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ISSN : 1229-6457(Print)
ISSN : 2466-040X(Online)
The Korean Journal of Vision Science Vol.24 No.3 pp.307-317
DOI : https://doi.org/10.17337/JMBI.2022.24.3.307

The Association between Sodium Intake and Refractive Error in Korea Children and Adolescents: The Korea National Health and Nutrition Examination Survey

Hye-Won Park1), Tae-Hun Kim2),3)
1)Dept. of Optometry, Kookje University, Professor, Pyeongtaek
2)Dept. of Visual Optics, Graduate School of Health and Welfare, Baekseok University, Professor, Seoul
3)Dept. of Visual Optics, Division of Health Science, Baekseok University, Professor, Cheonan
* Address reprint requests to Tae-Hun Kim (https://orcid.org/0000-0002-2840-650X) Dept. of Visual Optics, Baekseok University, Cheonan TEL: +82-41-550-2259, E-mail: kth@bu.ac.kr
August 18, 2022 September 22, 2022 September 26, 2022

Abstract


Purpose : This study investigated whether the association between sodium intake and refractive error among children and adolescents.



Methods : This study used the data from 968 children and adolescents aged 5~18 years, obtained from the Korea National Health and Nutrition Examination Survey in 2016. Sodium intake was measured by data for food items in the 24-hour diet recall. For refractive error, the spherical equivalent (SE) of the right eye was applied. The association between a sodium intake and refractive error was analyzed using logistic regression analysis, after adjusting for potential confounding factors.



Results : There is a correlation between sodium intake and refractive error (p<0.001), and the higher the sodium intake, the higher the refractive error (p<0.001). In particular, sodium intake was shown significantly higher in myopia. After adjusting for potential confounding factors, in multivariate logistic regression analysis it was found that high sodium intake was significantly associated with an increased risk of myopia (OR 1.62; 95% CI 1.12-2.33, p=0.010).



Conclusion : Our results revealed a significant association between sodium intake and refractive error in Korean children and adolescents. Being high sodium intake was also found to be associated with myopia.


Adolescent , Children , Myopia , Refractive error , Sodium

국내 소아·청소년의 나트륨 섭취와 굴절 이상과의 연관성: 국민건강영양조사

박 혜원1), 김 태훈2),3)
1)국제대학교 안경광학과, 교수, 평택
2)백석대학교 보건복지대학원 안경광학과, 교수, 서울
3)백석대학교 안경광학과, 교수, 천안

    Ⅰ. Introduction

    Refractive error is common causes of visual impairment worldwide. Its prevalence of myopia has sharply increased especially in East Asia.1) It is estimated that, by 2020, approximately one third of the world’s population, or 2.5 billion people, will be affected by myopia alone.2) It has a significant impact on the prevalence of refractive errors in children and adolescence, particularly on vision development. Vision development begins after birth and continues until the age of 13 years, with most children reaching the full development between 6 and 8 years of age.

    Pediatric ophthalmologic studies exploring refractive error have provided useful perspicacity into the early development of refractive error.3-5) Additionally, the nutritional intake plays important roles, especially among children and adolescents, who are experiencing a period of growth that has important implications. The minerals play an important role in the body in small amounts. Among the mineral, sodium is an essential substance that plays a role in maintaining homeostasis in the body and transmitting nerve impulses.6) However, high sodium intake has been associated with an increased risk of various health risks, such as hypertension,7) cardiovascular disease,8) obesity,9) and type 2 diabetes.10) In addition, sodium also affects the eyes. If you consume a lot of sodium, the body loses water and the cells of the eye become dry. These factors have been reported to affect vision and eye diseases, such as reducing vision by inducing retinopathy.11)

    Previous studies have reported that sodium intake has an effect on visual acuity, but there are few studies on the effect of sodium on vision. Therefore, the purpose of this study is to investigate the relationship between sodium intake and refractive error associated with the risk of prevalent refractive error among participants 5~18 years of age in the 2016 Korea National Health and Nutrition Examination Survey (KNHANES).

    Ⅱ. Subjects and Methods

    1. Subjects

    We used data from the 7th Korea National Health and Nutrition Examination Survey (KNHANES VII-1) of 2016. A total of 8,150 individuals participated in KNHANES. Among the participants, there were 1,323 children and adolescents aged 5~18 years. Those with missing data on refractive error were excluded. Of these, 968 were included in the analysis who underwent ophthalmologic examination. The 2016 KNHANES data used in this study was exempted from the IRB as it was a study conducted by the government directly for the public welfare according to the bioethics law by the Korea CDC (centers for disease control and prevention).

    2. Ophthalmic examinations

    The objective refraction was recorded as the subject’s refractive data. The degree of refractive error was measured by non-cycloplegic refraction using an auto-refractor (KR-8800, Topcon, Tokyo, Japan). The spherical equivalent (spherical+cylinder/2) was used to calculate the refractive error. Myopia was defined as the spherical equivalent of greater than –0.50 D (diopters).12) More specifically, myopia was classified as low (-0.50 D to -3.00 D), moderate (-3.00 D to -6.00 D), or high (-6.00 D or more).13,14) Hypermetropia was defined as the spherical equivalent of more than +0.50 D, and emmetropia was defined as the spherical equivalent of +0.50 D or less but –0.50 D.

    3. Health status variables

    Height was measured using a stadiometer, while weight was measured using a scale. Height and weight were recorded to one decimal place. BMI (Body mass index) was calculated by dividing the body weight by height squared.15) Childhood and adolescence obesity were defined according to clinical guidelines, underweight (<5th percentile), normal weight (5th≤percentile<85th), overweight (85th≤percentile<95th), and obese (≥95th percentile).15) Based on the responses to parental history of myopia, those classified as unsure, not applicable, or no response were excluded, leaving those classified as ‘yes’ or ‘no’. Time spent on near work was determined based on <1, 1~2, 3, and ≥4 hours, on responses classified as average time spent on near work hours per day, such as computer work, reading, etc.

    4. Measurement of sodium intake

    The nutrition survey data were collected using ‘food intake survey questionnaire’. To calculate the sodium intake, we established a sodium database by the data on food items from the 24-hour diet recall in the NHANES database with the established sodium value for each food item.16,17) The sodium intake group was classified as 1,500 mg or less and 1,500 mg or more.18)

    5. Statistical analyses

    In analyzing KNHANES data, a two-step stratified sampling method was applied using the survey district and household as primary and secondary sampling units, separately. The KNHANES 7th included weights to compensate for its complex sampling design and to allow approximations of the Korean population, weighted analyses were performed using SPSS ver. 18.0 (SPSS Inc., Chicago, IL, USA). The KNHANES sampling was weighted by adjusting for oversampling and nonresponses.19) The representative refractive error was defined based on the subject’s right eye.20) All values were presented as mean±standard deviation.

    The analysis of variance (ANOVA) or chi-square test was used to compare the general characteristics. In addition, the correlation between each of the sodium intake and refractive error was analyzed using Pearson correlation, and only variables with a variance inflation factor less than 10 due to correlation were used (all Variance Inflation Factor score=1.00<10). The association between sodium intake and refractive error was identified using linear regression analysis for the complex survey. The relationship between sodium intake and refractive error was analyzed using complex sample logistic regression. p-value of p<0.050 was considered statistically significant.

    Ⅲ. Results

    1. General characteristics of study participants

    General characteristics of the study subjects according to refractive error are shown in Table 1. This study included 968 children aged 5~18 years. Among those 968 children, the prevalence of myopia, emmetropia, and hyperopia was 616 (63.6%), 278 (28.7%), and 74 (7.7%), respectively. The mean subject age was 12.05±0.40 years. The average age in each of the groups was 12.67±3.52, 8.70±3.49, and 9.09±4.35 years for the myopia, emmetropia, and hyperopia group, respectively. In the age group of 5~8 years, emmetropia was the highest, and myopia was the highest in the 15~18 years of age. Moreover, there were no significant differences in gender groups with myopia, emmetropia, and hyperopia group. Average height was significantly higher in the myopia groups compared to the emmetropia and hyperopia group, at 154.70±16.80 cm, respectively. The average weight in each of the groups was 49.59±17.05, 33.08±15.40, and 32.80±16.26 kg for the myopia, emmetropia, and hyperopia group, respectively, differed significantly. Similarly, the average BMI of 20.06±4.04, 17.58±3.53, and 17.37±3.36 in the myopia, emmetropia, and hyperopia group, respectively, differed significantly. Statistical differences between BMI and refractive error were observed in each group. For myopia especially, the predictive value (%) in overweight and obese group was 54 (72.0%), 76 (67.3%) in each of the groups, respectively. Differences in the predictive value for each group, by refractive error, in levels of near-work activities were also statistically significant. When the parents had myopia, the number was 661 (68.3%). There were 436 (66.0%) myopia parents in children with myopia, 187 (28.3%) myopia parents in emmetropia children, and 38 (5.7%) myopia parents in hyperopia children. Of these, 171 children with myopia had both parents (data not shown).

    2. Sodium intake by refractive error classification

    The mean refractive error was –1.94±2.35 D, and refractive error in each of the groups was – 3.12±-0.05, -0.05±0.28, and 1.33±0.90 D for the myopia, emmetropia, and hyperopia group, respectively (Table 2). Among the myopia group, 329 (53.4%) had low myopia, 218 (35.4%) had moderate myopia, and 69 (11.2%) had high myopia. The average sodium intake was 2,756.95±1,587.20 mg. The comparison by the refractive error of myopia, emmetropia, and hyperopia according to sodium intake revealed that an estimated 2,945.51±1,603.53, 2,469.46±1,377.16, and 2,203.04±1,048.27 mg with significant differences between groups (p<0.001). On the other hand, significant differences were observed between groups in the severity of myopia, in particular moderate myopia.

    3. Association between refractive error and sodium intake

    Table 2 shows the results of correlation analyses. There was a statistically significant negative correlation between refractive error and sodium intake (r=-0.17, p<0.001).

    4. Risk analysis of refractive error by sodium intake logistic regression

    The higher the sodium intake, the higher the refractive error, resulting in myopia (F=24.92, p<0.001). The results of multivariate logistic regression analysis comparing the odds ratios (ORs) and 95% confidence intervals (CIs) for sodium and refractive error are shown in Table 3. The covariates included in the comparison of the emmetropia group and the myopia group and the covariates included in the comparison of the emmetropia group and the hyperopia group included variables of age, gender, height, weight, BMI, and near work time. Although emmetropia group was not significantly associated with sodium when compared with hyperopia group (OR 0.85; 95% CI 0.46-1.57, p=0.608), myopia group was significantly associated with sodium when compared with emmetropia group (OR 1.62; 95% CI 1.12-2.33, p=0.010). Moreover, there was a significant association with moderate myopia among myopia groups (OR 2.05; 95% CI 1.23-3.43, p=0.004).

    Ⅳ. Discussion and Conclusion

    This study documented the refractive error in the South Korean population 5~18 years of age based on a nationwide health survey and effects of sodium intake for each refractive error. A total of 968 in Korean children and adolescents, the prevalence of myopia was 63.6%, emmetropia was 28.7%, and that hyperopia was 7.7%. The mean age was 12.05±0.40 years. Age was the highest in myopia, and there was a significant difference in age according to refractive error. The present study showed that the prevalence of myopia was high, particularly in the age group of 15~18 years. In that study, there was no significant difference between male and female according to refractive error. According to a study by Vitale et al.,21) there was no gender difference in refractive error, and similar results were found in this study. In an urban Malay population in Singapore study, there was no gender difference in hyperopia and astigmatism except for myopia.22) On the other hand, Richter et al.23) reported that there was a relationship between gender and refractive error in a study on the Chinese-American population, and Ferraz et al.24) on a Brazilian population also reported that there was a strong relationship between the gender of refractive error. It is difficult to conclusion regarding the association between gender and refractive error considering the inconsistent results reported in previous studies.25) In our study, it was found to be associated with physical development and refractive errors in children and adolescents.

    In the myopia group, they were taller, overweight, and had higher BMI. Previous studies have reported that the risk of developing high or moderate myopia is 1.03 times greater in individuals with a higher BMI in a Korean population.26) In addition, a Dutch study on children demonstrated an association between myopia and high BMI.27) Nevertheless, the effect of BMI on development of myopia is still controversial. Saw et al.28) reported that children who were heavier or who had a higher BMI tended to have more hyperopia with shorter vitreous chambers.

    The proportion of subjects who spent more time on near work of greater than 4 hours per day was sequentially increased with increased refractive error. After decades of investigation, the role of near work in myopia remains unresolved, with some studies reporting no relationship. Recently, several reports have found that shorter working distances of less than 30 cm and continuous near-work of more than 30 min are risk factors for myopia onset and progression.29) There are many studies examining the risk factors for the refractive errors, and it has been reported that there is a significant association between refractive error and several different factors such as age,22,30) BMI31) etc.. Additionally, all degrees of parental myopia were strongly associated with the prevalence of myopic children.26,32) Similar results were found in this study. Children with two myopic parents who are myopic are more likely to be myopic than if one myopic parent, and if their myopic parents are more likely to be myopic than children with no myopic parents.

    The correlation between the risk of refractive error according to the sodium intake. The sodium intake was the highest in the myopia group, and especially, the sodium intake was high in the moderate myopia group. Also, our study results showed high sodium intake might be associated with the risk of myopia in children and adolescents (OR 1.62; 95% CI 1.12-2.33, p=0.010). In the results of this study, moderate myopia was affected by high sodium. The rate of myopia was increasing from more than the age of 9 years, and when they are exposed to factors that cause myopia, myopia progresses to an unconfirmed state as a result, finds focus, and causes axial elongation and moderate myopia in late adolescence.33) The subjects of this study, had a high rate of myopia, and it is thought that this moderate myopia was influenced by high sodium intake in adolescents. The average sodium intake in this study was 2,756.95±1,587.20 mg. The average daily sodium intake of Koreans was 4,789.2 mg in 2010, but it has decreased from 3,189.3 mg in 2020. However, Koreans are consuming more than twice the sufficient intake, which is the nutrient intake standard.34) WHO (World Health Organization) recommends a daily sodium intake of 5 g, which is equivalent to 2,000 mg of sodium, but the intake exceeds this.35,36) Therefore, the WHO recommends a 30% reduction in sodium intake to prevent chronic diseases.37) Myopia that develops in childhood progresses into adulthood, so it is maintained like a chronic disease. Therefore, it is thought that reducing sodium intake will reduce myopia.

    In a previous study, it was reported that high sodium intake stimulates dopamine receptors through animal experiments.38) Many neurotransmitters present in the retina are involved in eye growth. Among the neurotransmitters present in the retina, dopamine is closely related to eye growth. Dopamine exists between the inner granular layer and the inner reticular layer of the retina and increases or decreases depending on the photostimulation delivered to the retina.39,40) Min et al.41) reported that dopamine receptors are closely related to eye growth and myopia progression, and reported that bromocriptine, which selectively acts on dopamine-2 receptors, was instilled into chick eyes to inhibit eye growth and myopia development. In addition, an imbalance in serum sodium levels caused by a high sodium led to a rise in the lens sodium concentrations, which might exceed the capacity of the Na+/K+ channels in the lens and result in the expansion of extracellular fluid volume.42) The expansion of the volume of the lens can lead to myopia.43) As such, it has been reported that the mechanism of myopia progression is the change in the thickness of the lens in relation to accommodation, the expansion of the sclera and the increase in the axial length.43) Abraham44) said that the lens thickens, causing pseudomyopia, if this condition persists, permanent changes occur and myopia is induced. However, studies on how these mechanisms change the development of myopia are still lacking. There are some important limitations of this study. First, the cross-sectional characteristic of this study allows us to assess the association between sodium intake and refractive error, but the causal relationship could not be identified. Second, refractive error was considered as manifest refraction. Because we did not use cycloplegics, there is a possibility of transient accommodative error.45) However, while lack of cycloplegia leads to an overestimation of myopia, the problem with non-cycloplegic refraction situation the underestimation of hyperopia and consequently errors in estimating of mean spherical equivalence. Third, there was a limitation in the accurate determination of sodium intake by the food intake method.

    The dietary sodium intake is calculated by the sodium content of the food ingredient composition and measured using the 24-hour dietary recall method, food record, and food frequency questionnaire. Nevertheless, the 24-hour dietary recall and food record methods do not accurately reflect the amount of sodium in soup stocks such as salt. It is difficult to accurately measure the sodium status. Despite these limitations, the 24-hour recall method is still valid for measuring dietary sodium intake in individuals. The present study had limitations and further studies are necessary to identify the mechanisms and assess the association between sodium intake and refractive error in children. Hence, this is the first study in Korea to assess the relationship between sodium intake and refractive error in children and adolescents. Although mechanisms of the association between sodium intake and refractive error have not been well established, the role of eye growth has been expected and shown to be significant among children and adolescents. Also, we confirmed that the risk of myopia creased with increasing sodium intake. When eating food period of growth, it can adversely affect refractive error if sodium intake is not restricted.

    In conclusion, the association was clearer between sodium intake and refractive error in Korea children and adolescents. Therefore, if you do not limit your sodium intake when eating food in the period of the growth, it may have a negative effect on refractive error.

    Figure

    Table

    The general characteristics of the subjects according to refractive error (n=968)

    Sodium intake by refractive error classification

    Odds ratio of sodium intake and refractive error in multivariable logistic regression analysis

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