title: Definitions of Normal Liver Fat and the Association of Insulin Sensitivity with Acquired and Genetic NAFLD-A Systematic Review.
abstract: Non-alcoholic fatty liver disease (NAFLD) covers a spectrum of disease ranging from simple steatosis (NAFL) to non-alcoholic steatohepatitis (NASH) and fibrosis. "Obese/Metabolic NAFLD" is closely associated with obesity and insulin resistance and therefore predisposes to type 2 diabetes and cardiovascular disease. NAFLD can also be caused by common genetic variants, the patatin-like phospholipase domain-containing 3 (PNPLA3) or the transmembrane 6 superfamily member 2 (TM6SF2). Since NAFL, irrespective of its cause, can progress to NASH and liver fibrosis, its definition is of interest. We reviewed the literature to identify data on definition of normal liver fat using liver histology and different imaging tools, and analyzed whether NAFLD caused by the gene variants is associated with insulin resistance. Histologically, normal liver fat content in liver biopsies is most commonly defined as macroscopic steatosis in less than 5% of hepatocytes. In the population-based Dallas Heart Study, the upper 95th percentile of liver fat measured by proton magnetic spectroscopy (¹H-MRS) in healthy subjects was 5.6%, which corresponds to approximately 15% histological liver fat. When measured by magnetic resonance imaging (MRI)-based techniques such as the proton density fat fraction (PDFF), 5% macroscopic steatosis corresponds to a PDFF of 6% to 6.4%. In contrast to "Obese/metabolic NAFLD", NAFLD caused by genetic variants is not associated with insulin resistance. This implies that NAFLD is heterogeneous and that "Obese/Metabolic NAFLD" but not NAFLD due to the PNPLA3 or TM6SF2 genetic variants predisposes to type 2 diabetes and cardiovascular disease.
authors.affiliation: Minerva Foundation Institute for Medical Research, 00290 Helsinki, Finland. elina.petaja@helsinki.fi.|Department of Medicine, University of Helsinki and Helsinki University Central Hospital, 00290 Helsinki, Finland. elina.petaja@helsinki.fi.
authors.forename: Elina M
authors.initials: EM
authors.lastname: Petäjä
authors.affiliation: Minerva Foundation Institute for Medical Research, 00290 Helsinki, Finland. hannele.yki-jarvinen@helsinki.fi.|Department of Medicine, University of Helsinki and Helsinki University Central Hospital, 00290 Helsinki, Finland. hannele.yki-jarvinen@helsinki.fi.
authors.forename: Hannele
authors.initials: H
authors.lastname: Yki-Järvinen
mesh_terms: D006801:Humans; D007328:Insulin / Q000378:metabolism*; D007333:Insulin Resistance; D008049:Lipase / Q000235:genetics / Q000378:metabolism; D008099:Liver / Q000000981:diagnostic imaging / Q000473:pathology; D008279:Magnetic Resonance Imaging; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism; D065626:Non-alcoholic Fatty Liver Disease / Q000000981:diagnostic imaging / Q000235:genetics / Q000473:pathology*; D009765:Obesity / Q000473:pathology; D066244:Proton Magnetic Resonance Spectroscopy; D000217:Acyltransferases; D054512:Phospholipases A2, Calcium-Independent
chemical_list: D007328:Insulin; D008049:Lipase; D008565:Membrane Proteins; C419195:TM6SF2 protein, human; C533682:PNPLA3 protein, human; D000217:Acyltransferases; D054512:Phospholipases A2, Calcium-Independent
keywords: PNPLA3; TM6SF2; insulin resistance; liver fat; obesity