Effect of a ketogenic diet on hepatic steatosis and hepatic mitochondrial metabolism in nonalcoholic fatty liver disease - PubMed (original) (raw)

Effect of a ketogenic diet on hepatic steatosis and hepatic mitochondrial metabolism in nonalcoholic fatty liver disease

Panu K Luukkonen et al. Proc Natl Acad Sci U S A. 2020.

Abstract

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.

Keywords: carbohydrate restriction; citrate synthase; insulin resistance; pyruvate carboxylase; redox.

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Conflict of interest statement

The authors declare no competing interest.

Figures

Fig. 1.

Study design. (A) Before and after the 6-d KD, participants visited an imaging center for measurement of IHTG content and liver stiffness (days −1 and 6) and underwent metabolic studies at the Clinical Research Unit (days 0 and 7). Participants wore portable accelerometers between days 0 and 7 for determination of physical activity and recorded 3-d food intake starting at days −3 and 4 for determination of dietary composition and compliance. (B) During metabolic study visits, 180-min tracer infusions of lactate, β-OHB, and glucose were given for determination of rates of substrate fluxes. Indirect calorimetry was performed to measure energy expenditure and rates of substrate oxidation. An “X” denotes blood sample.

Fig. 2.

The study diet was ketogenic and participants were compliant. (A) Macronutrient intakes, (B) plasma β-OHB and (C) plasma AcAc concentrations, and (D) body weight before (orange bars) and after (yellow bars) the 6-d KD (n = 10). Data are shown as mean ± SEM. P values were determined using paired Student’s t tests.

Fig. 3.

KD decreased IHTG content. (A) IHTG content (n = 8), (B) liver stiffness (n = 10), (C) plasma GGT (n = 10), and (D) plasma ALP (n = 9) before (orange bars) and after (yellow bars) the 6-d KD. Data are shown as mean ± SEM. P values were determined using paired Student’s t tests.

Fig. 4.

KD improved plasma glucose, TGs, and insulin sensitivity. (A) Plasma glucose, (B) plasma NEFA, (C) plasma TG, (D) serum insulin, (E) serum C-peptide concentrations, and (F) HOMA-IR before (orange bars) and after (yellow bars) the 6-d KD (n = 10). Data are shown as mean ± SEM. P values were determined using paired Student’s t tests.

Fig. 5.

KD altered hepatic mitochondrial fluxes. Rates of endogenous (A) glucose, (B) lactate, and (C) β-OHB production; (D) ratio of hepatic _V_PC and _V_CS fluxes, (E) hepatic _V_CS flux, and (F) hepatic _V_PC flux before (orange bars) and after (yellow bars) the 6-d KD (n = 10). Data are shown as mean ± SEM. P values were determined using paired Student’s t tests.

Fig. 6.

Potential mechanisms underlying the reduction in _V_CS. (A) Ratio of plasma β-OHB and AcAc concentrations, which reflect mitochondrial redox state, (B) plasma leptin concentrations, and (C) plasma total T3 concentrations before (orange bars) and after (yellow bars) the 6-d KD (n = 10). Data are shown as mean ± SEM. P values were determined using paired Student’s t tests.

Fig. 7.

Model of the antisteatotic mechanisms of KD. Glucose production decreased during the 6-d KD, which could be attributed to hepatic glycogen depletion. The decrease in glucose production was accompanied by reduced serum insulin concentrations, which promoted net hydrolysis of TGs in the liver and adipose tissue. This increased hepatic availability of fatty acids underwent β-oxidation to produce acetyl-CoA. Consequently, the hepatic mitochondrial redox state increased, which inhibited _V_CS and diverted mitochondrial acetyl-CoA toward ketogenesis: That is, production of AcAc and β-OHB rather than into oxidation to carbon dioxide (CO2). The reduction in _V_CS was also associated with decreased plasma leptin and T3 concentrations. Mitochondrial _V_PC remained unchanged during the KD, despite decreases in availability of substrates (glucose, lactate, and alanine), likely due to allosteric activation by acetyl-CoA. The increase in _V_PC/_V_CS diverted mitochondrial oxaloacetate (OAA) toward gluconeogenesis rather than oxidation. In addition, by limiting the availability of the primary substrate (citrate) and decreasing insulin concentrations, KD resulted in reduced hepatic DNL. Taking these data together, we find that KD improves steatosis by markedly altering hepatic mitochondrial fluxes and redox state to promote partitioning of FA to ketogenesis rather than re-esterification and lipogenesis.

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Effect of a ketogenic diet on hepatic steatosis and hepatic mitochondrial metabolism in nonalcoholic fatty liver disease - PubMed (original) (raw)