In the context of the general implementation of online and offline blended teaching mode in China's high schools, it was necessary to understand the changes in body mass index (BMI), dietary knowledge acquisition level and sedentary time of high school students. However, there was a lack of multicenter, large-sample studies focusing on the relationship between the three.

To understand the BMI, dietary knowledge acquisition level and sedentary time of high school students in China, and to analyze their influencing factors, in order to provide empirical evidence for the promotion of physical and mental health development of high school students.

From January to March 2019, 6 213 high school freshmen and sophomores were selected as study subjects in 23 provinces in seven regions of China, namely east China, south China, north China, central China, northwest China, southwest China and northeast China, by using stratified random whole cluster sampling method. A self-designed questionnaire was used to conduct the survey, which involved 4 parts: students' personal and family information, BMI, dietary pagoda knowledge test (DPKT), and sedentary time. A total of 6 038 valid questionnaires were collected, and the effective recovery rate of the questionnaires was 98.35%.

Comparison of BMI among high school students with different regions, provinces, grades, number of family members, and father's education showed statistically significant differences (P<0.05). Comparison of DPKT among high school students with different regions, provinces, number of family members, annual income of the family, father's education, and mother's education showed statistically significant differences (P<0.05). Comparison of sedentary time among high school students with different regions, provinces, urban/rural distributions, age, family population, annual family income, father's education, mother's education, the difference is statistically significant (P<0.05). The results of the multiple linear regression analysis indicate that BMI (β=-0.05), sedentary time (β=0.07), family size (β=0.03), father's education level (β=0.05), and family income (β=0.05) were influencing factors of DPKT scores among high school students (P<0.05). Additionally, DPKT scores (β=0.07), family size (β=-0.03), and father's education level (β=0.04) were influencing factors of sedentary time among high school students (P<0.05) .

The median BMI levels of high school students in urban and rural areas in seven regions of China were generally in the normal range, but the overall dietary knowledge acquisition level were low, and the problem of overall longer sedentary time was prominent. Dietary dietary knowledge acquisition level and sedentary time were the most influential factors on each other, and father's education was the main social factor influencing dietary DPKT score and sedentary time.