This systematic review showed that evidence on the prevalence of obesity among school age children and adolescents in the GCC states is limited. Only one nationally representative survey was identified, and only 3/6 GCC states had any eligible data from the past 11 years, with multiple gaps in the evidence (e.g. for certain age groups) and weaknesses in the evidence (e.g. reliance on non-representative samples, lack of national surveys). More extensive and higher quality surveillance of obesity among school age children and adolescents in the GCC is required in future if the GCC states are to address the obesity epidemic effectively . Regular high quality surveillance is essential to assess the scale of the obesity problem, to identify trends and inequalities, to drive obesity prevention and control measures, and to assess the impact of policy measures aimed at obesity prevention and control .
Despite limitations in the evidence base on obesity prevalence in the GCC nations noted above, some trends were apparent from the 11 eligible studies. First, the prevalence of obesity according to BMI-for-age was very high. For example, prevalence of obesity in UAE according to the WHO definition exceeded one third of the sample in the secondary school-age participants, and increased with increasing age. One-quarter to one third of participants were obese according to the WHO definition in the Kuwaiti national survey. Moreover, BMI-for-age substantially underestimates the prevalence of obesity (excessive fatness) in children [12, 13] so ‘true’ prevalence of obesity in these studies in the GCC would have been even higher if this bias arising from use of the BMI had been accounted for. None of the eligible studies or surveys acknowledged that their prevalence estimates were subject to this source of bias, or attempted to adjust for it. A large recent study  across Africa found that the WHO-BMI-for-age definition of obesity only identified around one third of children with excessive body fatness measured by a reference method (Total Body Water). Second, in most of the eligible studies the prevalence of obesity was higher in boys than girls, suggesting that this is a real difference in susceptibility to paediatric obesity in the GCC states. It should be noted that prevalence of obesity among the girls would also be regarded as very high relative to other nations [10, 11]. Third, the eligible studies and surveys which compared prevalence estimates by the different definitions based on BMI-for-age found consistently that prevalence was substantially lower when the IOTF definition of obesity was used compared to definitions based on the CDC or WHO, consistent with previous evidence .
There are no previous systematic reviews of the prevalence of child or adolescent obesity from the GCC, and so the results of the present study cannot be compared easily with other evidence. Comparisons of prevalence of obesity among children and adolescents in the GCC with those living in other countries is also difficult because of differences in the timing of the studies, differences in the definitions of obesity used, and whether or not obesity prevalence estimates (as distinct from overweight prevalence estimates, or prevalence of overweight and obesity combined) can be found in published studies.
The present review found major limitations of obesity surveillance in the GCC, notably the apparent lack of any recent surveillance data from 3 of the 6 GCC countries, the availability of nationally representative sample data from only 1/6 GCC countries, the small sample sizes and scarcity of power calculations (and confidence intervals around prevalence estimates), and the fact that bias in the use of BMI-for-age to generate prevalence estimates was not considered by any of the 11 eligible studies/surveys. In addition, eligible study and survey response rates were often very low (under 50%), and not reported in all of the eligible studies and surveys. It should be noted that many of the studies did not set out to obtain nationally representative samples, and estimating obesity prevalence was not a primary aim of all of the eligible studies. In addition, a checklist for guiding/assessing the quality of prevalence studies  only became available after many of the eligible studies were conducted. Future studies and surveys of child and adolescent obesity prevalence in the GCC states and elsewhere may find it useful to refer to the checklist for assessment of prevalence study quality used in the present study .
This review had a number of strengths. First, it focused on obesity-rather than overweight and obesity. While obesity and overweight are often combined somewhat casually in paediatric prevalence studies they are not equivalent clinically or biologically in children, as in adults: there is currently a very large body of consistent evidence of adverse health effects of obesity in childhood and adolescence [23, 25], but the adverse health impact of overweight in childhood and adolescence is much less clear at present. Second, the present review attempted to provide evidence of most relevance and highest quality, by including only relatively recent studies, and only those which used acceptable objective measures of obesity (rather than self-or parent reports), and by formal appraisal of study quality. The conduct of the present systematic review was also intended to follow best practice, by using the AMSTAR tool as a guide to the process, and reporting of the review followed PRISMA guidance. Finally, by making use of extensive expert contacts in all of the 6 GCC states, the probability that eligible studies and surveys (including grey literature) were not identified by the conventional literature search was reduced.
The present review also had a number of limitations. The number of eligible studies was relatively small due in part to our decision to exclude studies which collected data prior to 2007. The rationale for this is that we included only recent studies to provide up to date information, especially important given likely recent rapid increases in obesity prevalence in the GCC [1,2,3,4, 8, 9]. Including older studies would have increased the size of the evidence base, but also made it much less generalisable to contemporary GCC populations. The literature search was limited to English language for practical reasons, but any grey literature or other studies suggested by expert contacts in the GCC published in Arabic would also have been considered if identified. The first author is from Kuwait, which may have biased the grey literature searching towards Kuwaiti sources of evidence. However, author connections in relevant institutions in the rest of the GCC are good, and responses from those contacts were generally informative (Additional file 2). It therefore seems unlikely that useful sources of evidence from the other GCC states, such as recent nationally representative surveys, were missed.