OBJECTIVE: Nonalcoholic fatty liver disease (i.e., increased intrahepatic triglyceride [IHTG] content), predisposes to type 2 diabetes and cardiovascular disease. Adipose tissue lipolysis and hepatic de novo lipogenesis (DNL) are the main pathways contributing to IHTG. We hypothesized that dietary macronutrient composition influences the pathways, mediators, and magnitude of weight gain-induced changes in IHTG. RESEARCH DESIGN AND METHODS: We overfed 38 overweight subjects (age 48 ± 2 years, BMI 31 ± 1 kg/m2, liver fat 4.7 ± 0.9%) 1,000 extra kcal/day of saturated (SAT) or unsaturated (UNSAT) fat or simple sugars (CARB) for 3 weeks. We measured IHTG (1H-MRS), pathways contributing to IHTG (lipolysis ([2H5]glycerol) and DNL (2H2O) basally and during euglycemic hyperinsulinemia), insulin resistance, endotoxemia, plasma ceramides, and adipose tissue gene expression at 0 and 3 weeks. RESULTS: Overfeeding SAT increased IHTG more (+55%) than UNSAT (+15%, P < 0.05). CARB increased IHTG (+33%) by stimulating DNL (+98%). SAT significantly increased while UNSAT decreased lipolysis. SAT induced insulin resistance and endotoxemia and significantly increased multiple plasma ceramides. The diets had distinct effects on adipose tissue gene expression. CONCLUSIONS: Macronutrient composition of excess energy influences pathways of IHTG: CARB increases DNL, while SAT increases and UNSAT decreases lipolysis. SAT induced the greatest increase in IHTG, insulin resistance, and harmful ceramides. Decreased intakes of SAT could be beneficial in reducing IHTG and the associated risk of diabetes.

The use of relative abundance data from next generation sequencing (NGS) can lead to misinterpretations of microbial community structures, as the increase of one taxon leads to the concurrent decrease of the other(s) in compositional data. Although different DNA- and cell-based methods as well as statistical approaches have been developed to overcome the compositionality problem, and the biological relevance of absolute bacterial abundances has been demonstrated, the human microbiome research has not yet adopted these methods, likely due to feasibility issues. Here, we describe how quantitative PCR (qPCR) done in parallel to NGS library preparation provides an accurate estimation of absolute taxon abundances from NGS data and hence provides an attainable solution to compositionality in high-throughput microbiome analyses. The advantages and potential challenges of the method are also discussed.

Weight loss by ketogenic diet (KD) has gained popularity in management of nonalcoholic fatty liver disease (NAFLD). KD rapidly reverses NAFLD and insulin resistance despite increasing circulating nonesterified fatty acids (NEFA), the main substrate for synthesis of intrahepatic triglycerides (IHTG). To explore the underlying mechanism, we quantified hepatic mitochondrial fluxes and their regulators in humans by using positional isotopomer NMR tracer analysis. Ten overweight/obese subjects received stable isotope infusions of: [D₇]glucose, [¹³C₄]β-hydroxybutyrate and [3-¹³C]lactate before and after a 6-d KD. IHTG was determined by proton magnetic resonance spectroscopy (¹H-MRS). The KD diet decreased IHTG by 31% in the face of a 3% decrease in body weight and decreased hepatic insulin resistance (−58%) despite an increase in NEFA concentrations (+35%). These changes were attributed to increased net hydrolysis of IHTG and partitioning of the resulting fatty acids toward ketogenesis (+232%) due to reductions in serum insulin concentrations (−53%) and hepatic citrate synthase flux (−38%), respectively. The former was attributed to decreased hepatic insulin resistance and the latter to increased hepatic mitochondrial redox state (+167%) and decreased plasma leptin (−45%) and triiodothyronine (−21%) concentrations. These data demonstrate heretofore undescribed adaptations underlying the reversal of NAFLD by KD: That is, markedly altered hepatic mitochondrial fluxes and redox state to promote ketogenesis rather than synthesis of IHTG. Significance Ketogenic diet is an effective treatment for nonalcoholic fatty liver disease (NAFLD). Here, we present evidence that hepatic mitochondrial fluxes and redox state are markedly altered during ketogenic diet-induced reversal of NAFLD in humans. Ketogenic diet for 6 d markedly decreased liver fat content and hepatic insulin resistance. These changes were associated with increased net hydrolysis of liver triglycerides and decreased endogenous glucose production and serum insulin concentrations. Partitioning of fatty acids toward ketogenesis increased, which was associated with increased hepatic mitochondrial redox state and decreased hepatic citrate synthase flux. These data demonstrate heretofore undescribed adaptations underlying the reversal of NAFLD by ketogenic diet and highlight hepatic mitochondrial fluxes and redox state as potential treatment targets in NAFLD.