Glucoregulatory responses to intense exercise performed in the postprandial state

Metformin and exercise effects on postprandial insulin sensitivity and glucose kinetics in pre-diabetic and diabetic adults

The potential interaction between metformin and exercise on glucose-lowering effects remains controversial. We studied the separated and combined effects of metformin and/or exercise on fasting and postprandial insulin sensitivity in individuals with pre-diabetes and type 2 diabetes (T2D). Eight T2D adults (60 ± 4 yr) with overweight/obesity (32 ± 4 kg·m-2) under chronic metformin treatment (9 ± 6 yr; 1281 ± 524 mg·day-1) underwent four trials; 1) taking their habitual metformin treatment (MET), 2) substituting during 96 h their metformin medication by placebo (PLAC), 3) placebo combined with 50 min bout of high-intensity interval exercise (PLAC + EX), and 4) metformin combined with exercise (MET + EX). Plasma glucose kinetics using stable isotopes (6,6-2H2 and [U-13C] glucose), and glucose oxidation by indirect calorimetry, were assessed at rest, during exercise, and in a subsequent oral glucose tolerance test (OGTT). Postprandial glucose and insulin concentrations were analyzed as mean and incremental area under the curve (iAUC), and insulin sensitivity was calculated (i.e., MATSUDAindex and OGISindex). During OGTT, metformin reduced glucose iAUC (i.e., MET and MET + EX lower than PLAC and PLAC + EX, respectively; P = 0.023). MET + EX increased MATSUDAindex above PLAC (4.8 ± 1.4 vs. 3.3 ± 1.0, respectively; P = 0.018) and OGISindex above PLAC (358 ± 52 vs. 306 ± 46 mL·min-1·m-2, respectively; P = 0.006). Metformin decreased the plasma appearance of the ingested glucose (Ra OGTT; MET vs. PLAC, -3.5; 95% CI -0.1 to -6.8 µmol·kg-1·min-1; P = 0.043). Metformin combined with exercise potentiates insulin sensitivity during an OGTT in individuals with pre-diabetes and type 2 diabetes. Metformin's blood glucose-lowering effect seems mediated by decreased oral glucose entering the circulation (gut-liver effect) an effect partially blunted after exercise.NEW & NOTEWORTHY Metformin is the most prescribed oral antidiabetic medicine in the world but its mechanism of action and its interactions with exercise are not fully understood. Our stable isotope tracer data suggested that metformin reduces the rates of oral glucose entering the circulation (gut-liver effect). Exercise, in turn, tended to reduce postprandial insulin blood levels potentiating metformin improvements in insulin sensitivity. Thus, exercise potentiates metformin improvements in glycemic control and should be advised to metformin users.

Pulmonary ventilation and gas exchange during prolonged exercise in humans: Influence of dehydration, hyperthermia and sympathoadrenal activity

NEW FINDINGS

What is the central question of the study? Ventilation increases during prolonged intense exercise, but the impact of dehydration and hyperthermia, with associated blunting of pulmonary circulation, and independent influences of dehydration, hyperthermia and sympathoadrenal discharge on ventilatory and pulmonary gas exchange responses remain unclear. What is the main finding and its importance? Dehydration and hyperthermia led to hyperventilation and compensatory adjustments in pulmonary CO2 and O2 exchange, such that CO2 output increased and O2 uptake remained unchanged despite the blunted circulation. Isolated hyperthermia and adrenaline infusion, but not isolated dehydration, increased ventilation to levels similar to combined dehydration and hyperthermia. Hyperthermia is the main stimulus increasing ventilation during prolonged intense exercise, partly via sympathoadrenal activation.

ABSTRACT

The mechanisms driving hyperthermic hyperventilation during exercise are unclear. In a series of retrospective analyses, we evaluated the impact of combined versus isolated dehydration and hyperthermia and the effects of sympathoadrenal discharge on ventilation and pulmonary gas exchange during prolonged intense exercise. In the first study, endurance-trained males performed two submaximal cycling exercise trials in the heat. On day 1, participants cycled until volitional exhaustion (135 ± 11 min) while experiencing progressive dehydration and hyperthermia. On day 2, participants maintained euhydration and core temperature (Tc ) during a time-matched exercise (control). At rest and during the first 20 min of exercise, pulmonary ventilation (V ̇ E ${\skew2\dot V_{\rm{E}}}$ ), arterial blood gases, CO2 output and O2 uptake were similar in both trials. At 135 ± 11 min, however,V ̇ E ${\skew2\dot V_{\rm{E}}}$ was elevated with dehydration and hyperthermia, and this was accompanied by lower arterial partial pressure of CO2 , higher breathing frequency, arterial partial pressure of O2 , arteriovenous CO2 and O2 differences, and elevated CO2 output and unchanged O2 uptake despite a reduced pulmonary circulation. The increasedV ̇ E ${\skew2\dot V_{\rm{E}}}$ was closely related to the rise in Tc and circulating catecholamines (R2  ≥ 0.818, P ≤ 0.034). In three additional studies in different participants, hyperthermia independently increasedV ̇ E ${\skew2\dot V_{\rm{E}}}$ to an extent similar to combined dehydration and hyperthermia, whereas prevention of hyperthermia in dehydrated individuals restoredV ̇ E ${\skew2\dot V_{\rm{E}}}$ to control levels. Furthermore, adrenaline infusion during exercise elevated both Tc andV ̇ E ${\skew2\dot V_{\rm{E}}}$ . These findings indicate that: (1) adjustments in pulmonary gas exchange limit homeostatic disturbances in the face of a blunted pulmonary circulation; (2) hyperthermia is the main stimulus increasing ventilation during prolonged intense exercise; and (3) sympathoadrenal activation might partly mediate the hyperthermic hyperventilation.

Effects of metabolic syndrome on fuel utilization during exercise on middle-aged moderately trained individuals

People with the metabolic syndrome (MetS) may have blunted exercise stimulation of metabolism explaining their resistance to lower blood glucose and triglycerides with exercise training. Glycerol and glucose rate of appearance (Ra) in plasma and substrate oxidation were determined at rest and during cycle ergometer exercise at three increasing intensities (55, 80 and 95% of maximal heart rate) in 9 middle-aged (61±7 yr) individuals with MetS. Data were compared to 8 healthy-younger (29±10 yr) individuals matched for habitual exercise training and fat free mass (Healthy-young). At rest, fasting plasma triglycerides (TG), blood glucose and insulin were higher in MetS than in Healthy-young (38%, 42% and 85%, respectively; all p<0.05). At rest, and during low intensity exercise (32-43% VO2MAX), plasma glycerol Ra (index of whole-body lipolysis) and glucose Ra and Rd (index of glucose appearance and disposal) were similar in MetS and Healthy-young. Fat oxidation peaked at low intensity exercise similarly in MetS and Healthy-young (0.273±0.082 vs 0.272±0.078 g·min-1, respectively; p = 0.961). Ra glycerol increased with exercise intensity but was lower in MetS at moderate and high exercise intensities (i.e., 60-100% VO2MAX; p<0.05). Metabolic clearance rate of glucose at high intensity (85-100% VO2MAX) was lower in MetS compared to Healthy-young (p = 0.029). The MetS that develops in middle adulthood, reduces exercise lipolysis and plasma glucose clearance at high exercise intensities, but does not blunt fat or carbohydrate metabolism at low exercise intensity.

Five Evidence-Based Lifestyle Habits People With Diabetes Can Use

Several evidence-based lifestyle habits focusing on the composition, timing, and sequence of meals and on pre- and postmeal exercise can improve diabetes management. Consuming low-carbohydrate, balanced meals and eating most carbohydrates early in the day are helpful habits. Eating the protein and vegetable components of a meal first and consuming the carbohydrates 30 minutes later can moderate glucose levels. Postmeal glucose surges can be blunted without precipitating hypoglycemia with moderate exercise 30-60 minutes before the anticipated peak. Short-duration, high-intensity exercise could also be effective. Premeal exercise can improve insulin sensitivity but can also cause post-exertion glucose elevations. Moreover, high-intensity premeal exercise may precipitate delayed hypoglycemia in some people. Glycemia benefits can be enhanced by eating a light, balanced breakfast after premeal exercise.

Postexercise Glucose–Fructose Coingestion Augments Cycling Capacity During Short-Term and Overnight Recovery From Exhaustive Exercise, Compared With Isocaloric Glucose

During short-term recovery, postexercise glucose–fructose coingestion can accelerate total glycogen repletion and augment recovery of running capacity. It is unknown if this advantage translates to cycling, or to a longer (e.g., overnight) recovery. Using two experiments, the present research investigated if postexercise glucose–fructose coingestion augments exercise capacity following 4-hr (short experiment; n = 8) and 15-hr (overnight experiment; n = 8) recoveries from exhaustive exercise in trained cyclists, compared with isocaloric glucose alone. In each experiment, a glycogen depleting exercise protocol was followed by a 4-hr recovery, with ingestion of 1.5 or 1.2 g·kg−1·hr−1 carbohydrate in the short experiment (double blind) and the overnight experiment (single blind), respectively. Treatments were provided in a randomized order using a crossover design. Four or fifteen hours after the glycogen depletion protocol, participants cycled to exhaustion at 70% Wmax or 65% Wmax in the short experiment and the overnight experiment, respectively. In both experiments there was no difference in substrate oxidation or blood glucose and lactate concentrations between treatments during the exercise capacity test (trial effect, p > .05). Nevertheless, cycling capacity was greater in glucose + fructose versus glucose only in the short experiment (28.0 ± 8.4 vs. 22.8 ± 7.3 min, d = 0.65, p = .039) and the overnight experiment (35.9 ± 10.7 vs. 30.6 ± 9.2 min, d = 0.53, p = .026). This is the first study to demonstrate that postexercise glucose–fructose coingestion enhances cycling capacity following short-term (4 hr) and overnight (15 hr) recovery durations. Therefore, if multistage endurance athletes are ingesting glucose for rapid postexercise recovery then fructose containing carbohydrates may be advisable.

Effect of combining pre-exercise carbohydrate intake and repeated short sprints on the blood glucose response to moderate-intensity exercise in young individuals with Type 1 diabetes

AIMS

To determine whether pre-exercise ingestion of carbohydrates to maintain stable glycaemia during moderate-intensity exercise results in excessive hyperglycaemia if combined with repeated sprints in individuals with Type 1 diabetes.

METHODS

Eight overnight-fasted people with Type 1 diabetes completed the following four 40-min exercise sessions on separate days in a randomized counterbalanced order under basal insulinaemic conditions: continuous moderate-intensity exercise at 50% V ˙ O 2 peak; intermittent high-intensity exercise (moderate-intensity exercise interspersed with 4-s sprints every 2 min and a final 10-s sprint); continuous moderate-intensity exercise with prior carbohydrate intake (~10 g per person); and intermittent high-intensity exercise with prior carbohydrate intake. Venous blood was sampled during and 2 h after exercise to measure glucose and lactate levels.

RESULTS

The difference in marginal mean time-averaged area under the blood glucose curve between continuous moderate-intensity exercise + prior carbohydrate and intermittent high-intensity exercise + prior carbohydrate during exercise and recovery was not significant [0.2 mmol/l (95% CI -0.7, 1.1); P = 0.635], nor was the difference in peak blood glucose level after adjusting for baseline level [0.2 mmol/l (95% CI -0.7, 1.1); P = 0.695]. The difference in marginal mean time-averaged area under the blood glucose curve between continuous moderate-intensity and intermittent high-intensity exercise during exercise and recovery was also not significant [-0.2 mmol/l (95% CI -1.2, 0.8); P = 0.651].

CONCLUSIONS

When carbohydrates are ingested prior to moderate-intensity exercise, adding repeated sprints is not significantly detrimental to glycaemic management in overnight fasted people with Type 1 diabetes under basal insulin conditions.

Exercising Tactically for Taming Postmeal Glucose Surges

This review seeks to synthesize data on the timing, intensity, and duration of exercise found scattered over some 39 studies spanning 3+ decades into optimal exercise conditions for controlling postmeal glucose surges. The results show that a light aerobic exercise for 60 min or moderate activity for 20-30 min starting 30 min after meal can efficiently blunt the glucose surge, with minimal risk of hypoglycemia. Exercising at other times could lead to glucose elevation caused by counterregulation. Adding a short bout of resistance exercise of moderate intensity (60%-80%  VO2max) to the aerobic activity, 2 or 3 times a week as recommended by the current guidelines, may also help with the lowering of glucose surges. On the other hand, high-intensity exercise (>80%  VO2max) causes wide glucose fluctuations and its feasibility and efficacy for glucose regulation remain to be ascertained. Promoting the kind of physical activity that best counters postmeal hyperglycemia is crucial because hundreds of millions of diabetes patients living in developing countries and in the pockets of poverty in the West must do without medicines, supplies, and special diets. Physical activity is the one tool they may readily utilize to tame postmeal glucose surges. Exercising in this manner does not violate any of the current guidelines, which encourage exercise any time.

Effect of Low Frequency Neuromuscular Electrical Stimulation on Glucose Profile of Persons with Type 2 Diabetes: A Pilot Study

The purpose of this study was to examine the effect of low-frequency neuromuscular electrical stimulation (NMES) on glucose profile in persons with type 2 diabetes mellitus (T2DM). Eight persons with T2DM (41 to 65 years) completed a glucose tolerance test with and without NMES delivered to the knee extensors for a 1-hour period at 8 Hz. Three blood samples were collected: at rest, and then 60 and 120 minutes after consumption of a glucose load on the NMES and control days. In NMES groups glucose concentrations were significantly lower (P<0.01) than in the control conditions. Moreover, a significant positive correlation (r=0.9, P<0.01) was obtained between the intensity of stimulation and changes in blood glucose. Our results suggest that low-frequency stimulation seem suitable to induce enhance glucose uptake in persons with T2DM. Moreover, the intensity of stimulation reflecting the motor contraction should be considered during NMES procedure.

Cardiovascular, metabolic and hormonal responses to the progressive exercise performed to exhaustion in patients with type 2 diabetes treated with metformin or glyburide

OBJECTIVES

To evaluate the effects of Metformin and Glyburide on cardiovascular, metabolic and hormonal parameters during progressive exercise performed to exhaustion in the post-prandial state in women with type 2 diabetes (T2DM).

DESIGN AND METHODS

Ten T2DM patients treated with Metformin (M group), 10 with Glyburide (G group) and 10 age-paired healthy subjects exercised on a bicycle ergometer up to exercise peak. Cardiovascular and blood metabolic and hormonal parameters were measured at times -60 min, 0 min, exercise end, and at 10 and 20 minutes of recovery phase. Thirty minutes before the exercise, a standard breakfast was provided to all participants. The diabetic patients took Metformin or Glyburide before or with meal.

RESULTS

Peak oxygen uptake (VO(2)) was lower in patients with diabetes. Plasma glucose levels remained unchanged, but were higher in both diabetic groups. Patients with diabetes also presented lower insulin levels after meals and higher glucagon levels at exercise peak than C group. Serum cortisol levels were higher in G than M group at exercise end and recovery phase. Lactate levels were higher in M than G group at fasting and in C group at exercise peak. Nor epinephrine, GH and FFA responses were similar in all 3 groups.

CONCLUSION

Progressive exercise performed to exhaustion, in the post-prandial state did not worsen glucose control during and after exercise. The administration of the usual dose of Glyburide or Metformin to T2DM patients did not influence the cardiovascular, metabolic and hormonal response to exercise.

Combined infusion of epinephrine and norepinephrine during moderate exercise reproduces the glucoregulatory response of intense exercise

Intense exercise (IE) (>80% O(2max)) causes a seven- to eightfold increase in glucose production (R(a)) and a fourfold increase in glucose uptake (R(d)), resulting in hyperglycemia, whereas moderate exercise (ME) causes both to double. If norepinephrine (NE) plus epinephrine (Epi) infusion during ME produces the plasma levels and R(a) of IE, this would prove them capable of mediating these responses. Male subjects underwent 40 min of 53% O(2max) exercise, eight each with saline (control [CON]), or with combined NE + Epi (combined catecholamine infusion [CCI]) infusion from min 26-40. In CON and CCI, NE levels reached 7.3 +/- 0.7 and 33.1 +/- 2.9 nmol/l, Epi 0.94 +/- 0.08 and 7.06 +/- 0.44 nmol/l, and R(a) 3.8 +/- 0.4 and 12.9 +/- 0.8 mg. kg(-1). min(-1) (P < 0.001), respectively, at 40 min. R(d) increased to 3.5 +/- 0.4 vs. 11.2 +/- 0.8 mg. kg(-1). min(-1) and glycemia 5.2 +/- 0.2 mmol/l in CON vs. 6.5 +/- 0.2 mmol/l in CCI (P < 0.001). The glucagon-to-insulin ratio did not differ. Comparing CCI data to those from 14-min IE (n = 16), peak NE (33.6 +/- 5.1 nmol/l), Epi (5.32 +/- 0.93 nmol/l), and R(a) (13.0 +/- 1.0 mg. kg(-1). min(-1)) were comparable. The induced increments in NE, Epi, and R(a), all of the same magnitude as in IE, strongly support that circulating catecholamines can be the prime regulators of R(a) in IE.

Intense exercise has unique effects on both insulin release and its roles in glucoregulation: implications for diabetes

In intense exercise (>80% VO(2max)), unlike at lesser intensities, glucose is the exclusive muscle fuel. It must be mobilized from muscle and liver glycogen in both the fed and fasted states. Therefore, regulation of glucose production (GP) and glucose utilization (GU) have to be different from exercise at <60% VO(2max), in which it is established that the portal glucagon-to-insulin ratio causes the less than or equal to twofold increase in GP. GU is subject to complex regulation by insulin, plasma glucose, alternate substrates, other humoral factors, and muscle factors. At lower intensities, plasma glucose is constant during postabsorptive exercise and declines during postprandial exercise (and often in persons with diabetes). During such exercise, insulin secretion is inhibited by beta-cell alpha-adrenergic receptor activation. In contrast, in intense exercise, GP rises seven- to eightfold and GU rises three- to fourfold; therefore, glycemia increases and plasma insulin decreases minimally, if at all. Indeed, even an increase in insulin during alpha-blockade or during a pancreatic clamp does not prevent this response, nor does pre-exercise hyperinsulinemia due to a prior meal or glucose infusion. At exhaustion, GU initially decreases more than GP, which leads to greater hyperglycemia, requiring a substantial rise in insulin for 40--60 min to restore pre-exercise levels. Absence of this response in type 1 diabetes leads to sustained hyperglycemia, and mimicking it by intravenous infusion restores the normal response. Compelling evidence supports the conclusion that the marked catecholamine responses to intense exercise are responsible for both the GP increment (that occurs even during glucose infusion and postprandially) and the restrained increase of GU. These responses are normal in persons with type 1 diabetes, who often report exercise-induced hyperglycemia, and in whom the clinical challenge is to reproduce the recovery period hyperinsulinemia. Intense exercise in type 2 diabetes requires additional study.

Exercise and diabetes

As rates of diabetes mellitus and obesity continue to increase, physical activity continues to be a fundamental form of therapy. Exercise influences several aspects of diabetes, including blood glucose concentrations, insulin action and cardiovascular risk factors. Blood glucose concentrations reflect the balance between skeletal muscle uptake and ambient concentrations of both insulin and counterinsulin hormones. Difficulties in predicting the relative impact of these factors can result in either hypoglycemia or hyperglycemia. Despite the variable impact of exercise on blood glucose, exercise consistently improves insulin action and several cardiovascular risk factors. Beyond the acute impact of physical activity, long-term exercise behaviors have been repeatedly associated with decreased rates of type 2 diabetes. While exercise produces many benefits, it is not without risks for patients with diabetes mellitus. In addition to hyperglycemia, from increased hepatic glucose production, insufficient insulin levels can foster ketogenesis from excess concentrations of fatty acids. At the opposite end of the glucose spectrum, hypoglycemia can result from excess glucose uptake due to either increased insulin concentrations, enhanced insulin action or impaired carbohydrate absorption. To decrease the risk for hypoglycemia, insulin doses should be reduced prior to exercise, although some insulin is typically still needed. Although precise risks of exercise on existing diabetic complications have not been well studied, it seems prudent to consider the potential to worsen nephropathy or retinopathy, or to precipitate musculoskeletal injuries. There is more substantive evidence that autonomic neuropathy may predispose patients to arrhythmias. Of clear concern, increased physical activity can precipitate a cardiac event in those with underlying CAD. Recognizing these risks can prompt actions to minimize their impact. Positive actions that are part of exercise programs for diabetic patients emphasize SMBG, foot care and cardiovascular functional assessment. SMBG provides critical information on the impact of exercise and is recommended for all patients before, during and after exercise. More frequent monitoring (and for longer periods following exercise) is recommended for those with hypoglycemia unawareness or those performing high-intensity exercise. Preventing the sequelae of an exercise-induced severe hypoglycemic reaction can be as simple as carrying glucose tablets or gel, a diabetic identification bracelet or card, or exercising with an individual who is aware of the circumstances. In addition to blood glucose concentrations, proper foot care is critical to people with diabetes who exercise and includes considering type of shoe, type of exercise, inspection of skin surfaces and appropriate evaluation and treatment of lesions (calluses and others). Those with severe neuropathy can consider alternatives to weight-bearing exercises. Precipitation of clinical CAD is of great concern for all diabetic patients participating in exercise activities. Although a sufficiently sensitive and specific screening test for coronary disease has not been identified, those planning an exercise program of moderate intensity or greater should be evaluated. Initial cardiac assessment should include exercise testing as well as identifying risk for autonomic neuropathy. In addition to noting maximal heart rate and blood pressure as well as ischemic changes, exercise tolerance testing can identify anginal thresholds and patients with asymptomatic ischemia. Those without symptoms should be counseled regarding target pulse rates to avoid inducing ischemia. Ischemic changes need to be evaluated for either further diagnostic testing or pharmacological intervention. For patients with diabetes mellitus, the overall benefits of exercise are clearly significant. Clinicians and patients must work together to maximize these benefits while minimizing risks for negative consequences. Identifying and preventing potential problems beforehand can reduce adverse outcomes and promote this important approach to healthy living.

Epinephrine infusion during moderate intensity exercise increases glucose production and uptake

The glucoregulatory response to intense exercise [IE, >80% maximum O(2) uptake (VO(2 max))] comprises a marked increment in glucose production (R(a)) and a lesser increment in glucose uptake (R(d)), resulting in hyperglycemia. The R(a) correlates with plasma catecholamines but not with the glucagon-to-insulin (IRG/IRI) ratio. If epinephrine (Epi) infusion during moderate exercise were able to markedly stimulate R(a), this would support an important role for the catecholamines' response in IE. Seven fit male subjects (26 +/- 2 yr, body mass index 23 +/- 0.5 kg/m(2), VO(2 max) 65 +/- 5 ml x kg(-1) x min(-1)) underwent 40 min of postabsorptive cycle ergometer exercise (145 +/- 14 W) once without [control (CON)] and once with Epi infusion [EPI (0.1 microg x kg(-1) x min(-1))] from 30 to 40 min. Epi levels reached 9.4 +/- 0.8 nM (20x rest, 10x CON). R(a) increased approximately 70% to 3.75 +/- 0.53 in CON but to 8.57 +/- 0.58 mg x kg(-1) x min(-1) in EPI (P < 0.001). Increments in R(a) and Epi correlated (r(2) = 0.923, P </= 0.01). In EPI, peak R(d) (5.55 +/- 0.54 vs. 3.38 +/- 0.46 mg x kg(-1) x min(-1), P = 0.006) and glucose metabolic clearance rate (MCR, P = 0.018) were higher. The R(a)-to-R(d) imbalance in EPI caused hyperglycemia (7.12 +/- 0.22 vs. 5.59 +/- 0.22 mM, P = 0.001) until minute 60 of recovery. A small and late IRG/IRI increase (P = 0.015 vs. CON) could not account for the R(a) increase. Norepinephrine (approximately 4x increase at peak) did not differ between EPI and CON. Thus Epi infusion during moderate exercise led to increments in R(a) and R(d) and caused rises of plasma glucose, lactate, and respiratory exchange ratio in fit individuals, supporting a regulatory role for Epi in IE. Epi's effects on R(d) and MCR during exercise may differ from its effects at rest.