Waist circumference cut-off values for the prediction of cardiovascular risk factors clustering in Chinese school-aged children: a cross-sectional study

The global prevalence of overweight and obesity has increased dramatically in North America, some European countries, and Australia in recent decades [1–4], however evidence suggests that a greater potential problem exists for China, South America and some countries in North Africa [4, 5].

Higher than desirable levels of body fat pose an increased risk of ill-health, however the location of excess fat appears to have particular implications [6–8]. For example, a greater concentration of adipose tissue in the abdomen, specifically in the visceral area, is directly related to metabolic and cardiovascular risk in adults [9]. Visceral adiposity is best quantified using sophisticated imaging techniques however such approaches are not feasible at the population level [10].

Recent attention has been paid to the applicability of anthropometric markers to measure abdominal obesity and the waist circumference is consistently identified as a better measure of cardiovascular risk than the body mass index [6, 11, 12]. Waist circumference is also recognized as a key component of the metabolic syndrome in both children and adults [13, 14]. Waist circumference cut-off points associated with increased risk have been developed for adult men and women, however relatively less work has been undertaken in children and adolescents. A further shortcoming of research to date is that reference standards have more commonly been developed on Caucasian populations and may have limited usefulness to people from different ethnic and racial backgrounds [15, 16].

An increasing body of research has explored ethnic differences in body composition in both children and adults [10], but considerably more work is needed. For example, the International Diabetes Federation uses the 90th percentile as a cut-off for waist circumference to define the pediatric metabolic syndrome but has recommended the development of ethnic-, age- and gender-specific normal ranges for waist circumference based on healthy values. In short, the percentiles used as cut-offs for waist circumference should be reassessed when more data are available [14].

A number of studies have developed reference waist circumference percentiles for children and adolescents in different countries [17–23]. To date, three studies have reported age- and gender-specific waist circumference cut-offs in Chinese children and adolescents, two in Hong Kong Chinese children and adolescents [24], and the third in children from Xinjiang province [25]. However, there are regional differences in the body composition of Chinese. People living in North China are taller and heavier than those living in the South due to a combination of genetic and environmental factors [26]. The development of waist circumference percentiles and cut-offs for different groups would be particularly valuable.

Therefore, the purpose of the present study was to develop waist circumference percentiles for Chinese children from both the North and South of the country, and secondly, to explore the optimal waist circumference cut-off values for predicting cardiovascular risk factors clustering in this population.

This study provides the age- and gender-specific waist circumference reference percentiles for Chinese children aged 6-12 years living in the North and South of the country. Consistent with findings in previous studies [17–21], waist circumference increases with age and boys have a higher value than girls at each age. The age- and gender-related variation of waist circumference also shows similarity to other body dimensions.

It is interesting to compare our data with those previously reported in British [21], Turkish [17], Australian [18], Mexican [23], and Chinese children in Hong Kong [24] and the Xinjiang Province [25] (see Figure 3). Boys in the present study had higher waist circumference values than Australian, Turkish, and British boys, and lower values than Mexican boys aged 6-12 years, however were similar to British boys aged 6-7 years. Girls in the present study had higher waist circumference values than British, Australian and Turkish girls, and lower values than Mexican girls in all age groups except lower values than Australian girls at 7-9 years. Compared with Caucasians, Asians are generally smaller [29], however waist circumference results from the present study contradict this expectation. A number of factors may contribute to this inconsistency. Firstly, the site of waist circumference measurement has varied between studies and previous research has reported significant differences in waist circumference at different sites [30]. International agreement regarding waist circumference measurement site is required if meaningful comparisons regarding central obesity are to be made between children from different countries and regions. Secondly, considerable changes in body size and shape, including waist circumference, have occurred in recent decades in both children and adults [26, 31, 32] and this may be a consideration when comparing data collected in different periods. For example, Utter et al. indicated that the mean waist circumference in New Zealand adolescents increased from 76.2 cm in 1997/1998 to 89.4 cm in 2005, with increases in waist circumference measurements at all points in the distribution [31]. Data from Britain and Australia were collected in 1985 and 1990, respectively, a gap of approximately twenty years from the present study. Compared with other studies of Chinese children, both boys and girls in the present study have higher waist circumference values in all age groups however had similar values to 7-year-old children from Xinjiang Province. It is widely accepted that body composition is influenced by the environment in addition to age, gender and ethnicity [10]. Compared with the population in Northern China, both children and adults in Southern China have a smaller body size [26]. This study also demonstrated differences in body size among children in Guangzhou (Southern China), and Liaoning and Tianjin (Northern China) (data not shown). Accordingly, further study is required in more districts of China to generate more representative waist circumference percentiles.

In a sub-sample, the present study also evaluated the threshold value of waist circumference to predict cardiovascular risk factors clustering using receiver-operating characteristic analysis. The 90th and 84th percentiles were identified as the thresholds for diagnosing a higher clustering of cardiovascular risk factors in Chinese boys and girls, respectively. The threshold of waist circumference for boys is consistent with the International Diabetes Federation recommendation [14] and the findings of Maffeis et al. and Ng et al. [15, 16], but higher than the two previous studies in Chinese boys. The threshold of waist circumference for girls is lower than the International Diabetes Federation recommendation and similar to the two studies in Chinese girls [24, 25]. The thresholds of waist circumference in the present study for both boys and girls are higher than those for Caucasian children [19].

The threshold of waist circumference in previous studies varies according to the definition of cardiovascular risk factors clustering. The two studies from Hong Kong [24] and the study from Xinjiang province [25], proposed the 85th waist circumference percentile as the appropriate threshold for predicting cardiovascular risk. However, Katzmarzyk et al. [19] proposed the 50-57th percentiles for white and black boys and girls. A number of factors may explain these differences. Firstly, waist circumference is ethnic-independent [33, 34], as well as the sensitivity to cardiovascular risk factors [35, 36]. Secondly, the definition of cardiovascular risk factors clustering has varied between studies with different groupings of cardiovascular risk factors used. Sung et al. reported the 85th percentile was the optimal threshold for diagnosing the presence of cardiovascular risk factors clustering which was defined as 4 or more of 6 risk factors, including elevated systolic and/or diastolic blood pressure, high triglycerides, low high-density lipoprotein cholesterol, high low-density lipoprotein cholesterol, glucose and insulin. However, if cardiovascular risk factors clustering was defined as 3 or more of the 6 risk factors, the cut-offs were the 74th and 69th percentiles for boys and girls, respectively. Furthermore, the level of each cardiovascular risk factor to define an abnormal level also varies between studies, for example, the 75th percentile [37], outer quintiles [19], and 85th percentiles [24]. In the study by Sung et al. [24], the lower the level to define each individual cardiovascular risk factor, the lower the waist circumference percentile. Risk factors tend to cluster together for individuals among both children and adults. The cluster of risk factors and thresholds with the strongest predictive relationship to cardiovascular disease should be identified for both clinical practice and prevention-oriented research and practice for the whole population, especially when developing universal cut-off points to predict it.

The present study provides waist circumference percentiles for Chinese children aged 6-12 years living in the North and South of the country. Optimal age- and gender-specific cut-off points for waist circumference to predict cardiovascular risk factors clustering are also proposed. Waist circumference percentiles should be updated in different countries consistent with changes in overall body size due to changes in environmental factors. Most importantly, standardized methodology should be used for the development of international waist circumference cut-off points to diagnose cardiovascular risk factors clustering.