |
||||||||
|
|
||||||||
|
Tim J Cole, Professor of medical statistics UCL Institute of Child Health, London WC1N 1EH, UK
Send response to journal:
|
It has been pointed out that the paper is unclear, in the Abstract and Table 4,
about the BMI range at age 18 that corresponds to each thinness grade. To
avoid confusion we repeat the grading here, which corresponds to the WHO
adult classification:
So grade 1 is less
severe
than grade 3. Competing interests: None declared |
||||||||