Nonalcoholic fatty liver disease (NAFLD) has emerged as a significant national concern owing to its increasing prevalence over the past decade.^1-3 According to recent meta-analyses on Korean studies, the estimated prevalence of NAFLD in Korea ranges from 20% to 40%.¹ Furthermore, concerning trends indicate a projected prevalence of approximately 30% among young adults aged <30 years by the year 2030.⁴ These consistent findings underscore the growing awareness of the risk of NAFLD in Korean society.

NAFLD constitutes a primary cause of liver-related complications and hepatocellular carcinoma. Emerging evidence suggests that NAFLD can progress to hepatocellular carcinoma even in the absence of cirrhosis.⁵ Moreover, NAFLD is intricately linked to a diverse spectrum of extrahepatic diseases, including cardiovascular disease, chronic kidney disease, and extrahepatic malignancies.⁶ Such associations are correlated with poorer outcomes and heightened mortality risk among individuals with NAFLD.⁷ Notably, a recent study conducted in Sweden revealed that young patients with NAFLD face over five times the mortality risk compared to individuals without NAFLD, emphasizing the importance of proactive management strategies among the youth population affected by NAFLD.⁸

Recent attention in the Korean society has also been directed towards metabolic syndrome, a condition intricately linked to NAFLD. Metabolic syndrome encompasses a cluster of risk factors for cardiovascular diseases, including hypertension (HTN), dyslipidemia, impaired fasting glucose, and abdominal obesity.⁹ A recent survey indicates that the prevalence of metabolic syndrome has nearly reached 30% among the male population, heightening concerns regarding the societal burden imposed by its increasing prevalence.¹⁰

In South Korea, mandatory military service for adult males is a prominent aspect of the societal structure, with exceptions granted only in cases of physical or mental incapacity. Consequently, the duration of military service, which typically ranges from 18 to 21 months, holds potential significance for an individual’s metabolic health status. Moreover, the military environment often induces substantial changes in lifestyle patterns. Factors such as heightened emotional stress, increased physical activity, and prohibition of alcohol consumption may exert either provocative or protective effects on the development of NAFLD. This underscores the relevance of exploring the potential impact of military service on NAFLD and metabolic syndrome.^11-14 Although some studies have conducted comparative analyses of the prevalence of metabolic syndrome between military personnel and the general population, none have investigated individual-level changes in metabolic status during military service.¹⁵

In the Korean military, individuals undergo a pre-enlistment physical examination, also known as the military service determination examination, conducted by the Military Manpower Administration. Additionally, soldiers are required to undergo at least one mandatory health check-up during their military service. These examinations typically encompass measurements of height, weight, blood pressure, and various blood tests. While there are overlapping elements in both pre-enlistment and in-service health check-ups that facilitate a comparative analysis, no study has been conducted within the South Korean military context to compare the results of these two examinations. Moreover, concerning terminology, metabolic dysfunction-associated steatotic liver disease (MASLD) has been proposed through a Delphi process as a substitute for NAFLD.¹⁶ Steatotic liver disease (SLD) is a broader term encompassing all causes of hepatic steatosis, of which the majority are composed of MASLD. ¹⁷ Here, we aim to elucidate the effect of military service on SLD and the components of metabolic syndrome by comparing the findings of these distinct health assessments.

1. Study population

In Korea, young men aged between 18 and 35 years are obligated to serve in the military for a period ranging from 18 to 21 months. Upon reaching the age of 19, Korean men are mandated to undergo health check-ups before entering military service. Additionally, soldiers are required to undergo health check-ups upon promotion to the rank of corporal. This retrospective study analyzed data from “pre-enlistment health check-ups” conducted between 2019 and 2022 and “in-service health check-ups” conducted between 2020 and 2022. Data were collected from the Armed Forces Medical Command database and the Military Manpower Administration. Subsequently, personal identification information such as military affiliation, service number, and other identifiable details was anonymized. Data integration was then performed in collaboration with the Korea Data Agency and the Korea Internet and Security Agency. This study was approved by the Institutional Review Board of the Korean Armed Forces Medical Command (IRB number: AFMC-2023-07-015-003) and conducted in accordance with the Declaration of Helsinki. The requirement for informed consent was waived due to the retrospective nature of the study.

2. Inclusion and Exclusion criteria

The study included paired data from pre-enlistment and in-service health check-ups for analysis. Exclusion criteria comprised individuals with missing data for crucial variables such as alanine aminotransferase (ALT), aspartate aminotransferase (AST), body mass index (BMI), systolic blood pressure (SBP), and diastolic blood pressure (DBP). Additionally, data points were excluded if they were identified as outliers according to the following thresholds: height <10 cm or >230 cm; weight <25 kg or >300 kg; SBP <60 mm Hg or >400 mm Hg; and DBP <30 mm Hg or >250 mm Hg.

3. Included variables

The study collected laboratory variables associated with metabolic dysfunction, including AST, ALT, triglyceride, and total cholesterol levels, for analysis. Additionally, SBP and DBP data were collected to diagnose HTN in the study population. Anthropometric measurements such as height, weight, and BMI were also collected and incorporated into the analysis.

4. Definition

SLD was defined using the modified hepatic steatosis index (HSI), calculated as follows: HSI=8×ALT/AST+BMI (+2 if female).^18,19 HSI score ≥36 was considered indicative of hepatic steatosis. Additionally, other concomitant diseases displaying features of metabolic dysfunction were assessed. BMI was categorized according to guidelines published by the Korean Society for the Study of Obesity.²⁰ A BMI of ≥25 kg/m² was classified as obese, while a BMI of ≥30 kg/m² was categorized as severe obesity. A BMI between 23 and 25 kg/m² was classified as overweight, and a BMI <18.5 kg/m² was categorized as underweight. HTN was defined as a SBP of ≥140 mm Hg or DBP of ≥90 mm Hg, in accordance with the guidelines provided by the 2022 updated Korean Hypertension Society.²¹ Hypertriglyceridemia was defined as a serum triglyceride level of 200 mg/dL or higher.²¹

5. SLD resolution and development

The HSI value of 30 serves as the cutoff point for ruling out steatosis.¹⁹ Under these parameters, the resolution of SLD was defined as achieving an HSI of 30 or below among individuals who initially presented with an HSI of 36 or higher. Conversely, SLD development was assessed among individuals with an initial HSI of 30 or below, and was considered to have occurred if the HSI subsequently increased to 36 or above.

6. Statistical analyses

All statistical analyses were performed using Python software (version 3.11.6. R Foundation Inc., Vienna, Austria) and R statistical software (version 4.0.3. R Foundation for Statistical Computing, Vienna, Austria). The Student t-test was used for continuous variables, and the results are reported as mean values with standard deviations. Categorical variables were analyzed using the chi-square test. Logistic regression analysis was employed to identify factors associated with SLD resolution and/or development, with the results presented as odds ratio (OR), 95% confidence interval (CI), and corresponding p-values. A two-tailed p-value <0.05 was considered statistically significant.

1. Demographics of study participants

Of the 916,638 cases of pre-enlistment health check-ups and 610,071 cases of in-service health check-ups, 520,439 were identified as paired data. After excluding patients with missing data and outliers, 503,136 patients were included in the analysis (Fig. 1). The median interval between the two health check-ups was 672.0 days. Among the study cohorts, 413,404 (82.2%) individuals were assigned to the army, 44,737 (8.9%) to the navy, 43,609 (8.7%) to the air force, and 1,386 (0.3%) to the marine branch (Table 1). At the time of the pre-enlistment health check-ups, the mean age was 18.6 years, and the mean BMI was 23.3 kg/m². For in-service health check-ups, the mean age and BMI were 21.5 years and 24.0 kg/m², respectively. Blood pressure measurements showed a decrease between the two time periods, with SBP decreasing from 126.0 to 119.6 mm Hg (p<0.001) and DBP decreasing from 74.8 to 71.5 mm Hg (p<0.001). Regarding laboratory variables, AST and ALT levels increased over time, whereas gamma-glutamyl transferase levels decreased during the study period (p<0.001). Total cholesterol levels increased from 165.3 to 173.4 mg/dL (p<0.001), while triglyceride levels decreased from 115.1 to 83.4 mg/dL. In terms of body weight classification, the proportions of individuals categorized as underweight (8.2% vs 1.4%, p<0.001) and severely obese (6.1% vs 4.9%, p<0.001) decreased over time, while the proportions of overweight and obese increased between the two health check-ups.

Figure 1. Patient selection flowchart.

Table 1 . Comparison of Baseline Characteristics between Pre-enlistment and In-Service Health Check-ups.

Characteristic	Pre-enlistment (n=503,136)	In-service (n=503,136)	p-value
Types of branches
Army	413,404 (82.2)	413,404 (82.2)
Navy	44,737 (8.9)	44,737 (8.9)
Air force	43,609 (8.7)	43,609 (8.7)
Marine	1,386 (0.3)	1,386 (0.3)
Age, yr	18.6±1.0	21.5±1.3	<0.001
Height, cm	173.9±5.6	174.5±5.6	<0.001
Weight, kg	70.8±12.5	73.2±10.9	<0.001
BMI, kg/m2	23.3±3.8	24.0±3.2	<0.001
SBP, mm Hg	126.0±12.2	119.6±12.4	<0.001
DBP, mm Hg	74.8±8.6	71.5±9.6	<0.001
AST, IU/L	21.3±17.2	25.1±17.4	<0.001
ALT, IU/L	23.5±24.4	24.4±19.9	<0.001
GGT, IU/L	24.9±18.2	21.8±13.0	<0.001
Creatinine, mg/dL	0.9±0.1	0.9±0.1	<0.001
Total cholesterol, mg/dL	165.3±29.1	173.4±30.2	<0.001
Triglycerides, mg/dL	115.1±73.9	83.4±48.9	<0.001
BMI category*
Underweight	41,128 (8.2)	7,122 (1.4)	<0.001
Normal	224,590 (44.6)	211,341 (42.0)	<0.001
Overweight	92,580 (18.4)	130,286 (25.9)	<0.001
Obese	144,838 (28.8)	154,387 (30.7)	<0.001
Severe obese	30,520 (6.1)	24,683 (4.9)	<0.001

Data are presented as number (%) or mean±SD..

BMI, body mass index; SBP, systolic blood pressure; DBP, diastolic blood pressure; AST, aspartate aminotransferase; ALT, alanine aminotransferase; GGT, gamma-glutamyl transferase..

*BMI category: underweight (<18.5 kg/m²), normal (18.5 to <23.0 kg/m²), overweight (23.0 to <25.0 kg/m²), obese (25.0 to <30 kg/m²), severe obese (≥30.0 kg/m²)..

2. Changes in metabolic parameters by service branches

Changes in metabolic parameters were compared among the different service branches (Supplementary Table 1). For BMI, all service branches exhibited an increase over time, with delta values ranging from +0.5 to +0.8 kg/m². SBP and DBP decreased across all branches, with changes ranging from –6.7 to –4.2 mm Hg for SBP and from –3.5 to –2.1 mm Hg for DBP. AST and ALT levels increased in all service branches except for ALT in the air force, where it decreased by –0.2 IU/L (p<0.001). Regarding lipid profile, total cholesterol increased in all service branches, whereas triglyceride levels decreased regardless of the service branch.

3. Prevalence of SLD and other concomitant diseases

The prevalence of SLD, HTN, and hypertriglyceridemia were compared between the two timelines (Fig. 2). In the pre-enlistment health check-ups, the prevalence of SLD was 22.2%, which decreased to 17.6% during the in-service health check-ups (p<0.001). Similarly, the prevalence of HTN decreased from 7.6% in the pre-enlistment period to 4.3% in the in-service health check-ups (p<0.001). Additionally, the prevalence of hypertriglyceridemia decreased substantially from 8.1% to 2.9% (p<0.001).

Figure 2. Changes in the prevalence. (A) Steatotic liver disease. (B) Hypertension. (C) Hypertriglyceridemia.

Furthermore, changes in the prevalence of SLD were compared among different service branches (Fig. 3). For individuals in the army, the prevalence of SLD decreased from 22.5% to 17.7% (p<0.001). Similarly, the prevalence for those serving in the navy, decreased from 20.0% to 16.9% (p<0.001), while it decreased from 21.9% to 16.8% (p<0.001) for those in the air force. In the marine branch, the prevalence of SLD decreased from 20.6% in the pre-enlistment period to 14.4% in the in-service health check-ups (p<0.001).

Figure 3. Changes in the prevalence of steatotic liver disease according to service branch type. (A) Army. (B) Navy. (C) Air force. (D) Marines.

4. Factors associated with SLD development and resolution

Individuals with a HSI of 30 or below at pre-enlistment were assessed for the development of SLD. Among 242,539 individuals with HSI ≤30 at pre-enlistment, 8,605 developed SLD during a mean follow-up period of 1.90±0.65 years, resulting in an incidence rate of 18.7 cases per 1,000 person-years. The factors associated with SLD development among those with HSI ≤30 at pre-enlistment were investigated (Fig. 4). In terms of service branch type, serving in the navy was associated with an increased incidence of SLD (OR, 1.105; 95% CI, 1.028 to 1.188; p=0.007), while serving in the marine (OR, 0.509; 95% CI, 0.287 to 0.901; p=0.021) was found to be a protective factor against SLD development. Age at the time of the in-service health check-up was identified as an inducible factor for SLD development (OR, 1.146; 95% CI, 1.127 to 1.165; p<0.001), whereas an interval of <450 days between the two health check-ups was associated with a decreased risk of SLD development (OR, 0.746; 95% CI, 0.694 to 0.801; p<0.001).

Figure 4. Forest plot depicting the ORs for the SLD development (A) and SLD resolution (B). SLD, steatotic liver disease; OR, odds ratio; CI, confidence interval.

Patients with SLD at pre-enlistment were evaluated for SLD resolution. Among the 111,765 patients diagnosed with SLD at pre-enlistment health check-ups, 9,961 showed resolution of SLD at in-service health check-ups. The mean follow-up period for SLD patients was 1.92±0.63 years, and the annual resolution rate was 46.4 cases per 1,000 person-years. Additionally, factors associated with SLD resolution were evaluated (Fig. 4). Regarding military service branch type, serving in the army (OR, 0.879; 95% CI, 0.833 to 0.927; p<0.001) and the navy (OR, 0.857; 95% CI, 0.791 to 0.928; p<0.001) were associated with a decreased likelihood of SLD resolution, while serving in the air force was a favorable factor for SLD resolution (OR, 1.406; 95% CI, 1.316 to 1.501; p<0.001). Serving in the marine, age at the time of in-service health check-up and interval of <450 days between health check-ups did not show statistical significance for SLD resolution.

Furthermore, changes in HSI over time among individuals initially presenting with SLD were compared across service branches (Supplementary Fig. 1). The results indicated a substantial decrease in HSI across the total group (41.6±4.6 vs 36.9±5.9, p<0.001), as well as within each service branch.

5. Subgroup analysis for health check-up interval <450 days

Soldiers are required to undergo in-service health check-ups approximately 8 to 10 months after entering military service. To minimize the bias resulting from the relatively long interval between pre-enlistment health check-ups and military service entrance, a subgroup analysis was conducted on individuals with a health check-up interval of <450 days. This subgroup spent a longer duration in military service compared to the time elapsed between pre-enlistment health check-up and military entrance, thereby facilitating the reduction of bias associated with the pre-enlistment period. Comparisons of demographics between the subgroups at pre-enlistment and in-service health check-ups revealed consistent trends compared to the entire cohort, except for the proportion of obesity, which did not exhibit a significant change in the subgroup analysis (31.0% vs 31.3%, pre-enlistment and in-service health check-ups, respectively; p=0.280) (Supplementary Table 2).

The prevalence of SLD, HTN, and hypertriglyceridemia were assessed in this subgroup (Fig. 5). The prevalence of SLD decreased substantially from 24.1% to 17.2% (p<0.001). HTN and hypertriglyceridemia also significantly decreased from 8.0% to 3.7% and from 10.2% to 2.6%, respectively (p<0.001 for both).

Figure 5. Change in prevalence in the subgroup of paired health check-ups with intervals less than 450 days. (A) Steatotic liver disease. (B) Hypertension. (C) Hypertriglyceridemia.

To the best of our knowledge, this study is the first to investigate the impact of the Korean military service on SLD and components of metabolic syndrome. The findings demonstrate a substantial decrease in SLD prevalence from 22.2% to 17.6% during the military service, consistent with the reported prevalence of 16.4% among Korean young men in previous studies.⁴ This decline in SLD prevalence was paralleled by significant reductions in HTN and hypertriglyceridemia rates during military service. A subgroup analysis focusing on individuals with a health check-up interval of <450 days provided further insights into the impact of military service. This analysis revealed an even steeper decline in the prevalence of SLD, from 24.1% to 17.2%, along with significant reductions in the HTN and hypertriglyceridemia rates.

Interestingly, while BMI increased overall during military service, further examinations based on the Asian-Pacific guidelines for BMI classification showed a different trend.²² As a result, the proportion of underweight men decreased from 8.2% to 1.4%, and the proportion of severe obesity men decreased from 6.1% to 4.9%. While obesity appeared to increase during military service (from 28.8% to 30.7%), our subgroup analysis, aimed at better understanding the impact of service on these parameters, revealed no significant differences between the pre-enlistment and in-service periods in terms of the prevalence of obesity (31.0% to 31.3%). Overall, military service improved the BMI status by reducing the prevalence of both underweight and severely obese individuals.

Improvements in metabolic status and reduction of SLD during military service could result from various factors. The increased level of physical activity inherent in military service likely plays a pivotal role in resolving SLD and improving metabolic parameters.²³ A recent systematic review on soldiers' physical activity found that soldiers typically exceed 10,000 steps per day, surpassing activity levels observed in the civilian population.²⁴ This aligns with a growing body of evidence indicating that increased exercise levels are associated with SLD resolution, independent of changes in body weight, which may help explain the observed improvements in health status despite an increase in BMI during the same period.^13,25 Furthermore, soldiers generally adhere to relatively healthier eating habits compared to civilians, which could contribute to the positive metabolic outcomes observed during military service.²⁶ Additionally, maintaining a regular sleep pattern may serve as another plausible factor underlying these outcomes, as adequate sleep duration and regular sleep patterns have been reported as protective factors against the development of metabolic syndrome and hepatic steatosis.^27-29 The prohibition from alcohol consumption for the majority of the military service duration may also play a role in improving SLD outcomes among soldiers.³⁰

In the present study, the factors associated with SLD resolution and development were explored. Age was found to be associated with an increased risk of SLD development. This finding aligns with those of previous studies demonstrating a higher incidence of SLD among older age groups than among younger individuals.^31,32 Additionally, a time interval of <450 days between paired health check-ups was associated with a decreased risk of SLD development. Because in-service health check-ups are routinely conducted 8 to 10 months after joining the military, a prolonged interval between paired health check-ups may not accurately reflect the impact of military service on SLD. Consequently, our results suggest that the influence of time before joining military service might hinder the improvement of SLD. Interestingly, intriguing results emerged regarding service branches, with the air force showing a clear tendency towards improving the SLD status for both SLD development and resolution, whereas serving in the navy exhibited negative outcomes in relation to SLD status.

Our study has several strengths, including the relatively high merging rate of 85.3% achieved through the data combination. This high merging rate was facilitated by the provision of clear personal identification information, including military branch and service number, which served as the basis for combining the data from the two institutions. Consequently, our study can be considered as a nearly complete enumeration, highlighting the reliability of our results.

Despite its strengths, this study has several limitations. First, several important variables including fasting glucose, high-density lipoprotein cholesterol, and low-density lipoprotein cholesterol were not available in the current database. The absence of these variables limited our ability to perform a comprehensive evaluation of our results, such as determining the prevalence of MASLD and other metabolic syndrome components. Second, hepatic steatosis was assessed using a non-invasive serological tool, HSI, rather than ultrasonography or liver biopsy. While the HSI has been validated in several studies and has shown a high predictive value for SLD, ultrasonography and biopsy are preferable for defining SLD with higher reliability.^33,34 Furthermore, the absence of data on lifestyle differences between service branches limits the interpretation of our study results. Future studies comparing lifestyle differences between service branches could provide a deeper understanding of our findings. Finally, insufficient data were provided to differentiate other chronic liver diseases such as alcohol consumption history or viral hepatitis. Although alcohol consumption is prohibited until the age of 19 years and chronic hepatitis B is reported to occur in <1% of patients under 20 years of age, additional information on these factors may have facilitated a more comprehensive analysis of our results.³⁵

In conclusion, this study is the first to statistically demonstrate the improvement in SLD and other metabolic parameters during military service. Through meticulous analysis utilizing nearly complete enumeration data, this study revealed a substantial decrease in the prevalence of SLD and other metabolic parameters during military service. Given that our results indicate a positive role of joining military service in SLD, further research on factors associated with SLD improvements during service could contribute to the establishment of practical policies aimed at alleviating the socioeconomic burden of SLD.

This research was supported by the Korean Military Medical Research Project, funded by the Republic of Korea Ministry of National Defense (ROK-MND-2023-KMMRP-022).

No potential conflict of interest relevant to this article was reported.

Study concept and design: J.L., J.H.J., J.A.Y., S.H.B. Data collection: J.L., J.H.J., S.J.C., B.H., J.A.Y. Data analysis and interpretation: J.L., J.H.J., S.H.B. Drafting of the manuscript: J.L., J.H.J. Administrative, technical, or material support; study supervision: H.Y., P.S.S., S.H.B. Approval of final manuscript: all authors.

Supplementary materials can be accessed at https://doi.org/10.5009/gnl240077.

The original contributions presented in the study are included in the article/Supplemental Material. Further inquiries can be directed to the corresponding author.