Glucagon, not insulin, may play a secondary role in defense against hypoglycemia during exercise

Association of HbA1c with VO2max in Individuals with Type 1 Diabetes: A Systematic Review and Meta-Analysis

The aim of this systematic review and meta-analysis was to evaluate the association between glycemic control (HbA1c) and functional capacity (VO2max) in individuals with type 1 diabetes (T1DM). A systematic literature search was conducted in EMBASE, PubMed, Cochrane Central Register of Controlled Trials, and ISI Web of Knowledge for publications from January 1950 until July 2020. Randomized and observational controlled trials with a minimum number of three participants were included if cardio-pulmonary exercise tests to determine VO2max and HbA1c measurement has been performed. Pooled mean values were estimated for VO2max and HbA1c and weighted Pearson correlation and meta-regression were performed to assess the association between these parameters. We included 187 studies with a total of 3278 individuals with T1DM. The pooled mean HbA1c value was 8.1% (95%CI; 7.9−8.3%), and relative VO2max was 38.5 mL/min/kg (37.3−39.6). The pooled mean VO2max was significantly lower (36.9 vs. 40.7, p = 0.001) in studies reporting a mean HbA1c > 7.5% compared to studies with a mean HbA1c ≤ 7.5%. Weighted Pearson correlation coefficient was r = −0.19 (p < 0.001) between VO2max and HbA1c. Meta-regression adjusted for age and sex showed a significant decrease of −0.94 mL/min/kg in VO2max per HbA1c increase of 1% (p = 0.024). In conclusion, we were able to determine a statistically significant correlation between HbA1c and VO2max in individuals with T1DM. However, as the correlation was only weak, the association of HbA1c and VO2max might not be of clinical relevance in individuals with T1DM.

Post Meal Exercise May Lead to Transient Hypoglycemia Irrespective of Glycemic Status in Humans

During exercise, there is coordination between various hormonal systems to ensure glucoregulation. This study examined if hypoglycemia occurs during moderate-intensity exercise in non-obese and obese individuals with and without type 2 diabetes (T2D). Eighteen non-obese, 18 obese, and 10 obese with T2D completed 2 study days that included a meal at 1,800 h followed by rest (NOEX) or exercise (PMEX; 45 min/55% of VO₂ max 2 h post meal). Glucose, insulin, and glucagon concentrations were measured throughout this 5.5 h period. Subjects with T2D had elevated glucose responses to the meal on both study days, compared to non-obese and obese subjects (P < 0.05). During evening exercise (PMEX), subjects with T2D had a greater drop in glucose concentration (-98.4 ± 13.3 mg/dL) compared to obese (-44.8 ± 7.1 mg/dL) and non-obese (-39.3 ± 6.1 mg/dL; P < 0.01) subjects. Glucose levels decreased more so in females than males in both conditions (P < 0.01). Nadir glucose levels <70 mg/dL were observed in 33 subjects during NOEX and 39 subjects during PMEX. Obese males had a larger exercise-induced insulin drop than obese females (P = 0.01). During PMEX, peak glucagon concentrations were elevated compared to NOEX (P < 0.001). Male participants with T2D had an increased glucagon response during NOEX and PMEX compared to females (P < 0.01). In conclusion, in individuals with varying glucose tolerance, there is a dramatic drop in glucose levels during moderate-intensity exercise, despite appropriate insulin concentrations prior to exercise, and glucagon levels rising during exercise. Moderate-intensity exercise can result in low glucose concentrations (<60 mg/dL), and yet many of these individuals will be asymptomatic.

Blood glucose regulation during prolonged, submaximal, continuous exercise: a guide for clinicians

Management of many chronic diseases now includes regular exercise as part of a viable treatment plan. Exercise in the form of prolonged, submaximal, continuous exercise (SUBEX; i.e., approximately 30 min to 1 h, approximately 40-70% of maximal oxygen uptake) is often prescribed due to its relatively low risk, the willingness of patients to undertake, its efficacy, its affordability, and its ease of prescription. Specifically, patients who are insulin resistant or that have type 2 diabetes mellitus may benefit from regular exercise of this type. During this type of exercise, muscles dramatically increase glucose uptake as the liver increases both glycogenolysis and gluco-neogenesis. While a redundancy of mechanisms is at work to maintain blood glucose concentration ([glucose]) during this type of exercise, the major regulator of blood glucose is the insulin/glucagon response. At exercise onset, blood [glucose] transiently rises before beginning to decline after approximately 30 min, causing a subsequent decline in blood [insulin] and rise in blood glucagon. This leads to many downstream effects, including an increase in glucose output from the liver to maintain adequate glucose in the blood to fuel both the muscles and the brain. Finally, when analyzing blood [glucose], consideration should be given to nutritional status (postabsorptive versus postprandial) as well as both what the analyzer measures and the type of sample used (plasma versus whole blood). In view of both prescribing exercise to patients as well as designing studies that perturb glucose homeostasis, it is imperative that clinicians and researchers alike understand the controls of blood glucose homeostasis during SUBEX.

Hyperinsulinaemia during exercise does not suppress hepatic glycogen concentrations in patients with type 1 diabetes: a magnetic resonance spectroscopy study

AIMS/HYPOTHESIS

We compared in vivo changes in liver glycogen concentration during exercise between patients with type 1 diabetes and healthy volunteers.

METHODS

We studied seven men with type 1 diabetes (mean +/- SEM diabetes duration 10 +/- 2 years, age 33 +/- 3 years, BMI 24 +/- 1 kg/m(2), HbA(1c) 8.1 +/- 0.2% and VO(2) peak 43 +/- 2 ml [kg lean body mass](-1) min(-1)) and five non-diabetic controls (mean +/- SEM age 30 +/- 3 years, BMI 22 +/- 1 kg/m(2), HbA(1c) 5.4 +/- 0.1% and VO(2) peak 52 +/- 4 ml [kg lean body mass](-1) min(-1), before and after a standardised breakfast and after three bouts (EX1, EX2, EX3) of 40 min of cycling at 60% VO(2) peak. (13)C Magnetic resonance spectroscopy of liver glycogen was acquired in a 3.0 T magnet using a surface coil. Whole-body substrate oxidation was determined using indirect calorimetry.

RESULTS

Blood glucose and serum insulin concentrations were significantly higher (p < 0.05) in the fasting state, during the postprandial period and during EX1 and EX2 in subjects with type 1 diabetes compared with controls. Serum insulin concentration was still different between groups during EX3 (p < 0.05), but blood glucose concentration was similar. There was no difference between groups in liver glycogen concentration before or after the three bouts of exercise, despite the relative hyperinsulinaemia in type 1 diabetes. There were also no differences in substrate oxidation rates between groups.

CONCLUSIONS/INTERPRETATION

In patients with type 1 diabetes, hyperinsulinaemic and hyperglycaemic conditions during moderate exercise did not suppress hepatic glycogen concentrations. These findings do not support the hypothesis that exercise-induced hypoglycaemia in patients with type 1 diabetes is due to suppression of hepatic glycogen mobilisation.

Suppression of endogenous glucose production by mild hyperinsulinemia during exercise is determined predominantly by portal venous insulin

Hyperinsulinemia during exercise in people with diabetes requiring exogenous insulin is a major clinical problem. The aim of this study was to assess the significance of portal vein versus arterial insulin to hepatic effects of hyperinsulinemia during exercise. Dogs had sampling (artery, portal vein, and hepatic vein) and infusion (vena cava and portal vein) catheters and flow probes (hepatic artery and portal vein) implanted >16 days before a study. Protocols consisted of equilibration (-130 to -30 min), basal (-30 to 0 min), and treadmill exercise (0-150 min) periods. Somatostatin was infused and glucagon and insulin were replaced in the portal vein to achieve basal arterial and portal vein levels at rest and simulated levels during the first 60 min of exercise. From 60 to 150 min of exercise, the simulated insulin infusion was sustained (C; n = 7), modified to selectively create a physiologic increment in arterial insulin (Pe; n = 7), or altered to increase arterial insulin as in Pe but with a concomitant increase in portal insulin (PePo; n = 7). Euglycemic clamps were performed in all studies. Portal and arterial insulin were 15 +/- 2 and 4 +/- 1 micro U/ml (mean +/- SE of all groups), respectively, at t = 60 min in all groups. Insulin levels were unchanged for the remainder of the exercise period in C. Arterial insulin was increased from 3 +/- 1 to 14 +/- 2 micro U/ml, whereas portal insulin did not change in Pe after t = 60 min. Arterial insulin was increased from 3 +/- 1 to 15 +/- 2 micro U/ml, and portal insulin was increased from 16 +/- 3 to 33 +/- 3 micro U/ml in PePo after t = 60 min. Endogenous glucose production (R(a)) rose similarly from basal during the first 60 min of exercise in all groups (mean +/- SE of all groups was from 2.2 +/- 0.1 to 6.8 +/- 0.5 mg. kg(-1). min(-1)). The increase in R(a) was sustained for the remainder of the exercise period in C. R(a) was suppressed by approximately 40%, but only after 60 min of hyperinsulinemia, and by approximately 20% after 90 min of hyperinsulinemia in Pe. In contrast, the addition of portal venous hyperinsulinemia caused approximately 90% suppression of R(a) within 20 min and for the remainder of the experiment in PePo. Measurements of net hepatic glucose output were similar to R(a) responses in all groups. Arterial free fatty acids (FFAs), a stimulus of R(a), were increased to 1,255 +/- 258 micro mol/l in C but were only 459 +/- 67 and 312 +/- 42 micro mol/l in Pe and PePo, respectively, by 150 min of exercise. Thus, during exercise, the exquisite sensitivity of R(a) to hyperinsulinemia is due entirely to portal venous hyperinsulinemia during the first 60 min, after which peripheral hyperinsulinemia may control approximately 20-40%, possibly as a result of inhibition of the exercise-induced increase in FFA.

Decreased hepatic glucagon responses in type 1 (insulin-dependent) diabetes mellitus

The effect of glucagon infusion on hepatic glucose production during euglycaemia was evaluated in seven Type 1 (insulin-dependent) diabetic patients and in ten control subjects. In the diabetic subjects normoglycaemia was maintained during the night preceding the study by a variable intravenous insulin and glucose infusion. During the study endogenous insulin secretion was suppressed by somatostatin (450 micrograms/h) and replaced by insulin infusion (0.15 mU.kg-1.min-1). 3H-glucose was infused for isotopic determination of glucose turnover. Plasma glucose was clamped at 5 mmol/l for 2 h 30 min and glucagon (1.5 ng.kg-1.min-1) was then infused for the following 3 h. Hepatic glucose production and glucose utilisation were measured during the first, second and third hour of the glucagon infusion. Basal hepatic glucose production (just prior to glucagon infusion) was similar in diabetic (1.2 +/- 0.3 mg.kg-1.min-1) and control (1.6 +/- 0.1 mg.kg-1.min-1) subjects. In diabetic patients hepatic glucose production rose slowly to 2.1 +/- 0.5 mg.kg-1.min-1 during the first hours of glucagon infusion and stabilized at this level (2.4 +/- 0.5 mg.kg-1.min-1) in the third hour. In control subjects hepatic glucose production increased sharply to higher levels than in the diabetic subjects (3.4 +/- 0.3 mg.kg-1.min-1) during the first and second hour of glucagon infusion (p less than 0.05) and then gradually fell (2.9 +/- 0.4 mg.kg-1.min-1) during the third hour. In conclusion, when stimulated with glucagon at a physiologic plasma concentration diabetic patients had 1) an overall reduced hepatic glucose production response and 2) an abnormal sluggish response pattern.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma glucose metabolism during exercise in humans

Plasma glucose is an important energy source in exercising humans, supplying between 20 and 50% of the total oxidative energy production and between 25 and 100% of the total carbohydrate oxidised during submaximal exercise. Plasma glucose utilisation increases with the intensity of exercise, due to an increase in glucose utilisation by each active muscle fibre, an increase in the number of active muscle fibres, or both. Plasma glucose utilisation also increases with the duration of exercise, thereby partially compensating for the progressive decrease in muscle glycogen concentration. When compared at the same absolute exercise intensity (i.e. the same VO2), reliance on plasma glucose is also greater during exercise performed with a small muscle mass, i.e. with the arms or just 1 leg. This may be due to differences in the relative exercise intensity (i.e. the %VO2peak), or due to differences between the arms and legs in their fitness for aerobic activity. The rate of plasma glucose utilisation is decreased when plasma free fatty acid or muscle glycogen concentrations are very high, effects which are probably mediated by increases in muscle glucose-6-phosphate concentration. However, glucose utilisation is also reduced during exercise following a low carbohydrate diet, despite the fact that muscle glycogen is also often lower. When exercise is performed at the same absolute intensity before and after endurance training, plasma glucose utilisation is lower in the trained state. During exercise performed at the same relative intensity, however, glucose utilisation may be lower, the same, or actually higher in trained than in untrained subjects, because of the greater absolute VO2 and demand for substrate in trained subjects during exercise at a given relative exercise intensity. Although both hyperglycaemia and hypoglycaemia may occur during exercise, plasma glucose concentration usually remains relatively constant. Factors which increase or decrease the reliance of peripheral tissues on plasma glucose during exercise are therefore generally accompanied by quantitatively similar increases or decreases in glucose production. These changes in total glucose production are mediated by changes in both hepatic glycogenolysis and hepatic gluconeogenesis. Glycogenolysis dominates under most conditions, and is greatest early in exercise, during high intensity exercise, or when dietary carbohydrate intake is high. The rate of gluconeogenesis is increased when exercise is prolonged, preceded by a restricted carbohydrate intake, or performed with the arms. Both glycogenolysis and gluconeogenesis appear to be decreased by endurance exercise training. These effects are due to changes in both the hormonal milieu and in the availability of hepatic glycogen and gluconeogenic precursors.(ABSTRACT TRUNCATED AT 400 WORDS)

Physiological bases for the treatment of the physically active individual with diabetes

Substrate utilisation and glucose homoeostasis during exercise is controlled by the effects of precise changes in insulin, glucagon and the catecholamines. The important role these hormones play is clearly seen in people with diabetes, as the normal endocrine response is often lost. In individuals with insulin-dependent diabetes (IDDM), there can be an increased risk of hypoglycaemia during or after exercise or, conversely, there can be a worsening of the diabetic state if insulin deficiency is present. In contrast, it appears that people with non-insulin-dependent diabetes (NIDDM) can generally exercise without fear of a deleterious metabolic response. The exercise response both in healthy subjects and in those with diabetes is dependent on many factors such as age, nutritional status and the duration and intensity of exercise. Since there are so many variables which govern individual response to exercise, an exact exercise prescription for all people with diabetes cannot be made. There are many adjustments to the therapeutic regimen which an individual with IDDM can make in order to avoid hypoglycaemia during or after exercise. In general, a reduction in insulin dosage and the added ingestion and continual availability of carbohydrates are wise precautions. On the other hand, exercise should be postponed if blood glucose is greater than 2500 mg/L and ketones are present in the urine. As more is understood about the regulation of substrate metabolism during exercise, more refined therapeutic strategies can be defined. An understanding of the metabolic response to exercise is critical for generating an effective and safe training programme for all diabetic individuals who wish to be physically active.