Is BMI the best measure of obesity?
BMJ 2018; 360 doi: https://doi.org/10.1136/bmj.k1274 (Published 29 March 2018) Cite this as: BMJ 2018;360:k1274
All rapid responses
Let’s not give up too quickly on anthropometric indicators that describe abdominal adiposity. The supine sagittal abdominal diameter (SAD; also called “abdominal height”) correlates more specifically with visceral fat rather than with abdominal subcutaneous fat. Thus, variation in the SAD, compared to the waist circumference, better reflects levels of cardiometabolic risk. For persons able to lie supine, the SAD can be measured reliably by a portable, sliding-beam caliper (http://www.cdc.gov/nchs/data/nhanes/nhanes_13_14/2013_Anthropometry.pdf).
Population-based comparisons of SAD:height ratio (SADHtR), waist:height ratio (WHtR) and BMI confirm the advantages of SADHtR for estimating adult risk assessment [1]. Contemporary reference values for SADHtR have been published [2].
Competing interests: No competing interests
Henry S Kahn
Professor Emeritus, Emory University School of Medicine
Atlanta, Georgia USA
= = = = = = = = =
[1] Kahn HS, Cheng YJ. Comparison of adiposity indicators associated with fasting-state insulinemia, triglyceridemia, and related risk biomarkers in a nationally representative, adult population. Diabetes Res Clin Pract. 2018;136:7-15.
[2] Kahn HS, Bullard KM. Indicators of abdominal size relative to height associated with sex, age, socioeconomic position and ancestry among US adults. PLoS One [Internet] 2017; 12(3): e0172245. Available from: http://dx.doi.org/10.1371/journal.pone.0172245.
Competing interests: No competing interests
The current childhood obesity thresholds for clinical and epidemiological purposes in the UK are the 98th and 95th centiles, respectively, using the UK1990 reference curves in UK (not the other way around as was stated in the article).
The rationale for these thresholds is described in the linked report: https://www.gov.uk/government/publications/sacn-statement-defining-child...
Competing interests: No competing interests
Although useful, the BMI is a poor measure of fatness. For adults, this is partly because BMI values show a positive correlation with the ratio of sitting height to total height that is usually ignored. For children and adolescents, the BMI is even less satisfactory since it needs to be interpreted in terms of age and reference populations. This is mainly because height and bodily proportions alter with growth. However, there is a related anthropometric index that theory and statistical findinss show to be superior to the BMI from age two to adulthood (Burton, 2018)*. This is the ratio (body mass)/(sitting height)3. Relationships with fatness and health have yet to be quantified, but should be closer than for the BMI. Incidentally, because this ‘sitting height index of build’ is independent of relative leg length, the latter can be treated separately as an indicator of nutritional history and health risks.
* Burton RF. The sitting-height index of build, (body mass)/(sitting height)3, as an improvement on the body mass index for children, adolescents and young adults. Children 2018;5: 30. doi:10.3390/children5020030
Competing interests: No competing interests
Patrick Woodburn's suggestion of using self-reported trouser size unfortunately does not consider how 'vanity sizing' has normalised obesity and that trousers marked as, for example '34 inches' will now accommodate a rather more rotund belly with a waist of up to 37 inches.
I'm afraid direct measurement is needed.
Competing interests: No competing interests
May I suggest that an advantage of waist:height ratio that this article overlooks is that, for men in the UK at least, it can reasonably be estimated from a man's trouser size and self-reported height without any direct measurement? For instance, if you are 5'10 (70 inches), this means that your ideal trouser size (between 40 and 50% of your height) should be between size 28 and size 35. Below that and you have too little central adiposity; above that and you have too much.
Competing interests: No competing interests
Re: Is BMI the best measure of obesity?
The various responses to the editorial on the best measure of obesity highlights the varying opinions and range of alternative measures that are in existence. Dr Woodburn, Dr Burton and Professor Kahn all suggest alternative measurements and rightly emphasise the limitations of BMI as the perfect measure. However, whilst these alternative measurement methods have some validity and are useful for some practical situations, they have not been as widely evaluated as BMI and not always as reliable.
Professor Ong has pointed out an error in the article, and correctly has stated that the actual thresholds for childhood obesity for clinical and epidemiological purposes in the UK are the 98th and 95th centiles, respectively, using the UK1990 reference curves. We apologise for our mistake in having written this the wrong way round in the editorial and not picking up on this error during proof reading.
Competing interests: No competing interests