Comparison of glucose tolerance tests to detect the insulin sensitizing effects of a bout of continuous exercise

Glucose volume of distribution affects insulin sensitivity measured by intravenous glucose tolerance test

AIM

To determine whether glucose volume of distribution (VdGLUCOSE ) affects the diagnosis of impaired insulin sensitivity (IS) when using an intravenous glucose tolerance test (IVGTT).

METHODS

Individuals with distinct levels of IS underwent IVGTT after an overnight fast. The prediabetic group (Prediab; n = 33) differed from the healthy group (Healthy; n = 14) in their larger glycosylated hemoglobin (HbA1c of 5.9 ± 0.3 vs. 5.4 ± 0.1%; 41 ± 4 vs. 36 ± 1 mmol/mol; p < 0.001), percent body fat (37 ± 6 vs. 24 ± 3%; p < 0.001) and cardiovascular fitness level (VO2MAX 22 ± 5 vs. 44 ± 5 mL of O2 ·kg-1 ·min-1 ; p < 0.001). Ten minutes after intravenous infusion of the glucose bolus (i.e., 35 g in a 30% solution), VdGLUCOSE was assessed from the increases in plasma glucose concentration. IS was calculated during the next 50 min using the slope of glucose disappearance and the insulin time-response curve.

RESULTS

VdGLUCOSE was higher in Healthy than in Prediab (230 ± 49 vs. 185 ± 21 mL·kg-1 ; p < 0.001). VdGLUCOSE was a strong predictor of IS (β standardized coefficient 0.362; p = 0.004). VO2MAX was associated with VdGLUCOSE and IS (Pearson r = 0.582 and 0.704, respectively; p < 0.001). However, body fat was inversely associated with VdGLUCOSE and IS (r = -0.548 and -0.555, respectively; p < 0.001).

CONCLUSIONS

Since fat mass is inversely related to VdGLUCOSE and in turn, VdGLUCOSE affects the calculations of IS, the IV glucose bolus dose should be calculated based on fat-free mass rather than body weight for a more accurate diagnosis of impaired IS.

Exercise improves metformin 72-h glucose control by reducing the frequency of hyperglycemic peaks

PURPOSE

To determine the separated and combined effects of metformin and exercise on insulin sensitivity and free-living glycemic control in overweight individuals with prediabetes/type 2 diabetes (T2DM).

METHODS

We recruited 16 adults with BMI of 32.7 ± 4.3 kg m^-2 and insulin resistance (HOMA-IR 3.2 ± 0.4) under chronic metformin treatment (1234 ± 465 g day^-1) enrolled in a high-intensity interval training (HIIT) program. Participants underwent four 72-h experimental trials in a random-counterbalanced order: (1) maintaining their habitual metformin treatment (MET); (2) replacing metformin treatment by placebo (CON); (3) placebo plus two HIIT sessions (EX + CON), and (4) metformin plus two HIIT sessions (MET + EX). We used intermittently scanned continuous glucose monitoring (isCGM) during 72 h in every trial to obtain interstitial fluid glucose area under the curve (IFG_AUC) and the percentage of measurements over 180 mg dL^-1 (% IFG_PEAKS). Insulin sensitivity was assessed on the last day of each trial with HOMA-IR index and calculated insulin sensitivity (C_SI) from intravenous glucose tolerance test.

RESULTS

IFG_AUC was lower in MET + EX and MET than in CON (P = 0.011 and P = 0.025, respectively). In addition, IFG_AUC was lower in MET + EX than in EX + CON (P = 0.044). %IFG_PEAKS were only lower in MET + EX in relation to CON (P = 0.028). HOMA-IR and C_SI were higher in CON in comparison with MET + EX (P = 0.011 and P = 0.022, respectively) and MET (P = 0.006 and P < 0.001, respectively). IFG_AUC showed a significant correlation with HOMA-IR.

CONCLUSION

Intense aerobic exercise in patients with diabetes and prediabetes under metformin treatment reduces free-living 72-h blood hyperglycemic peaks. This may help to prevent the development of cardiovascular complications associated with diabetes.

Effectiveness of Aerobic Exercise Programs for Health Promotion in Metabolic Syndrome

PURPOSE

Continuous and interval are the two types of aerobic exercise training commonly used for health promotion. We sought to determine which aerobic exercise training program results in larger health improvements in metabolic syndrome (MetS) individuals.

METHODS

One hundred twenty-one MetS patients (age, 57 ± 8 yr; weight, 92 ± 15 kg; and MetS factors, 3.8 ± 0.8 components) with low initial cardiorespiratory fitness (CRF) (V˙O2peak, 24.0 ± 5.5 mL·kg·min) were randomized to undergo one of the following 16-wk exercise program: (a) 4 × 4-min high-intensity interval training at 90% of HRMAX (4HIIT group; n = 32), (b) 50-min moderate-intensity continuous training at 70% of HRMAX (MICT group; n = 35), (c) 10 × 1-min HIIT at 100% of HRMAX (1HIIT group; n = 32), or (d) no exercise control group (CONT; n = 22). We measured the evolution of all five MetS components (i.e., MetS Z Score) and CRF (assessed by V˙O2peak) before and after intervention.

RESULTS

MetS Z score decreased 41% after 4HIIT (95% confidence interval [CI], 0.25-0.06; P < 0.01) and 52% in MICT (95% CI, 0.24-0.06; P < 0.01), whereas it did not change in 1HIIT (decreased 24%; 95% CI, -0.16 to 0.03; P = 0.21) and CONT (increased 20%; 95% CI, -0.19 to 0.04; P = 0.22). However, the three exercise groups improved similarly their V˙O2peak (4HIIT, 11%; 95% CI, 0.14-0.33; MICT, 12%; 95% CI, 0.18-0.36; and 1HIIT, 14%; 95% CI, 0.21-0.40 L·min; all P < 0.001).

CONCLUSIONS

Our findings suggest that in sedentary individuals with MetS and low initial CRF level any aerobic training program of 16 wk with a frequency of three times per week is sufficient stimulus to raise CRF. However, the more intense but shorter 1HIIT training program is not effective on improving MetS Z score, and thus we caution its recommendation for health promotion purposes in this population.

Effects of Consuming Almonds on Insulin Sensitivity and Other Cardiometabolic Health Markers in Adults With Prediabetes

Objective: This study was designed to assess the effects of replacing high-carbohydrate (CHO) foods with raw almonds on insulin sensitivity and cardiometabolic health markers in overweight or obese adults with prediabetes.Method: This randomized crossover study consisted of two 6-week dietary intervention periods, separated by a ≥ 4-week washout. Subjects incorporated 1.5 oz of raw almonds twice daily or isocaloric CHO-based foods into their diets, with instructions to maintain body weight. Dietary intakes as well as insulin sensitivity, CHO metabolism indices, lipoprotein lipids and particles, and inflammatory markers were assessed.Results: Thirty-three subjects (17 male, 16 female), mean age 48.3 ± 2.2 years and body mass index 30.5 ± 0.7 kg/m², provided evaluable data. Compared to CHO, almonds resulted in significantly (p < 0.01) higher intakes of protein, fat (unsaturated fatty acids), fiber, and magnesium and significantly (p < 0.001) lower intakes of CHO and sugars. No differences were observed between diet conditions for changes from baseline in the insulin sensitivity index from a short intravenous glucose tolerance test or other indices of glucose homeostasis. No significant differences were observed in biomarkers of cardiovascular risk except that the CHO intervention led to a shift toward a higher concentration of cholesterol in small, dense low-density lipoprotein subfraction 3+4 (LDL3 + 4) particles (p = 0.024 vs almonds).Conclusions: Intake of 3.0 oz/d raw almonds, vs energy-matched CHO foods, improved the dietary nutrient profile, but did not significantly affect insulin sensitivity and most markers of cardiometabolic health in overweight and obese men and women with prediabetes.

Exercise intervention alters HDL subclass distribution and function in obese women

BACKGROUND

Obesity is associated with a change in high-density lipoprotein (HDL) function and subclass. Exercise training reduces cardiovascular risk in obese patients. We aimed to explore the effect of an exercise training stimulus on HDL functionality and subclass in obese women.

METHODS

Thirty-two obese black South African women were randomly assigned to exercise (combined aerobic and resistance exercise) or control (no exercise) conditions for 12-weeks. Pre- and post-testing included venous blood sampling for analysis of lipid profile and HDL functionality, by measuring cellular cholesterol efflux capacity, reduction in endothelial vascular cell adhesion molecule (VCAM) expression (anti-inflammatory function), paraoxonase (PON) (antioxidative function) and platelet activating factor acetylhydrolase (PAF-AH) activities (anti-thrombotic function). PON-1 and PAF-AH expression were determined in serum and in isolated HDL using Western blotting. Levels of large, intermediate and small HDL subclasses were measured using the Lipoprint® system.

RESULTS

Exercise training resulted in a decrease in body mass index (- 1.0 ± 0.5% vs + 1.2 ± 0.6%, p = 0.010), PON activity (- 8.7 ± 2.4% vs + 1.1 ± 3.0%, p = 0.021), PAF-AH serum expression (- 22.1 ± 8.0% vs + 16.9 ± 9.8, p = 0.002), and the distribution of small HDL subclasses (- 10.1 ± 5.4% vs + 15.7 ± 6.6%, p = 0.004) compared to controls. Exercise did not alter HDL cellular cholesterol efflux capacity and anti-inflammatory function.

CONCLUSIONS

These results demonstrate the potential for exercise training to modify HDL subclass distribution and HDL function in obese women.

TRIAL REGISTRATION

Clinical trials number: PACTR201711002789113 .

Changes in markers for cardio-metabolic disease risk after only 1-2 weeks of a high saturated fat diet in overweight adults

Purpose

Diets high in saturated fat acids (SFA) have been linked with cardio-metabolic disease risk. The purpose of this study was to determine whether only 1–2 weeks of a high SFA diet could impact disease risk factors in overweight adults who normally eat a relatively low proportion of SFA (i.e., <40% of dietary fat).

Methods

Twelve overweight (BMI: 27±1 kg/m²) young adults were studied before and after a 2-week diet that increased the proportion of SFA (<40% to 60% of dietary fat), while maintaining their daily intake of total fat, carbohydrate, protein, and calories. Insulin resistance, blood pressure, plasma markers of liver damage, total plasma cholesterol concentrations, and fatty acid profile within plasma and skeletal muscle lipid pools were assessed before and after the intervention.

Results

Total plasma cholesterol concentration increased (148±5 vs. 164±8 mg/dl; P<0.05) after only one week, due exclusively to an increase in LDL-cholesterol (78±4 vs. 95±7 mg/dl; P<0.05). After two weeks, plasma aspartate amino transferase (AST) concentration increased (P<0.05) but we found no change in insulin resistance, or resting blood pressure. The diet increase the proportion of SFA in plasma (35±1% vs. 39±2%; P<0.05) and the intramyocellular triglyceride pool (32±1% vs. 37±1%; P<0.05) suggesting the fatty acids in these pools may readily exchange.

Conclusions

Although blood lipids remain within normal clinical range, increasing saturated fat in diet for only 2 weeks raises plasma markers of cardiovascular risk (LDL-cholesterol) and liver damage (AST). In overweight, but healthy-young adults SFA accumulate in plasma and muscle after only 1–2 weeks of dietary increase.

An Exercise Intervention to Unravel the Mechanisms Underlying Insulin Resistance in a Cohort of Black South African Women: Protocol for a Randomized Controlled Trial and Baseline Characteristics of Participants

Background

The pathogenesis of type 2 diabetes (T2D) in black African women is complex and differs from that in their white counterparts. However, earlier studies have been cross-sectional and provide little insight into the causal pathways. Exercise training is consistently used as a model to examine the mechanisms underlying insulin resistance and risk for T2D.

Objective

The objective of the study was to examine the mechanisms underlying the changes in insulin sensitivity and secretion in response to a 12-week exercise intervention in obese black South African (SA) women.

Methods

A total of 45 obese (body mass index, BMI: 30-40 kg/m²) black SA women were randomized into a control (n=22) or experimental (exercise; n=23) group. The exercise group completed 12 weeks of supervised combined aerobic and resistance training (40-60 min, 4 days/week), while the control group maintained their typical physical activity patterns, and both groups were requested not to change their dietary patterns. Before and following the 12-week intervention period, insulin sensitivity and secretion (frequently sampled intravenous glucose tolerance test) and its primary and secondary determinants were measured. Dietary intake, sleep quality and quantity, physical activity, and sedentary behaviors were measured every 4 weeks.

Results

The final sample included 20 exercise and 15 control participants. Baseline sociodemographics, cardiorespiratory fitness, anthropometry, cardiometabolic risk factors, physical activity, and diet did not differ between the groups (P>.05).

Conclusions

The study describes a research protocol for an exercise intervention to understand the mechanisms underlying insulin sensitivity and secretion in obese black SA women and aims to identify causal pathways underlying the high prevalence of insulin resistance and risk for T2D in black SA women, targeting specific areas for therapeutic intervention.

Trial Registration

Pan African Clinical Trial Registry PACTR201711002789113; http://www.pactr.org/ATMWeb/ appmanager/atm/atmregistry?_nfpb=true&_pageLabel=portals_app_atmregistry_portal_page_13 (Archived by WebCite at http://www.webcitation.org/6xLEFqKr0)

Dietary supplementation with omega-3 fatty acids and oleate enhances exercise training effects in patients with metabolic syndrome

OBJECTIVE

We studied the effects of exercise training alone or combined with dietary supplementation of omega-3 polyunsaturated fatty acids (Ω-3PUFA) and oleate on metabolic syndrome (MSyn) components and other markers of cardiometabolic health.

METHODS

Thirty-six patients with MSyn underwent 24 weeks of high-intensity interval training. In a double-blind randomized design, half of the group ingested 500 mL/day of semi-skim milk (8 g of fat; placebo milk) whereas the other half ingested 500 mL/day of skim milk enriched with 275 mg of Ω-3PUFA and 7.5 g of oleate (Ω-3 + OLE).

RESULTS

Ω-3 + OLE treatment elevated 30% plasma Ω-3PUFA but not significantly (P = 0.286). Improvements in VO2peak (12.8%), mean blood pressure (-7.1%), waist circumference (-1.8%), body fat mass (-2.9%), and trunk fat mass (-3.3%) were similar between groups. However, insulin sensitivity (measured by intravenous glucose tolerance test), serum concentration of C-reactive protein, and high-density lipoprotein improved only in the Ω-3 + OLE group by 31.5%, 32.1%, and 10.3%, respectively (all P < 0.05). Fasting serum triacylglycerol, glucose, and plasma fibrinogen concentrations did not improve in either group after 24 weeks of intervention.

CONCLUSIONS

Diet supplementation with Ω-3PUFA and oleate enhanced cardiometabolic benefits of intense aerobic exercise training in patients with MSyn.

Twelve Weeks of Sprint Interval Training Improves Indices of Cardiometabolic Health Similar to Traditional Endurance Training despite a Five-Fold Lower Exercise Volume and Time Commitment

Aims

We investigated whether sprint interval training (SIT) was a time-efficient exercise strategy to improve insulin sensitivity and other indices of cardiometabolic health to the same extent as traditional moderate-intensity continuous training (MICT). SIT involved 1 minute of intense exercise within a 10-minute time commitment, whereas MICT involved 50 minutes of continuous exercise per session.

Methods

Sedentary men (27±8y; BMI = 26±6kg/m²) performed three weekly sessions of SIT (n = 9) or MICT (n = 10) for 12 weeks or served as non-training controls (n = 6). SIT involved 3x20-second ‘all-out’ cycle sprints (~500W) interspersed with 2 minutes of cycling at 50W, whereas MICT involved 45 minutes of continuous cycling at ~70% maximal heart rate (~110W). Both protocols involved a 2-minute warm-up and 3-minute cool-down at 50W.

Results

Peak oxygen uptake increased after training by 19% in both groups (SIT: 32±7 to 38±8; MICT: 34±6 to 40±8ml/kg/min; p<0.001 for both). Insulin sensitivity index (CS_I), determined by intravenous glucose tolerance tests performed before and 72 hours after training, increased similarly after SIT (4.9±2.5 to 7.5±4.7, p = 0.002) and MICT (5.0±3.3 to 6.7±5.0 x 10⁻⁴ min^-1 [μU/mL]^-1, p = 0.013) (p<0.05). Skeletal muscle mitochondrial content also increased similarly after SIT and MICT, as primarily reflected by the maximal activity of citrate synthase (CS; P<0.001). The corresponding changes in the control group were small for VO₂peak (p = 0.99), CS_I (p = 0.63) and CS (p = 0.97).

Conclusions

Twelve weeks of brief intense interval exercise improved indices of cardiometabolic health to the same extent as traditional endurance training in sedentary men, despite a five-fold lower exercise volume and time commitment.

High intensity interval exercise is an effective alternative to moderate intensity exercise for improving glucose tolerance and insulin sensitivity in adolescent boys

OBJECTIVES

High-intensity interval exercise (HIIE) may offer a time efficient means to improve health outcomes compared to moderate-intensity exercise (MIE). This study examined the acute effect of HIIE compared to a work-matched bout of MIE on glucose tolerance, insulin sensitivity (IS), resting fat oxidation and exercise enjoyment in adolescent boys.

DESIGN

Within-measures design with counterbalanced experimental conditions.

METHODS

Nine boys (14.2 ± 0.4 years) completed three conditions on separate days in a counterbalanced order: (1) HIIE; (2) work matched MIE, both on a cycle ergometer; and (3) rest (CON). An oral glucose tolerance test (OGTT) was performed after exercise or rest and the area under curve (AUC) responses for plasma [glucose] and [insulin] were calculated, and IS estimated (Cederholm index). Energy expenditure and fat oxidation were measured following the OGTT using indirect calorimetry. Exercise enjoyment was assessed using the Physical Activity Enjoyment Scale.

RESULTS

The incremental AUC (iAUC) for plasma [glucose] was reduced following both MIE (-23.9%, P = 0.013, effect size [ES] = -0.64) and HIIE (-28.9%, P=0.008, ES = -0.84) compared to CON. The iAUC for plasma [insulin] was lower for HIIE (-24.2%, P = 0.021, ES = -0.71) and MIE (-29.1%, P = 0.012, ES = -0.79) compared to CON. IS increased by 11.2% after HIIE (P = 0.03, ES = 0.76) and 8.4% after MIE (P = 0.10, ES = 0.58). There was a trend for an increase in fat oxidation following HIIE (P = 0.097, ES = 0.70). Both HIIE and MIE were rated as equally enjoyable (P > 0.05, ES < 0.01).

CONCLUSION

A single bout of time efficient HIIE is an effective alternative to MIE for improving glucose tolerance and IS in adolescent boys immediately after exercise.

Metformin does not attenuate the acute insulin-sensitizing effect of a single bout of exercise in individuals with insulin resistance

Combining metformin and exercise is recommended for the treatment of insulin resistance. However, it has been suggested that metformin blunts the insulin-sensitizing effects of exercise. We evaluated in a group of insulin-resistant patients the interactions between exercise and their daily dose of metformin. Ten insulin-resistant patients underwent insulin sensitivity assessment using intravenous glucose tolerance test after an overnight fast in the following conditions: (1) after taking their habitual morning dose of metformin (MET), (2) after 45 min of high intensity interval exercise having withheld metformin during 24 h (EX), and (3) after their habitual metformin dose plus an identical exercise bout (MET + EX). During the exercise trials (EX and MET + EX), energy expenditure and substrate oxidation were assessed by indirect calorimetry. In addition, 12-h postprandial blood glucose was measured in all three trials. Insulin sensitivity was similar between MET and EX [4.0 ± 0.8 and 4.1 ± 0.7 × 10(-4) min(-1) (μU mL)(-1); P = 0.953] but was 43 % higher than both MET and EX after MET + EX (NS; P = 0.081). Blood glucose disappearance rate was higher after MET + EX than after MET or EX trials (1.7 ± 0.2, 1.0 ± 0.1, and 1.2 ± 0.1 % min(-1), respectively; P = 0.020). There was no difference in postprandial blood glucose concentration after the three meals that followed the trials (P = 0.446). Exercise energy expenditure was 9 % higher during MET + EX than during EX (P = 0.015) partly due to increased carbohydrate oxidation. Our data suggest that habitual metformin treatment in insulin-resistant patients does not blunt the acute insulin-sensitizing effects of a single bout of exercise that on the contrary, tends to enhance it.