Central obesity in both adults [1–3] and children [4–6] is related to cardiovascular (CV) risk factors (hyperlipidemia, insulin resistance and hypertension). Waist circumference (WC) and WC-derived indices such as waist-to-hip ratio and waist-to-height ratio (WHTR) have been used as proxy measures of central obesity, the former being widely used in adults [7, 8]. There is growing evidence however that waist circumference (WC) alone correlates more strongly than waist-to-height ratio (WHTR) with CV risk, in adults [2] as well as in children [9–11]. Previous studies have shown that adults from a number of South and East Asian countries are predisposed to central obesity and insulin resistance [12, 13]. This ethnic difference of increased body fat distribution in the trunk region has also been observed in adolescents [14] and prepubertal children [15] of mainly East Asian origin, highlighting the importance of ethnic-specific studies.

WHTR has been proposed as a convenient alternative measure for assessing central fatness in children, on the basis that it is relatively age-independent and that in normalising for growth it might obviate the need for age-related reference charts [28–30]. "Keep your waist circumference to less than half your height" (i.e. WHTR 0.5) is a seductively simple target [31]. However WHTR percentiles have not been reported and the generality of its independence of ethnic and age differences is questionable.

In 1993 a growth survey of 25,000 Hong Kong children from birth to 18 y was undertaken and growth charts for weight, height, weight-for-height and body mass index were developed [32]. WC was not measured. We now report a further growth study of 14,842 ethnic Chinese children aged 6 to 18 y undertaken in 2005/2006, using similar methods but including WC measurement to provide reference values for WC and WHTR.

Sex and age-specific WC and WHTR percentiles, developed from data collected during 2005/6 on a large and representative population of 14,842 Chinese children aged six to 18 y old, are presented. These charts are based on WC measured midway between lowest rib and superior iliac crest. Comparisons of WC data between different studies need to be undertaken with caution since WC may be measured at different sites and currently there is no agreement on which site is optimal [34]. In children, WC has been measured at five different sites: (i) midway between lowest rib and superior iliac crest (as in the present study) [20, 22, 25, 27]; (ii) at the umbilical level [19, 21, 28]; (iii) at the narrowest point of the torso [23]; (iv) at the level of the right upper iliac crest [24]; and (v) at the level of 2 cm above the umbilicus [35]. WC measures at these different sites in children have not been formally compared. WC measurements in adults taken at four different sites have been compared and shown to provide slightly different, albeit highly reproducible data, that each correlate significantly with total and central body fat [34]. If WC is to become an important public health assessment tool of central obesity in both adults and children, international agreement about measurement site is required.

In the study, we compared Hong Kong children's WC with data from Britain and Iran where the method of measuring WC was the same as our study. The Iranian children's 50^th and 90^th percentile curves were much higher than those from Hong Kong and Britain. A previous report has shown that Iranian children have a high prevalence of high blood triglyceride (20%) and low HDL-C (28%) [6]. In contrast, our unpublished data has shown much lower prevalence of these risk factors in Hong Kong children. The Iranian paediatric population may provide an important opportunity to further study the relation between central obesity and dyslipidaemia.

Our study challenges the proposal that WHTR is a relatively age-independent measure that could obviate the need for age and growth-related reference standards in children. Our data show that WHTR is not independent of age in children under 14 y old. Kahn and colleagues reported that WHTR appears to be independent of age and sex [36]. Ashwell and colleagues have proposed the use of an age-independent universal cut-off for WHTR of 0.5 [29]. This is despite their finding that the overall mean WHTR (with WC measured at the same site as in the present study) of 8135 British children aged 5 to 16 y decreased with age from 0.47 to 0.42 in boys and from 0.46 to 0.41 in girls [31, 37]. WHTR measures obtained in our study were slightly less than the 1988 British study. Our WHTR decreased with age up to 14 y and then remained almost constant (Table 1, Figure 2). WHTR has now been correlated with CV risk markers in children in at least six studies [6, 28, 30, 35, 36, 38] and different WHTR cut-offs for predicting high CV risks in children at different ages were reported in one of them [6]. Ashwell's proposed 0.5 cut-off point for predicting CVS risk has yet to be tested by direct correlation with CV risk markers in children. Notably, few of our older children would be categorized as being at CV risk if this single 0.5 WHTR cut-off were used (Figure 2). This would conflict with our previous findings that a WC cut-off at the 85^th percentile for children under the age of 12 y and at the 76^th percentile for those aged 13 to 18 y would predict CV risk [11, 39]. For a WHTR cut-off of 0.5 to become a universal predictor of CV risk in children, further direct confirmatory evidence from comparative correlations with CV risk factors in children of different age and ethnic origins will be required.

BMI is the most widely used index to define children's weight status. Our finding that WC correlates considerably more closely than does WHTR with BMI in children is unsurprising given that WC and BMI increase, whereas WHTR decreases, with age. CV risk factors found in our previous study of around 2500 children drawn from the same population correlated in descending order of strength with WC, BMI and WHTR [11]. Others have reported that WC [28, 39–41] and WHTR [28, 30, 36] correlate more strongly than does BMI with CV risk factors in children. No difference in the ability of WHTR and BMI-for-age to predict CV risk was found in one study [38]. These inconsistencies may reflect different methods of analysis, different ethnic populations and the relatively small differences between the related parameters.

A limitation of our study is that 7% of the primary and 10% of the secondary school students declined to participate. Reasons for non-participation were not recorded so it is not possible to determine whether non-participation resulted in any systematic bias.

Reference values and percentile curves for WC and WHTR of Chinese children and adolescents are provided in the study. Both WC and WHTR were age dependent. As a public health tool, WHTR does not on present evidence offer major advantages over WC for children. WC, measured at an agreed site and interpreted against appropriate established reference standards, remains the simplest clinical measure of central obesity for the prediction of CV risk in children. For standards based on WC and WC-derived parameters to be of general public health and research use for international comparisons, it will be important that a single defined site be agreed at which to measure WC.