Disposition of a Glucose Load into Hepatic Glycogen by Direct and Indirect Pathways in Juvenile Seabass and Seabream

Determining the contribution of a high-fructose corn syrup formulation to hepatic glycogen synthesis during ad-libitum feeding in mice

Excessive sugar intake including high-fructose corn syrup (HFCS) is implicated in the rise of obesity, insulin resistance and non-alcoholic fatty liver disease. Liver glycogen synthesis is influenced by both fructose and insulin signaling. Therefore, the effect of HFCS on hepatic glycogenesis was evaluated in mice feeding ad-libitum. Using deuterated water: the fraction of glycogen derived from triose-P sources, Krebs cycle substrates, and direct pathway + cycling, was measured in 9 normal-chow fed mice (NC) and 12 mice fed normal chow plus a 55% fructose/45% glucose mix in the drinking water at 30% w/v (HFCS-55). This was enriched with [U-¹³C]fructose or [U-¹³C]glucose to determine the contribution of each to glycogenesis. For NC, direct pathway + cycling, Krebs cycle, and triose-P sources accounted for 66 ± 0.7%, 23 ± 0.8% and 11 ± 0.4% of glycogen synthesis, respectively. HFCS-55 mice had similar direct pathway + cycling (64 ± 1%) but lower Krebs cycle (12 ± 1%, p < 0.001) and higher triose-P contributions (24 ± 1%, p < 0.001). HFCS-55-fructose contributed 17 ± 1% via triose-P and 2 ± 0% via Krebs cycle. HFCS-55-glucose contributed 16 ± 3% via direct pathway and 1 ± 0% via Krebs cycle. In conclusion, HFCS-55 supplementation resulted in similar hepatic glycogen deposition rates. Indirect pathway contributions shifted from Krebs cycle to Triose-P sources reflecting HFCS-55-fructose utilization, while HFCS-55-glucose was incorporated almost exclusively by the direct pathway.

Limitations to Starch Utilization in Barramundi (Lates calcarifer) as Revealed by NMR-Based Metabolomics

Practical diets for commercial barramundi production rarely contain greater than 10% starch, used mainly as a binding agent during extrusion. Alternative ingredients such as digestible starch have shown some capacity to spare dietary protein catabolism to generate glucose. In the present study, a carnivorous fish species, the Asian seabass (Lates calcarifer) was subjected to two diets with the same digestible energy: Protein (P) – with high protein content (no digestible starch); and Starch (S) – with high digestible (pregelatinized) starch content. The effects of a high starch content diet on hepatic glycogen synthesis as well as the muscle and liver metabolome were studied using a complementary approach of ¹H and ²H NMR. The hepatosomatic index was lower for fish fed high starch content diet while the concentration of hepatic glycogen was similar between groups. However, increased glycogen synthesis via the direct pathway was observed in the fish fed high starch content diet which is indicative of increased carbohydrate utilization. Multivariate analysis also showed differences between groups in the metabolome of both tissues. Univariate analysis revealed more variations in liver than in muscle of fish fed high starch content diet. Variations in metabolome were generally in agreement with the increase in the glycogen synthesis through direct pathway, however, this metabolic shift seemed to be insufficient to keep the growth rate as ensured by the diet with high protein content. Although liver glycogen does not make up a substantial quantity of total stored dietary energy in carnivorous fish, it is a key regulatory intermediate in dietary energy utilization.

Impact of dietary starch on extrahepatic tissue lipid metabolism in farmed European (Dicentrarchus labrax) and Asian seabass (Lates calcarifer)

In aquaculture, there is high interest in substituting marine-derived with vegetable-based ingredients as energy source. Farmed carnivorous fish under high carbohydrate diets tend to increase adiposity but it remains unclear if this happens by increased lipid retention/accumulation, promotion of lipogenic pathways, or both. In order to determine the response of extrahepatic tissue to dietary starch, European (Dicentrarchus labrax) and Asian (Lates calcarifer) seabass were fed a control (low starch; LS) or experimental (high starch; HS) diet, for at least 21 days and then transferred for 6 days to saltwater enriched with deuterated water ²H₂O. Incorporation of ²H-labelling follows well-defined metabolic steps, and analysis of triacylglycerols (TAG) ²H-enrichment by ²HNMR allowed evaluation of de novo lipogenesis (DNL) in muscle and visceral adipose tissue (VAT). Fractional synthetic rates for TAG-bound fatty acids and glycerol were quantified separately providing a detailed lipogenic profile. The FA profile differed substantially between muscle and VAT in both species, but their lipogenic fluxes revealed even greater differences. In European seabass, HS promoted DNL of TAG-bound FA, in muscle and VAT. High ²H-enrichment also found in muscle TAG-bound glycerol was indicative of its role on lipid cycling. In Asian seabass, HS had no effect on muscle FA composition and lipogenic flux, with no ²H-enriched TAG being detected. VAT on the other hand revealed a strong enhancement of DNL in HS-fed fish along with high TAG-bound glycerol cycling. This study consolidated the use of ²H₂O as tracer for fish lipid metabolism in different tissues, under different dietary conditions and suitable to use in different fish models.

Sources of hepatic glycogen synthesis in mice fed with glucose or fructose as the sole dietary carbohydrate

PURPOSE

The positional analysis of hepatic glycogen enrichment from deuterated water (² H₂ O) by ² H NMR has been applied previously to resolve the contributions of glucose and fructose to glycogen synthesis in rodents fed a high sucrose diet. To further validate this method, this analysis was applied to mice fed with synthetic diets whose carbohydrate components consisted solely of either glucose or fructose.

METHODS

Eight glucose-fed and 12 fructose-fed mice were given ² H₂ O followed by ad libitum feeding overnight. Mice were then euthanized, hepatic glycogen was isolated and derivatized to monoacetone glucose, and ² H-enrichment of positions 2, 5, and 6_S were measured by ² H NMR. From these data, the fraction of overnight glycogen appearance from the direct pathway and/or glycogen cycling and indirect pathway were estimated. Indirect pathway fractions were resolved into Krebs cycle and triose-phosphate sources-the latter including contributions from fructose metabolism.

RESULTS

After overnight feeding, the fraction of overnight glycogen appearance derived from direct pathway and/or glycogen cycling in glucose-fed-mice was 63 ± 1%. For the indirect pathway, Krebs cycle and triose-phosphate sources contributed 22 ± 1% and 15 ± 1%, respectively. For fructose-fed-mice, glycogen appearance was dominated by triose-phosphate sources (60 ± 2%) with lesser contributions from Krebs cycle (14 ± 1%) and direct and/or glycogen cycling (26 ± 2%).

CONCLUSIONS

² H NMR analysis of hepatic glycogen ² H enrichment from ² H₂ O provides realistic profiles of dietary glucose and fructose contributions to hepatic glycogen synthesis in mice fed with diets containing 1 or the other sugar as the sole carbohydrate source.

Response to dietary carbohydrates in European seabass (Dicentrarchus labrax) muscle tissue as revealed by NMR-based metabolomics

INTRODUCTION

Feed optimization is a key step to the environmental and economic sustainability of aquaculture, especially for carnivorous species. Plant-derived ingredients can contribute to reduce costs and nitrogenous effluents while sparing wild fish stocks. However, the metabolic use of carbohydrates from vegetable sources by carnivorous fish is still not completely understood.

OBJECTIVES

We aimed to study the effects of diets with carbohydrates of different digestibilities, gelatinized starch (DS) and raw starch (RS), in the muscle metabolome of European seabass (Dicentrarchus labrax).

METHODS

We followed an NMR-metabolomics approach, using two sample preparation procedures, the intact muscle (HRMAS) and the aqueous muscle extracts (¹H NMR), to compare the variations in muscle metabolome between the two diets.

RESULTS

In muscle, multivariate analysis revealed similar metabolome shifts for DS and RS diets, when compared with the control diet. HRMAS of intact muscle, which included both hydrophobic and hydrophilic metabolites, showed increased lipid in DS-fed fish by univariate analysis. Regardless of the nature of the starch, increased glycine and phenylalanine, and decreased proline were observed when compared to the Ctr diet. Combined univariate analysis of intact muscle and aqueous extracts indicated specific diet related changes in lipid and amino acid metabolism, consistent with increased dietary carbohydrate supplementation.

CONCLUSIONS

Due to differential sample processing, outputs differ in detail but provide complementary information. After tracing nutritional alterations by profiling fillet components, DS seems to be the most promising alternative to fishmeal-based diets in aquaculture. This approach should be reproducible for other farmed fish species and provide valuable information on nutritional and organoleptic properties of the final product.