Detrimental effects of branched-chain amino acids in glucose tolerance can be attributed to valine induced glucotoxicity in skeletal muscle.

OBJECTIVE: Current data regarding the roles of branched-chain amino acids (BCAA) in metabolic health are rather conflicting, as positive and negative effects have been attributed to their intake. METHODS: To address this, individual effects of leucine and valine were elucidated in vivo (C57BL/6JRj mice) with a detailed phenotyping of these supplementations in high-fat (HF) diets and further characterization with in vitro approaches (C2C12 myocytes). RESULTS: Here, we demonstrate that under HF conditions, leucine mediates beneficial effects on adiposity and insulin sensitivity, in part due to increasing energy expenditure-likely contributing partially to the beneficial effects of a higher milk protein intake. On the other hand, valine feeding leads to a worsening of HF-induced health impairments, specifically reducing glucose tolerance/insulin sensitivity. These negative effects are driven by an accumulation of the valine-derived metabolite 3-hydroxyisobutyrate (3-HIB). Higher plasma 3-HIB levels increase basal skeletal muscle glucose uptake which drives glucotoxicity and impairs myocyte insulin signaling. CONCLUSION: These data demonstrate the detrimental role of valine in an HF context and elucidate additional targetable pathways in the etiology of BCAA-induced obesity and insulin resistance.

Effect of Microbial Status on Hepatic Odd-Chain Fatty Acids Is Diet-Dependent.

Odd-chain fatty acids (OCFA) are inversely associated with type-2-diabetes in epidemiological studies. They are considered as a biomarker for dairy intake because fermentation in ruminants yields high amounts of propionate, which is used as the primer for lipogenesis. Recently, we demonstrated endogenous OCFA synthesis from propionate in humans and mice, but how this is affected by microbial colonization is still unexplored. Here, we investigated the effect of increasing microbiota complexity on hepatic lipid metabolism and OCFA levels in different dietary settings. Germ-free (GF), gnotobiotic (SIH, simplified human microbiota) or conventional (CONV) C3H/HeOuJ-mice were fed a CHOW or high-fat diet with inulin (HFI) to induce microbial fermentation. We found that hepatic lipogenesis was increased with increasing microbiota complexity, independently of diet. In contrast, OCFA formation was affected by diet as well as microbiota. On CHOW, hepatic OCFA and intestinal gluconeogenesis decreased with increasing microbiota complexity (GF > SIH > CONV), while cecal propionate showed a negative correlation with hepatic OCFA. On HFI, OCFA levels were highest in SIH and positively correlated with cecal propionate. The propionate content in the CHOW diet was 10 times higher than that of HFI. We conclude that bacterial propionate production affects hepatic OCFA formation, unless this effect is masked by dietary propionate intake.

The branched-chain amino acids valine and leucine have differential effects on hepatic lipid metabolism.

Dairy intake, as a source of branched-chain amino acids (BCAA), has been linked to a lower incidence of type-2-diabetes and increased circulating odd-chain fatty acids (OCFA). To understand this connection, we aimed to investigate differences in BCAA metabolism of leucine and valine, a possible source of OCFA, and their role in hepatic metabolism. Male mice were fed a high-fat diet supplemented with leucine and valine for 1 week and phenotypically characterized with a focus on lipid metabolism. Mouse primary hepatocytes were treated with the BCAA or a Pparα activator WY-14643 to systematically examine direct hepatic effects and their mechanisms. Here, we show that only valine supplementation was able to increase hepatic and circulating OCFA levels via two pathways; a PPARα-dependent induction of α-oxidation and an increased supply of propionyl-CoA for de novo lipogenesis. Meanwhile, we were able to confirm leucine-mediated effects on the inhibition of food intake and transport of fatty acids, as well as induction of S6 ribosomal protein phosphorylation. Taken together, these data illustrate differential roles of the BCAA in lipid metabolism and provide preliminary evidence that exclusively valine contributes to the endogenous formation of OCFA which is important for a better understanding of these metabolites in metabolic health.

Specific Wheat Fractions Influence Hepatic Fat Metabolism in Diet-Induced Obese Mice.

Low whole grain consumption is a risk factor for the development of non-communicable diseases such as type 2 diabetes. Dietary fiber and phytochemicals are bioactive grain compounds, which could be involved in mediating these beneficial effects. These compounds are not equally distributed in the wheat grain, but are enriched in the bran and aleurone fractions. As little is known on physiological effects of different wheat fractions, the aim of this study was to investigate this aspect in an obesity model. For twelve weeks, C57BL/6J mice were fed high-fat diets (HFD), supplemented with one of four wheat fractions: whole grain flour, refined white flour, bran, or aleurone. The different diets did not affect body weight, however bran and aleurone decreased liver triglyceride content, and increased hepatic n-3 polyunsaturated fatty acid (PUFA) concentrations. Furthermore, lipidomics analysis revealed increased PUFA concentration in the lipid classes of phosphatidylcholine (PC), PC-ether, and phosphatidylinositol in the plasma of mice fed whole grain, bran, and aleurone supplemented diets, compared to refined white flour. Furthermore, bran, aleurone, and whole grain supplemented diets increased microbial α-diversity, but only bran and aleurone increased the cecal concentrations of short-chain fatty acids. The effects on hepatic lipid metabolism might thus at least partially be mediated by microbiota-dependent mechanisms.

Control of hepatic gluconeogenesis by Argonaute2.

OBJECTIVE: The liver performs a central role in regulating energy homeostasis by increasing glucose output during fasting. Recent studies on Argonaute2 (Ago2), a key RNA-binding protein mediating the microRNA pathway, have illustrated its role in adaptive mechanisms according to changes in metabolic demand. Here we sought to characterize the functional role of Ago2 in the liver in the maintenance of systemic glucose homeostasis. METHODS: We first analyzed Ago2 expression in mouse primary hepatocyte cultures after modulating extracellular glucose concentrations and in the presence of activators or inhibitors of glucokinase activity. We then characterized a conditional loss-of-function mouse model of Ago2 in liver for alterations in systemic energy metabolism. RESULTS: Here we show that Ago2 expression in liver is directly correlated to extracellular glucose concentrations and that modulating glucokinase activity is adequate to affect hepatic Ago2 levels. Conditional deletion of Ago2 in liver resulted in decreased fasting glucose levels in addition to reducing hepatic glucose production. Moreover, loss of Ago2 promoted hepatic expression of AMP-activated protein kinase α1 (AMPKα1) by de-repressing its targeting by miR-148a, an abundant microRNA in the liver. Deletion of Ago2 from hyperglycemic, obese, and insulin-resistant Lepob/ob mice reduced both random and fasted blood glucose levels and body weight and improved insulin sensitivity. CONCLUSIONS: These data illustrate a central role for Ago2 in the adaptive response of the liver to fasting. Ago2 mediates the suppression of AMPKα1 by miR-148a, thereby identifying a regulatory link between non-coding RNAs and a key stress regulator in the hepatocyte.

Short-chain fatty acids and inulin, but not guar gum, prevent diet-induced obesity and insulin resistance through differential mechanisms in mice.

The role of dietary fibre and short-chain fatty acids (SCFA) in obesity development is controversially discussed. Here, we investigated how various types of dietary fibre and different SCFA ratios affect metabolic syndrome-related disorders. Male mice (B6) were fed high-fat diets supplemented with dietary fibres (either cellulose, inulin or guar gum) or different Ac:Pr ratios (high acetate (HAc) or propionate (HPr)) for 30 weeks. Body-fat gain and insulin resistance were greatly reduced by inulin, but not by guar gum, and completely prevented by SCFA supplementation. Only inulin and HAc increased body temperature, possibly by the induction of beige/browning markers in WAT. In addition, inulin and SCFA lowered hepatic triglycerides and improved insulin sensitivity. Both, inulin and HAc reduced hepatic fatty acid uptake, while only inulin enhanced mitochondrial capacity and only HAc suppressed lipogenesis in liver. Interestingly, HPr was accompanied by the induction of Nrg4 in BAT. Fermentable fibre supplementation increased the abundance of bifidobacteria; B. animalis was particularly stimulated by inulin and B. pseudolongum by guar gum. We conclude that in contrast to guar gum, inulin and SCFA prevent the onset of diet-induced weight gain and hepatic steatosis by different mechanisms on liver and adipose tissue metabolism.

Odd-chain fatty acids as a biomarker for dietary fiber intake: a novel pathway for endogenous production from propionate.

Background: The risk of type 2 diabetes is inversely correlated with plasma concentrations of odd-chain fatty acids [OCFAs; pentadecanoic acid (15:0) and heptadecanoic acid (17:0)], which are considered as biomarkers for dairy fat intake in humans. However, rodent studies suggest that OCFAs are synthesized endogenously from gut-derived propionate. Propionate increases with dietary fiber consumption and has been shown to improve insulin sensitivity.Objective: We hypothesized that OCFAs are produced in humans from dietary fibers by a novel endogenous pathway.Design: In a randomized, double-blind crossover study, 16 healthy individuals were supplemented with cellulose (30 g/d), inulin (30 g/d), or propionate (6 g/d) for 7 d. In addition, human hepatoma cells were incubated with different propionate concentrations. OCFAs were determined in plasma phospholipids and hepatoma cells by gas chromatography.Results: Cellulose did not affect plasma OCFA levels, whereas inulin and propionate increased pentadecanoic acid by ∼17% (P < 0.05) and 13% (P = 0.05), respectively. The effect on heptadecanoic acid was even more pronounced, because it was elevated in almost all participants by inulin (11%; P < 0.01) and propionate (13%; P < 0.001). Furthermore, cell culture experiments showed a positive association between propionate and OCFA levels (R2 = 0.99, P < 0.0001), whereas palmitate (16:0) was negatively correlated (R2 = 0.83, P = 0.004).Conclusions: Our data show that gut-derived propionate is used for the hepatic synthesis of OCFAs in humans. The association of OCFAs with a decreased risk of type 2 diabetes may therefore also relate to dietary fiber intake and not only dairy fat. This trial was registered at www.germanctr.de as DRKS00010121.

Importance of propionate for the repression of hepatic lipogenesis and improvement of insulin sensitivity in high-fat diet-induced obesity.

SCOPE: The SCFA acetate (Ac) and propionate (Pr) are major fermentation products of dietary fibers and provide additional energy to the host. We investigated short- and long-term effects of dietary Ac and Pr supplementation on diet-induced obesity and hepatic lipid metabolism. METHODS AND RESULTS: C3H/HeOuJ mice received high-fat (HF) diets supplemented with 5% SCFA in different Ac:Pr ratios, a high acetate (HF-HAc; 2.5:1 Ac:Pr) or high Pr ratio (HF-HPr; 1:2.5 Ac:Pr) for 6 or 22 weeks. Control diets (low-fat (LF), HF) contained no SCFA. SCFA did not affect body composition but reduced hepatic gene and protein expression of lipogenic enzymes leading to a reduced hepatic triglyceride concentration after 22 weeks in HF-HPr mice. Analysis of long-chain fatty acid composition (liver and plasma phospholipids) showed that supplementation of both ratios led to a lower ω6:ω3 ratio. Pr directly led to increased odd-chain fatty acid (C15:0, C17:0) formation as confirmed in vitro using HepG2 cells. Remarkably, plasma C15:0 was correlated with the attenuation of HF diet-induced insulin resistance. CONCLUSION: Dependent on the Ac:Pr ratio, especially odd-chain fatty acid formation and insulin sensitivity are differentially affected, indicating the importance of Pr.

Effects of dietary inulin on bacterial growth, short-chain fatty acid production and hepatic lipid metabolism in gnotobiotic mice.

In literature, contradictory effects of dietary fibers and their fermentation products, short-chain fatty acids (SCFA), are described: On one hand, they increase satiety, but on the other hand, they provide additional energy and promote obesity development. We aimed to answer this paradox by investigating the effects of fermentable and non-fermentable fibers on obesity induced by high-fat diet in gnotobiotic C3H/HeOuJ mice colonized with a simplified human microbiota. Mice were fed a high-fat diet supplemented either with 10% cellulose (non-fermentable) or inulin (fermentable) for 6 weeks. Feeding the inulin diet resulted in an increased diet digestibility and reduced feces energy, compared to the cellulose diet with no differences in food intake, suggesting an increased intestinal energy extraction from inulin. However, we observed no increase in body fat/weight. The additional energy provided by the inulin diet led to an increased bacterial proliferation in this group. Supplementation of inulin resulted further in significantly elevated concentrations of total SCFA in cecum and portal vein plasma, with a reduced cecal acetate:propionate ratio. Hepatic expression of genes involved in lipogenesis (Fasn, Gpam) and fatty acid elongation/desaturation (Scd1, Elovl3, Elovl6, Elovl5, Fads1 and Fads2) were decreased in inulin-fed animals. Accordingly, plasma and liver phospholipid composition were changed between the different feeding groups. Concentrations of omega-3 and odd-chain fatty acids were increased in inulin-fed mice, whereas omega-6 fatty acids were reduced. Taken together, these data indicate that, during this short-term feeding, inulin has mainly positive effects on the lipid metabolism, which could cause beneficial effects during obesity development in long-term studies.