Nonalcoholic fatty liver disease (NAFLD) is characterized by the accumulation of excess liver triacylglycerol (TAG), inflammation, and liver damage. The goal of the present study was to directly quantify the biological sources of hepatic and plasma lipoprotein TAG in NAFLD. Patients (5 male and 4 female; 44 ± 10 years of age) scheduled for a medically indicated liver biopsy were infused with and orally fed stable isotopes for 4 days to label and track serum nonesterified fatty acids (NEFAs), dietary fatty acids, and those derived from the de novo lipogenesis (DNL) pathway, present in liver tissue and lipoprotein TAG. Hepatic and lipoprotein TAG fatty acids were analyzed by gas chromatography/mass spectrometry. NAFLD patients were obese, with fasting hypertriglyceridemia and hyperinsulinemia. Of the TAG accounted for in liver, 59.0% ± 9.9% of TAG arose from NEFAs; 26.1% ± 6.7%, from DNL; and 14.9% ± 7.0%, from the diet. The pattern of labeling in VLDL was similar to that in liver, and throughout the 4 days of labeling, the liver demonstrated reciprocal use of adipose and dietary fatty acids. DNL was elevated in the fasting state and demonstrated no diurnal variation. These quantitative metabolic data document that both elevated peripheral fatty acids and DNL contribute to the accumulation of hepatic and lipoprotein fat in NAFLD.

Background & Aims There have been few studies of the role of de novo lipogenesis in the development of nonalcoholic fatty liver disease (NAFLD). We used isotope analyses to compare de novo lipogenesis and fatty acid flux between subjects with NAFLD and those without, matched for metabolic factors (controls). Methods We studied subjects with metabolic syndrome and/or levels of alanine aminotransferase and aspartate aminotransferase >30 mU/L, using magnetic resonance spectroscopy to identify those with high levels (HighLF, n = 13) or low levels (LowLF, n = 11) of liver fat. Clinical and demographic information was collected from all participants, and insulin sensitivity was measured using the insulin-modified intravenous glucose tolerance test. Stable isotopes were administered and gas chromatography with mass spectrometry was used to analyze free (nonesterified) fatty acid (FFA) and triacylglycerol flux and lipogenesis. Results Subjects with HighLF (18.4% ± 3.6%) had higher plasma levels of FFAs during the nighttime and higher concentrations of insulin than subjects with LowLF (3.1% ± 2.7%; P = .04 and P < .001, respectively). No differences were observed between groups in adipose flux of FFAs (414 ± 195 μmol/min for HighLF vs 358 ± 105 μmol/min for LowLF; P = .41) or production of very-low-density lipoprotein triacylglycerol from FFAs (4.06 ± 2.57 μmol/min vs 4.34 ± 1.82 μmol/min; P = .77). In contrast, subjects with HighLF had more than 3-fold higher rates of de novo fatty acid synthesis than subjects with LowLF (2.57 ± 1.53 μmol/min vs 0.78 ± 0.42 μmol/min; P = .001). As a percentage of triacylglycerol palmitate, de novo lipogenesis was 2-fold higher in subjects with HighLF (23.2% ± 7.9% vs 10.1% ± 6.7%; P < .001); this level was independently associated with the level of intrahepatic triacylglycerol (r = 0.53; P = .007). Conclusions By administering isotopes to subjects with NAFLD and control subjects, we confirmed that those with NAFLD have increased synthesis of fatty acids. Subjects with NAFLD also had higher nocturnal plasma levels of FFAs and did not suppress the contribution from de novo lipogenesis on fasting. These findings indicate that lipogenesis might be a therapeutic target for NAFLD.

Current trends in health promotion emphasize the importance of reducing dietary fat intake. However, as dietary fat is reduced, the dietary carbohydrate content typically rises and the desired reduction in plasma cholesterol concentrations is frequently accompanied by an elevation of plasma triacylglycerol. We review the phenomenon of carbohydrate-induced hypertriacylglycerolemia, the health effects of which are among the most controversial and important issues in public health nutrition today. We first focus on how seminal observations made in the late 1950s and early 1960s became the basis for subsequent important research questions and areas of scientific study. The second focus of this paper is on the current knowledge of biological mechanisms that contribute to carbohydrate-induced hypertriacylglycerolemia. The clinical rationale behind mechanistic studies is this: if carbohydrate-induced hypertriacylglycerolemia shares a metabolic basis with endogenous hypertriacylglycerolemia (that observed in subjects consuming high-fat diets), then a similar atherogenic risk may be more likely than if the underlying metabolic mechanisms differ. The third focus of the paper is on both the positive metabolic changes that occur when high-carbohydrate diets are consumed and the potentially negative health effects of such diets. The review concludes with a summary of some important research questions that remain to be addressed. These issues include the level of dietary carbohydrate that induces carbohydrate-induced hypertriacylglycerolemia, whether the phenomenon is transient or can be avoided, whether de novo lipogenesis contributes to the phenomenon, and what magnitude of triacylglycerol elevation represents an increase in disease risk.

Mitochondria are critical for respiration in all tissues; however, in liver, these organelles also accommodate high-capacity anaplerotic/cataplerotic pathways that are essential to gluconeogenesis and other biosynthetic activities. During nonalcoholic fatty liver disease (NAFLD), mitochondria also produce ROS that damage hepatocytes, trigger inflammation, and contribute to insulin resistance. Here, we provide several lines of evidence indicating that induction of biosynthesis through hepatic anaplerotic/cataplerotic pathways is energetically backed by elevated oxidative metabolism and hence contributes to oxidative stress and inflammation during NAFLD. First, in murine livers, elevation of fatty acid delivery not only induced oxidative metabolism, but also amplified anaplerosis/cataplerosis and caused a proportional rise in oxidative stress and inflammation. Second, loss of anaplerosis/cataplerosis via genetic knockdown of phosphoenolpyruvate carboxykinase 1 (Pck1) prevented fatty acid–induced rise in oxidative flux, oxidative stress, and inflammation. Flux appeared to be regulated by redox state, energy charge, and metabolite concentration, which may also amplify antioxidant pathways. Third, preventing elevated oxidative metabolism with metformin also normalized hepatic anaplerosis/cataplerosis and reduced markers of inflammation. Finally, independent histological grades in human NAFLD biopsies were proportional to oxidative flux. Thus, hepatic oxidative stress and inflammation are associated with elevated oxidative metabolism during an obesogenic diet, and this link may be provoked by increased work through anabolic pathways.

Low-fat, high-carbohydrate (LF/HC) diets commonly elevate plasma triglyceride (TG) concentrations, but the kinetic mechanisms responsible for this effect remain uncertain. Subjects with low TG (normolipidemic [NL]) and those with moderately elevated TG (hypertriglyceridemic [HTG]) were studied on both a control and an LF/HC diet. We measured VLDL particle and TG transport rates, plasma nonesterified fatty acid (NEFA) flux, and sources of fatty acids used for the assembly of VLDL-TG. The LF/HC diet resulted in a 60% elevation in TG, a 37% reduction in VLDL-TG clearance, and an 18% reduction in whole-body fat oxidation, but no significant change in VLDL-apo B or VLDL-TG secretion rates. Significant elevations in fasting apo B-48 concentrations were observed on the LF/HC in HTG subjects. In both groups, fasting de novo lipogenesis was low regardless of diet. The NEFA pool contributed the great majority of fatty acids to VLDL-TG in NL subjects on both diets, whereas in HTG subjects, the contribution of NEFA was somewhat lower overall and was reduced further in individuals on the LF/HC diet. Between 13% and 29% of VLDL-TG fatty acids remained unaccounted for by the sum of de novo lipogenesis and plasma NEFA input in HTG subjects. We conclude that (a) whole-food LF/HC diets reduce VLDL-TG clearance and do not increase VLDL-TG secretion or de novo lipogenesis; (b) sources of fatty acids for assembly of VLDL-TG differ between HTG and NL subjects and are further affected by diet composition; (c) the presence of chylomicron remnants in the fasting state on LF/HC diets may contribute to elevated TG levels by competing for VLDL-TG lipolysis and by providing a source of fatty acids for hepatic VLDL-TG synthesis; and (d) the assembly, production, and clearance of elevated plasma VLDL-TG in response to LF/HC diets therefore differ from those for elevated TG on higher-fat diets.