Chapter 55: Gastrointestinal and Liver Disorders

INTRODUCTION

Gastrointestinal and liver diseases are a prevalent group of disorders, and include chronic and acute conditions that are infectious, metabolic, autoimmune, and result from a complex interplay of environmental and genetic influences. In this chapter, we discuss two of the most prevalent gastrointestinal and liver disorders that occur globally, nonalcoholic fatty liver disease (NAFLD) and irritable bowel syndrome (IBS).

NONALCOHOLIC FATTY LIVER DISEASE

Introduction and Definitions

Nonalcoholic fatty liver disease is a chronic liver disease characterized by fat infiltration in the liver (steatosis). It is closely related to obesity and the metabolic syndrome of diabetes, hyperlipidemia, and hypertension.¹ The liver injury can progress to varying degrees of severity over several years, varying from mild liver disease in most, to cirrhosis and liver cancer in a significant minority. By many estimates it is the most prevalent cause of chronic liver disease in the United States and worldwide.²

NAFLD comprises a spectrum of liver disease spanning from nonalcoholic fatty liver (NAFL) to nonalcoholic steatohepatitis (NASH) and cirrhosis (Fig. 55-1). NAFL signifies presence of isolated hepatic steatosis without evidence of liver cell (hepatocellular) injury and carries minimal-risk of progression to cirrhosis. NASH signifies steatosis with inflammation and hepatocyte injury and has a higher likelihood of progress to cirrhosis, liver failure, and liver cancer. Although NASH is clearly the more progressive form of the disease, emerging data suggest that NAFL may not be completely benign and itself can progress to NASH in around 25% of patients over a 2-year follow-up period.³

Epidemiology of NAFLD

Incidence of NAFLD

Although there are no true population-based incidence data for NAFLD in the United States, a study based on liver enzyme levels in veterans under the age of 45 has shown an increase in incidence from 2% to 4% between 2003 and 2011.⁴ True population incidence of NAFLD (based on imaging studies) has been reported from Asia (China and Japan) and Israel with NAFLD incidence reported as 28/1000 person-years from Israel, 34 per/1000 person-years from China and 87 per 1000 person-year from Japan.² One of the few reported studies regarding incidence is a prospective, observational study from Japan. Using ALT levels and ultrasonography of the liver in patients without significant alcohol use, they found that at baseline, 812 of 4401 (18%) participants had NAFLD. During the mean follow-up period of 414 days (SD, 128), the authors observed 308 new cases (10%) of NAFLD among 3147 participants who were disease-free at baseline.⁵

NAFLD Prevalence

The determination of population prevalence of NAFLD has been challenging due to need for an ...