This is the first study to our knowledge that shows a relationship between NAFLD (defined using FLI) and cardiovascular events, particularly MI and stroke, among young adults 20–39 years old. Risk for MI and stroke increased significantly in young adults with NAFLD compared with a control group, independent of conventional cardiovascular risk factors. Also, there was a dose-dependent increase in MI and stroke risk according to steatosis grade.
Youth-onset NAFLD is thought to be a more progressive disease, leading to advanced fibrosis, compared with adult-onset NAFLD,[26] and it has a stronger association with CVD.[27] However, evaluations of the association between NAFLD and cardiovascular events in adults under 40 years are limited. In this study, high HRs for MI and stroke were observed in subjects younger than 30 years as well as in subjects 30 years and older (HR 1.43 and 1.80 for MI and HR 1.63 and 1.35 for stroke, respectively), suggesting an adverse relationship between NAFLD and CVD risk even at a relatively young age. When we categorized FLI values into three groups (< 30, 30–59, and ≥ 60), there was a dose-dependent increase MI and stroke risk. These results suggest FLI is an independent predictor of CV events and, thus, FLI may be of clinical value for identifying young-adult patients who require intense lifestyle modification. Additionally, there was an increased risk of MI and stroke, even at lower FLI levels (30–59) that generally do not meet the criteria for NAFLD, implying a need for early detection of high-risk subjects via FLI score.
The mechanisms linking NAFLD with CV events in young adults are not yet fully elucidated, but several hypotheses exist. First, obesity is a strong risk factor for NAFLD in young people and is considered an initial stage of chronic and metabolic inflammatory disease. In adipose tissue, leukocytes generate sustained pro-inflammatory processes that have negative effects on adipocyte insulin sensitivity and contribute to insulin resistance,[28] which contributes to myocardial damage.[29] Indeed, younger MI patients are reported to have more central obesity and higher BMI compared with control subjects.[30] Second, the rising prevalence of conventional cardiovascular risk profiles such as dyslipidemia [31], high blood pressure [32], and increased carotid intimal medial thickness [33] are strongly associated with NAFLD in children and adolescents, implying a close link between early atherosclerosis and NAFLD in young adults. When we adjusted for traditional cardiovascular risk factors, including obesity, the independent association between NAFLD and CVD remained. Third, CVD risk may be determined by genetic or metabolic factors in young adults with NAFLD.[34] Stroke onset younger than 35 years is likely to have other underlying mechanisms including non-atherosclerotic arteriopathy, changes in hemostatic balance, vasospasm and coagulation disorder,[35] or advanced liver disease, which are associated with imbalances in pro- and anti-coagulation.[36] However, further prospective studies are needed to confirm the independent role of NAFLD in CVD pathogenesis in young adults.
In this study, analyses stratified by baseline variables showed no strong effect modifiers between FLI and outcome variables, indicating that the results are generally consistent, regardless of baseline characteristics such as hypertension, dyslipidemia, obesity, or regular exercise. Although not statistically significant, the association between NAFLD and incident MI or stroke was slightly higher among people without regular exercise, suggesting the potential for regular exercise to reduce CVD risk.
This study provides new insights for understanding the relationship between CVD and NAFLD in young adults and presents a strategy for early identification of individual risk factors for appropriate CVD prevention. For asymptomatic young people, especially those under 40 years, it is difficult to evaluate CVD risk. For example, the ASCVD risk algorithm only applies to individuals 40–75 years old. Because the parameters included in the FLI are easily accessible in clinical practice, our results can be used to establish a strategy to identify NAFLD in young-adult patients at higher risk of early onset CVD and to reduce their future risk of CVD.
This study has some limitations. First, because of its population-based observational design, our study cannot establish a causal relationship. Second, using FLI as a surrogate marker of fatty liver cannot accurately quantify steatosis presence and severity.[37] It was impossible to differentiate simple steatosis from steatohepatitis, and various severities of NAFLD may affect CVD events differently. However, the association with multiple-site atherosclerosis and cardiovascular mortality was well defined in a large cohort study,[38] and the use of FLI is practical for screening the general population in epidemiologic studies.[39] Third, because MI and stroke diagnoses were based on claims data using the ICD-10 code, it is possible that these conditions were under- or overestimated. However, the definition we used in this study has been validated in several previous studies.[34, 24, 40] Also, we excluded patients who had only one diagnosis in an outpatient clinic to avoid overestimation. Lastly, because the NHIS health exam is provided to young adult workers and householders, only half of young adults were eligible. Further replicative research using more accurate measures to diagnose cardiovascular events is needed to validate our results.