Introduction

Nutrition in children and adolescents plays an important role in physical, mental, and emotional development. Under-nutrition is a major public health problem affecting a significant number of school age children, particularly in developing countries [6, 12]. On the other hand, there has been a dramatic increase in overweight and obesity among children and adolescents worldwide in recent years. Even in developing countries, obesity rates among children and adolescents are growing markedly. For example, in China, the prevalence rates of overweight and obesity increased between 1992 and 2002 by 17.9% among students aged 7 to 17 years [20]. Moreover, obesity is a major risk factor for the development of hypertension. In accompany with the high prevalence of overweight and obesity, the prevalence of hypertension is also increasing. The coexistence of under-nutrition and obesity among children and adolescents is becoming increasingly prevalent in many developing countries.

China is a large country with 56 nationalities. Hainan province is located on the southern coast of China. Li ethnicity inhabited Hainan for over 3,000 years. Li minority and the Han ethnicity are the two most populous ethnic groups in Hainan. However, the nutritional status of Han and Li ethnicity students residing in Hainan is not well known. So far, no studies have been performed to measure their blood pressure and assess the association of nutritional status with blood pressure. The purpose of the present study is therefore to examine the prevalence of thinness, overweight, and obesity among children and adolescents in Hainan province, China. We also want to assess the prevalence of pre-hypertension and hypertension and their relationship with nutritional status.

Materials and methods

Study subjects

A school-based cross-sectional study was conducted during the period March 2009 to December 2009 in Hainan province, China. Two-stage cluster sampling technique was used to choose the study sample. Students under 7 or over 18 years of age were excluded from this study. A total sample of 5,456 children and adolescents from the selected ten schools constituted the subjects of the study. Written informed consent form was obtained from adolescents and their parents. This study was approved by The Ethical Committee of School of Public Health, Sun Yat-sen University.

Anthropometric measurements

Body weight was determined to the nearest 0.1 kg on standard physician's beam scales with the children and adolescents wearing only the underwear and no shoes. Height was measured to the nearest 0.1 cm on standardized, wall-mounted height boards according to the following protocol: no shoes, heels together, and student's heels, buttocks, shoulders, and head touching the vertical wall surface with line of sight aligned horizontally. Each of the standard physician's beam scales and wall-mounted height boards used to measure were calibrated previously, using three different weights and one reference tape. Body mass index (BMI) was computed by dividing weight (kg) by height squared (m2). The age and gender-specific BMI cutoffs newly developed by the working group on obesity in China (WGOC) were used to define overweight and obesity [15]. Overweight is defined as a BMI at or above the 85th percentile and lower than the 95th percentile for children and adolescents of the same age and sex. And obesity is defined as BMI greater than the 95th percentile. The new international age and gender-specific BMI cutoff points proposed by Cole et al. were utilized to identify thinness [4]. In this definition, thinness is defined as a BMI of less than 18.5 by age and sex. Thinness was also graded as mild (BMI in the range of 17–18.5), moderate (BMI in the range of 16–17), or severe (BMI less than 16), respectively.

Blood pressure measurement and high blood pressure definition

Blood pressure was measured by two trained physicians using a standard mercury sphygmomanometer at the right arm with students in the seated position after at least 5 min of rest. The cuff size was based on the length and circumference of the upper arm and was chosen to be as large as possible without having the elbow skin crease obstruct the stethoscope [25]. Blood pressure values were approximated to the nearest 2 mm Hg. Systolic blood pressure (SBP) was defined by the first Korotkoff sound (appearance of sounds), and diastolic blood pressure (DBP) was defined by the fifth Korotkoff sound (disappearance of sounds). In order to make the children comfortable in a relaxed environment, measurements were taken in the classroom in the presence of their classmates and teachers without specifying that doctors were performing the activity and doctors wore casual clothes. The age and gender-specific blood pressure cutoffs in Chinese children and adolescents [23] were used to define pre-hypertension and hypertension. In this definition, hypertension was defined as SBP and/or DBP above the 95th percentile for age and gender. Pre-hypertension was defined as SBP and/or DBP between the 90th and 95th percentile for age and gender.

Statistical analysis

The continuous variables, including age, weight, height, BMI, and blood pressure, were summarized by the median and range according to gender. Other categorical variables were summarized by count and percentage. Chi square test was performed to compare the difference between boys and girls and Han and Li ethnicity groups. Logistical regression model was used to estimate the odds ratios (ORs) and 95% confidence intervals (CIs) between nutritional status and blood pressure after adjustment for age and gender. Statistical tests were set with a significance level of 0.05. All data analyses were conducted by using SPSS 13.0 (SPSS Inc., Chicago, Illinois, USA).

Results

Table 1 displays the study population characteristics by gender. The median age (range) of the students was 13.0 years (range 7–18 years), and 2,759 (50.6%) were boys. Forty-nine percent (n = 2,658) were Han ethnicity, and 51% (n = 2,798) were Li ethnicity. The median height was 148.8 cm, weight 37.4 kg, and BMI 16.6 for all students. The median (range) systolic blood pressure was 98 mmHg (56–164) and diastolic blood pressure was 60 mmHg (40–110).

Table 1 Characteristics of 5,456 studied children and adolescents. Data are given as percentages or as medians and ranges
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As shown in Table 2, the prevalence of thinness, overweight, and obesity among male students was 34.0%, 3.2%, and 1.3%, while it was 34.3%, 2.2%, and 1.3% among female students, respectively. No significant difference was observed in the prevalence of thinness and obesity between genders, but boys had a higher prevalence of overweight than girls. Figures 1 and 2 present the prevalence of thinness, overweight, and obesity by ethnicity and age among children and adolescents. Mild thinness was the most common type of thinness, followed by moderate thinness. The prevalence of mild thinness was 23.9% among boys and 24.2% among girls, respectively.

Table 2 Prevalence of thinness, overweight, obesity, and high blood pressure among studied children and adolescents
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Fig. 1
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Frequency of thinness, overweight, and obesity among Han ethnicity students according to age (years)

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Fig. 2
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Frequency of thinness, overweight, and obesity among Li ethnicity students according to age (years)

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The prevalence of pre-hypertension and hypertension was also shown in Table 2 and Figs. 3 and 4. Pre-hypertension and hypertension were detected in 3.9% and 3.3% of children and adolescents, respectively. A proportion of 3.7% of male students presented pre-hypertension and 3.5% hypertension. The corresponding proportion was 4.2% and 3.1% among female students. There was no significant difference in the prevalence of hypertension between Han and Li ethnicity students and genders.

Fig. 3
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Means of systolic blood pressure of both Han and Li ethnicity boys and girls aged 7–18 years

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Fig. 4
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Means of diastolic blood pressure of both Han and Li ethnicity boys and girls aged 7–18 years

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The association of nutritional status with blood pressure was examined. Of 148 overweight students, 10 (6.8%) had pre-hypertension, and 18 (12.2%) had hypertension. Pre-hypertension and hypertension were detected in 6 (8.7%) and 10 (14.5%) obese students, respectively. Even among thinness students, there were also 67 (3.6%) and 38 (2.0%) students with pre-hypertension and hypertension. Logistical regression analysis showed that obesity was associated with both pre-hypertension and hypertension. In comparison with normal weight students, obese students had 44% higher risk of pre-hypertension (OR = 1.44, 95%CI = 1.08–1.92, P = 0.014) and 100% higher risk of hypertension (OR = 2.00, 95%CI = 1.53–2.60, P < 0.001) after adjustment for age and gender. Overweight was also associated with both pre-hypertension (adjusted OR = 1.43, 95%CI = 1.02–2.00, P = 0.038) and hypertension (adjusted OR = 1.67, 95%CI = 1.31–2.11, P < 0.001). These associations were observed across both male and female students. However, compared to normal weight students, no significant association between thinness and pre-hypertension and hypertension was found (adjusted OR = 0.91, 95%CI = 0.67–1.27, P = 0.533 for pre-hypertension and OR = 0.80, 95% CI = 0.51–1.27, P = 0.346 for hypertension) (Fig. 5).

Fig. 5
figure 5

Association of nutritional status with high blood pressure among studied children and adolescents. OR odds ratio, CI confidence interval. OR was adjusted for age and gender

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Discussion

This cross-sectional study assessed the nutritional status and blood pressure of Han and Li ethnicity students in Hainan province, China. Our results showed that the prevalence of thinness, especially mild thinness, is high in the studied students. Overweight and obesity were significantly and strongly associated with pre-hypertension and hypertension.

Different definitions have been used to describe under-nutrition, such as weight below a specified percentage of median weight for the child's age [5], use of z scores for the definitions of under-weight, wasting (low weight for height), and stunting (low height for age). The recent study by Cole et al. [4] has stated that under-nutrition is better assessed as thinness (low BMI for age) than as wasting. The level of thinness in our study population using age and gender-specific cutoff points based on BMI was much higher (34.0% for boys and 34.3% for girls) than those conducted in Shanghai, China (18.3%) [16] using weight less than 90% of the ideal weight for height. The differences in prevalence rates of thinness could partially be explained by the different standards for measurement of thinness. Another reason may be that over 60% participants were from rural areas in the present study. However, a higher prevalence of thinness observed in this study should be noted. This indicated that the nutritional status of the studied population is not satisfactory although the standard of living in China has increased dramatically in the past three decades. Several studies have shown that dietary and environmental factors are major determinants of the nutritional status in children and adolescents. Therefore, these provided additional justification for improvement of the nutritional status of children and adolescents among these areas in the form of supplementary nutrition or school-based nutrition education and nutrition programs.

On the other hand, compared to other areas in China and other countries, the prevalence rates of overweight and obesity (2.7% and 1.3%, respectively) of this study are relatively low. Based on data from the 2002 China National Nutrition and Health Survey [20], the prevalence of overweight and obesity in children and adolescents aged 7 to 17 years was 4.5% and 2.1%, respectively. In Tianjin, China, the recent study reported that the prevalence of overweight and obesity in children and adolescents aged 7–18 years was 12.5% and 15.7%, respectively [1]. The overall prevalence of obesity among 7–18 years students was 4.11% in Xi'an, China [30]. For other countries, the overweight prevalence is 30.8% in Spain schoolchildren, 31% in Greece, 36% in Italy, 19% in France, 16% in Germany, 15% in Denmark, and 25.6% in USA [19]. The result of the present study seemed that the problem of overweight and obesity may not be a serious problem in the studied population.

Fewer studies have examined the prevalence of pre-hypertension and hypertension in children and adolescents in China. One study conducted in Changsha city, China [17] showed that the prevalence of pre-hypertension and hypertension among children and adolescents aged 12–17 years was 6.0% and 6.7%, respectively. Another study done in Hangzhou city [29] reported that 13.63% of children and adolescents had hypertension. The prevalence of hypertension is 10.26% among children aged 6 to 12 years in Shenzhen city [14]. In the current study, the overall prevalence of pre-hypertension and hypertension was 3.9% and 3.3%, which was relatively lower than those reported from other areas of China. However, one study showed that the rate of progression of pre-hypertension to hypertension was ~7% per year [8]. Several longitudinal studies [2, 3, 18] have reported that children with higher pressure levels tend to develop hypertension in later life. These data suggested that appropriate measures should be taken to prevent pre-hypertension from progressing to hypertension and to lower hypertension among studied children and adolescents.

In this study, a statistically significant positive association was demonstrated between overweight and obesity and elevated blood pressure among studied students. Compared to normal weight students, overweight and obese students had higher risk of hypertension and pre-hypertension. The result that overweight and obesity were associated with hypertension is in accordance with previous studies conducted in children and adolescents [10, 13, 22, 26, 28] and adults [24]. To date, few studies have evaluated the association of overweight and obesity with pre-hypertension, and the results are inconsistent. Some studies found that pre-hypertension was positively associated with overweight and obesity in adolescents [7, 8, 22]. One study reported that only hypertension but not pre-hypertension was related to overweight among younger subjects who lived in a rural community in Canada [27]. The strict relationship between pre-hypertension and overweight/obesity found in this study suggested the importance of changing diet, exercises, and other lifestyle-related factors to control obesity and high blood pressure.

The most striking finding of this study is that there were 67 (3.6%) and 38 (2.0%) students with pre-hypertension and hypertension among thinness students, respectively, although there was no significant association found in this study between thinness and high blood pressure. In agreement with our findings, one study done in Brazil showed that the prevalence of hypertension was elevated among malnourished adolescents [9]. Another study conducted in India showed that hypertension had a high prevalence in under-weight children (4.3%) and in normal-weight children (6.9%) [11]. This indicated that early detection and treatment of nutritional deficiencies is necessary to prevent future high blood pressure for children and adolescents. However, unlike the definition of hypertension in adults that is based on the level of blood pressure that is linked with an increase in the risk of cardiovascular events, hypertension in children and adolescents is defined statistically due to the absence of these data. Therefore, with this statistical definition, the prevalence of pre-hypertension and hypertension is 3.6% and 2% in thinness children and adolescents, respectively.

This study had some limitations. First, the study sample only included Han and Li ethnicity students in Hainan province; therefore, the generalization of these findings to other ethnicities or other areas should be cautious. Second, due to the use of cross-sectional data, the causal relationship between nutritional status and elevated blood pressure cannot be concluded. Further studies with longitudinal study design would help to solve this question. Third, although the BMI has well-known limitations [21], the use of BMI to define the nutritional status is an acceptable and valid measure for quantifying childhood obesity for research purposes.

In conclusion, the present study showed a high prevalence of thinness among Han and Li ethnicity students in Hainan province of China. Overweight and obesity were strongly associated with pre-hypertension and hypertension in the studied population.