This cross-sectional study of 500 Iranian students showed that 42.6% of the students have a sufficient HL level and 57.4% have an inadequate and problematic HL. The use and then appraisal dimensions were the lowest aspect of health literacy in Iranian high school students.

Our findings are consistent with other studies from different geographic regions which indicated that most adolescents have an insufficient level of HL and the rate of inadequate HL is estimated at 13–50%6,7,8,9,21. Different instruments have been employed in different studies to measure health literacy. However, in the study by Vashe et al.22 and Khajouei et al.9, which used the HELMA tool, the dimensions of use and appraisal had the lowest scores, respectively. Due to adolescents’ limited experience interacting with the health system, they may not to find the appropriate source of health information and also face difficulties in understanding, and evaluating information when they obtain it. They may also be unable to apply health measures correctly without sufficient health knowledge and skills. Therefore, they require health-related advice and guidance. School-based education and health information campaigns that aim to improve HL should incorporate in adolescent health promotion programs. The sustainability of these interventions over time and cooperation between educational teachers, parents and health experts is necessary. In addition, in present study higher HL was observed in female students and those whose parents had higher levels of education. Similar results are also observed in other studies. For example, according to Vardavas et al., a large number of Greek adolescents aged 12–18 years old, especially boys, have insufficient knowledge about major health issues23. Also, based on the results of research conducted by Jafari et al.24 and Khajouei and Salehi9, health literacy has been associated with the education of the student’s parents. The association between parental occupation or income with HL was not observed in our study, however, some studies have mentioned the association between HL and parental income, or other demographic factors such as race, culture, environmental and communication factors24,25.

These differences may be due to the social and educational environment in which the student lives. In addition, health literacy can vary between studies and may be due to factors such as sample size, or the items assessed in the questionnaire. However, it is important that HL interventions are delivered more frequently to boys than girls. Special attention should also be paid to students from lower socioeconomic backgrounds by providing social support, as they may have less access to information resources and healthcare due to lower parental education.

Here, the HL score of students who used hookah was lower than others. The relationship between hookah use in adolescents has not been directly studied, but in some studies, level of HL was related to performing or not performing other risky behaviors in adolescents. For example, Yangyuen et al. reported adolescents in Northeast Thailand with inadequate HL were more likely to drink alcohol and smoke26. Furthermore, Park et al. focused on American adolescents aged 13–17 years old, declaring that lower HL was related to, unhealthy diet, higher weight, greater involvement in problematic and sexual behaviors and increased substance use over time27. It’s possible that adolescents especially with lower score of HL are unaware of the long-term consequences of risky behaviors. Therefore, they need to be educated about risky behaviors in school, where they spend a lot of time. Curriculum planners and school administrators can integrate educational content on healthy lifestyle into students’ curricula.

24. Also, Students who were more interested in health issues scored higher in HL. This finding are similar to study of Saeedi et al.28. Adolescents who are interested in learning about health issues are more likely to participate in their own health decisions27. This leads to greater engagement with the health system, increased knowledge and skills, and ultimately greater health literacy.

Here, in general, students with a BMI less than 25 achieved a higher HL score and the mean of BMI in dimensions of understanding, appraisal, self-efficacy, access, reading and use, except numeracy, is significantly lower in students with a sufficient and excellent HL level.

Adewole et al. reported a significant relationship between HL level and obesity among Nigerian adolescents, indicating that adolescents with normal weight showed better HL in all of the areas compared to obese ones29. These results have been replicated in other studies14,15. It means that students who easily read, understand, and evaluate health information and guidelines are more likely to apply them to a healthy lifestyle, which results in better health status such as a normal BMI. Children and adolescents who have an increase in BMI are more likely to experience long-term health-related consequences, including non-communicable diseases such as cardiovascular disease, type 2 diabetes mellitus, metabolic syndrome, cancer and mental health problems11,12,13. Therefore, it is important that they can access the right information and use it appropriately. Adolescents who have a high level of health literacy may be able to set specific health goals (such as losing weight or maintaining a healthy body weight) and also have the self-efficacy to achieve these predetermined goals30.

In the current study, the odds of obesity (versus overweight and normal groups) in students with inadequate HL is 2.1 times higher as compared to those with sufficient/excellent HL.

after adjustment for demographic variables such as age, gender, field, parents’ education, job status, economic status, fast food consumption and physical activity. Few studies have been conducted on HL and BMI among high school adolescents using multivariable analyses and our study is innovative within the field. In some other studies, a number of confounding factors related to health literacy and BMI were examined. For example Lam and Yang evaluated Chinese adolescents aged 12–18 years old and showed that after adjusting for potential confounding factors (such as school performance and parental health problems) low HL is significantly related to overweight or obesity (with OR = 1.88)15. In addition, Shih et al. assessed adolescents aged 11–12 years old and argued that the probability of being underweight or obese was lower in those with the highest quartile of HL after controlling for gender, ethnicity, as well as health status and health behaviors such as physical activity and sugar-sweetened beverage intake (Relative Risk Ratio = 0.84)14. Considering the number of confounding factors can increase the strength of the results. In the current study, fast food consumption and physical activity were considered in the adjusted model in addition to demographic and socioeconomic variables. Exercise habits and dietary habits are two important factors influencing adolescent obesity31, especially in high school adolescents who have more independence in choosing food and daily activities. It is recommended that future studies examine a range of factors affecting health literacy and body mass index using literature reviews. Some studies have also reported results that differ from our study. Sharif and Blank reported a weak negative correlation between HL and BMI scores among overweight children32. Further, some other studies Some other studies found no significant association between health literacy and body mass index17,19,33. This could be for a variety of reasons. One possible explanation is that.

differences in age group of participants in various studies which can affect BMI and HL differently. The students in our study were between 16 and 18 years old. While in Sharif and Blank study32, a large range of age groups participated, from 6 to 19 years old, and in Rademakers et al. 11–15 years old19. Since the nutrition of children and adolescents at a younger age is usually influenced by the parent’s selections. In addition, understanding and applying health information differ at different ages due to various experiences in dealing with health issues. The diversity or homogeneity of adolescents in the grouping in terms of BMI and gender is regarded as another factor to create differences in studies. In our study, samples were selected from both female and male students within a population in the community. This is while in Sharif and Blank study32, the sample size included overweight children and in Motamedi et al.33consisted only female students. Also, in study of Zare-Zardini et al.17, the students were categorized into 4 levels in terms of body mass index, with about 22% of students also being underweight. This difference in weight distribution could affect the relationship with HL.

Finally, using different HL instrument can present different results as Sharif and Blank32, was used Short Test of Functional Health Literacy (STOFHLA), Motamedi et al. study was used Newest Vital Sign (NVS) health literacy33 and Rademakers et al.19 was used Dutch version of the HLS-Child-Q15. There are various well-known and valid instruments regarding adult HL34. Most of the utilized measurement instruments are considered as local and limited to specific geographical areas due to the small number of HL studies regarding adolescents around the world7,19,35, which may not provide comprehensive information about the level of overall HL in different regions of the world. However, HL can influence the adolescents’ behavior and should be widely assessed36.

In this study, there are several strengths and limitations. This study is among the few studies worldwide and the first one in Iran which discusses the association between adolescent HL and BMI by considering multiple confounding factors. HL represents an emerging area of research that offers valuable insights to health planners and policymakers regarding the access, evaluation, and utilization of health information within the community, particularly among adolescents. To prevent obesity among adolescents, systematic and multilevel approaches are needed to provide adequate nutrition education, encourage regular physical activity, and how to maintain a normal body weight. Tailored health education programs appear to improve health literacy and, consequently, a wide range of future health outcomes. In order for adolescents to participate in their own health management, they need to be educated over a long period of time. School is one of the settings that has been proven to be effective for long-term health interventions. These interventions will be strengthened by the participation of teachers, administrators, and parents. To gain deeper insights into adolescents’ understanding and application of health literacy in their daily lives, particularly regarding weight management and choosing healthy diet qualitative research methodologies are also suggested. Also, the present study can be strengthened through collecting information in wider geographical environments and considering the parents’ HL and other psychological factors related to the adolescents’ health. The cross-sectional nature is regarded as the limitation of this study, and causal associations cannot be inferred with such a study design. Longitudinal studies should be conducted to understand the factors affecting HL prospectively, and to what extent HL affects various risk factors. Another limitation of this study was the use of a questionnaire to collect data from students, where respondent circumstances may have influenced the reported responses. However, students were urged to provide truthful responses. It is also recommended that adolescent health literacy researches use universal health literacy instruments that have been validated across diverse ethnic and racial populations.