Text box 1. Contributions to the literature

•This study provides evidence that adolescents with medium-score dietary knowledge are less likely to be overweight/obese and highlights the need for nutrition education programs that promote healthy eating habits.

•The study also suggests that parental dietary knowledge may not be significantly associated with adolescents’ BMI or overweight/obesity.

•The results underscore the importance of targeting adolescent nutrition education to prevent and address the increasing prevalence of overweight/obesity in this population.

•Future research could explore the effectiveness of various implementation strategies in promoting healthy eating habits among adolescents.

Introduction

In recent years, overweight and obesity in children and adolescents have become a major public health concern globally due to the alarming increase in prevalence [1]. In China, the rapid economic development and changes in dietary patterns and lifestyles have led to a substantial increase in the prevalence of overweight and obesity in children and adolescents. The latest national estimates indicate a prevalence of 11.1% for overweight and 7.9% for obesity in children and adolescents aged 6–17 years in 2015–2019 [2, 3]. This represents a significant increase in prevalence compared to monitoring data in 1992, where prevalence was 3.9% for overweight and 1.8% for obesity [2, 3]. Childhood and adolescent overweight are likely to lead to lifelong obesity [4], and being overweight during this period is associated with a higher risk of chronic disorders, such as type 2 diabetes [5,6,7].

Effective prevention strategies of childhood obesity remain a public health priority [8]. Previous studies have shown that enhancing nutrition knowledge can help individuals adopt healthy eating and exercise habits to achieve energy balance [9, 10]. The World Health Organization’s Commission on Ending Childhood Obesity has also emphasized the importance of promoting nutrition knowledge among adolescents and parents or caregivers [11].Previous research has primarily focused on the relationship between nutrition or dietary knowledge and body mass index (BMI) in adults [12,13,14], with limited attention given to adolescents and their parents. Findings on the associations between dietary knowledge and BMI have been mixed. O’Dea et al. found no association between nutrition knowledge and BMI in Australian children and adolescents aged 6–18 years [15], while Said et al. reported a significant positive correlation between BMI z-scores and total dietary knowledge scores in Lebanese adolescents aged 15–18 years and their parents [16]. Conversely, Kakinami et al. observed an inverse association between nutrition knowledge and adiposity among Canadian children aged 10–15 years [17]. Similarly, a cross-sectional study in China also found that adolescents aged 8–18 years with high levels of dietary knowledge may be negatively associated with overweight and obesity, but it did not explore the effect of parental dietary knowledge on children’s overweight and obesity [18]. In another study, Subih et al. found no significant association between maternal dietary knowledge and mean BMI among children and adolescents aged 6–18 years in north Jordan, but a significantly decreased mean waist circumference among those whose mothers had a moderate and high level of nutritional knowledge compared with those whose mothers had a low level of nutritional knowledge [19]. Given the increasing prevalence of overweight and obesity among children and adolescents and the crucial role of parents in sha** their dietary habits and nutritional status, it is crucial to understand the impact of adolescents’ and parental dietary knowledge on adolescents’ BMI and overweight/obesity. However, limited studies have investigated this topic, and longitudinal evidence is still lacking. Therefore, this study aims to investigate the impact of dietary knowledge of Chinese adolescents aged 12–17 years and their parents on adolescents’ BMI and the prevalence of overweight and obesity, using longitudinal data from the China Health and Nutrition Survey (CHNS) conducted in 2004, 2006, 2009, 2011, and 2015.

Methods and materials

Study design and study sample

The China Health and Nutrition Survey (CHNS) is an ongoing international collaborative project designed to investigate the impact of social and economic changes on nutrition and health outcomes in China [20]. Surveys were conducted every 2–4 years. More details of the CHNS design, sampling and cohort profile information have been described in previous researches [21, 22]. The study was approved by the Institutional Review Boards of the University of North Carolina at Chapel Hill and the National Institute of Nutrition and Food Safety, China Center for Disease Control and Prevention. Each participant signed informed consent by their parents or caregivers.

The dietary knowledge survey began in 2004, and available CHNS data from surveys conducted in 2004, 2006, 2009, 2011, and 2015 was utilized in this study. Individuals above the age of 12 were invited to answer the section on dietary knowledge. Hence, 3,620 adolescents aged 12–17 years participated in at least one of the five waves of surveys. Furthermore, we excluded adolescents who had no height and weight information or implausible BMI, those who had no dietary knowledge information, and whose parents had no dietary knowledge information. Finally, 2,035 adolescents were included in our data analysis (Fig. 1), numbers of adolescents extracted were 630 (2004), 423 (2006), 367 (2009), 410 (2011), 205 (2015).

Fig. 1
figure 1

Participant flow chart

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Outcome variables: BMI and overweight/obesity

The weight and height of adolescents were measured by at least two trained health professionals who followed standard protocol and techniques. One professional took the measurements, while another recorded the reading data. Weight was measured in light indoor clothing without shoes to the nearest tenth of a kilogram using a beam balance scale, and height was measured without shoes to the nearest tenth of a centimeter using a portable stadiometer [23]. BMI, defined as the body weight in kilograms divided by the squared body height in meters, is used here as the indicator of adolescents’ overweight and obesity. Moreover, overweight and obesity were defined based on the International Obesity Task Force (IOTF) recommended age-sex-specific BMI cut-off-points [24].

Exposure variables: dietary knowledge of adolescents and their parents

The description of 17 questions in terms of dietary knowledge is presented in Supplementary Tables 1, which includes 12 questions from the 2004, 2006, 2009, and 2011 surveys and five additional questions added in 2015. The five new questions added in 2015 were: “eating salty foods can cause hypertension”, “refined grains (rice and wheat flour) contain more vitamins and minerals than unrefined grains”, “lard is healthier than vegetable oils”, “vegetables contain more starch than staple foods (rice or wheat flour)”, “eggs and milk are the important sources of high-quality protein”. And these 17 questions have been validated in previous studies [14, 18]. Responses to the dietary knowledge questions were categorized as correct, wrong, neutral, or unknown, with a score of 1 assigned for a correct answer and 0 for a wrong, neutral, or unknown answer. To ensure comparability, we transformed the scores into percentages for each survey wave, and then divided the scores into tertiles for both adolescents and their parents for the analysis.

Covariates

Adolescents’ age, sex (boys/girls), residence (urban/rural areas), per capita annual family income (tertiles: low, medium, and high), education level of adolescents and their parents (low: primary school or below; medium: secondary school/secondary vocational school; high: high school or above), overweight and obesity status of adolescents’ parents (no/yes), physical activity (metabolic equivalent of task (MET), h /week), parental diagnosed hypertension (no/yes), and parental diagnosed diabetes (no/yes), were considered as covariates in this study. And the classifications of these variables were based on previous studies [36]. Story et al. mentioned that as adolescents go through significant changes during this period, such as growing independence and eating away from home, parents have less control over what their children eat [37]. Additionally, Asakura et al. pointed out that communication between caregivers and children about nutrition and foods might be insufficient, thus, the nutrition knowledge of caregivers may have a slight effect on the dietary intake of their children [38], not to mention those adolescents who were boarding at school. Williams et al. demonstrated that parents with high dietary knowledge may provide children and adolescents with a healthy diet, but adolescents might be able to alter a variety of foods offered from their parents frequently by refusing foods they would not like to consume [39]. And Räsänen et al. reported that when a nutrition counseling intervention was given to parents, the nutrition knowledge score of children was not improved in the intervention group compared with the control group [40]. Moreover, in many countries, especially in China, due to the traditional family structure of three-generation households, grandparental child care is more prevalent and plays a significant role in child care [41, 42]. As grandparents assume the role of parents in children’s lives, they may exert more influence over children’s dietary behaviors and nutritional status compared to parents. Therefore, further studies should explore the impact of grandparents’ dietary knowledge on the nutritional status of children and adolescents. These findings suggest that food education programs targeting adolescents may be more effective in improving their nutritional status than programs aimed at parents in China.

Our study found a significant interaction between adolescents’ dietary knowledge score and education in relation to adolescents’ BMI. Specifically, among adolescents with high school education or above, a high dietary knowledge score was negatively associated with BMI. However, no such association was found in participants with low or medium levels of education. These results suggest that adolescents with higher levels of education may be more likely to consistently apply their dietary knowledge in practice. Thus, educational level should be taken into consideration when analyzing the association between dietary knowledge and adolescents’ nutritional status.

The prevalence of overweight and obesity among Chinese adolescents is a critical issue emphasized in our study. Yaru et al. reported an overall upward trend of overweight/obesity among Chinese children and adolescents from 1991 to 2015 in a recent meta-analysis [43]. Our results confirmed that overweight/obesity of adolescents are becoming an increasingly serious matter in China. The prevalence of overweight has increased from 6.8% in 2004 to 15.1% in 2015, and the prevalence of obesity remarkably increased approximately thirteen times from 0.6% in 2004 to 7.8% in 2015. Some measures, such as improving the level of adolescents’ dietary knowledge according to the results of this study should be considered. In addition, it is worth noting that adolescents’ dietary knowledge score are 77.2, 71.0, 72.6, 75.0 in 2004, 2006, 2009 and 2011, respectively, while it dropped to 63.3 in 2015. The different difficulty degree between five new questions added in the survey of 2015 and the questions in previous surveys may be one possible reason. More importantly, it reflects that dietary knowledge of Chinese adolescents needs to be improved over recent years in China. However, dietary education is not yet a part of the curriculum for Chinese students leaving few scientific and rational channels for students to obtain accurate and systematic dietary knowledge [13]. Systematic dietary nutrition education is urgently needed, in China and what is more importantly is that incorporating dietary/nutrition courses into the current compulsory education system, and consequently, decreasing the overweight/obesity rate of Chinese children and adolescents.

The results from the present study should be interpreted with caution as there are some limitations. Firstly, we only measured general dietary knowledge, which may not fully reflect the entire spectrum of dietary knowledge. Thus, there may be some biases in accurately and objectively assessing the level of adolescents’ and parental dietary knowledge. Secondly, the use of self-reported data may introduce biases caused by dishonesty and measurement flaws. Thirdly, we were unable to consider the impact of confounding factors such as TV programs, applications or video games on our results, due to the lack of relevant data in the CHNS. Despite these limitations, our study addresses the existing gap in the literature regarding the influence of adolescents’ and parents’ dietary knowledge on adolescents’ weight status and aims to contribute to the effective prevention and control of obesity among this population. Additionally, the longitudinal data provided stronger evidence than a cross-sectional study.

Conclusion

Our study confirms the increasing seriousness of overweight and obesity among Chinese adolescents aged 12–17 years. The results of our mixed-effect models suggest that adolescents with medium-score dietary knowledge were less likely to be overweight/obese compared to those with low-score dietary knowledge, while no significant association was found between parental dietary knowledge and adolescents’ BMI or overweight/obesity. Our findings highlight the need to promote nutrition education programs that improve dietary knowledge and healthy eating habits among adolescents, as a preventative measure for overweight/obesity.