Burden of NAFLD in the U.S.
NAFLD is a manifestation of systemic metabolic abnormalities, including insulin resistance, dyslipidemia, central obesity, and hypertension. In this short review, we summarize data on the burden of NAFLD in the U.S. and its prognostic determinants and review what clinical and public health approaches may be needed to mitigating its impact.
Epidemiology of NAFLD
Worldwide, the prevalence of NAFLD is estimated at 6% to 35%, with biopsy-based studies reporting NASH in 3% to 5%.1 U.S. estimates for the prevalence of NAFLD range from 10% to 46%.2 In our own analysis of the National Health and Nutrition Examination Survey (NHANES) data, transient elastography-detected steatosis was found in 36%, which projected to a minimum of 73 million American adults.3
Dr. Sedki
Dr. Mai Sedki
NAFLD represents a spectrum of disorders ranging from simple steatosis to nonalcoholic steatohepatitis (NASH), the latter leading, in some cases, to progressive hepatic fibrosis and cirrhosis.4 Out of a large number of subjects with NAFLD, the proportions of NASH patients that develop severe liver problems such as end-stage liver disease (ESLD) or hepatocellular carcinoma (HCC) are progressively smaller. For example, we recently reported that less than 2,000 liver-related deaths are attributable to NAFLD in the U.S. per annum, which corresponds to a crude case fatality rate of < 0.005% per year.5
According to the Centers for Disease Control and Prevention (CDC), there have been substantial increases in liver-related deaths over the last 2 decades. Mortality from liver disease including hepatobiliary cancers more than doubled from 41,966 deaths (including 15,321 women and 26,645 men) in 2000 to 85,884 deaths (33,000 women and 52,884 men) in 2020. The proportion of deaths specifically attributed to NAFLD among liver-related deaths was miniscule in 2000, accounting for 1.1% in women and 0.7% in men. By 2020, the proportions increased several folds in both sexes (7.4% in women and 2.7% in men).6 Moreover, it is likely that a substantial portion of deaths from chronic liver disease from unknown causes (“cryptogenic”) are likely end-stage NAFLD, making these figures underestimates of the true impact of NAFLD in the U.S.
From a comparative epidemiologic perspective, there are significant racial and ethnic and socioeconomic disparities in NAFLD prevalence, wherein Hispanic persons and individuals experiencing food insecurity – independent of poverty status, education level, race and ethnicity – are disproportionately more affected by NAFLD.7,8 Furthermore, these disparities persist when examining long-term complications of NAFLD, such as developing HCC.
Prognosis in NAFLD: NASH versus fibrosis
Given the enormous prevalence and increasing public health burden of NAFLD, systematic interventions to mitigate its impact are urgently needed. Clearly, patients who already have developed advanced liver disease need to be directed to specialty care so the disease progression may be halted and complications of ESLD may be prevented or managed. On the other hand, in order to mitigate the future impact of ESLD, prompt identification of at-risk patients and proactive interventions to improve liver health are needed.
Stanford University
Dr. W. Ray Kim
In the assessment of disease progression, prior data have shown that the presence of NASH and increasing stages of liver fibrosis are important predictors of disease progression. Fibrosis is a component of NASH, while NASH is thought to be a prerequisite for fibrosis. In a prospective, multicenter follow-up study of NAFLD evaluated by liver biopsies (n = 1,773), over a median follow-up of 4 years, 37 (2%) developed hepatic decompensation, while 47 (3%) died from any cause, which included ESLD (n = 12), cardiovascular complications (n = 4), and malignancies (n = 12), including HCC (n = 9).9 It is not entirely surprising that advanced fibrosis and cirrhosis was highly associated with the development of hepatic decompensation. In their multivariable analysis, patients with F3-4 had a 13.8-fold (95% confidence interval [CI]: 4.6, 41.0) increase in the hazard of reaching a MELD score of 15 compared to those with F0-2. In addition, all-cause mortality was 17.2-fold (95% CI: 5.2, 56.6) higher with F3-4 compared to F0-2.
These data have been borne out by a larger body of literature on the topic. In a recent meta-analysis assessing the relation between liver fibrosis and future mortality, which included 17,301 subjects with NAFLD, patients with at least stage 2 fibrosis experience a significantly increased risk of liver-related and overall mortality, a trend that accelerates at higher fibrosis stages.10 These point to liver fibrosis as the singular determinant of long-term prognosis, in comparison, for example, with the diagnosis of NASH. Hagström conducted a retrospective cohort study of patients with biopsy-proven NAFLD in Sweden. When fibrosis stage and histological diagnosis of NASH were considered together, NASH did not have an impact on overall mortality (hazard ratio [HR] = 0.83, P = .29) or liver morbidity (HR = 0.62, P = .25).11
On an individual level, factors that affect fibrosis progression are not as well studied. It is commonly believed that demographic factors (e.g., age, sex and race), genetic polymorphisms (e.g., PNPLA3, TM6SF2), clinical comorbidities (e.g., obesity, DM, and sleep apnea), and environmental factors (e.g., smoking) may accelerate fibrosis and disease outcomes, although prospective data are sparse to estimate the extent these individual variables affect progression.12 Recent guidelines remain silent about whether and how these data may be incorporated in screening for NAFLD in the population.
Assessment of liver fibrosis
The traditional means to detect liver fibrosis is liver histology, which also assesses steatosis, individual components of NASH and, often importantly, other concomitant liver pathology. In reality, however, liver biopsies have several limitations including the risk of complications, patient discomfort, economic costs, and sampling variability. Increasingly, “noninvasive” methods have been used to estimate liver fibrosis in patients with NAFLD. Liver elastography estimates the physical stiffness of the organ, which may be measured by MRI or ultrasound. Among ultrasound-based technologies, vibration-controlled transient elastography (VCTE) is more widely accepted and affordable although it may not be as accurate as MR elastography.13
In general, these elastographic tests are not readily accessible to most physicians outside hepatology specialty practices. Instead, blood test-based markers have been developed and widely recommended as the initial modality to assess liver fibrosis. Figure 1 represents a partial list of blood test-based markers. Traditionally, FIB-4 and NFS have been considered the most widely recommended by society guidelines. The AGA Pathway for evaluation of patients with NAFLD recommends first to apply the FIB-4 score and, in patients considered to be at intermediate risk of fibrosis for advanced fibrosis (stage 3 or 4, FIB-4 = 1.3-2.67), to assess liver stiffness by VCTE.14
More recently, the accumulating natural history data have highlighted the inflection in the risk of future outcomes coinciding with F2 and therapeutic trials that target patients with “at risk NASH,” thus more attention has been paid to the identification of patients with stage 2 (or higher). The steatosis-associated fibrosis estimator (SAFE) was developed for this specific purpose. The score has been validated in multiple data sets, in all of which SAFE outperformed FIB-4 and NFS (Figure 1). When the score was applied to assess overall survival in participants of the NHANES, patients with NAFLD deemed to be high risk (SAFE > 100) had significantly lower survival (37% Kaplan-Meier survival at 20 years), compared to those with intermediate (SAFE 0-100, 61% survival) and low (SAFE < 0, 86% survival). In comparison, the 20-year survival of subjects without NAFLD survival was 79%.15
Regardless of the modality for initial stratification, it is widely accepted that mechanical elastography constitutes the next step in prognosticating the patient. In the AGA Pathway, liver stiffness of < 8 kPa is considered low risk, which corresponds in most analysis with lack of stage 2 fibrosis, whereas stiffness of > 12 kPa may be indicative of stage 3 or 4. These recommendations are consistent with those from the latest Baveno Consensus Conference (“Baveno 7”). Figure 2 expands on the so-called “rule of 5” from the consensus document and correlates liver stiffness (by VCTE) with progression of liver fibrosis as well as clinical presentation. For example, liver stiffness < 15 kPa is associated with a low risk of clinically significant portal hypertension (CSPH). Similarly, in patients with a normal platelet count (>150,000/mm3) and liver stiffness < 20 kPa, the probability of gastroesophageal varices is sufficiently low that a screening endoscopy may be avoided. On the other hand, liver stiffness > 25 kPa is associated with increasing risk of decompensated cirrhosis and portal hypertension.16
Partnership between primary care and specialty
The insights expressed in Figure 2 can be utilized to guide management decisions. In patients without evidence of liver fibrosis, emphasis may primarily be on screening, stratification and management of metabolic syndrome. For patients with evidence of incipient liver fibrosis, medical management of NAFLD needs to be implemented including lifestyle changes and pharmacological interventions as appropriate. For patients unresponsive to medical therapy, an endoscopic or surgical bariatric procedure should be considered. Management of patients with evidence of cirrhosis includes screening for portal hypertension, surveillance for HCC, medical management of cirrhosis, and finally, in suitable cases, referral for liver transplant evaluation. The reader is referred to the latest treatment guidelines for detailed discussion of these individual management modalities [ref, AGA and AASLD guidelines].14,17
Given the spectrum of management modalities needed to successfully manage patients with NAFLD, it is unrealistic to expect that hepatologists and gastroenterologists are able to manage the large number of patients with NAFLD. In general, clinical activities on the left side of the figure are in the domain of primary care providers, whereas management of patients with progressive liver fibrosis is conducted by the specialist. An important aspect of the overall management of these patients is risk management in terms of the metabolic syndrome, including cardiovascular risk reduction and diabetes management, as appropriate. Many patients with NAFLD are burdened with several comorbidities and likely to benefit from a multidisciplinary team consisting of primary care, endocrinology, preventive cardiology, pharmacy, nutrition/dietetics, social services, and addiction specialists, as well as hepatology and gastroenterology. Prospective, high-quality data to define these teams and their function are yet to be generated.
Conclusion
NAFLD is an important and increasing public health concern in the U.S. Once diagnosed, assessing liver fibrosis and evaluating the presence of the components of metabolic syndrome in these patients, constitute the key components in the care in terms of risk stratification, medical management, and referral decisions. Noninvasive tests have been increasingly utilized including liver stiffness measurements and various blood test-based indicators. For patients in specialty GI/hepatology care, transient elastography is a widely accepted tool, with which standardized recommendations may be made for screening, stratification, and medical and surgical interventions in patients with NAFLD.
Mai Sedki, MD, MPH, is a doctoral candidate at the University of California, San Francisco. W. Ray Kim, MD, is professor of medicine (gastroenterology and hepatology) at Stanford (Calif.) University. Address correspondence to: wrkim@stanford.edu. The authors disclosed no conflicts of interest. Twitter: @SedkiMD and @WRayKimMD.
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