Honey I shrunk the Kids! Daily variation in Height and Weight and it’s Implications for BMI Based Public Health Surveillance

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Travis’ Note: Today’s post comes from PhD Student Ash Routen.  You can find out more about Ash and his work at the bottom of this post.

Consistent with the majority of developed countries, a significant proportion of children here in the UK are overweight or obese (around 30% of 10-11 year olds as of 2010). How do we know this? Well, since 2005 the UK Department of Health have been operating the ‘National Child Measurement Programme’ (NCMP) a nationwide public health surveillance initiative, which to the best of my knowledge is the biggest in Europe. Annually over one million children (aged 4-5 and 10-11 years) have their height and weight measured by teams of school nurses, these measures are then used to calculate body mass index (BMI), and then BMI determined weight status. This data is used to inform local planning and delivery of weight intervention/healthy lifestyle services, and to produce national overweight and obesity prevalence figures.

As BMI differs between genders and increases with age during childhood, the NCMP categorise the children’s weight status as underweight, healthyweight, overweight or obese, by comparing their BMI to children the same age and gender using a BMI growth reference chart. So, children above the 91st percentile (91% of all children used in the reference sample) are defined as overweight, and above the 98th as obese. In most regions, the child’s weight status is fed back to the parent’s and children by letter, with information on local weight management initiatives if they are categorised as underweight, overweight, or obese. Of course you can imagine the furore of some parent’s and thus the NCMP has attracted quite a lot of negative media attention (e.g.http://www.telegraph.co.uk/health/children_shealth/7514267/Letter-to-fat-four-year-old-prompts-complaint-from-obesity-group.html) as a result. As such there is great onus on ensuring the quality of data and identifying any sources of potential ‘error’, which include human (i.e. reliability of the nurses measurements), technical (i.e. reliability of the scales/height measuring device) and biological ‘error’(i.e. both daily and monthly variation in BMI).

I was interested in the impact of the time of day when the measurements are conducted. The nurses follow a standardised protocol, but can take the measurements at any time of day (and indeed the month of measurement may also vary). We know that our weight fluctuates throughout the day, and that we shrink a little after rising due to gravity pushing us back down! What we didn’t know was if combined variation in these measures would result in a change in BMI. Who cares right? they would be at no more or less risk of adverse health if their BMI shifts a little…but could it be enough for those whom are on the cusp of a BMI weight category (e.g. 90.5 percentile) to be differently classified due to the time of day they are measured?

What did we do?

To investigate this issue we took a sample of 74 children (aged 10-11 years) and measured their height and weight in the morning (0900-1045 hr) and again in the afternoon (1300-1500). From this we calculated their BMI, BMI percentile and weight status category using two set’s of BMI percentile cut-off’s, namely clinical cut-off’s (overweight: 91st and obese: 98th) as used by the NCMP and clinicians, and population monitoring cut-off’s as used mainly by researchers (85th and 95th centiles).

What did we find?

Not surprisingly in the afternoon all the children were shorter (-0.5 cm), however only girls were heavier (+0.1 kg), and BMI (+0.12 kg.m2), and BMI percentile was greater (+2.5 centiles) in all children. In relation to weight status categories there were no shifts in the number of people in each category from morning to afternoon, but on an individual level there were some interesting findings. When applying the clinical BMI cut-off’s we saw that one girl moved from healthyweight to overweight, and using the population monitoring cut-off’s , two girls moved from the healthyweight to overweight category, and one moved from the overweight to obese category with BMI increases of only 0.30, 0.55 and 0.26 kg/m2, respectively.

What are the implications?

We saw that it only takes a height loss of about 1 cm and an increase in weight of about 150g to shift a girls BMI category if they are near to the cut-off threshold. As only a few individuals changed (and this was a small sample) the results may seem inconsequential. However both on an individual and national level there may be some impact. Nationally, comparison of prevalence data (and thus future direction of resources) between schools and regions (and this is where I speculate) may be may be clouded if they measure a greater proportion of their children either in the morning or the afternoon. Whilst the extrapolation of the present observations using the clinical BMI cut-off’s (which the NCMP use in parental feedback) to the potential impact on the national NCMP data is tenuous, it is worthy of consideration. As the time of day when measurements are taken is not standardised, or recorded by the NCMP it could be supposed that 50% of the measurements taken are performed in the morning and 50% in the afternoon. If one in every 27 (3.7%) healthy weight girls (as we found in our sample) were on the cusp of overweight in samples measured in the morning they could well have been categorised as overweight had they been measured in the afternoon. Out of the 162,640 healthyweight girls measured by the NCMP in 2009/10 this would represent 6017 girls  being classified as overweight instead of healthyweight, which hinders a ‘true’ assessment of prevalence data.

Arguably of more importance is that potentially 6017 parents and children would be informed that their child is overweight, due to their misfortune of being measured in the afternoon as opposed to the morning. We know that children labelled as overweight may be at greater risk of stigmatisation, teasing and anxiety; it is not unimaginable therefore that such a letter could trigger unhealthy activity and dietary habits and unnecessary parental intervention. For all the useful information such screening programmes provide us researchers, we must be cognisant of the discourse surrounding the issue of childhood obesityand consider the impact of such surveillance programmes on individual children and families (see Michael Gard’s work for a thought provoking viewpoint: http://bod.sagepub.com/content/13/4/118.extract).

What can we do?

In our paper we conclude that arguably, to increase data reliability the time of day in which the measurements are performed should be standardized (to either morning or afternoon) by the NCMP and indeed any public health surveillance programme that does not standardise measurement – this would at least ensure that all children are treated equitably. We do not have one ‘true BMI’, but fluctuate about a mean value on a daily and weekly basis. Therefore for the purposes of comparison, and analysis of trends year-on year, we should choose either to measure in the morning or afternoon. However, on the individual level it appears wise to ensure that children are measured in the morning to avoid unfavourable shifts in weight category and associated psychosocial implications of labelling. Standardisation of the timing of taking the measurements is one simple revision to the procedures of such surveillance programmes that could help to limit the impact of at least one potential ‘error’ variable.

Ash Routen

About the author: Ash Routen is in the final months of his doctoral studies at the University of Worcester, UK examining the impact of pedometer interventions on habitual PA in kids, with an interest in the assessment of body composition and objective physical activity measurement in kids.  He can be found on Twitter @AshRouten.

Routen, A., Edwards, M., Upton, D., & Peters, D. (2011). The impact of school-day variation in weight and height on National Child Measurement Programme body mass index-determined weight category in Year 6 children Child: Care, Health and Development, 37 (3), 360-367 DOI: 10.1111/j.1365-2214.2010.01204.x

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9 Responses to Honey I shrunk the Kids! Daily variation in Height and Weight and it’s Implications for BMI Based Public Health Surveillance

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  3. WRG says:

    My first reaction is: why is the BMI still the arbiter of all that is healthy?

    If a child classified as “overweight” gets sent home with “nutritional and lifestyle” advice, the assumption is that this child’s parents are somehow doing something wrong. Do we still deny the fact that not all people are born slim? I’m sure there are plenty of families that eat healthy, balanced diets, low in additives and high in fresh fruit and veg whose members nevertheless have a higher than “normal” BMI.

    Let’s now look at the child classified as “normal weight” who lives in a home where Coke is served with the evening meal, many meals are cheap take-out (high in sodium, low in nutrients), white bread is the norm, chocolate milk is seen as a health food, etc. No one’s going to send nutritional information to that child’s family. That child is “healthy” (lol).

    I’m a firm believer in physical activity and healthy food. But BMI information is not going to help target the kids and families who really need help in developing a healthier lifestyle. It’s just going to shame kids even more and push them yet further to the margins. Who wants a fat kid on their baseball team, I ask you?

    It’s interesting that you would post this article today. Arya Sharma, who’s away on vacation just re-posted a couple of articles on the BMI and why it’s basically useless. They make a nice counterpoint to this article.

    Isn’t it time to move on to health promotion and leave weight shaming in the dust, where it belongs?

    Hope my tone is less abrasive than usual. I’m trying. And BTW, as they say where I come from, “mazal tov” on your upcoming wedding!

    • Travis says:

      Thanks for the kind words :)

      I can’t really speak to the purpose of the survey itself, and I think it’s somewhat tangential to the paper’s findings – that info is only there to give people some background on the dataset. I will tell you that we put some thought and effort into how to describe the study while minimizing possibilities for misinterpretation (I’m trying too!). Frankly I’m not sure how I feel about sending parents letters about their child’s weight status as I’ve heard good arguments on both sides of the issue. I’ll agree that there is the potential for harm with these types of programs, but that’s really a separate issue from these analyses (e.g. if we deleted all of the text above “What did we do”, it wouldn’t alter the take-home message from this post in any way).

      I actually think the take-home message from this paper is quite similar to Arya’s recent posts on BMI – it is a measure with important limitations, and people who ignore those limitations do so at their own peril. I certainly wouldn’t argue that BMI is a perfect measure (my MSc supervisor has built his career around the importance of other measures), but so long as BMI is the most widely used method of assessing weight status it’s important to know whether it is doing so reliably.

    • Ash Routen says:

      Hi Wrg,

      Thanks for your comments. Of course BMI has it’s limitations (one of which is clearly outlined by my paper) and should not be the sole judge of whether an individual is healthy but, it does still remain a simple and cheap tool for monitoring population level trends.

      I don’t think anyone knows as yet what the ramifications are of such programmes, but I know there is a project under-way at UCL (London) investigating this issue in the NCMP. One part of me thinks yes it is a good idea as it will likely find those who are overweight, and help with preventive efforts. The other side of the picture is that those who are not truly overweight may be miss-classified as we found; again with unknown consequences. Further, as you state even for those who are overweight, is it appropriate for them to be labelled at such a young age? This is a question for a psychologist really and one which I could not comment upon. As I said, we should be careful of the ‘obesity discourse’ and reflect upon the benefit-harm ratio of such weight feedback initiatives at regular time-periods.

      Ideally we would have wider ranging programmes covering cardiovascular fitness, blood pressure, metabolic measures and other health-related fitness markers as these may be just as important for disease risk (see the Fat vs. Fit debate-http://news.bbc.co.uk/1/hi/health/4778274.stm)

      • ksol says:

        Alas, it’s used more often as a be-all, end-all measure of individual fitness instead of at population-level. And it’s a highly imperfect measure of individual health, anyway. I’m in the high overweight range with good lipids, fasting glucose and aerobic capacity. It gets irritating to hear that I’m going to die, die, die if I don’t lose 25 pounds.

  4. Trindel Maine says:

    It seems to me that the real problem addressed here is that hard cut-offs are seldom truly meaningful. I seriously doubt there is a identifiable health risk difference between the child that is in the 90.5 reference percentile and the child that is in the 91.5 reference percentile. I don’t have the expertise to know how big that buffer should be but I’m sure its there. A simple fix would be to present the data as a bar graph with reference lines at the level borders, possibly giving said reference line width to indicate this buffer. Then one can easily see that their child is just barely above or below a cut-off. So you neither foster complacency for the child that is just below a border nor panic for the child just above it. Both children probably should lose a few pounds or grow a few inches (lots of kids seem to bulk up just before shooting up), and there is little reason to suppose that the child just above the cut-off is significantly worse off than the child just below it.

  5. Liisa says:

    I always wonder how anyone in the know, and healthcare professionals should the hell be in the know, can use BMI as anything else than it was designed for – a statistical tool. I’m one of those outlying folks, being simply tall, big and heavy. Obviously, I got my fair share of crap because when I was around 10, my paediatrician looked in some tables, explained my mom that normal for girls my age is this-and-that and I’m three kilos above and that mom should do something about it. Fast forward some eight or ten years when I was eating twice a week. I still haven’t really recovered from eating disorders and I keep oscillating between anorexic behaviour and binge eating. Alright, I tend to binge tomatoes but that’s improvement only in the physical bit. Borked mind is still there.

    Based on my experience, I agree with Trindel Maine and everyone else who is wary of the hard lines. I guess it wouldn’t be too difficult to tweak the guides and have an area of ‘beware, close to being overweight/obese” or some such.

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