Assessments of nutrient intake and metabolic profiles in Korean adolescents according to exercise regularity using data from the 2008 Korean National Health and Nutrition Examination Survey

In Korea, current economic growth and improvements of standard of living have lead to decreases in various infectious diseases, but have led to large increases in chronic degenerative diseases due to lack of physical activity, excessive nutrient intake and high stress [1, 2]. Among the chronic degenerative diseases, the frequency of obesity caused by lack of exercise has increased gradually whereas age at diagnosis has decreased, thus obesity in adolescents has risen quickly and becomes a critical health issue during adolescence [3]. According to the Korean National Health and Nutrition Examination Survey (KNHANES), obese adults having a BMI of 25 or within the criterion of the Korean Society for the Study of Obesity (KSSO) in Asians [4], increased continuously in Korea from 29.0% in 1998 to 31.7% in 2007 and the number of obese adolescents also increased approximately four times from 2.3% to 9.6% in the same period indicating that obesity has become a serious concern in adolescents [5].

Exercise as physical activity implies all physical movement promoting actual energy consumption through the contraction of skeletal muscle [6]. Physical inactivity in adults provokes various chronic diseases and problems related to skeletal muscle, mental health, and the immune system along with obesity [7] and lack of physical activity in children and adolescents has been reported to increase relative risks of many diseases including obesity and metabolic syndrome [8]. Generally, physical activity declines continuously with age [9]. Therefore, helping children and adolescents, who do not have established patterns of physical activity, to acquire healthy habits for increasing activity is useful in terms of public health, by leading to individual efforts to reduce occurrences diseases such as obesity, hypertension, and diabetes in adulthood [10]. As a basic condition for performing physical activity, it is important to acquire a habit of exercising regularly [11]. Regular exercise has been proven to decrease occurrence rates of chronic degenerative diseases, and it cam not be denied that exercise has such effects by being reciprocally related to other risk factors such as hypertension, obesity, diabetes, and hyperlipidemia [12-14]. In particular, since exercise during adolescence as well as adulthood is an important factor affecting occurrence rates of diabetes, hypertension, and cardiovascular disease along with having an influence on one entire life, it is therefore a major health variable [15].

Because adolescence is a time of experiencing dramatic physical and emotional development, as the periodic transition phase from childhood to adulthood, it demands high amounts of nutrients, thus a sufficient nutrient supply based on appropriate lifestyle and dietary habits is critical. However, most of Korean adolescents not only undergo psychological stress due to academic performance and examinations in middle and high schools, they also may adopt excessive weight control and unbalanced dietary habits due to attention on appearance, provoking failure to thrive following underweight and malnutrition, thus studies on adolescent health are desperately needed [5, 16]. Because childhood and adolescence are important periods that require behavior adjustments and health policies to prevent chronic diseases, many efforts to investigate and analyze the health behaviors of adolescents have been made worldwide by dividing subjects into physical activity and dietary behavior groups, thus offering information to correct poor habits. However, most previous studies on adolescent health had a limitation of analyzing practice rates of recommendations of physical activity and dietary behavior independently [2, 10]. Therefore, to enhance health by improving behavioral factors for health, a comprehensive rather than an independent approach considering both physical activity and dietary behavior is necessary, and strategies with a comprehensive approach to enhance health behavioral factors as well as mediate them need to be established.

Therefore, this study evaluated metabolic factors and nutritional quality according to exercise regularity and also analyzed correlations between metabolic factors and nutritional quality by investigating the practice of regular exercise in adolescents using raw data from the forth KNHANES performed in 2008. In addition, the results of this study can be expected to suggest new health approaches for Korean adolescents by helping them understand the importance of regular exercise as well as reconsidering the effects of exercise on ones metabolic profile and nutrient intake.

KNHANES is a nationally approved statistical survey on health and nutritional condition conducted according to the law for national health promotion. The raw data used in this study were from the second year of the fourth KNHANES from January to December, 2008. Although the number of adolescents in the original raw data was 900, this study analyzed data from 834 of these subjects, consisting of 440 boys and 394 girls who answered all of the questions in the questionnaire for adolescents aged 12-18 years and participated in all surveys, in order to increase the reliability of the study results.

Out of the total subjects in this study, 75.2% of the boys (331) and 48.7% of the girls (192) were found to perform regular exercise that expended over 1,000 kcal per week. This finding is similar to other studies reporting that male adolescents did more exercise than female adolescents [21-23] and suggests that research on active participation of girls in exercise and measures to promote their participation are necessary.

The average age of the subjects was 14.6 years, and average height, body weight and BMI were 167.3 cm, 58.8 kg and 20.9 kg/m², respectively in the male adolescents, and 158.8 cm, 52.0 kg and 20.5 kg/m², respectively in the female adolescents. As compared to levels of height, body weight and BMI for boys (164.3 cm, 56.7 kg, and 21.0 kg/m², respectively) and girls (158.4 cm, 51.5 kg and 20.5 kg/m², respectively) aged 14 years reported by the Ministry of Education and Human Resources Development and the Korean Educational Development Institute (KEDI) [24], the boys' height and body weight were 3.0 cm and 2.1 kg higher, respectively and the girls' height and body weight were recorded as 0.4 cm and 0.5 kg higher, respectively. Interestingly, although the average BMIs of the boys and girls observed in this study were similar to those determined by the Ministry of Education and Human Resources Development and KEDI, the physiques of the subjects in this study, including height and body weight were found to be improved. This study showed that EM recorded significantly higher levels of body weight, waist circumference, and BMI than NM. In addition, EF had significantly higher levels of height, body weight, waist circumference and BMI than NF. The results indicate that the physiques of adolescents who exercised regularly were better than those of adolescents who did not. However, this is contrary to the results of previous studies revealing that in exercising groups, body weight was significantly lower than in non-exercising groups [5, 25]. Considering that the normal range of BMI reported by KSSO was 18.5-25.0 for both males and females, and a standard physique as determined by the KDRI's was a BMI of 22.0 kg/m², the adolescents doing regular exercise had significantly higher body weights and BMIs than the other subjects, however, their levels were within the normal range and rather was nearer to the standard. Therefore, regular exercise can be helpful in maintaining and managing a standard physique for both of boys and girls.

Most cross-sectional studies [26, 27] have not shown any significant correlations between a regular exercise and variables of metabolic syndrome in children and adolescents. In the third survey, KNHANES investigated physical activity with an International Physical Activity Questionnaire (IPAQ) according to frequency and time of walking, and moderate-intensity and severe-intensity physical activity. This study, which calculated calories by designating intense and moderate physical activity and walking, also revealed there were no significant differences according to exercise in hemoglobin and hematocrit levels or glucose, TC and TG. This is considered to be because the adolescents in all groups of this study were within normal ranges for metabolic profile data. Meanwhile, HDL-C level could not be analyzed in this study since it was not included in the available raw data of KNHANES.

When the nutrient intakes of all subjects were investigated according to exercise, there were no significant differences between groups, except for higher niacin and potassium intakes of EM compared to NM. Major sources of niacin are poultry, wheat bran, tuna and other fish, whereas fresh fruits, vegetables, milk, whole grains, and meats are sources of potassium [18]. In the present study, the adolescents who exercised showed greater niacin and potassium intake than the non-exercising adolescents. Although it was not presented in this study, this result was due to the exercising adolescents having dietary habits of greater milk and meat intake. These results are similar to previous studies. According to previous investigations on dietary intake in Korean adolescents [10, 15], the rate of exercising adolescents who practiced recommendations for daily intakes of meat, milk, and milk products was significantly higher than that of the non-exercising adolescents. NAR is used to evaluate the dietary adequacy of individual nutrients, and MAR is a method to assess dietary adequacy of overall nutrients. NAR and MAR values of over 1.0, respectively, indicate adequate intake conditions [1, 3, 19, 20]. To evaluate dietary adequacy properly, both NAR and MAR should be calculated and used for assessment, thus this study utilized both values. MAR assessing the appropriateness of overall nutrients, indicated that all groups except EM had inadequate intake, and therefore the qualitative dietary intake of Korean adolescents is considered to be a problem. NAR analysis showed that in all groups riboflavin, vitamin C and Ca intakes were inadequate. Regardless of exercise regularity, NAR vitamin A and NAR Fe intakes were assessed inadequate in female adolescents. According to other reports on dietary intake in adolescents [2, 17, 21], the rate of boys practicing recommendations for daily intake of fruits and vegetables was significantly lower than that of girls, and the rate of boys practicing recommendations for daily intakes of meat and protein was significantly higher than that of girls. However, this study showed that in both male and female adolescents, protein intake was assessed as adequate but vitamin C intake was not.

When correlations between metabolic profile values and nutritional adequacy were analyzed, a significantly positive correlation was observed between height and NAR Fe in the exercising groups of both boys and girls. This finding indicates that exercise worked as a factor promoting increased height and intake of iron. Similar to the results of other studies revealing that exercise positively affected physical development in adolescents [28-31], this study also found that exercise produced positive effects on physical development. The rate of participation in exercise was higher in male students than in female students at 75.2% and 48.7%, respectively, and EM was observed to be good in terms of height and body weight and in the appropriateness of MAR. Among the female adolescents, physical development was significantly poorer in NF than in EF, and NF recorded the lowest level of 0.91 in MAR analysis. This result indicates that inactivity in girls negatively influenced physical development and nutrient intakes and suggests that efforts to provoke physical development and improvements in nutrient intake by inducing their participation in exercise are necessary. Similar to the findings of previous studies reporting that female students had a high interest in appearance but tended not to practice healthy behaviors in terms of physical activity and dietary behavior [2, 17, 23], this study revealed that the rate of girls not practicing healthy behaviors in terms of physical activity and dietary behavior was high and that they needed to practice these behaviors. According to a study by Lee and Lee [32] on dietary habits and exercise in underweight and obese adolescents, when adolescents have the habit of enjoying exercise rather than focusing on consuming calories through exercise, obesity can be prevented thus physical training should be a required subject rather than an optional one and class time should be extended to promote and enhance physical strength. This study also indicates that practice of dietary behaviors needs to be induced through practice of regular physical activity and exercise in Korean adolescents. In particular exercise programs and nutrient education programs are necessary for girls to help them create habits toward enjoying exercise.

According to the above results, regular exercise by Korean adolescents is considered to affect physical characteristics and nutritional adequacy, and efforts to induce the active participation of female students in exercise are desperately needed. To promote active participation by girls, appropriate recognitions of appearance and health must be made and strategic health education with comprehensive approach strategies including practices of voluntary and healthy exercise and dietary habits should be provided.