The findings of the present study showed that mean score in none of the FNL domains and dimensions was above the adequate level (≥60) which indicates that the FNL status of Iranian youth needs improvement. Considering the fact that the study participants were senior high-school students who had completed formal education, their FNL status conveys key messages for the educational system and could reflect the weakness points of current school curricula in improving food and nutrition literacy among students.
There were no significant differences between boys and girls scores in overall FNL and its dimensions, except for functional and food label reading skill scores. The mean score of functional skills was slightly higher in girls compared to boys; however, after adjusting for other factors in the multivariate analysis, gender was not a significant predictor of functional skills anymore. On the other hand, with regard to food label reading skill, gender was a strong predictor even after adjusting the effect of all other possible predictors in multivariate analysis. The results showed that boys scored higher in reading and interpreting food labels. Reviewing the available literature indicate that there is no consistent gender difference in food label use or interpreting skills. Some studies did not show any gender differences [24,25,26], while some indicated that females more frequently used or correctly interpreted food labels compared to males [27,28,29]. According to the literature, women seem to use food labels more frequently than men [25, 27, 28]; however, inconsistency between studies exist regarding interpreting and understanding food labels [24,25,26, 29], suggesting that other factors such as age, education level, nutrition knowledge, etc., may affect gender differences.
Based on bivariate analysis, mother education level, studying in private school and higher SES score were significantly associated with higher food and nutrition knowledge score. Multivariate analysis confirmed these results; as increasing SES score was associated with a higher likelihood of higher knowledge score. Consistent results have been reported in several studies [12, 30,31,32,33]. Aihara et al. indicated that higher educational level and economic status was associated with adequate nutrition literacy in elderly Japanese . Although they used the term “nutrition literacy”, but their questionnaire only assessed nutrition knowledge. Similarly, other studies have shown higher education level [12, 31,32,33] and job position [12, 31, 32] were positively associated with nutrition knowledge. The necessity of food and nutrition knowledge as a prerequisite for dietary changes , although not sufficient, calls the need for more emphasis on nutrition educational programs targeted at lower SES groups.
Academic performance was also associated with higher knowledge score, but surprisingly with lower functional and interactive scores. This may be due to the fact that current high school curriculums and textbooks in the country have relatively little on food and nutrition which is dominantly focused on knowledge aspects . Besides, students with better academic performance due to heavy school workload, may have limited time or interest to develop their food and nutrition-related skills i.e. food shopping, preparation, and cooking (functional skills) or interact with others about food and nutrition (interactive skills). This may be especially more relevant in our study participants who were senior high-school students preparing for the university entrance exam. Further research is needed to make a more reliable conclusion in this regard.
The possibility of higher knowledge score was significantly higher in students who studied Natural Sciences compared to those whose major were Literature and Humanities. Food and nutrition-related topics are more likely to be discussed in the Natural Sciences courses than other majors. A recent analysis of the content of high school textbooks in Iran showed that food and nutrition-related topics have been addressed more frequently in textbooks of Natural Sciences than other majors  which confirmed the results of the current study.
The weight and health status of the subjects were also examined as possible determinants of FNL and its dimensions. Higher BMI was correlated with higher functional score in bivariate analysis. However, after controlling for the effect of other possible predictors in multivariate analysis, this association was not significant anymore. The relationship between weight status and FNL has been addressed in a number of studies [12, 15, 20, 30, 34, 35]; however, the results have not been consistent. In some studies, people with higher BMI have had a lower level of FNL [20, 34], while in some others non-significant [12, 15, 30] or positive  association between BMI and FNL has been reported. These investigations have been conducted among different age and sex groups that may partly explain this inconsistency in findings. In a study conducted by Kubiet et al. among adolescents , multivariate analysis showed no significant association between weight status and FNL, which is consistent with our findings. However, the limited number of studies, all with cross-sectional design makes it hard to make a conclusion.
In the current study, the presence of nutrition-related diseases in a family member predicted the possibility of higher food label reading skill of the students. Previous reports have also been indicated that people who suffer from nutrition-related diseases e.g. hypertension, diabetes, cardiovascular disease, etc., pay more attention to food labels . People with nutrition-related chronic diseases and their families have more concerns about diet and may want to limit the consumption of some specific dietary components like calories, sugar, fat, salt, etc. These concerns could explain higher food label interpreting skills among people with chronic diseases and their families.
To the best of our knowledge, this is the first study assessing the FNL status of Iranian senior high-school students by a valid multidimensional tool. However, this study had some limitations that need to be taken into consideration. First, its cross-sectional design makes it impossible to interpret the direction of the associations. Moreover, the determinant factors examined in the current study could not explain the variation in score of the skill domain and its dimensions well. It seems that more complex factors affect FNL related skills which had not been included in our study. For example, food skills may be affected by socio-cultural norms which were not assessed in the present study. Therefore, in order to explore possible determinant factors of the FNL skill domain, further research especially with qualitative design could provide more insights. Finally, this study conducted among senior high-school students in Tehran; therefore, its results may not be generalized to other age groups or different populations.
In conclusion, the present study showed that Iranian senior high-school students have relatively low food and nutrition-related knowledge and skills. Among possible determinant factors examined, study major, academic performance and SES were significant predictors of youth’s food and nutrition knowledge; and male gender and having nutrition-related diseases in family members were determinant factors of higher food label reading skill. Further studies are recommended to identify other possible factors related to youth’s FNL. The findings reemphasize the need for evaluating current formal education curriculums with regard to food and nutrition knowledge and skill development as an important competent of life skills. Also, relatively low FNL level among senior high-school students highlighted the need for future studies focusing on FNL promoting interventions in high school students in Iran.