Effect of physical training on insulin uptake by the perfused rat liver

Dietary weight loss-induced improvements in metabolic function are enhanced by exercise in people with obesity and prediabetes

The additional therapeutic effects of regular exercise during a dietary weight loss program in people with obesity and prediabetes are unclear. Here, we show that whole-body (primarily muscle) insulin sensitivity (primary outcome) was 2-fold greater (P = 0.006) after 10% weight loss induced by calorie restriction plus exercise training (Diet+EX; n = 8, 6 women) than 10% weight loss induced by calorie restriction alone (Diet-ONLY; n = 8, 4 women) in participants in two concurrent studies. The greater improvement in insulin sensitivity was accompanied by increased muscle expression of genes involved in mitochondrial biogenesis, energy metabolism and angiogenesis (secondary outcomes) in the Diet+EX group. There were no differences between groups in plasma branched-chain amino acids or markers of inflammation, and both interventions caused similar changes in the gut microbiome. Few adverse events were reported. These results demonstrate that regular exercise during a diet-induced weight loss program has profound additional metabolic benefits in people with obesity and prediabetes.Trial Registration: ClinicalTrials.gov (NCT02706262 and NCT02706288).

Evolution of the diagnostic value of "the sugar of the blood": hitting the sweet spot to identify alterations in glucose dynamics

In this paper, we provide an overview of the evolution of the definition of hyperglycemia during the past century and the alterations in glucose dynamics that cause fasting and postprandial hyperglycemia. We discuss how extensive mechanistic, physiological research into the factors and pathways that regulate the appearance of glucose in the circulation and its uptake and metabolism by tissues and organs has contributed knowledge that has advanced our understanding of different types of hyperglycemia, namely prediabetes and diabetes and their subtypes (impaired fasting plasma glucose, impaired glucose tolerance, combined impaired fasting plasma glucose, impaired glucose tolerance, type 1 diabetes, type 2 diabetes, gestational diabetes mellitus), their relationships with medical complications, and how to prevent and treat hyperglycemia.

Worksite-based intensive lifestyle therapy has profound cardiometabolic benefits in people with obesity and type 2 diabetes

Lifestyle therapy (energy restriction and exercise) is the cornerstone of therapy for people with type 2 diabetes (T2D) but is difficult to implement. We conducted an 8-month randomized controlled trial in persons with obesity and T2D (17 women and 1 man) to determine the therapeutic effects and potential mechanisms of intensive lifestyle therapy on cardiometabolic function. Intensive lifestyle therapy was conducted at the worksite to enhance compliance and resulted in marked (17%) weight loss and beneficial changes in body fat mass, intrahepatic triglyceride content, cardiorespiratory fitness, muscle strength, glycemic control, β cell function, and multi-organ insulin sensitivity, which were associated with changes in muscle NAD+ biosynthesis, sirtuin signaling, and mitochondrial function and in adipose tissue remodeling. These findings demonstrate that intensive lifestyle therapy provided at the worksite has profound therapeutic clinical and physiological effects in people with T2D, which are likely mediated by specific alterations in skeletal muscle and adipose tissue biology.

Acute Exercise Improves Insulin Clearance and Increases the Expression of Insulin-Degrading Enzyme in the Liver and Skeletal Muscle of Swiss Mice

The effects of exercise on insulin clearance and IDE expression are not yet fully elucidated. Here, we have explored the effect of acute exercise on insulin clearance and IDE expression in lean mice. Male Swiss mice were subjected to a single bout of exercise on a speed/angle controlled treadmill for 3-h at approximately 60-70% of maximum oxygen consumption. As expected, acute exercise reduced glycemia and insulinemia, and increased insulin tolerance. The activity of AMPK-ACC, but not of IR-Akt, pathway was increased in the liver and skeletal muscle of trained mice. In an apparent contrast to the reduced insulinemia, glucose-stimulated insulin secretion was increased in isolated islets of these mice. However, insulin clearance was increased after acute exercise and was accompanied by increased expression of the insulin-degrading enzyme (IDE), in the liver and skeletal muscle. Finally, C2C12, but not HEPG2 cells, incubated at different concentrations of 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR) for 3-h, showed increased expression of IDE. In conclusion, acute exercise increases insulin clearance, probably due to an augmentation of IDE expression in the liver and skeletal muscle. The elevated IDE expression, in the skeletal muscle, seems to be mediated by activation of AMPK-ACC pathway, in response to exercise. We believe that the increase in the IDE expression, comprise a safety measure to maintain glycemia at or close to physiological levels, turning physical exercise more effective and safe.

Increased insulin-stimulated Akt pSer473 and cytosolic SHP2 protein abundance in human skeletal muscle following acute exercise and short-term training

The purpose of the present study was to determine in human skeletal muscle whether a single exercise bout and 7 days of consecutive endurance (cycling) training 1) increased insulin-stimulated Akt pSer(473) and 2) altered the abundance of the protein tyrosine phosphatases (PTPases), PTP1B and SHP2. In healthy, untrained men (n = 8; 24 +/- 1 yr), glucose infusion rate during a hyperinsulinemic euglycemic clamp, when compared with untrained values, was not improved 24 h following a single 60-min bout of endurance cycling but was significantly increased ( approximately 30%; P < 0.05) 24 h following completion of 7 days of exercise training. Insulin-stimulated Akt pSer(473) was approximately 50% higher (P < 0.05) 24 h following the acute bout of exercise, with this effect remaining after 7 days of training (P < 0.05). Insulin-stimulated insulin receptor and insulin receptor substrate-1 tyrosine phosphorylation were not altered 24 h after acute exercise and short-term training. Insulin did not acutely regulate the localization of the PTPases, PTP1B or SHP2, although cytosolic protein abundance of SHP2 was increased (P < 0.05; main effect) 24 h following acute exercise and short-term training. In conclusion, insulin-sensitive Akt pSer(473) and cytosolic SHP2 protein abundance are higher after acute exercise and short-term training, and this effect appears largely due to the residual effects of the last bout of prior exercise. The significance of exercise-induced alterations in cytosolic SHP2 and insulin-stimulated Akt pSer(473) on the improvement in insulin sensitivity requires further elucidation.

Fatty acids in the portal vein of the rat regulate hepatic insulin clearance

The effects of FFA on hepatic insulin clearance were studied in the in situ perfused rat liver. Clearance decreased with increasing body weight (age) of the rats. When FFA were added to the perfusate a 40% reduction of hepatic removal of insulin was found over the normal, physiological range (less than 1,000 mumol/liter), less pronounced in heavier rats. When perfusion was started with high concentrations of FFA, inhibition was rapidly reversible, a phenomenon again blunted in heavier rats. In contrast to FFA, different glucose concentrations in the perfusate did not affect the hepatic insulin uptake in the presence of FFA within physiological concentrations. Thus, hepatic clearance of insulin is proportional to rat weight (age) and portal FFA concentrations. Other studies have recently shown that fatty acids inhibit insulin binding, degradation, and function in isolated rat hepatocytes, and that hepatic clearance is inversely dependent on hepatic triglyceride concentrations, both inhibitions reversible by prevention of fatty acid oxidation. It is suggested that the diminished hepatic clearance of insulin in heavier (older) rats is at least partly due to their relative obesity and increased hepatic triglyceride contents. This effect as well as that of portal FFA is probably mediated via fatty acid oxidation in the liver. This mechanism may have implications for the regulation of hepatic metabolism, and peripheral insulin concentrations.

Effect of training on response to a glucose load adjusted for daily carbohydrate intake

From responses to identical absolute glucose loads in trained (T) and untrained (UT) subjects, it has been inferred that training promotes health by reducing glucose levels and insulin secretion. To mimic daily living conditions, we studied responses to oral glucose loads making up identical fractions of daily carbohydrate intake (i.e., same relative glucose load) in seven T and eight UT males [maximal O2 uptake (VO2max) 76 +/- 2 vs. 48 +/- 1 (SE) ml.min-1.kg-1; age 24 +/- 1 vs. 25 +/- 1 yr]. Daily energy intake was higher in T than in UT subjects (18,607 +/- 835 vs. 12,493 +/- 720 kJ/day, P less than 0.05), reflecting a 2.3 times higher carbohydrate intake (678 +/- 34 vs. 294 +/- 18 g/day, P less than 0.05). After 1 g/kg body wt glucose, C-peptide and insulin responses were lower in T than in UT subjects (P less than 0.05). However, after identical relative glucose loads [high: 2.3 +/- 0.2 (T) vs. 1 (UT) g/kg; low: 1 (T) vs. 0.4 +/- 0.03 (UT) g/kg], glucose [incremental areas 300 +/- 56 (T) vs. 304 +/- 35 (UT) mM.180 min and 148 +/- 30 (T) vs. 124 +/- 22 (UT)] and C-peptide [181 +/- 18 (T) vs. 171 +/- 27 (UT) nM.180 min, and 100 +/- 13 (T) vs. 71 +/- 12 (UT)] responses did not differ between groups, while insulin responses were lower in T [at low relative load 15 +/- 4 (T) vs. 20 +/- 2 (UT) nM.180 min, P less than 0.05].(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of training and detraining on dose-response relationship between glucose and insulin secretion

We studied the effect of training and detraining on the dose-response relationship between plasma glucose and beta-cell secretion in seven trained young men using sequential hyperglycemic clamp technique (7, 11, and 20 mM). Experiments were performed in the habitual state 15 h after last training session (T) as well as after 5 days of detraining (DT). Results were compared to data from seven untrained subjects (UT). Glucose-stimulated insulin, proinsulin, and C-peptide levels were lower in T than in UT. They increased during detraining but not to levels seen in UT. Furthermore, in T and DT, but not in UT, increases in C-peptide and proinsulin leveled off with increasing glucose concentrations. Estimated by C-peptide-to-insulin ratios, clearance of endogenous insulin was not influenced by T. Glucose uptake in tissue was the same in T, DT, and UT during clamps, despite lower insulin levels in T and DT. Differences between groups in counterregulatory hormones, fat metabolites, alanine, or electrolytes did not account for these findings. Oxygen consumption was higher in the basal state in T and DT compared with UT but increased similarly in all groups in response to glucose.

CONCLUSIONS

regular physical activity causes an adaptive decrease in glucose-mediated beta-cell secretion in humans. The training-induced decrease in glucose-stimulated insulin secretion is accurately matched to increased insulin action, keeping glucose disposal constant at any given plasma glucose concentration. Finally, training increases basal metabolic rate but does not influence glucose-induced thermogenesis or clearance of endogenous insulin.

The glucose stimulus-response curve of the beta-cell in physically trained humans, assessed by hyperglycemic clamps

In order to examine the effect of habitual exercise on beta-cell responses over a wide range of plasma glucose levels, plasma insulin and C-peptide responses to 2 1/2-hour hyperglycemic clamps at 7.5, 10, and 15 mmol/L glucose were assessed in six trained athletes and six age- and weight-matched sedentary controls. Athletes were significantly fitter than controls (estimated maximal oxygen uptake [VO2 max] mean 44 v 30 mL.kg-1.min-1, P less than .05) and were more sensitive to insulin as assessed by dividing the mean glucose infusion rate over the last 20 minutes of the clamp by the steady-state plasma insulin (mean 0.44 v 0.19 mg.min-1.kg-1.nmol-1. L, respectively, P less than .01). Plasma C-peptide responses were lower in the athletes, both fasting (geometric mean 0.28 v 0.62 nmol/L, P less than .05), and at the end of all clamps (at 7.5, 10, and 15 mmol/L plasma glucose, respectively, 0.65 v 1.43, 1.25 v 2.85, and 2.40 v 4.46 nmol/L each, P less than .05). First-phase plasma C-peptide responses were lower in the athletes at the 10 and 15 mmol clamp levels. The slope of the glucose-C-peptide stimulus-response curve was approximately linear over the range examined, the slope being significantly shallower in athletes than controls for both first phase (P less than .01) and second phase (P less than .01). Plasma insulin responses were similar to C-peptide responses. The attenuation of beta-cell responsiveness over a wide glucose range may be an adaptation to the enhanced peripheral insulin sensitivity seen in athletes.

Synergistic improvement of glucose tolerance by sucrose feeding and exercise training

The interactive effects of exercise training (5-7 wk) and sucrose consumption (ad libitum feeding of a 32% sucrose solution and Purina chow) on intravenous glucose tolerance and plasma insulin levels were investigated using a 2 X 2 experimental design. Rats were divided in Purina-sedentary, Purina-trained, sucrose-sedentary, and sucrose-trained groups. Sucrose feeding of sedentary animals significantly increased basal and glucose-stimulated insulin levels and improved basal glycemia and glucose tolerance. On the other hand, exercise training of Purina-fed animals significantly reduced basal as well as glucose-stimulated insulinemia without altering basal glycemia or glucose tolerance. Such a sparing effect of exercise training on insulin requirements was not as evident in rats consuming sucrose. These animals displayed a reduced basal glycemia (P less than 0.01) with normal basal insulin levels. Their glucose tolerance was markedly improved (P less than 0.01) but their insulin response during intravenous glucose tolerance test remained as high as in sucrose-sedentary animals. Results from these studies indicate that sucrose feeding of sedentary animals leads to hyperinsulinemia without compensatory insulin resistance, resulting in an improvement of glucose tolerance, exercise training increases the sensitivity of peripheral tissues to insulin, and the marked improvement of glucose tolerance observed in sucrose-trained animals results from a synergistic combination of the above two factors, i.e., increased insulinemia (induced by diet) and enhanced insulin sensitivity (induced by training).

Reduced hepatic insulin clearance in rats with dietary-induced obesity

Insulin uptake in the in situ perfused liver from rats that were moderately obese after overfeeding was diminished in comparison with controls. The obese rats had higher levels of portal free fatty acids (FFA) and liver triglyceride contents but not of insulin concentration in the portal vein. There were strong negative correlations between hepatic triglyceride and insulin clearance (r approximately 0.8-0.9). The perfusions were performed with lower FFA concentrations than those in vivo in the portal vein. It is suggested that the inhibited insulin uptake in the obese rats was due to exposure of these livers in vivo to elevated FFA concentrations, and that this inhibition remained during the experiment and was associated with the triglyceride contents of the livers. It is also suggested that this mechanism was responsible for the moderate peripheral hyperinsulinemia seen in these rats. A mechanism of regulation of insulin uptake in the liver via FFA and liver triglyceride might be of importance in several conditions with hyperinsulinemia and known elevation of portal FFA, and liver triglyceride contents.

Physical training and fasting serum insulin levels in sedentary men

The present study examined fasting serum insulin levels in relation to body composition and dietary intake during the initial 4 weeks of a 12-week physical training programme in 26 previously sedentary men. Fasting serum insulin concentrations decreased markedly during the first 4 weeks of training and remained at these reduced levels for the rest of the study. The early fall in serum insulin concentration was significantly correlated with the concomitant decrease in body fat, the increase in lean body weight and the age of the subjects. Body weight and reported dietary intake on the other hand, did not change significantly over this period. These results indicate that the decrease in fasting serum insulin in previously sedentary men with physical training is associated with the concomitant changes in body composition. Increased muscle tissue in particular may contribute to this training-induced decrease in serum insulin.

Relationship of physical activity, glucose tolerance, and weight management

Diabetes mellitus and hyperinsulinemia have been demonstrated by population studies to be risk factors for ischemic heart disease (IHD). But, the relationship of glucose intolerance to IHD is still unclear. Restricted physical activity and obesity both result experimentally in glucose intolerance and cell insulin insensitivity with hyperinsulinemia and may contribute to IHD. Prolonged moderate-intensity exercise lowers plasma insulin levels in obese hyperinsulinemic subjects as well as in nonobese subjects. Increased insulin binding to cell receptors (and number of insulin receptors) and a decrease in insulin secretion from the pancreas are believed to be contributing mechanisms. The independent effect of exercise on glucose tolerance is less clear, although there are strong indications favoring the positive effects of physical activity. There is also suggestive evidence that exercise may be beneficial in improving metabolic control in type II diabetes. Regular exercise together with appropriate diet and correction of obesity seems to be a more promising approach to improving glucose tolerance and controlling diabetes than does exercise alone.