Metabolism and oxidation of U- 14 C-glucose, xylitol, fructose and sorbitol in the fasted and in the streptozotocin-diabetic rat

Hepatic metabolomics and transcriptomics to study susceptibility to ketosis in response to prepartal nutritional management

Background

Ketosis in dairy cows is associated with body fat mobilization during the peripartal period. Sub-clinical and clinical ketosis arise more frequently in cows that are overfed energy during the entire dry (last 50 to 45 days prior to parturition) or close-up period (last ~ 28 days prepartum).

Methods

A retrospective analysis was performed on 12 cows from a larger cohort that were fed a higher-energy diet [1.54 Mcal/kg of dry matter (DM); 35.9% of DM corn silage and 13% of DM ground corn] during the close-up dry period, of which 6 did not develop clinical ketosis (OVE, 0.83 mmol/L plasma hydroxybutyrate; BHB) and 6 were diagnosed with clinical ketosis (KET, 1.4 mmol/L BHB) during the first week postpartum. A whole-transcriptome bovine microarray (Agilent Technologies) and metabolomics (GC-MS, LC-MS; Metabolon® Inc.) were used to perform transcript and metabolite profiling of liver tissue harvested at − 10 days relative to parturition which allowed to establish potential associations between prepartal transcriptome/metabolome profiles and susceptibility to clinical ketosis postpartum.

Results

Cows in KET had greater (P = 0.01) overall body weight between − 2 and 1 week around parturition, but similar body condition score than OVE. Although dry matter intake (DMI) did not differ prepartum, KET cows had lower (P < 0.01) DMI and similar milk yield as OVE cows during the first week postpartum. Transcriptome analysis revealed a total of 3065 differentially expressed genes (DEG; P ≤ 0.05) in KET. Metabolomics identified 15 out of 313 total biochemical compounds significantly affected (P ≤ 0.10) in KET. Among those, greater concentrations (P ≤ 0.06, + 2.3-fold) of glycochenodeoxycholate in KET cows also have been detected in humans developing non-alcoholic fatty liver disease. Bioinformatics analysis using the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway database and the DEG revealed that, among the top 20 most-impacted metabolic pathway categories in KET, 65% were overall downregulated. Those included ‘Metabolism of cofactors and vitamins’, ‘Biosynthesis of other secondary metabolites’, ‘Lipid’, ‘Carbohydrate’, and ‘Glycan biosynthesis and metabolism’. The lower relative concentration of glucose-6-phosphate and marked downregulation of fructose-1,6-bisphosphatase 2 and pyruvate dehydrogenase kinase 4 support a strong impairment in gluconeogenesis in prepartal liver of cows developing KET postpartum. Among the top 20 most-impacted non-metabolic pathways, 85% were downregulated. Pathways such as ‘mTOR signalling’ and ‘Insulin signalling’ were among those. ‘Ribosome’, ‘Nucleotide excision repair’, and ‘Adherens junctions’ were the only upregulated pathways in cows with KET.

Conclusions

The combined data analyses revealed more extensive alterations of the prepartal liver transcriptome than metabolome in cows overfed energy and developing ketosis postpartum. The causative link between these tissue-level adaptations and onset of clinical ketosis needs to be studied further.

Importance of substrate competition in the mechanism of insulin resistance in man

Carbohydrate (CHO) oxidation induced by a glucose or fructose (0.5 g/kg X h) infusion over two hours was compared for 160 minutes by means of continuous indirect calorimetry in seven normal subjects without or with a concomitant infusion of Intralipid, a neutral fat emulsion. The glucose infusion was accompanied by a rise over basal values in both glucose (99 +/- 10 mg/dL) and insulin (36 +/- 7 microU/mL) plasma levels, with a further rise of both curves during the Intralipid infusion (140 +/- 7 mg/dL and 53 +/- 12 microU/mL). By contrast, plasma glucose and insulin rose only minimally during the fructose infusion (3.5 +/- 2.9 mg/dL and 5.3 +/- 1.4 microU/mL, respectively, without Intralipid, and 10.6 +/- 2.1 mg/dL and 9.6 +/- 2.0 microU/mL with Intralipid). During the two-hour sugar infusion, a mean quantity of 68.7 g glucose or fructose was infused. The total CHO oxidation was 15.6 +/- 1.2 g for glucose and 21.6 +/- 2.6 for fructose infusion for the 160 minutes of the test. During the Intralipid infusion, CHO oxidation was inhibited with values of 5.9 +/- 1.3 g for glucose (P less than .005) and 13.8 +/- 1.8 g (P less than .05) for fructose infusion. Lipid oxidation was increased in both cases during the Intralipid infusion. These results show that the lipid-induced inhibition of CHO oxidation observed with glucose infusion also occurs to some extent with fructose, suggesting that insulin might not be primarily involved. They suggest a metabolic origin for insulin resistance during elevated fat metabolism.

Intestinal absorption of sorbitol and effects of its acute administration on glucose homeostasis in normal rats

Intestinal absorption of sorbitol was studied in a duodeno-jejunal loop of anaesthetized rats. The acute effects of exogenous sorbitol on glucose homeostasis were also evaluated in male and female rats. In the presence of lumen concentrations of sorbitol ranging from 1 microM to 200 mM, a fairly constant low percentage (about 12%) of the loop's contents was absorbed after 30 min. This amount increased only slightly with time, but this was not due to sorbitol accumulation in the mucosal layer of the loop. 3-O-methylglucose was absorbed much more quickly than sorbitol, but did not interfere with sorbitol absorption. The latter was not impaired by omission of lumen sodium ions nor by phloridzin, both of which inhibited 3-O-methylglucose absorption. Gastric administration of sorbitol did not affect plasma glucose or insulin levels. Glucose or sucrose administration caused a similar rise in plasma glucose, but the increase in plasma insulin levels was larger after glucose than after sucrose administration. Intravenous administration of sorbitol slightly increased plasma glucose and insulin levels. These changes were, however, considerably smaller than those occurring after glucose administration. In the normal rat, intestinal absorption of sorbitol is passive and proceeds at a low rate. Acute oral administration of sorbitol does not affect glucose homeostasis, which is only slightly disturbed by a large intravenous load of sorbitol.

Comparison of glucose, fructose, sorbitol, and xylitol utilization in humans during insulin suppression

During fructose, sorbitol, and xylitol perfusions, carbohydrate utilization was studied by continuous indirect calorimetry and compared with glucose utilization during pharmacologic inhibition of endogenous insulin secretion. The experiment was performed in 28 normal volunteers divided into 5 groups (glucose, fructose, sorbitol, xylitol, and saline), each subject being its own control. Insulin suppression was obtained by means of a constant infusion of epinephrine (6 microgram/min) and propranolol (0.08 mg/min). After 90 min, during plasma insulin steady state, each sugar or polyol was infused at a rate of 6 mg/kg/min for 120 min. In contrast with a rise in plasma glucose from 161 +/- 6 mg/dl) to 291 +/- 14 mg/dl during glucose infusion, glucose levels remained unchanged during infusion of the glucose substitutes. Carbohydrate oxidation showed a rise of 24, 65, 76, and 44 mg/min during infusions of glucose, fructose, sorbitol, and xylitol, respectively. Lipid oxidation rates decreased by 7, 20, 33, and 23 mg/min during the same infusions. These results indicate that fructose, sorbitol, and xylitol are oxidized at a higher rate than glucose during suppression of endogenous insulin secretion, without any significant rise in glycemia.

[Comparison of side effects of infusion of glucose and glucose substitutes at different doses]

Glucose, fructose, sorbitol or xylitol were infused for four hours at different dose levels to metabolically healthy volunteers. The metabolic effects of the so-called glucose substitutes were compared to that of glucose. Even at very high doses (2.0 g/kg bodyweight per hour) of infusion of glucose or fructose a steady state was attained. This, however, was not the case with xylitol or sorbitol at lower doses (i.e. 0.5 g/kg bodyweight per hour), where no steady state was reached. The blood glucose concentration is not influenced by any of the glucose substitutes. During infusion of very high doses of fructose a small increase in serum insulin level is found, however, without any alteration in blood glucose concentration. Glucose as well as glucose substitutes cause an immediate suppression of free fatty acid concentrations in serum. In case of glucose there is a manifold increase in fatty acid concentration after the infusion is terminated. On the other hand, the free fatty acid concentration remains low even several hours following termination of the high-dosed fructose infusion. Theoretically one would expect an increase in triglyceride concentration, at least at the high dosed carbohydrate infusions. In contrast to this theoretical expectation, in the case of glucose and of xylitol a significant reduction of triglyceride concentration in serum was observed. Fructose and sorbitol did not exhibit this effect. Glucose and fructose are well utilized in metabolically healthy subjects. The maximum turnover rates for both polyols are lower. Unlike glucose, the glucose substitutes obviously do not cause any serious disturbation in hormonal regulations. Only in the case of glucose, counterregulation is seen following the termination of the infusion.