Nonalcoholic fatty liver disease (NAFLD) is the hepatic manifestation of the metabolic syndrome. NAFLD also is an independent risk factor for cardiovascular disease, type 2 diabetes mellitus (T2DM), chronic kidney disease, cirrhosis, liver cancer, and all-cause mortality.1-3 As the leading cause of liver disease in the US and globally, NAFLD is strongly associated with obesity and metabolic syndrome, with the rising prevalence of NAFLD closely mirroring the epidemic of obesity and T2DM.4,5 The unrelenting increase of NAFLD prevalence has led to a significant rise in associated health care and economic burdens, compounded by the boom in childhood obesity and an aging population. In this review, we will discuss the epidemiology and economic burden of NAFLD in the US and how it affects veteran health.
NAFLD is defined as the presence of > 5% of hepatic steatosis in the absence of excessive alcohol use, steatosis-inducing medication, or other concurrent chronic liver diseases.
Compared with patients with NAFL, patients with NASH are at a much higher risk of developing fibrosis (scarring of the liver) and cirrhosis (significant scarring with distorted liver architecture). Patients with either NAFL or NASH, with or without advanced fibrosis, also can develop hepatocellular carcinoma (HCC). Severity of liver fibrosis (ie, fibrosis stage) is the most important predictor of liver-associated mortality and all-cause mortality; those with significant fibrosis (≥ F2 stage of fibrosis) are more likely to die of liver disease or to undergo a liver transplant compared with those with earlier stages of disease (ie, stages 0 to F1). Those with advanced scarring or cirrhosis (≥ F3 stage of fibrosis) exhibit an even higher risk of death or liver transplantation.6
NAFLD is a slow and often progressive disease. Time to progression between each stage of fibrosis is about 7 years; however, there has been a documented subset of patients with rapid progression to advanced fibrosis.7 The risk factors associated with this increased risk of fibrosis progression remain poorly understood.
The prevalence of NAFLD in the US is about 24% to 26%—about 85 million Americans. Up to 20% to 30% of these cases (about 17-25 million Americans) are thought to have NASH (Figure 2).
Although liver biopsy remains the current gold standard for diagnosis and histopathologic staging of NAFLD, alternatives to liver biopsy include elastography techniques (ie, transient elastography using Fibroscan[Paris, France], shear wave elastography using Supersonic Image Aixplorer [Weston, FL], and magnetic resonance elastography), magnetic resonance spectroscopy, liver enzymes, and noninvasive simple and complex (serologic) scoring systems such as the Fatty Liver Index. Among these, liver enzymes and serologic scores are most likely to underestimate NAFLD disease burden. Transient elastographyis widely used because the test is easy to perform, noninvasive, and reliably estimates the degree of liver fibrosis in patients with NAFLD by measuring the speed of a mechanically induced shear wave using pulse-echo ultrasonic acquisitions in a much larger portion of the tissue (about 100 times more than a liver biopsy core). Transient elastography also can objectively quantify the amount of liver fat by measuring a 3.5 MHz ultrasound coefficient of attenuation or controlled attenuation parameter (CAP). This correlates with the degree of liver fat, and a higher CAP level reflects a greater degree of steatosis.
The largest study of US veterans utilized abnormal (ie, elevated) liver enzymes as the diagnostic criteria and reviewed nearly 10 million veterans who were followed between 2003 and 2011. Investigators reported a NAFLD prevalence of 13.6% in this population and noted an overall increase in NAFLD prevalence from 6.3% in 2003 to 17.6% in 2011, which highlights the continued growth in NAFLD clinical burden.10 This study, however, is likely to have underestimated the prevalence of NAFLD among veterans because liver enzymes are often normal among those with NAFLD (ie, low sensitivity), and the prevalence of obesity and T2DM are significantly higher in the veteran population vs the general population.