Updated reference ranges for aminotransferase levels of Korean children and young adolescents based on the risk factors for metabolic syndrome

Abstract

We investigated the reference values of liver enzymes based on cardiometabolic risks among children and adolescents using the Korea National Health and Nutrition Examination Survey (KNHANES). A total of 8,091 subjects aged 10–18 years were included from data from 2007–2017. Overall, Aspartate aminotransferase (AST), alanine aminotransferase (ALT), and the AST/ALT ratio varied with sex and age. AST levels tended to decrease with age, but ALT levels had a U-shaped curve, which resulted in a gradual increase in the AST/ALT ratio after age 13. The prevalence of MetS was strongly associated with elevated AST or ALT and a decreased AST/ALT ratio. The odds ratios of the development of MetS were elevated in groups with high levels of AST and ALT and a low AST/ALT ratio. Particularly in the combined ALT and AST/ALT analyses, borderline-high levels also showed a high odds ratio of MetS. Liver enzymes were also involved in the increase in the adjusted mean values for each risk factor for MetS. Here, we provided updated reference values for liver enzymes based on the analysis between population-based data and cardiometabolic risk factors; AST, ALT and the AST/ALT ratio might be useful in the early diagnosis and treatment of the MetS.

Introduction

Rates of childhood obesity and metabolic syndrome are increasing worldwide ¹. Metabolic syndrome (MetS) is a pathophysiological state that has been used to associate overweight and obesity and their consequences such as cardiovascular diseases and diabetes mellitus. Since National Cholesterol Education Program (NCEP) defined metabolic syndrome in the adult population in 2001, subsequent studies have been conducted using modified criteria for MetS for children and adolescents and reported an increasing prevalence of MetS. Although many different criteria resulted in difficulty estimating the exact prevalence in pediatric populations, a recent systematic review reported that the estimated prevalence of pediatric MetS ranged from 0.3–26.4% according to different geographic locations and populations ². In addition, the prevalence rates of MetS according to the modified NCEP-ATP III criteria have been reported to range from 5.7–10.9% in Korean studies ^3,4.

Hepatic involvement of MetS is commonly observed as hepatic steatosis in nonalcoholic fatty liver disease (NAFLD); pathological fat accumulation associated with chronic inflammation in the liver results in detrimental consequences, including impairment of glucose and lipid metabolism and an increase in cardiovascular events combined with oxidative stress, endothelial dysfunction and hypercoagulability ⁵. Therefore, the recent current consensus recommends screening for NAFLD in children with accompanying MetS ⁶. However, there is wide variability in the upper limit of normal for AST or ALT across different ages and between the sexes. The distinction of liver disease usually depends on the cutoff values that define an abnormal test, which is also critical for sensitivity and specificity ⁷; this is the reason it is necessary to analyze population based data to establish reference intervals.

Several lines of studies have reported the upper limits of normal for liver enzymes in children and adolescents. In the United States, a population-based study suggested that the upper limits of normal foe ALT in metabolically normal individuals without liver disease were 26 mg/dl for boys and 22 mg/dl for girls ⁸. Furthermore, in a Canadian study, the cutoff value for ALT was suggested to be 30 mg/dl in children between 1 and 12 years of age and 24 mg/dl in those between 13 and 19 years ⁹. In other regions, including Europe ^10,11, the United Kingdom ¹², Mexico ¹³, Sweden ¹⁴, Iran ¹⁵, Taiwan ¹⁶ and China ¹⁷, the reference ranges of liver enzymes in children were evaluated in a similar manner. However, in Korean children and adolescents, the similar studies were not population-based, and the association with metabolic syndrome was not analyzed ¹⁸ or adult data ¹⁹.

In this study, we aimed to establish sex- and age-stratified reference intervals based on a healthy non-hospitalized pediatric population using data from KNHANES 2007–2017. We analyzed liver enzyme levels and other biochemical, physical, and social data from KNHAES, to assess the association of AST, ALT and the AST/ALT ratio with MetS. Related components of MetS were also analyzed based on the normal, borderline-high and high levels of liver enzymes. Through the results of our population-based group study, we expect to add to our understanding of the roles of liver enzymes and a related ratio in MetS and provide a basis for novel liver enzyme reference ranges for children and adolescents in assessing the risk for MetS.

Materials And Methods

Results

Discussion

We found that clear differences in AST, ALT, and AST/ALT ratio curves between age- and sex- different subgroups; the AST levels tended to decrease as age, but the ALT curves were U-shaped, which resulted in trends of the AST/ALT ratio to reduce according to age and skewed to the left. In the cardiometabolic risk assessment based on our novel references, components of MetS significantly increased in high levels of AST, ALT, and AST/ALT ratio; some of the components were increased even in borderline high levels of liver enzymes in the analysis of ANCOVA with correction of age, sex, BMI, etc. Similarly, the odds ratio of metabolic risk factors also increased in the high AST, ALT, and the low AST/ALT ratio; some components of MetS were significantly increased even in borderline high levels.

In our findings, AST and ALT were higher in boys than in girls and differ over time; AST tended to decrease with age but ALT levels were U-shaped and increased with age. Those tendencies were consistent with previous reports in Korean subjects ¹⁸ and those of other ethnicities, for instance, an obvious increase in ALT levels was observed in male after the age of 11 years ¹⁷, and a sex difference with a continuous decrease in concentration from ages 2 to 14 years was observed for AST ^9,17,20,21. Percentile distributions of the AST/ALT ratio showed sex differences across all observed ages in our data; the ratio was significantly higher in girls than in boys. Despite conflicts with the data of the HELENA study ¹⁰, our results were similar to those of a recent Chinese study ²², which seems to be due to ethnic or geographic differences.

Regarding on MetS, an increase in the prevalence of MetS according to AST, ALT, and the AST/ALT ratio was more clearly observed as BMI increased (Figs. 3 and 4). This result suggested that although a strong correlation between high levels of AST, ALT, and the AST/ALT ratio and most cardiometabolic risk factors exist, the interpretation of liver markers in children and adolescents could be adapted differentially depending on the differential BMI values. Previously, AST or AST/ALT ratios were known to indicate progression to diabetes ²³. Similarly, in our findings, high serum AST, ALT, and the AST/ALT ratio were all associated with a high OR of elevated glucose (Table 4). These results strongly supported that a decreased AST/ALT ratio might effectively predict increased cardiometabolic risk, especially insulin resistance when high ALT exists. Of interest, our results showed that the associations of AST, ALT, and the AST/ALT ratio with cardiometabolic risk factors were significant both in boys and girls, which partially conflicts with a previous European study ¹⁰. The differential distribution of overweight or obese children in the study population might affect the analysis. In addition, differential body adiposity might also influence screening tests of liver markers. Therefore, detailed comparative studies on this point will be needed in future studies.

Based on the percentile distribution, we suggested that the upper limits of normal of AST, ALT and the AST/ALT ratio were 23 IU/L and 20 IU/L (< 75th percentile), 19 IU/L and 14 IU/L (< 75th percentile), and 1.1 and 1.35 (< 25 percentile) in boys and girls, respectively. Similar studies with non-overweight adolescents proposed sex-specific thresholds for ALT levels < 25 IU/L in males and < 22 IU/L in females to detect pediatric chronic liver disease ⁸. However, Labayen et al. suggested upper limits of normal for ALT of 24–25 (75th percentile) and 22–24 IU/L (75th percentile) and thresholds of the AST/ALT ratio associated with high cardiometabolic risk of 1.0-0.74 and 0.86–0.87 (ranging from 13-35th percentile) for boys and girls, respectively ¹⁰. In those studies, the estimated upper limits of normal of ALT in adolescents were higher than those in our report. This might be due to differences in ethnicities between the studies; other possibilities are differences in the proportion of subjects with obesity and severity of central adiposity levels. Although direct comparison of the AST/ALT ratio between our data and previous reports is difficult, it seems that stricter levels of the AST and ALT should be applied for Korean adolescents, especially for the overweight or obese subgroup for the precise estimation of cardiometabolic prognosis.

The main limitation of this study is the cross-sectional analysis, which cannot identify the temporal association of MetS with AST, ALT, and the AST/ALT ratio. A large population-based, longitudinal cohort study could address this limitation in the future by serial measurements of liver enzymes and follow-up for the occurrence of cardiometabolic events. The other limitation is that our data were from subjects of one ethnicity in a single country. Thus, comparisons and meta-analysis with other ethnic groups will be needed for the broad application of pediatric reference intervals. Despite of information about a family history of premature coronary heart disease, we could not exclude familial hyperlipidemia during subject selection due to limitations on laboratory tests. Combined familial hyperlipidemia frequently accompanies NAFLD in approximately 49–76% of cases ²⁴, which implies that possible effects of these comorbidities were not completely excluded in our data. Other liver markers such as gamma-GT have also been suggested to be strong predictors of cardiovascular disease and T2DM in adults ²⁵, and metabolic risks in adolescents ¹⁰; however, we did not analyze other possible markers in our current study.

In conclusion, we newly established reference values for AST, ALT, and the AST/ALT ratio based on the risk assessment of MetS components. High levels of AST and ALT and a low AST/ALT ratio were closely associated with the prevalence of MetS and its components. In particular, overweight and obese children and adolescents have a considerably higher prevalence of MetS and its components when liver enzymes exceed the upper limits of normal than do normal subjects. Both ALT and the AST/ALT ratio were effective in screening for metabolic risk in both sexes in a Korean population. Therefore, the age- and sex-specific reference values provided in this study may contribute to the early diagnosis and treatment of MetS.

Abbreviations

ALT: Alanine aminotransferase

AST: Aspartate aminotransferase

BP: Blood pressure

BMI: Body mass index  

CI: Confidence interval

HDL-C: High-density lipoprotein cholesterol

KCDC: Korean Centers for Disease Control and Prevention

KNHANES: The Korea National Health and Nutrition Examination Survey

LDL-C: Low-density lipoprotein cholesterol 

MetS: Metabolic syndrome 

NAFLD: Nonalcoholic fatty liver disease

NCEP-ATP III: National Cholesterol Education Program Adult Treatment Panel III 

NCEP: National Cholesterol Education Program 

ORs: Odds ratios  

SDS: Standard deviation scores 

T2DM: Type 2 diabetes mellitus 

TG: Triglyceride

TC: Total cholesterol 

WC:  Waist circumference

Declarations

Author contributions

Seo Y-J designed the study, drafted the manuscript, and analyzed the publicly available data set. Lee HS and Hwang JS reviewed and revised the manuscript and provided important intellectual content, including conceptualization of the study design. Shim YS supervised all aspects of manuscript preparation and assisted with the study formulation, data analysis, manuscript writing, and interpretation of the findings. All authors approved the final manuscript as submitted and agree to be accountable for all aspects of the work.

Conflict of interest 

No conflicts of interest are declared. 

Data availability statement

The data that support the findings of this study are available in [the KNHANES website] at [https://knhanes.kdca.go.kr]. The data that support the findings of this study are available from the corresponding author, [Shim YS], upon reasonable request.

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