Association between body composition and blood pressure in normal-weight Chinese children and adolescents

A cross-sectional study was designed to collect data from 1175 children and adolescents aged 10–18 years, who were selected from three junior schools and two high schools in China between 2017 and 2019 by cluster sampling. Schools were first chosen by a convenient sampling method, then grades, classes were randomly selected from each grade in the survey (a total of 11 whole classes from the three junior high schools and 29 from the three high schools) The sample size was calculated based on the following formula: \(n=\frac{{Z}_{\alpha }^{2}\times pq}{{d}^{2}}\)(α is the significance level, p is the prevalence of elevated BP [15], d is the tolerance error and Z is the significance test statistic; α = 0.05, p = 16.3%, d = 0.15p, q = 1-p), assuming a non-response rate of 15% and a final required sample size of approximately n = 1009, this study ultimately surveyed 1175 children and adolescents. All study subjects participated in questionnaires, anthropometric measurements and body composition examinations. Of the 1175 participants who participated in this study, a total of 898 normal-weight (52.2% boys) samples were remained in the final analysis after excluding 277 participants with overweight (n = 216) and obesity (n = 61). The study was conducted according to the standards of the Declaration of Helsinki and approved by the Ethics Committee of Ningxia Medical University (2021-G053), all methods were performed in accordance with the relevant guidelines and regulations. Informed consent was signed by all participants and their parents/ guardians.

Data for the study were collected by trained staff at each selected school according to the measurement criteria of each instrument. The questionnaire was self-filled and consisted mainly of demographic characteristics (sex, age, date of birth, smoking and drinking, etc.). It should be noted the classification of smoking were: 0. Never smoked; 1. Try to smoke (try to smoke, even if it's just one or two); 2. Recently smoked (have smoked at least 1 cigarette in the past 30 days), and the classification of drinking are:0. Never drink alcohol; 1. Try to drink alcohol (have drunk at least half a bottle or a can of beer, a small cup of liquor, etc.); 2. Drinking alcohol now (in the past 30 days, at least a glass of alcohol has been drunk one day). Height and weight were measured using a mechanical stadiometer (Model: ZH7082) and an electronic scale (Model: RGT-140). Both were measured twice and averaged for inclusion in the final analysis, to an accuracy of 0.1 cm and 0.1 kg for height and weight respectively. The height, weight and waist circumference were measured according to the requirements of the "2014 National Student Physical Fitness and Health Survey Manual" [16]. Height measurement: the subjects were barefoot, the torso was naturally straight, and the eyes were straight ahead. The upper limbs drooped naturally and the legs were straightened. Bring the heels together, and the toes were separated by about 45 degrees. The heel and sacrum and between the shoulder blades were in contact with the upright post, forming a "three points and one line" standing posture. Weight measurement: the subjects stood barefoot and naturally stood in the center of the weight scale pedal to keep the body stable. When measuring the waist circumference (WC), the subjects stood naturally with their feet 30–40 cm apart, and place a nylon tape measured between the upper hip bone and the lower thoracic cavity (usually the natural narrowest part of the waist), at the end of normal expiration, around the abdomen in a horizontal direction twice, took the average to an accuracy of 0.1 cm. BMI was calculated as weight divided by the squared of height (kg/m²). Normal weight was defined as the reference standard for overweight and obesity in children and adolescents as recommended by the International Obesity Task Force (IOTF): BMI ≤ international cut off points for BMI for overweight and obesity by age and sex [17].

Blood pressure (BP) was measured using a calibrated electronic sphygmomanometer (Model: OMRON HEM-7012, Omron Healthcare, Kyoto, Japan) according to the standard method by the "American Hypertension Education Project Working Group" [18]. A suitable cuff was chosen for the measurement (7 cm, 9 cm, 12 cm, etc. for BP measurement in children and adolescents) and the subject was seated facing the measurer and BP was measured on the right upper arm with the elbow at the same level as the sphygmomanometer and the heart. Systolic BP (SBP) and diastolic BP (DBP) were measured three times at 1-min intervals, and the average of the last two readings was recorded for the final analysis (a third measurement was taken if the difference between the first two blood pressure values exceeded 10 mm Hg (1 mm Hg = 0.133 kPa)).

The participants were assessed for body composition using bioelectrical impedance analyzer (Model: InBody-370, Biospace of Korea, Seoul, Korea). Under the condition of fasting condition, the test subject stood barefoot on the instrument, stepped on the designated electrodes on both sides of the forefoot and heel respectively, held the handle and relaxed the whole body, waited for the instrument to automatically complete the test, and completed the calculation based on the measured resistance rate and generate a test report. In this study, body fat mass (BFM), fat mass percentage (FMP, calculated as BFM divided by total body mass), visceral fat level (VFL), VFL/FMP ratio (indicating the degree of visceral fat deposition in the body) and lean body mass (LBM) were included in the final analysis.

In this study, statistical analyses were accomplished by SPSS 26.0. Sex-stratified analyses were conducted. Continuous variables were described by mean (standard deviation), discrete variables by median (25th percentile,75th percentile), and categorical variables were calculated by frequencies and percentages. For normal variables, sex differences were tested by t-test and differences between quartiles by ANOVA; For skewed variables, differences between groups were tested by Mann–Whitney U-test, and categorical variables with χ² test. A binary logistic regression analysis was performed with FMP, VFL, VFL/FMP ratio and LBM as independent variables and abnormal BP as the dependent variable. Model 1 was a separate regression model for various body composition indexes and abnormal BP, and Model 2 to Model 5 were regression models with different body composition variables included simultaneously in the corresponding equations, respectively. All models were adjusted for age, smoking and drinking (with an additional adjustment for sex in the total population). The OR values and their 95% CIs for abnormal BP risk per 1-SD increase in the above indicators were estimated. P < 0.05 was regarded as statistically significant.