Synthesis of muscle glycogen in man after glucose and fructose infusion

Quantitation of muscle glycogen synthesis in normal subjects and subjects with non-insulin-dependent diabetes by 13C nuclear magnetic resonance spectroscopy

To examine the extent to which the defect in insulin action in subjects with non-insulin-dependent diabetes mellitus (NIDDM) can be accounted for by impairment of muscle glycogen synthesis, we performed combined hyperglycemic-hyperinsulinemic clamp studies with [13C]glucose in five subjects with NIDDM and in six age- and weight-matched healthy subjects. The rate of incorporation of intravenously infused [1-13C]glucose into muscle glycogen was measured directly in the gastrocnemius muscle by means of a nuclear magnetic resonance (NMR) spectrometer with a 15.5-minute time resolution and a 13C surface coil. The steady-state plasma concentrations of insulin (approximately 400 pmol per liter) and glucose (approximately 10 mmol per liter) were similar in both study groups. The mean (+/- SE) rate of glycogen synthesis, as determined by 13C NMR, was 78 +/- 28 and 183 +/- 39 mumol-glucosyl units per kilogram of muscle tissue (wet weight) per minute in the diabetic and normal subjects, respectively (P less than 0.05). The mean glucose uptake was markedly reduced in the diabetic (30 +/- 4 mumol per kilogram per minute) as compared with the normal subjects (51 +/- 3 mumol per kilogram per minute; P less than 0.005). The mean rate of nonoxidative glucose metabolism was 22 +/- 4 mumol per kilogram per minute in the diabetic subjects and 42 +/- 4 mumol per kilogram per minute in the normal subjects (P less than 0.005). When these rates are extrapolated to apply to the whole body, the synthesis of muscle glycogen would account for most of the total-body glucose uptake and all of the nonoxidative glucose metabolism in both normal and diabetic subjects. We conclude that muscle glycogen synthesis is the principal pathway of glucose disposal in both normal and diabetic subjects and that defects in muscle glycogen synthesis have a dominant role in the insulin resistance that occurs in persons with NIDDM.

Post-exercise glucose uptake and glycogen synthesis in human muscle during oral or i.v. glucose intake

Post-exercise muscle uptake and the intracellular fate of glucose was studied after oral or intravenous glucose administrations which caused similar plasma glucose concentrations, but high and moderate plasma insulin concentrations, respectively. Five male subjects participated in two experiments with 6-16 weeks in between. In the first experiment, the oral glucose experiment (OG), 1.4, 0.7 and 0.7 g.kg-1 body mass of glucose was given as oral loads at 0, 1 and 2 h of a 3-h post-exercise observation period (3hOP). In the second experiment, the glucose clamp experiment (GC), a glucose infusion clamp technique was employed. Based on repetitive, immediate plasma glucose measurements performed every 5th min, the rate of glucose infusion was adjusted to obtain the same temporal pattern of the plasma glucose concentration as in OG. The average plasma glucose concentrations during 3hOP were 9.2 +/- 1.1 and 9.3 +/- 1.2 mmol.1(-1) in OG and GC, respectively. The average arterio-femoral venous (a-v)f glucose differences were 1.0 +/- 0.3 and 0.5 +/- 0.2 mmol.1(-1) (p less than 0.001), while the average plasma insulin concentrations were 56 +/- 12 and 26 +/- 5 microU.ml-1 (p less than 0.001) for the two experiments. Increases in muscle glycogen concentrations were 28 +/- 4 and 25 +/- 3 mmol.kg-1 (NS) during 3hOP in OG and GC, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Absence of lactate accumulation on transurethral resection of the prostate using 2.5% sorbitol solution as an irrigating fluid

19 patients with benign hyperplasia of the prostate were studied. During transurethral resection of the prostate, 2.5% sorbitol solution was used as an irrigating fluid. Blood samples were taken preoperatively, immediately postoperatively, and 60 and 120 min postoperatively. Samples were analysed for sorbitol metabolites (fructose, glucose, lactate and pyruvate) and inorganic phosphate. The series was divided into two groups, one with low absorption and one with high absorption of irrigating fluid. The limit for the plasma sorbitol concentration immediately postoperatively, dividing the groups, was 1.0 mmol/l, corresponding to an absorbed fluid volume of about 0.1 l. There was a slight increase in lactate and a significant decrease in pyruvate in the blood in both groups postoperatively. Blood fructose was zero in the group with low absorption of irrigating fluid whereas there was a slight increase in the group with high absorption, with a maximum of 0.53 mmol fructose/l. Blood glucose did not show any significant changes postoperatively. Inorganic phosphate in serum showed a significant decrease postoperatively in both groups. There were no significant differences between the groups at the various postoperative sampling times with regard to lactate, pyruvate, glucose or inorganic phosphate in the blood. Thus, we did not observe any accumulation of lactate in the blood when using 2.5% sorbitol solution as an irrigating fluid with absorbed fluid volumes up to 1 litre (corresponding to 25 g sorbitol).

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.

Comparison of thermogenic effect of fructose and glucose in normal humans

After nutrient ingestion there is an increase in energy expenditure that has been referred to as dietary-induced thermogenesis. In the present study we have employed indirect calorimetry to compare the increment in energy expenditure after the ingestion of 75 g of glucose or fructose in 17 healthy volunteers. During the 4 h after glucose ingestion the plasma insulin concentration increased by 33 +/- 4 microU/ml and this was associated with a significant increase in carbohydrate oxidation and decrement in lipid oxidation. Energy expenditure increased by 0.08 +/- 0.01 kcal/min. When fructose was ingested, the plasma insulin concentration increased by only 8 +/- 2 microU/ml vs. glucose. Nonetheless, the increments in carbohydrate oxidation and decrement in lipid oxidation were significantly greater than with glucose. The increment in energy expenditure was also greater with fructose. When the mean increment in plasma insulin concentration after fructose was reproduced using the insulin clamp technique, the increase in carbohydrate oxidation and decrement in lipid oxidation were markedly reduced compared with the fructose-ingestion study; energy expenditure failed to increase above basal levels. To examine the role of the adrenergic nervous system in fructose-induced thermogenesis, fructose ingestion was also performed during beta-adrenergic blockade with propranolol. The increase in energy expenditure during fructose plus propranolol was lower than with fructose ingestion alone. These results indicate that the stimulation of thermogenesis after carbohydrate ingestion is related to an augmentation of cellular metabolism and is not dependent on an increase in the plasma insulin concentration per se.(ABSTRACT TRUNCATED AT 250 WORDS)

Which carbohydrates should we recommend for intravenous nutrition?

In a short review solutions of glucose, fructose and invertose have been compared in regard to protein sparing effects, side effects and infusion rates. It is concluded that fructose alone offers no advantage to glucose neither in the normal nor in the stressed organism. Compared with pure glucose and fructose solutions, higher amounts of carbohydrate can be administered as invertose, without increasing the risk of side effects.

Effect of beta and alpha adrenergic blockade on glucose-induced thermogenesis in man

After intravenous glucose/insulin infusion there is an increase in oxygen consumption and energy expenditure that has been referred to as thermogenesis. To examine the contribution of the beta and alpha adrenergic nervous system to this thermogenic response, 12 healthy volunteers participated in three studies: (a) euglycemic insulin (plasma insulin approximately 100 microunits/ml) clamp study (n = 12); (b) insulin clamp study after beta adrenergic blockade with intravenous propranolol for 1 h (n = 12); (c) insulin clamp study after alpha adrenergic blockade with phentolamine for 1 h (n = 5). During the control insulin clamp study total glucose uptake, glucose oxidation and nonoxidative glucose uptake averaged 7.85 +/- 0.47, 2.62 +/- 0.22, and 5.23 +/- 0.51 mg/kg X min. After propranolol infusion, insulin-mediated glucose uptake was significantly reduced, 6.89 +/- 0.41 (P less than 0.02). This decrease was primarily the result of a decrease in glucose oxidation (1.97 +/- 0.19 mg/kg X min, P less than 0.01) without any change in nonoxidative glucose metabolism. Phentolamine administration had no effect on total glucose uptake, glucose oxidation, or nonoxidative glucose disposal. The increments in energy expenditure (0.10 +/- 0.01 vs. 0.03 +/- 0.01 kcal/min) and glucose/insulin-induced thermogenesis (4.9 +/- 0.5 vs. 1.5 +/- 0.5%) were reduced by 70% during the propranolol/insulin clamp study. The increments in energy expenditure (0.12 +/- 0.03 kcal/min) and thermogenesis (5.0 +/- 1.5%) were not affected by phentolamine. These results indicate that activation of the beta adrenergic receptor plays an important role in the insulin/glucose-mediated increase in energy expenditure and thermogenesis. In contrast, the alpha adrenergic receptor does not appear to participate in this response.

Mechanism of sympathetic activation during prolonged physical exercise in dogs. The role of hepatic glucoreceptors

It seems likely that depletion of body carbohydrates may account for the rise in the sympathetic activity during prolonged exercise, since glucose given during or before exercise reduces the increase in plasma catecholamines. The aim of the present study was to find out whether the increase in plasma noradrenaline (NA) in response to exercise can be reduced by 1. increasing of the amount of carbohydrate available for metabolism without producing hyperinsulinemia and 2. by inhibition of afferent activity from hepatic glucoreceptors. The study was performed on dogs which exercised whilst receiving either the intravenous fructose infusion (2.2 mmol/min) or a slow glucose infusion (0.25 mmol/min) which was given either via the portal or a peripheral vein. Fructose infusion reduced the muscle glycogen depletion during exercise and reduced the increase in plasma NA and glycerol concentrations without altering the blood glucose or insulin levels. The exercise-induced increases in plasma NA and glycerol concentrations were significantly smaller with intraportal than with peripheral glucose infusion but there were no differences between these two cases in the concentration of glucose in the systemic circulation. These findings indicate that the reduction of the plasma NA response to physical effort under conditions of increased carbohydrate availability cannot be attributed to the inhibitory effect of insulin on sympathetic activity and provide evidence for the participation of hepatic glucoreceptors in the control of the sympathetic activity during exercise.

Effect of intravenous nutrition, with glucose as the only calorie source, on muscle glycogen

This study was undertaken to assess the clinical significance of changes in muscle glycogen in the calculation of the energy requirements of patients fed intravenously. The glycogen content of the vastus or rectus muscle of 27 ill surgical patients with serious complications of the gastrointestinal tract was determined before and after a course of intravenous nutrition with various quantities of glucose as the caloric source. The quantity of glycogen in muscle before intravenous nutrition for all patients was 37 +/- 23 mg/g dry muscle; this was increased significantly (p less than .01) to 69 +/- 44 mg/g dry muscle after 14 days of feeding. In 5 patients who received intravenous nutrition for another 2 weeks, the glycogen content of the vastus muscle increased from a mean value of 80 mg/g dry muscle on day 14 of feeding to 106 mg/g on day 28, but this was not significant. The administration of glucose is invariably associated with an increase in the stores of muscle glycogen but the calorie equivalent of this gain in glycogen is small and, by our calculations, is no more than 530 kcal over the 14 days. We suggest that this gain is small relative to the energy expenditure of this type of patient and is of little clinical significance in the determination of their calorie requirements.

[Fructose tolerance and utilization in healthy and chronically diseased liver]

In 12 patients with liver cirrhosis (LC) and 11 normal subjects (N) equimolar (0.75 g/kg/h for 4 hrs.) 14C-(1)-glucose and 14C-(1)-fructose infusions were administered. When given fructose, N and LC showed only a small increase of serum-glucose and -fructose concentrations in steady state. In N and LC the total fructose clearance was significantly higher than the total glucose clearance. No differences were found between N and LC. The metabolic clearance was equal in both groups when given fructose, whereas during glucose infusion lower in LC than in N. 30% of the infused 14C-fructose were recovered in 14C-glucose. The renal loss in both groups was higher for glucose than for fructose. A more extensive renal 14C-excretion during infusion of 14C-fructose was due to a higher 14C-lactate excretion.

Metabolic substrates, muscle fibre composition and fibre size in late walking and normal children

In 10 children with delayed walking, muscle specimens were obtained by needle biopsy from the vastus lateralis muscle for determination of ATP, creatine phosphate (CP), glycogen and lactate concentration, as well as fibre composition and fibre size. The values obtained are compared with corresponding results for a control group of 25 children aged 2 months--11 years. Details of the control series are given and its validity as a reference group is discussed. Six children (group I) who showed minor gross motor delay but otherwise normal development at follow-up were found to have lowered ATP, CP and glycogen concentrations, compared to 4 children (group II) who had general developmental delay, and to the control group. The children in group I were shown to have smaller fibre size and a greater difference between type 1 and 2 fibres than the children in group II and in the control group. No differences in the concentrations of metabolic substrates or the morphometric values could be detected between group II and the control group.

The plasma clearance of fructose and glucose during and after surgical operation

Fasting blood glucose is elevated and the rate of disappearance of a glucose load is reduced after major surgery. Resistance to insulin is considered to play a part in post-traumatic glucose intolerance. Fructose metabolism is partly independent of insulin. Glucose and fructose clearance were compared in two groups of 6 matched male patients with normal glucose tolerance who were studied before and after major vascular surgical operations of the same severity. Fructose or glucose (25 g) was given intravenously over a 2-min period before, during and at intervals for 8 days after surgery. The rate of clearance of fructose increased significantly during operation (P less than 0.01), but returned to the preoperative level by the first postoperative day. Glucose clearance, in contrast, was reduced during and throughout the 8 days of the study. The fructose load produced a brisk insulin response before operation which was diminished and delayed during surgery. These findings suggest that administered fructose may be removed more rapidly than glucose during and immediately after surgical operation.

Cyclic AMP-mediated activation of hepatic glycogenolysis by fructose

Isolated livers from fed and fasted rats were perfused for 30 min with recirculating blood-buffer medium containing no added substrate and then switched to a flow-through perfusion using the same medium for an additional 5, 10 and 30 min. Continuous infusion of fructose for the final 5, 10 or 30 min resulted in activation of glycogen phosphorylase, an increase in the activity of protein kinase, elevated levels of tissue adenosine 3', 5'-monophosphate (cyclic AMP), and no consistent effect on glycogen synthase. Infusion of glucose under the same conditions resulted in activation of glycogen synthase, inactivation of glycogen phosphorylase, no change in protein kinase, and no consistent change in tissue cyclic AMP. These results demonstrate that while glucose promotes hepatic glycogen synthesis, fructose promotes activation of the enzymatic cascade responsible for glycogen breakdown.

Effects of intravenously administered fructose and glucose on splanchnic amino acid and carbohydrate metabolism in hypertriglyceridemic men

Splanchnic metabolism was studied in the fed state during prolonged intravenous administration (30 g/h) of either fructose or glucose to hypertriglyceridemic men who had been maintained on a high-carbohydrate diet for 2 wk. Splanchnic exchange of amino acids and carbohydrates was quantified by measurement of splanchnic flow and of blood or plasma arteriohepatic venous concentration gradients. Results obtained in subjects receiving fructose were compared with those obtained in (a) similar subjects receiving glucose and (b) postabsorptive controls maintained on isocaloric, balanced diets. Mean arterial plasma levels of alanine, glycine, serine, threonine, methionine, proline, valine, leucine, histidine, lysine, and ornithine were significantly higher in subjects given fructose than in those give glucose (P less than 0.05). The mean arterial concentration and splanchnic uptake of alanine were significantly higher in subjects given fructose than in postabsorptive controls, despite a significantly lower fractional extraction of alanine in the former (P less than 0.05). The mean arterial plasma levels of serine and ornithine were significantly lower in subjects receiving fructose than in postabsorptive controls (P less than 0.05). About half of the administered fructose or glucose was taken up in the splanchnic region, where approximately 15% was converted to CO2 and 10% to lactate. Half of the fructose taken up in the splanchnic region was converted to glucose released from the liver. The amount of hexose carbon remaining for hepatic synthesis of liquids in subjects given fructose was less than half of that of subjects given glucose. These studies demonstrate that fructose and glucose have divergent effects on amino acid metabolism and that during hypercaloric infusion of glucose (as with fructose), the human liver is a major site of lactate production.

Fructose in medicine. A review with particular reference to diabetes mellitus

A review is given of the metabolism of fructose in the mammalian organism, and its significance in medicine. Emphasis is laid upon the absorption and assimilation of fructose through pathways not identical with those of glucose. The metabolism of fructose is largely insulin-independent, although the ultimate fate of fructose carbons is determined by the presence or the absence of insulin.

Clinical and experimental work has suggested that fructose may exert beneficial effects as a component of the diet for patients with mild and well-balanced diabetes. Fructose is absorbed slowly from the gut, and does not induce drastic changes in blood sugar levels. Secondly, fructose is metabolized by insulin-independent pathways in the liver, intestinal wall, kidney and adipose tissue.

As a consequence of the rapid and efficient utilization of fructose, it has been used widely for intravenous feeding in medicine and surgery. However, it has been shown that the rapid infusion of large amounts of fruetose may cause accumulation of lactic acid in the extracellular fluid. The possibility of lactate acidosis, with concomitant impairment of the acid-base balance, already disturbed, constitutes a relative contraindication to the use of intravenous fructose in the treatment of diabetic ketoacidosis.

Fructose is known to accelerate ethanol metabolism in the liver. No well-documented reports on the use of fructose in the treatment of ethanol intoxication have been published, although it has recently been suggested that fructose might be of value in the treatment of delirium tremens.

Fructose may be less cariogenic than sucrose, at least in short-term experiments. Long-term trials are lacking, and thus the potential advantages of fructose in preventive odontology have not been determined.

Fructose does not seem to have any side-effects when used in reasonable amounts. However, it has been reported that the administration of fructose in large amounts induces hyperlipidemia both in man and in experimental animals. Earlier suggestions concerned with the atherogenic properties of fructose have recently been challenged. The apparent increase in the incidence of coronary disease among sucrose users seems to be a statistical artefact, caused by the increased ingestion of coffee and soft drinks by cigarette smokers.

Insulin and corticoid response to intravenous fructose in relation to glucose tolerance

In 12 subjects with normal glucose tolerance fructose infusion was associated with a rise in plasma insulin and plasma corticoid levels. Similar but lesser changes were seen in seven maturity onset diabetics in whom there was also a considerable rise in blood glucose. It is suggested that a catabolite of fructose is responsible for these changes.