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Comorbidities and Nonalcoholic Fatty Liver Disease: The Chicken, the Egg, or Both?

Improvement in NAFLD may lead to improvement of metabolic syndrome, cardiovascular disease, and malignancy and vice versa.
Federal Practitioner. 2019 February;36(2)a:64-71
February 12, 2019|Federal Practitioner
Lisa M. Glass, MD;Christine M. Hunt, MD;Michael Fuchs, MD, PhD;Grace L. Su, MD
Author and Disclosure Information

Lisa Glass is a Staff Physician, and Grace Su is Chief of Gastroenterology and Associate Chief of Medicine, both at VA Ann Arbor Healthcare System. Christine Hunt is a Physician Affiliate and Director of the Cooperative Studies Program Epidemiology Centers at Durham VAMC in North Carolina. Michael Fuchs is Chief of Hepatology and Liver Transplantation at Hunter Holmes McGuire VAMC. Christine Hunt is an Adjunct Associate Professor of Medicine at Duke University Medical Center in Durham. Lisa Glass is an Assistant Professor and Grace Su is a Professor of Medicine, both at the University of Michigan Medical School in Ann Arbor. Michael Fuchs is Professor of Medicine at Virginia Commonwealth University in Richmond. Correspondence: Lisa Glass (lisaglas@med.umich.edu)

Author disclosures
The authors report no actual or potential conflicts of interest with regard to this article.

Disclaimer
The opinions expressed herein are those of the authors and do not necessarily reflect those of Federal Practitioner, Frontline Medical Communications Inc., the US Government, or any of its agencies. This article may discuss unlabeled or investigational use of certain drugs. Please review the complete prescribing information for specific drugs or drug combinations—including indications, contraindications, warnings, and adverse effects—before administering pharmacologic therapy to patients.

Chronic Kidney Disease

The prevalence of CKD, defined as estimated glomerular filtration rate (GFR) < 60 mL/min/1.72 m2, abnormal albuminuria, or proteinuria, is significantly increased in patients with NAFLD. Several epidemiologic studies have shown the prevalence of CKD in NAFLD patients ranges from 20% to 55% compared with 5% to 30% among patients without NAFLD.40 Overall, patients with NAFLD have a 2-fold increased risk of prevalent (OR 2.12; 95% CI, 1.69-2.66) or incident (hazard ratio 1.79; 95% CI, 1.65-1.95) CKD, even after adjusting for T2DM, visceral fat, and insulin resistance.40 There is an additional 2-fold increase in CKD risk in patients with NASH and advanced fibrosis compared with those with NASH and mild/no fibrosis. Additionally, advancing NASH fibrosis stage is independently associated with worsening stage of CKD.41

Data regarding the exact mechanism of kidney pathology in the setting of NAFLD is lacking. The accelerated atherogenesis in NAFLD likely contributes to renal damage. Another potential mechanism to explain the association between NASH and CKD involves the increased activation of the angiotensin-aldosterone system (RAAS) seen in NASH, which leads to increased hepatic fibrogenesis as well as kidney damage.42

Similar to the previously listed comorbidities, there is evidence that improvement in NAFLD can lead to improvements in renal disease. A prospective study of NASH patients undergoing 52 weeks of lifestyle modification found that the patients who had improvements in histologic NASH endpoints also had improvement in renal function.43

There are currently no specific recommendations on screening for CKD in professionalguidelines, but many experts propose monitoring for CKD yearly with serum creatinine and urinalysis and referring to nephrology if needed. Given the association between NASH and activation of the RAAS pathway that is associated with worsening hepatic fibrosis, RAAS-inhibitors should be a first-line agent in the treatment of hypertension in patients with NAFLD.

Obstructive Sleep Apnea

OSA is characterized by repeated pharyngeal collapse during sleep, which leads to chronic intermittent hypoxia and is associated with increased metabolic and cardiovascular morbidity and mortality. The cycle of intermittent hypoxia and reoxygenation in OSA results in inflammation and oxidative stress. Multiple studies have supported a link between NAFLD and OSA.

Hepatic fat content on ultrasound was increased in patients with OSA independent of BMI. There also has been evidence of a positive association between the severity of chronic intermittent hypoxia and increased hepatic fibrosis based on liver elastography.44 A meta-analysis using histologic NAFLD diagnosis showed that the presence of OSA was associated with a higher risk of fibrosis compared with that of patients with NAFLD without OSA (OR 2.6; 95% CI, 1.3-5.2).45

Based on animal models, hypoxia can drive fat accumulation and inflammation in the liver via multiple different pathways. Hypoxia can increase fasting glucose and systemic TG levels and induce hepatic lipogenesis by altering gene expression.45 Hypoxia also can increase oxidative stress and reduce β-oxidation, which leads to the production of lipotoxic lipids. These hypoxia-induced changes are typically more pronounced in subjects with obesity compared with that in subjects without obesity. Despite multiple adverse metabolic effects of OSA-induced hypoxia in the setting of NAFLD, preliminary, short-term studies have failed to find an association with OSA treatment with continuous positive airway pressure and improvement in NAFLD.45 Perhaps larger, long-term prospective trials will clarify this question.

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