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Re: Clinical significance vs. statistical significance in childhood obesity weight management programmes

Dear Editor,

Having developed a hospital-based multidisciplinary outpatient service for obese children (1), we read with interest the recent paper by Rudolf et al reporting early pilot data from the community-based “Watch It” programme (2). The authors report a significant mean reduction in Body Mass Index Standard Deviation Score (BMI SDS) of -0.07 in 48 children (mean age 12.2 years) enrolled into the programme for 6 months. This effect was mainly seen in girls, with little change being seen in boys. Using the British 1990 growth reference data produced by the Child Growth Foundation (3), and presuming normal longitudinal growth, this equates to a mean weight gain of approximately 3kg over 6 months for girls with a height on the 50th centile for the stated mean age of enrolment, and 4.5kg over 6 months for girls with a height on the 99.6th centile for age. This would indicate that this community-based programme is unable to meet the joint advice of the Royal College of Paediatrics and Child Health and National Obesity Forum (http://www.rcpch.ac.uk/publications), which recommends weight maintenance in young children and gradual weight loss of 0.5kg/month in children aged over 7 years with obesity and/or complications (4).

Furthermore, some distinction should be made between statistically significant and clinically significant changes in BMI SDS. Reinehr and colleagues have previously shown that a reduction of 0.5 BMI SDS is required to obtain tangible health benefits in terms of blood pressure, dyslipidaemia or insulin sensitivity (5). Moreover, our group has recently obtained preliminary data using a validated measure of body composition, suggesting that a reduction in BMI SDS of at least 0.5 is necessary to be certain of actual fat mass reduction in childhood. This change is required over at least one year, as seasonal variations in weight impact highly upon results reported from weight management programmes (6). A recent audit of our outpatient-based childhood obesity service (137 obese children aged 2.2 to 17.8 years, attending over 3.5 years) has revealed that in children seen in the service for more than one year (n=58), the mean [range] change in BMI SDS was -0.3 [-1.19 to +0.48] with younger children, particularly from non-obese families, gaining maximal benefit (7). Overall, 83% of children attained reductions in BMI SDS, with 28% achieving target reductions of 0.5 BMI SDS or more. Similarly, evidence from children’s residential weight loss programmes have demonstrated mean reductions in BMI SDS of 0.28 in camp attendees (8), although long-term data are still awaited. We would encourage caution in the interpretation of success (in terms of health benefits) in a community-based project which reports small, but statistically significant, reductions in BMI SDS in less than fifty children over a six month period.

Dr MA Sabin Dr JPH Shield

Clinical Sciences South Bristol, University of Bristol, Bristol, UK.

Competing interests: none declared.

References:

1. Sabin MA, Crowne EC, Shield JPH. Outpatient-based paediatric obesity intervention clinic - Simple advice and support are associated with significant reductions in weight in the first year. Arch Dis Child 2003;88 (Suppl 1):A66.

2. Rudolf MC, Christie D, McElhone S, Sahota P, Dixey R, Walker J, et al. Watch it: a community based programme for obese children and adolescents. Arch Dis Child 2006. In Press.

3. Cole TJ, Freeman JV, Preece MA. British 1990 growth reference centiles for weight, height, body mass index and head circumference fitted by maximum penalized likelihood. Stat Med 1998;17(4):407-29.

4. Gibson P, Edmunds L, Haslam DW, Poskitt E. An approach to weight management in children and adolescents (2-18 years) in primary care. J Fam Health Care 2002;12(4):108-9.

5. Reinehr T, Andler W. Changes in the atherogenic risk factor profile according to degree of weight loss. Arch Dis Child 2004;89(5):419- 22.

6. Gillis L, McDowell M, Bar-Or O. Relationship between summer vacation weight gain and lack of success in a pediatric weight control program. Eat Behav 2005;6(2):137-43.

7. Sabin M, Ford A, Hunt L, Jamal R, Crowne E, Shield J. Which factors are associated with a successful outcome in a weight management programme for obese children? J Eval Clin Prac 2006. In Press.