Sweet foods have become a favorite in all age groups, and the consumption of sugar-added processed foods has increased; thus, sugar intake among Korean people is spiking [12].

The result of analyzing the sugar intake of Korean people showed that sugar intake continuously increased from 2008 to 2010; in particular, adolescents' average daily sugar intake (69.6 g) was about 13% higher than Korean people's average daily sugar intake (61.4 g). When compared with all other age groups, adolescents' sugar intake was the highest. The main source of adolescents' sugar intake included beverages (14.3 g, 20.5%), fruits (10.9 g, 15.7%), and bread products · confectionary · rice cake (8.9 g, 12.8%). Of these, sugar intake from carbonated drinks was the highest among young people aged between 6 and 29 [3]. According to the Korean National Health and Nutrition Examination Survey, the group who drank carbonated drinks or soda had a higher calorie intake than the non-soda drinking group, while their intake levels of vitamin B2, niacin, vitamin C, calcium, and phosphorus were relatively low [4]. This indicates that too much soda intake can hinder the absorption of essential nutrients.

Sugar intake from various forms of foods can relieve fatigue, refresh the mood, and promote mental stability in adolescents; however, frequent consumption of sugary snack foods can lead to nutritional problems [56]. For example, over-consumption of sugar leads to the release of excessive insulin, which causes low blood sugar, affecting the central nervous system, which uses glucose as its main source of energy; hence, it can cause hyperactivity disorder in adolescents [67]. Moreover, when overweight adolescents consume too much sugar, they can become obese in adulthood and suffer from adult lifestyle diseases such as diabetes or cardiovascular diseases [8]. Therefore, it is necessary for adolescents particularly to make a continuous effort to control their sugar intake in order to maintain their health and prevent lifestyle diseases in adulthood.

Since 2007, Korea has continually conducted educational and promotional activities targeting processed food companies and restaurant businesses, to mark and monitor sugar contents and to lower sugar intake [28]. Despite this continuous effort, however, the awareness for sugar reduction and education is still insufficient and particularly sugar-reduction studies targeting high school students, who need to reduce their sugar intake, are extremely rare.

Several research have shown that the higher nutrition knowledge the people had, the more positive they behaved in terms of eating [91011]. On the other hand, some researcher reported that there was little correlation between nutrition knowledge and eating attitude or behavior [1213]. Carruth et al [14] also stated that high nutrition knowledge level was not always expected the proper eating behaviors. From those different points of views, it is difficult to reach a consistent conclusion about the relationship between nutrition knowledge and eating behaviors or attitudes. Nevertheless, right food choice and eating habits could start from accurate nutrition information and knowledge. That is why nutrition education for all generation has been emphasized. Especially, students can learn the right nutrition knowledge through official school education, and nutrition knowledge acquired in that time can steadily be utilized as an adult for good eating habits [15]. Therefore, more study of nutrition knowledge and behaviors among students should.be performed. In this context, this study was conducted in order to analyze the sugar-related nutrition knowledge level and the related eating behavior among high school students and investigate whether differences in nutrition knowledge levels make behavioral difference.

In doing so, it aims to suggest educational guidance for lowering sugar intake in homes and in schools.

The present study was conducted in order to provide baseline data for presenting a guideline for sugar reduction policy and education by examining current high school students' level of nutrition knowledge related to sugar and their status of sugar intake based on the knowledge levels.

The result shows a positive association between the knowledge level of sugar and the appropriate sugar intake behavior and sugary food choice. It was found that the high knowledge group displayed significantly high levels of awareness in all topics including “concept and characteristics of sugar,” “sugar and food,” “sugar and health,” and “sugar reduction”.

When examining the specific contents of questions in four topics, the rate of correct answers of both groups were below 50% with regard to the following contents: glucose as energy source, diabetes and sugar, sugar contained in processed beverages and digestion of sugar. Such results imply that high school students have simple knowledge about sugar, but the accuracy of their knowledge is questionable. Considering that right actions can be drawn only when the knowledge is accurate, the need for education on the insufficient sugar knowledge is at stake. Results of the study on the development of a sugar education program for elementary school students conducted by Han & Lee [19] show that continuous in-school education about sugar in detail not only increases students' knowledge on sugar, but also brought positive changes in their eating habits. Therefore, it is believed that detailed education should be conducted and planned focusing on the lesser-known topics about sugar, and planned and long-term eating habits program should be developed in order to promote high school students' proper sugar intake.

With regard to high school students' sugar intake behaviors based on the level of sugar-related nutrition knowledge, the results show that the high knowledge group tries a relatively desirable behavior related to sugar intake compared to the low knowledge group, regardless of significance. Results of this study were similar to those of Yoo & Jung's stating [20] that the higher middle school students' nutrition knowledge score was, the lower their sugar intake frequency, sugar-related preferences, sugar-related eating behavior scores were. Other studies indicate that knowledge levels of specific foods or nutrition are related to eating behavior; nutritional knowledge levels and degrees of prevention practices regarding high blood pressure were positively correlated [21], a fixed positive correlation existed between housewives' nutritional knowledge and practice [22]. This study revealed similar results to those of previous studies, and such results indicate that the less sugar-related nutritional knowledge is, the more lack of interest in nutritional education exists and leads to a negative effect on sugar-related eating behavior [2324]. Since nutritional knowledge affects eating behavior, inaccurate knowledge can lead to incorrect eating behavior [24]. Whether nutrition knowledge affects eating behavior is still controversial. However when considering the result of this study as well past studies which revealed the relationship between nutrition knowledge and eating behavior, it could be said that students who have more sugar-related nutrition knowledge, were tendency to show a desirable eating behaviors such as reducing sugar intake and paying careful attention to the selection of processed foods and beverages. Therefore, the cultivation of correct nutrition knowledge about sugar could be an important factor in having a healthy diet for high school students. As a result, proper eating behavior based on accurate knowledge should be encouraged to students by providing education on correct information that reflects possible adolescence nutritional health problems that can occur from sugar intake.

When examining the status of consuming the food groups that lead to too much sugar intake, the main sources were beverages, confectionary, and bakery goods in the corresponding order, irrespective of the level of nutrition knowledge related to sugar. In particular, the low nutrition knowledge group's frequency of consuming beverages was significantly higher than that of the high knowledge group (p < 0.05), as was the consumption of confectionary and bakery goods. According to the Korean Ministry of Food and Drug Safety [3], adolescents' daily average sugar intake (69.6g) is the highest of all age groups and 13% higher than Korean people's daily average sugar intake (61.4g). According to the survey result on the total sugar intake of adolescents (age 12~18) between 2008 and 2011, the most popular sugary foods for this age group were as follows: beverages (20.5%), fruits (15.7%), and baked goods · snack food · sweet rice cake (12.8%) in the corresponding order [3]. In other research, among the processed foods that children between 12 and 18 consume, the main sources of sugar intake were soda, snack food/baked goods, and beverages [8]; this result was similar to Kim's study that reported favorite snacks among middle school and high school students were baked goods, fast foods, and snack foods in the corresponding order [16].

Based on the present research results, the frequency of sugar intake was highest with beverages in the sugary processed food group, and especially sports (ion) drinks was consumed more frequently by the low knowledge group than the high knowledge group. Also, although not statistically significant, the low knowledge groups showed a higher intake frequency of soda, fruit juice · fruit flavored drink. Such results are in line with other studies that claim sugar-sweetened drinks to be the main cause of adolescents' sugar intake. According to Reedy et al, adolescents frequently consume sugar-sweetened drinks, and this is the main source of added sugar [25]. A study conducted in the United States by Wang and colleagues [26] also revealed that 84% of adolescents reported drinking more than one sugar-sweetened drink, and soda was the most frequently consumed one. Other previous studies asserted that much of the change in adolescents' added sugar intakes can be affected to changes in their beverage consumption patterns in other previous studies. As children progressed from childhood to early adolescence, both frequency and prevalence of milk or fresh fruit juice intake decreased, while at the same time, soft drink consumption has escalated [27,28,29].

Several researchers have reported that hyperactivity and weak mental health as well as obesity, dental problem and type 2 diabetes were related to the consumption of soft drinks [3031323334]. As types of sugar-sweetened drinks increase, recent studies show positive relationships between energy drink containing too much caffeine and sleeping disorder, between the increase in soft drinks intake frequency and increase in smoking or decline in grades [353637]. Sugar intake through beverages is also considered as a serious problem in other countries; some European countries and Singapore implement various sugar reduction policies such as soda or sugar taxation against beverage manufacturing companies in an attempt to improve beverage components [3839].

KFDA (Korea Food and Drug Administration) [40] has announced a sugar reduction policy including processed food nutrition labeling regulations, support for sugar-reduced recipes development, and restrictions of selling high sugar foods targeting children in 2016 in an aim to lower sugar intake by processed foods to under 10%. Such implements of nationwide strategies and citizens' individual awareness and appetites improvement should be reached together. Appropriate sugar intake guidelines for adolescents are especially in need, because adolescents need more energy than other age groups but have less interest regarding health issues and thus have less negative awareness levels regarding sugar intake [41]. Therefore, various ways to encourage choosing healthful activities and drinks such as milk and water should be prepared, along with efforts to reduce adolescents' sugar-sweetened drink intake. Results of this study especially point out that many students (77.3% of the high knowledge group, 64.2% of the low knowledge group) felt the need for learning about the sugar content in drink, regardless of their nutrition knowledge levels. This indicates that high school students were sensitive to the need for learning about the sugar content in drinks. Therefore, high school students should have continuing nutrition education so that they can become aware of and consume healthful drinks.

Do and colleagues [42] reported that high school students in Gyeonggi province used taste (76.7%) and price (10.5%) as criteria in selecting drinks. That is to say, regardless of age, everyone considered “taste” to be most important when selecting drinks. Based on this, it appears that school cafeteria and homes need to identify the favorite taste among students and develop many recipes using natural ingredients instead of processed foods. With regard to drinks, although it is unfeasible to make and offer healthy drinks with reduced sugar content, it is necessary to provide continuing education to encourage students to select and consume healthy drinks at school where they spend most of their time, because healthy drinks are essential for their health and nutritional status.

With regard to the interest in participating in education on sugar, 43.9% of the high knowledge group and 36.4% of the low knowledge group responded positively. Although the high knowledge group's desire to participate is higher than that of the low knowledge group, the percentages of interested students in both groups were lower than those of uninterested students. Considering the current high school students' life, voluntary participation in learning something outside their regular curriculum seems difficult. Nevertheless, education related to eating habits is a crucial agenda for development and health during adolescence; hence, it is necessary to remind them of the importance of reducing sugar and help them practice it in various ways. With regard to methods of learning sugar-related topics, students responded that it is most appropriate to learn from “nutrition class in school.” Therefore, it appears that education methods should be developed in connection with school classes. With regard to the content of sugar-related education, both groups expressed the strongest interest in learning about “foods that contain too much sugar” and “sugar and health: diseases caused by over-consumption of sugar.” That is, students wanted to learn about the foods they actually eat and about the health risk factors of consuming certain foods. Therefore, it is believed that necessary education programs for sugar reduction besides the specific interest areas can take place in stages if education for proper sugar intake is performed starting with the topics that students are interested in.

The major findings of this study and suggestions for future research are presented as follows. First, the result of examining sugar-related nutrition knowledge by topic showed that the level of awareness of the “concept and characteristics of sugar” was low. Based on this result, it is believed that sugar-related nutrition education targeting high school students regarding the topic of “concept and characteristics of sugar,” which is less known, should be conducted after specific information is taught on top of basic knowledge. Second, the group with higher levels of knowledge in sugar-related nutrition displayed desirable eating habits related to sugar intake. Therefore, it is necessary to become aware of the importance of continuing nutrition education among students and to conduct a consistent education inside and outside of schools. Third, the result of examining the status of sugar intake showed that sugar intake through beverages was the highest, followed by snack foods and baked goods. Thus, for the drinks at school cafeterias or homes, natural ingredients should be utilized as much as possible in making beverages while considering student preference. Fourth, the group with higher levels of knowledge in sugar had more experience in learning about sugar and had a stronger desire to participate in educational programs related to sugar. In short, high schools should also offer nutrition classes as a part of the regular curriculum as in elementary and middle schools, and provide more educational opportunities for students to participate through systematic education to reinforce positive eating habits and help them practice low-sugar diets in real life. Park et al. [43] encouraged a natural decrease in sugar intake for students via school meals. Therefore, inducements to decrease sugar intake should be made in diversified methods. Finally, research and education on sugar reduction is insufficient whereas lowering sodium is well-advocated. Therefore, academia needs to make an effort to conduct continuous research on education methods, and advocate and educate the down cut of sugar intake. Doing so will help students use their accurate knowledge of sugar reduction in real life.

The limitation of this study is that the results do not reflect attributes of the entire high school population, for the research focused on humanities high school students in Seoul. Therefore, future researches should look into cities other than Seoul.

Another limitation is that although the intake frequency of sugary processed foods and related eating behaviors can be caused by various reasons, this study did not consider all possible factors. In other words, this study was conducted under the assumption that increasing nutritional knowledge level of specific food products can change students' related eating behaviors and attitudes, and was focused only on this. However, adolescents' sugar intake behavior can differ according to many reasons. Therefore, if future studies explore multilateral factors that influence sugar intake and their relative influence levels, it could help establish political and educational plans to reach desired sugar intake behavior.