Impaired glucose tolerance after endurance exercise is associated with reduced insulin secretion rather than altered insulin sensitivity

Long distance running - Can bioprofiling predict success in endurance athletes?

The TransEuropeFootRace (TEFR) was one of the most extreme multistage competitions worldwide. The ultramarathon took the runners over a distance of 4487 km, from Bari, Italy, to the North Cape, Norway, in 64 days. The participating ultra-long-distance runners had to complete almost two marathons per day (~70 km). The race was accompanied by a research team analysing adaptations of different organ systems of the human body that were exposed to a chronic lack of regeneration time. Here, we analyzed runner's urine using mass spectrometric profiling of thousands of low-molecular weight compounds. The results indicated that pre-race molecular factors can predict finishers and separate them from nonfinishers already before the race. These observations were related to the training volume as finishers ran about twice as many kilometers per week before TEFR than nonfinishers, thus apparently achieving a higher performance level and resistance against overuse. While this hypothesis needs to be validated in future long-distance races, the bioprofiling experiments suggest that the competition readiness of the runners is measurable and might be adjustable.

Physiology and Pathophysiology in Ultra-Marathon Running

In this overview, we summarize the findings of the literature with regards to physiology and pathophysiology of ultra-marathon running. The number of ultra-marathon races and the number of official finishers considerably increased in the last decades especially due to the increased number of female and age-group runners. A typical ultra-marathoner is male, married, well-educated, and ~45 years old. Female ultra-marathoners account for ~20% of the total number of finishers. Ultra-marathoners are older and have a larger weekly training volume, but run more slowly during training compared to marathoners. Previous experience (e.g., number of finishes in ultra-marathon races and personal best marathon time) is the most important predictor variable for a successful ultra-marathon performance followed by specific anthropometric (e.g., low body mass index, BMI, and low body fat) and training (e.g., high volume and running speed during training) characteristics. Women are slower than men, but the sex difference in performance decreased in recent years to ~10–20% depending upon the length of the ultra-marathon. The fastest ultra-marathon race times are generally achieved at the age of 35–45 years or older for both women and men, and the age of peak performance increases with increasing race distance or duration. An ultra-marathon leads to an energy deficit resulting in a reduction of both body fat and skeletal muscle mass. An ultra-marathon in combination with other risk factors, such as extreme weather conditions (either heat or cold) or the country where the race is held, can lead to exercise-associated hyponatremia. An ultra-marathon can also lead to changes in biomarkers indicating a pathological process in specific organs or organ systems such as skeletal muscles, heart, liver, kidney, immune and endocrine system. These changes are usually temporary, depending on intensity and duration of the performance, and usually normalize after the race. In longer ultra-marathons, ~50–60% of the participants experience musculoskeletal problems. The most common injuries in ultra-marathoners involve the lower limb, such as the ankle and the knee. An ultra-marathon can lead to an increase in creatine-kinase to values of 100,000–200,000 U/l depending upon the fitness level of the athlete and the length of the race. Furthermore, an ultra-marathon can lead to changes in the heart as shown by changes in cardiac biomarkers, electro- and echocardiography. Ultra-marathoners often suffer from digestive problems and gastrointestinal bleeding after an ultra-marathon is not uncommon. Liver enzymes can also considerably increase during an ultra-marathon. An ultra-marathon often leads to a temporary reduction in renal function. Ultra-marathoners often suffer from upper respiratory infections after an ultra-marathon. Considering the increased number of participants in ultra-marathons, the findings of the present review would have practical applications for a large number of sports scientists and sports medicine practitioners working in this field.

Prior exercise does not alter the incretin response to a subsequent meal in obese women

Prior research has shown an increase in GLP-1 concentrations during exercise but this exercise bout was conducted postprandially. The purpose of this study was to examine the incretin response to a meal following an exercise bout of different intensities in obese subjects. Eleven women (BMI>37.3±7.0kg/m(2); Age 24.3±4.6year) participated in 3 counter- balanced study days, where a standardized meal was preceded by: (1) No exercise (NoEx), (2) ModEx (55% VO2max), and (3) IntEx (4min (80% VO2max)/3min (50% VO2max). Frequent blood samples were analyzed for glucose, lactate, insulin, glucagon, glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP), and C-peptide concentrations throughout 280min of testing. Glucose concentrations were not different between conditions during exercise or meals. There were no differences between conditions in insulin levels during exercise and recovery, but postprandial insulin incremental area under the curve was lower in ModEx vs. NoEx (p<0.01). GIP and GLP-1 levels were not different between conditions during exercise, but during exercise recovery, GLP-1 concentrations were higher in ModEx vs. NoEx (p=0.03). The meal increased the incretin responses (p<0.01) but this response was not affected by prior exercise. Glucagon concentrations increased with exercise (p<0.05) and continued to be elevated during recovery, with the greatest increase with IntEx compared with NoEx (p<0.05). No differences between conditions were detected for hepatic insulin extraction, insulin secretion, or insulin sensitivity. Exercise prior to an evening meal has no impact on the incretin response to the subsequent meal, yet insulin concentrations were lower during the meals that followed exercise. Exercise intensity had no impact on this response.

Impact of exercise and moderate hypoxia on glycemic regulation and substrate oxidation pattern

Objective

We examined metabolic and endocrine responses during rest and exercise in moderate hypoxia over a 7.5 h time courses during daytime.

Methods

Eight sedentary, overweight men (28.6±0.8 kg/m²) completed four experimental trials: a rest trial in normoxia (FiO₂ = 20.9%, NOR-Rest), an exercise trial in normoxia (NOR-Ex), a rest trial in hypoxia (FiO₂ = 15.0%, HYP-Rest), and an exercise trial in hypoxia (HYP-Ex). Experimental trials were performed from 8:00 to 15:30 in an environmental chamber. Blood and respiratory gas samples were collected over 7.5 h. In the exercise trials, subjects performed 30 min of pedaling exercise at 60% of VO₂max at 8:00, 10:30, and 13:00, and rested during the remaining period in each environment. Standard meals were provided at 8:30, 11:00, and 13:30.

Results

The areas under the curves for blood glucose and serum insulin concentrations over 7.5 h did not differ among the four trials. At baseline, %carbohydrate contribution was significantly higher in the hypoxic trials than in the normoxic trials (P<0.05). Although exercise promoted carbohydrate oxidation in the NOR-Ex and HYP-Ex trials, %carbohydrate contribution during each exercise and post-exercise period were significantly higher in the HYP-Ex trial than in the NOR-Ex trial (P<0.05).

Conclusion

Three sessions of 30 min exercise (60% of VO₂max) in moderate hypoxia over 7.5 h did not attenuate postprandial glucose and insulin responses in young, overweight men. However, carbohydrate oxidation was significantly enhanced when the exercise was conducted in moderate hypoxia.

The immediate effects of a single bout of aerobic exercise on oral glucose tolerance across the glucose tolerance continuum

We investigated glucose tolerance and postprandial glucose fluxes immediately after a single bout of aerobic exercise in subjects representing the entire glucose tolerance continuum. Twenty‐four men with normal glucose tolerance (NGT), impaired glucose tolerance (IGT), or type 2 diabetes (T2D; age: 56 ± 1 years; body mass index: 27.8 ± 0.7 kg/m², P > 0.05) underwent a 180‐min oral glucose tolerance test (OGTT) combined with constant intravenous infusion of [6,6‐²H₂]glucose and ingestion of [U‐¹³C]glucose, following 1 h of exercise (50% of peak aerobic power) or rest. In both trials, plasma glucose concentrations and kinetics, insulin, C‐peptide, and glucagon were measured. Rates (mg kg⁻¹ min⁻¹) of glucose appearance from endogenous (R_aEndo) and exogenous (oral glucose; R_aOGTT) sources, and glucose disappearance (R_d) were determined. We found that exercise increased R_aEndo, R_aOGTT, and R_d (all P < 0.0001) in all groups with a tendency for a greater (~20%) peak R_aOGTT value in NGT subjects when compared to IGT and T2D subjects. Accordingly, following exercise, the plasma glucose concentration during the OGTT was increased in NGT subjects (P < 0.05), while unchanged in subjects with IGT and T2D. In conclusion, while a single bout of moderate‐intensity exercise increased the postprandial glucose response in NGT subjects, glucose tolerance following exercise was preserved in the two hyperglycemic groups. Thus, postprandial plasma glucose responses immediately following exercise are dependent on the underlying degree of glycemic control.

This study shows that following an exercise bout, plasma glucose concentrations during an oral glucose tolerance test are increased in subjects with normal glucose tolerance, but unchanged in subjects with impaired glucose tolerance or type 2 diabetes. While rates of glucose disappearance and rates of glucose appearance from endogenous sources and from orally ingested glucose were all increased following exercise, there was a 20% greater peak value for the rate of orally ingested glucose appearance in normal glucose tolerant subjects, when compared to IGT and T2D subjects. In summary, postprandial plasma glucose responses immediately following exercise are dependent on the underlying level of glycemic control.

Breakfast and exercise contingently affect postprandial metabolism and energy balance in physically active males

The present study examined the impact of breakfast and exercise on postprandial metabolism, appetite and macronutrient balance. A sample of twelve (blood variables n 11) physically active males completed four trials in a randomised, crossover design comprising a continued overnight fast followed by: (1) rest without breakfast (FR); (2) exercise without breakfast (FE); (3) breakfast consumption (1859 kJ) followed by rest (BR); (4) breakfast consumption followed by exercise (BE). Exercise was continuous, moderate-intensity running (expending approximately 2·9 MJ of energy). The equivalent time was spent sitting during resting trials. A test drink (1500 kJ) was ingested on all trials followed 90 min later by an ad libitum lunch. The difference between the BR and FR trials in blood glucose time-averaged AUC following test drink consumption approached significance (BR: 4·33 (SEM 0·14) v. FR: 4·75 (SEM 0·16) mmol/l; P=0·08); but it was not different between FR and FE (FE: 4·77 (SEM 0·14) mmol/l; P=0·65); and was greater in BE (BE: 4·97 (SEM 0·13) mmol/l) v. BR (P=0·012). Appetite following the test drink was reduced in BR v. FR (P=0·006) and in BE v. FE (P=0·029). Following lunch, the most positive energy balance was observed in BR and least positive in FE. Regardless of breakfast, acute exercise produced a less positive energy balance following ad libitum lunch consumption. Energy and fat balance is further reduced with breakfast omission. Breakfast improved the overall appetite responses to foods consumed later in the day, but abrogated the appetite-suppressive effect of exercise.

Exercise prior to a freely requested meal modifies pre and postprandial glucose profile, substrate oxidation and sympathovagal balance

Background

The effects of exercise on glucose and metabolic events preceding and following a freely initiated meal have never been assessed. Moreover, the relationship between these events and sympathovagal balance is not known. The objective of this study was to determine whether exercise prior to a freely requested meal modifies the pre- and postprandial glucose profile, substrate oxidation and sympathovagal balance.

Methods

Nine young active male subjects consumed a standard breakfast (2298 ± 357 kJ). After 120 min, they either performed 75 min of exercise on a cycle ergometer (EX - 70% VO_2max) or rested (RT). Lunch was freely requested but eaten ad libitum only during the 1^st session, and then energy intake was fixed across conditions. Glucose and sympathovagal balance were assessed continuously using a subcutaneous glucose monitoring system and analysis of heart rate variability, respectively. Every 5 min, a mean value was calculated for both glucose and sympathovagal balance. Substrate oxidation was determined by calculating the gas exchange ratio when lunch was requested and 180 min after the onset of eating.

Results

Preprandial glucose profiles were found in 72% of the sessions and with a similar frequency under both conditions. Meals were requested after a similar delay (40 ± 12 and 54 ± 10 min in EX and RT respectively; ns). At meal request, sympathovagal balance was not different between conditions but CHO oxidation was lower and fat oxidation higher in EX than in RT (-46% and +63%, respectively; both p < 0.05). Glucose responses to the meal were higher in incremental (+ 48%) but not in absolute value in EX than in RT, with a higher fat oxidation (+ 46%, p < 0.05), and a greater vagal withdrawal (+ 15%, p < 0.05).

Conclusions

These results show that exercise does not impair preprandial glucose declines at the following meal freely requested, but leads to an increased postprandial glucose response and an elevated fat oxidation, an effect that vagal withdrawal may contribute to explain.

The minimal model of glucose disposal in the analysis of glucose effectiveness: importance of early insulin data

BACKGROUND

Glucose effectiveness (S(g)) is an important component in glucose tolerance. Values of S(g) using "open loop" glucose kinetic computer programs are usually higher compared to closed loop method (CLM) programs that incorporate insulin secretion modeling. We aimed to test whether these differences are caused by (1) inclusion of insulin secretion modeling or (2) the method of representing plasma insulin values in the first few minutes of the frequently sampled intravenous glucose tolerance test (FSIGT).

METHODS

FSIGTs without insulin supplementation were performed in six healthy volunteers, and the Bergman minimal model was fitted to the data using the simulation and modeling program SAAM.

RESULTS

The CLM, which represents the insulin data in the first few minutes by a best-fit curve extrapolated to the y-axis, yielded a significantly lower S(g) than the approach similar to the computer program MINMOD, where the first few minutes of insulin data are represented by a line joining the basal to the peak values (1.55 +/- 0.28 vs. 1.97 +/- 0.27 [SE] x 10(-2)/min, P < 0.05). This second analysis was then repeated while forcing the program to represent the insulin data after the insulin peak in the same way as in the CLM, obtaining an almost identical result for S(g) (1.99 +/- 0.29). Insulin sensitivity was not significantly affected.

CONCLUSIONS

The higher S(g) estimates are caused by the method of representing the first few minutes of insulin data rather than by the incorporation of insulin secretion modeling. It is, therefore, important to know how the early insulin data are represented when comparing results from different computer modeling programs.

The gastroenteroinsular response to glucose ingestion during postexercise recovery

This study examined gastrointestinal hormone and peptide responses when glucose was ingested after prolonged exercise. Six endurance-trained male athletes ran on a treadmill for 2 h at 60% VO2 max. Immediately after the run, the athletes consumed 75 g of glucose in 250 ml of water (ExGLU) or flavored water as a placebo control (ExPL). On a separate visit, the athletes rested for 2 h and then consumed glucose (ConGLU). During the first 60 min of recovery from exercise alone (ExPL), plasma vasoactive intestinal peptide (VIP), gastrin, and glucagon-like peptide-1 (GLP-1) all increased significantly, whereas glucose, insulin, and gastric inhibitory polypeptide (GIP) were unchanged from the immediate postexercise value. When glucose was ingested after exercise (ExGLU), glucose, insulin, VIP, gastrin, GLP-1, and GIP were all increased (P < 0.01). However, when glucose was ingested after resting for 2 h (ConGLU), VIP levels were unaffected, although glucose, insulin, gastrin, GLP-1, and GIP levels increased (P < 0.05). The plasma glucose response was greater (P < 0.03) and the plasma insulin response lower (P < 0.004) during ExGLU compared with ConGLU. There was a significantly higher (P < 0.01) VIP response during the initial period of recovery in ExGLU than there was with both ExPL and ConGLU. Plasma VIP showed a modest negative correlation with circulating glucose (r = -0.35, P < 0.03) and insulin (r = -0.37, P < 0.03) during the ExGLU recovery period. In summary, when glucose is ingested after prolonged exercise, there is mild insulin resistance and a corresponding rapid transitory increase in plasma VIP. These data suggest that VIP may play an important glucoregulatory role when glucose is ingested during the immediate postexercise recovery period.

Effect of moderate exercise training on peripheral glucose effectiveness, insulin sensitivity, and endogenous glucose production in healthy humans estimated by a two-compartment-labeled minimal model

For examining the effects of moderate exercise training on peripheral glucose effectiveness (S(g)(2)*), insulin sensitivity (S(i)(2)*), and endogenous glucose production (EGP), seven men and one woman (24.8 +/- 1.8 years) participated in cycle ergometer training at lactate threshold intensity for 60 min/day, 5 days/week for 12 weeks. Stable-labeled frequently sampled intravenous glucose tolerance tests were performed before and 16 h and 1 week after the last training session. S(g)(2)* (pre 0.71 +/- 0.03 x 10(-2), 16 h 0.85 +/- 0.02 x 10(-2) dl. kg(-1). min(-1)) and S(i)(2)* (pre 12.6 +/- 2.6 x 10(-4), 16 h 19.7 +/- 3.3 x 10(-4) dl. kg(-1). min(-1). [ micro U/ml](-1)), analyzed using the two-compartment minimal model, were significantly elevated 16 h after the last training session. The elevated S(g)(2)* remained higher despite the cessation of exercise training for 1 week (1.00 +/- 0.03 x 10(-2) dl. kg(-1). min(-1)). EGP was suppressed within 20 min after glucose bolus, and the suppression of EGP was followed by their overshoot. The time course of EGP during the intravenous glucose tolerance test remained similar after the training period. In conclusion, moderate exercise training at lactate threshold improves not only peripheral insulin sensitivity but also peripheral glucose effectiveness with no change in the effect of glucose and/or insulin to suppress EGP in healthy humans.

Effect of mild exercise training on glucose effectiveness in healthy men

OBJECTIVE

To detect whether mild exercise training improves glucose effectiveness (S(G)), which is the ability of hyperglycemia to promote glucose disposal at basal insulin, in healthy men.

RESEARCH DESIGN AND METHODS

Eight healthy men (18-25 years of age) underwent ergometer training at lactate threshold (LT) intensity for 60 min/day for 5 days/week for 6 weeks. An insulin-modified intravenous glucose tolerance test was performed before as well as at 16 h and 1 week after the last training session. S(G) and insulin sensitivity (S(I)) were estimated using a minimal-model approach.

RESULTS

After the exercise training, VO(2max) and VO(2) at LT increased by 5 and 34%, respectively (P < 0.05). The mild exercise training improves S(G) measured 16 h after the last training session, from 0.018 +/- 0.002 to 0.024 +/- 0.001 min(-1) (P < 0.05). The elevated S(G) after exercise training tends to be maintained regardless of detraining for 1 week (0.023 +/- 0.002 min(-1), P = 0.09). S(I) measured at 16 h after the last training session significantly increased (pre-exercise training, 13.9 +/- 2.2; 16 h, 18.3 +/- 2.4, x10(-5). min(-1). pmol/l(-1), P < 0.05) and still remained elevated 1 week after stopping the training regimen (18.6 +/- 2.2, x10(-5). min(-1). pmol/l(-1), P < 0.05).

CONCLUSIONS

Mild exercise training at LT improves S(G) in healthy men with no change in the body composition. Improving not only S(I) but also S(G) through mild exercise training is thus considered to be an effective method for preventing glucose intolerance.

Insulin and exercise differentially regulate PI3-kinase and glycogen synthase in human skeletal muscle

The purpose of this study was to determine the separate and combined effects of exercise and insulin on the activation of phosphatidylinositol 3-kinase (PI3-kinase) and glycogen synthase in human skeletal muscle in vivo. Seven healthy men performed three trials in random order. The trials included 1) ingestion of 2 g/kg body wt carbohydrate in a 10% solution (CHO); 2) 75 min of semirecumbent cycling exercise at 75% of peak O(2) consumption; followed by 5 x 1-min maximal sprints (Ex); and 3) Ex, immediately followed by ingestion of the carbohydrate solution (ExCHO). Plasma glucose and insulin were increased (P < 0.05) at 15 and 30 (Post-15 and Post-30) min after the trial during CHO and ExCHO, although insulin was lower for ExCHO. Hyperinsulinemia during recovery in CHO and ExCHO led to an increase (P < 0.001) in PI3-kinase activity at Post-30 compared with basal, although the increase was lower (P < 0. 004) for ExCHO. Furthermore, PI3-kinase activity was suppressed (P < 0.02) immediately after exercise (Post-0) during Ex and ExCHO. Area under the insulin response curve for all trials was positively associated with PI3-kinase activity (r = 0.66, P < 0.001). Glycogen synthase activity did not increase during CHO but was increased (P < 0.05) at Post-0 and Post-30 during Ex and ExCHO. Ingestion of the drink increased (P < 0.05) carbohydrate oxidation during CHO and ExCHO, although the increase after ExCHO was lower (P < 0.05) than CHO. Carbohydrate oxidation was directly correlated with PI3-kinase activity for all trials (r = 0.63, P < 0.001). In conclusion, under resting conditions, ingestion of a carbohydrate solution led to activation of the PI3-kinase pathway and oxidation of the carbohydrate. However, when carbohydrate was ingested after intense exercise, the PI3-kinase response was attenuated and glycogen synthase activity was augmented, thus facilitating nonoxidative metabolism or storage of the carbohydrate. Activation of glycogen synthase was independent of PI3-kinase.

Influence of mild exercise at the lactate threshold on glucose effectiveness

The effect of a single bout of mild exercise on glucose effectiveness (S(G)) and insulin sensitivity (S(I)) was studied in six young male subjects by using a minimal model. An intravenous glucose tolerance test was performed under two conditions as follows: 1) 25 min after a bout of exercise on a cycle ergometer at the lactate threshold level for 60 min (Ex) and 2) without any prior exercise (Con). Leg blood flow (LBF) was also measured by strain-gauge plethysmography simultaneously with blood sampling. S(I) did not significantly change after exercise (18.1 +/- 1.5 vs. 17.7 +/- 1.9 x 10-(5) min/pM), whereas S(G) significantly increased (0.016 +/- 0.002 vs. 0.025 +/- 0.002 min(-1), P < 0.01). The increased blood flow after exercise remained high during the time period for measurement of the glucose disappearance constant and may be a determinant of S(G). The incremental lactate area under the curve until insulin loading was also significantly higher in Ex than in Con (2.6 +/- 0.9 vs. -3.5 +/- 1.5 mM/min, P < 0.05). These results suggest that increased S(G) after mild exercise may be due, at least in part, to increased LBF and lactate production under a hyperglycemic state.

An overview of muscle glucose uptake during exercise. Sites of regulation

The uptake of blood glucose by skeletal muscle is a complex process. In order to be metabolized, glucose must travel the path from blood to interstitium to intracellular space and then be phosphorylated to glucose 6-phosphate (G6P). Movement of glucose from blood to interstitium is determined by skeletal muscle blood flow, capillary recruitment and the endothelial permeability to glucose. The influx of glucose from the interstitium to intracellular space is determined by the number of glucose transporters in the sarcolemma and the glucose gradient across the sarcolemma. The capacity to phosphorylate glucose is determined by the amount of skeletal muscle hexokinase II, hexokinase II compartmentalization within the cell, and the concentration of the hexokinase II inhibitor G6P. Any change in glucose uptake occurs due to an alteration in one or more of these steps. Based on the low calculated intracellular glucose levels and the higher affinity of glucose for phosphorylation relative to transport, glucose transport is generally considered rate-determining for basal muscle glucose uptake. Exercise increases both the movement of glucose from blood to sarcolemma and the permeability of the sarcolemma to glucose. Whether the ability to phosphorylate glucose is increased in the working muscle remains to be clearly shown. It is possible that the accelerated glucose delivery and transport rates during exercise bias regulation so that muscle glucose phosphorylation exerts more control on muscle glucose uptake. Conditions that alter glucose uptake during exercise, such as increased NEFA concentrations, decreased oxygen availability and adrenergic stimulation, must work by altering one or more of the three steps involved in glucose uptake. This review describes the regulation of glucose uptake during exercise at each of these sites under a number of conditions, as well as describing muscle glucose uptake in the post-exercise state.

The effect of norepinephrine on insulin secretion and glucose effectiveness in non-insulin-dependent diabetes

It has previously been shown that in normal subjects, physiological elevation of norepinephrine (NE) impairs insulin sensitivity (Si) but does not influence insulin secretion. The aim of this study was to determine the effect of short-term physiological elevation of NE on insulin secretion, Si, and glucose-mediated glucose disposal, or the glucose effectiveness index (Sg), in non-insulin-dependent diabetes mellitus (NIDDM). Two intravenous glucose tolerance tests (IVGTTs) were performed in eight well-controlled NIDDM patients, using a supplemental exogenous insulin infusion to achieve an approximation of normal endogenous insulin secretion. The IVGTTs were performed in random order after 30 minutes of either the saline (SAL) or NE (25 ng/kg/min) infusions, which were continued throughout the 3-hour IVGTT. Sg and Si were estimated by minimal model analysis of the IVGTT data as previously described. Plasma C-peptide was used to estimate insulin secretion rate using the ISEC program. NE infusion produced approximately a threefold increase in plasma NE, associated with (1) a significant reduction in glucose disposal ([KG] SAL v NE, 0.73 +/- 0.06 v 0.61 +/- 0.06 x 10(-2).min-1, P < .05), (2) no reduction in Si (2.33 +/- 0.8 v 2.62 +/- 0.9 x 10(-4).min-1/mU/L, NS), (3) a reduced mean second-phase insulin secretion rate (1.21 +/- 0.19 v 1.01 +/- 0.16 x 10(-3) pmol/kg/min per mmol/L glucose, P < .05), (4) a significant increase in Sg (0.89 +/- 0.08 v 1.63 +/- 0.2 x 10(-2).min-1, P < .05), and (5) a corresponding increase in glucose effectiveness at zero insulin ([GEZI] 0.55 +/- 0.13 v 1.30 +/- 0.33 x 10(-2).min-1, P < .05). These results show that in contrast to normal subjects, physiological elevation of NE in NIDDM does not result in a reduction in Si, but causes a reduction in glucose disposal related to inhibition of insulin secretion that is only partially compensated for by increased Sg.

Influence of moderate physical exercise on insulin-mediated and non-insulin-mediated glucose uptake in healthy subjects

To establish the relative importance of insulin sensitivity and glucose effectiveness during exercise using Bergman's minimal model, 12 nontrained healthy subjects were studied at rest and during 95 minutes of moderate exercise (50% maximum oxygen consumption [VO2max]). Each subject underwent two frequently sampled intravenous glucose tolerance tests (FSIGTs) for 90 minutes, at rest (FSIGTr) and during exercise (FSIGTe). Plasma glucose, insulin, and C-peptide were determined. Insulin sensitivity (S(I)), glucose effectiveness at basal insulin (S(G)), insulin action [X(t)], and first-phase (phi1) and second-phase (phi2) beta-cell responsiveness to glucose were estimated using both minimal models of glucose disposal (MMg) and insulin kinetics (MMi). Glucose effectiveness at zero insulin (GEZI), glucose tolerance index (K(G)), and the area under the insulin curve (AUC(0-90)) were also calculated. Intravenous glucose tolerance improved significantly during physical exercise. During exercise, S(I) (FSIGTr v FSIGTe: 8.5 +/- 1.0 v 25.5 +/- 7.2 x 10(-5) x min(-1) [pmol x L(-1)]-1, P < .01), S(G) (0.195 +/- 0.03 v 0.283 +/- 0.03 x 10(-1) x min(-1), P < .05), and GEZI (0.190 +/- 0.03 v 0.269 +/- 0.04 x 10(-1) x min(-1), P < .05) increased; however, no changes in phi1 and phi2 were found. Despite a significant decrease in the insulin response to glucose (AUC0-90, 21,000 +/- 2,008 v 14,340 +/- 2,596 pmol x L(-1) x min, P < .01), insulin action [X(t)] was significantly higher during the FSIGTe. These results show that physical exercise improves mainly insulin sensitivity, and to a lesser degree, glucose effectiveness. During exercise, the insulin response to glucose was lower than at rest, but beta-cell responsiveness to glucose did not change.

Effect of prior exercise on the partitioning of an intestinal glucose load between splanchnic bed and skeletal muscle

Exercise leads to marked increases in muscle insulin sensitivity and glucose effectiveness. Oral glucose tolerance immediately after exercise is generally not improved. The hypothesis tested by these experiments is that after exercise the increased muscle glucose uptake during an intestinal glucose load is counterbalanced by an increase in the efficiency with which glucose enters the circulation and that this occurs due to an increase in intestinal glucose absorption or decrease in hepatic glucose disposal. For this purpose, sampling (artery and portal, hepatic, and femoral veins) and infusion (vena cava, duodenum) catheters and Doppler flow probes (portal vein, hepatic artery, external iliac artery) were implanted 17 d before study. Overnightfasted dogs were studied after 150 min of moderate treadmill exercise or an equal duration rest period. Glucose ([14C]glucose labeled) was infused in the duodenum at 8 mg/kg x min for 150 min beginning 30 min after exercise or rest periods. Values, depending on the specific variable, are the mean +/- SE for six to eight dogs. Measurements are from the last 60 min of the intraduodenal glucose infusion. In response to intraduodenal glucose, arterial plasma glucose rose more in exercised (103 +/- 4 to 154 +/- 6 mg/dl) compared with rested (104 +/- 2 to 139 +/- 3 mg/dl) dogs. The greater increase in glucose occurred even though net limb glucose uptake was elevated after exercise (35 +/- 5 vs. 20 +/- 2 mg/min) as net splanchnic glucose output (5.1 +/- 0.8 vs. 2.1 +/- 0.6 mg/kg x min) and systemic appearance of intraduodenal glucose (8.1 +/- 0.6 vs. 6.3 +/- 0.7 mg/kg x min) were also increased due to a higher net gut glucose output (6.1 +/- 0.7 vs. 3.6 +/- 0.9 mg/kg x min). Adaptations at the muscle led to increased net glycogen deposition after exercise [1.4 +/- 0.3 vs. 0.5 +/- 0.1 mg/(gram of tissue x 150 min)], while no such increase in glycogen storage was seen in liver [3.9 +/- 1.0 vs. 4.1 +/- 1.1 mg/(gram of tissue x 150 min) in exercised and sedentary animals, respectively]. These experiments show that the increase in the ability of previously working muscle to store glycogen is not solely a result of changes at the muscle itself, but is also a result of changes in the splanchnic bed that increase the efficiency with which oral glucose is made available in the systemic circulation.

Influence of short-term submaximal exercise on parameters of glucose assimilation analyzed with the minimal model

After exercise, glucose uptake in tissues increases by insulin-dependent and -independent mechanisms. We evaluated whether these two effects of exercise on glucose disposal can be detected with the minimal model technique. Seven healthy volunteers were submitted at random order to two frequently sampled intravenous glucose tolerance test (FSIVGTTs), one at rest and the other 25 minutes after a 15-minute exercise test. This exercise included 5 minutes of increasing workload on a cycloergometer followed by 10 minutes at 85% of the maximal theoretic heart rate. Bergman's minimal model of insulin action was used to analyze the two FSIVGTTs and produced the following parameters: coefficient of glucose tolerance (Kg), ie, the slope of the exponential decrease in glycemia between 4 and 19 minutes after intravenous glucose; insulin sensitivity (Sl); and glucose effectiveness at basal insulin (Sg). Sg was divided into its two components: basal insulin effectiveness ([BIE] Sl x basal insulin) and glucose effectiveness at zero insulin ([GEZI] Sg-BIE). After the exercise bout, subjects had an increased Kg (3.44 +/- 0.44 v 2.06 +/- 0.28 x 10(-2).min-1, P < .02), Sl (11.43 +/- 1.27 v 6.23 +/- 0.97 x 10(-4) microU/mL.min-1, P < .01), and Sg (4.40 +/- 0.55 v 2.81 +/- 0.36 x 10(-2).min-1, P < .02). The increase in Sg was mainly explained by a 60% increase in GEZI (3.6 +/- 0.57 v 2.25 +/- 0.36 x 10(-2).min-1, P < .02), but also by an increase in BIE (0.80 +/- 0.12 v 0.47 +/- 0.08 x 10(-2).min-1, P < .05).(ABSTRACT TRUNCATED AT 250 WORDS)