Glucose kinetics differ between women and men, during and after exercise

Sex Differences in Energy Metabolism: A Female-Oriented Discussion

The purpose of this review is to delineate aspects of energy metabolism at rest and during exercise that may be subject to sex differences and the potential underlying mechanisms involved. It focuses on distinct aspects of female physiology with an oriented discussion following the reproductive life stages of healthy, eumenorrheic females, including premenopausal time frames, pregnancy, perimenopause, and menopause. Finally, this review aims to address methodological challenges surrounding sexual dimorphism in energy metabolism investigations and confounding factors in this field. During resting conditions, females tend to have higher rates of non-oxidative free fatty acid clearance, which could contribute to lower respiratory exchange ratio measures. At the same time, carbohydrate energy metabolism findings are mixed. In general, females favor lipid energy metabolism during moderate-intensity exercise, while men favor carbohydrate energy metabolism. Factors such as age, dietary intake, genetics, and methodological decisions confound study findings, including properly identifying and reporting the menstrual cycle phase when female subjects are eumenorrheic. Pregnancy presents a unique shift in physiological systems, including energy metabolism, which can be observed at rest and during exercise. Changes in body composition and hormonal levels during the post-menopausal period directly impact energy metabolism, specifically lipid metabolism. This change in physiological state factors into the evidence showing a reduction in our understanding of sex differences in lipid metabolism during exercise in older adults. This review reveals a need for a focused understanding of female energy metabolism that could help exercise and nutrition professionals optimize female health and performance across the lifespan.

Sexual Dimorphism in Substrate Metabolism During Exercise

During aerobic exercise, women oxidize significantly more lipids and less carbohydrates than men. This sexual dimorphism in substrate metabolism has been attributed, in part, to the observed differences in epinephrine and glucagon levels between men and women during exercise. To identify the underpinning candidate physiological mechanisms for these sex differences, we developed a sex-specific multi-scale mathematical model that relates cellular metabolism in the organs to whole-body responses during exercise. We conducted simulations to test the hypothesis that sex differences in the exercise-induced changes to epinephrine and glucagon would result in the sexual dimorphism of hepatic metabolic flux rates via the glucagon-to-insulin ratio (GIR). Indeed, model simulations indicate that the shift towards lipid metabolism in the female model is primarily driven by the liver. The female model liver exhibits resistance to GIR-mediated glycogenolysis, which helps maintain hepatic glycogen levels. This decreases arterial glucose levels and promotes the oxidation of free fatty acids. Furthermore, in the female model, skeletal muscle relies on plasma free fatty acids as the primary fuel source, rather than intramyocellular lipids, whereas the opposite holds true for the male model.

The Impact of Sex, Body Mass Index, Age, Exercise Type and Exercise Duration on Interstitial Glucose Levels during Exercise

The impact of age, sex and body mass index on interstitial glucose levels as measured via continuous glucose monitoring (CGM) during exercise in the healthy population is largely unexplored. We conducted a multivariable generalized estimating equation (GEE) analysis on CGM data (Dexcom G6, 10 days) collected from 119 healthy exercising individuals using CGM with the following specified covariates: age; sex; BMI; exercise type and duration. Females had lower postexercise glycemia as compared with males (92 ± 18 vs. 100 ± 20 mg/dL, p = 0.04) and a greater change in glycemia during exercise from pre- to postexercise (p = 0.001) or from pre-exercise to glucose nadir during exercise (p = 0.009). Younger individuals (i.e., <20 yrs) had higher glucose during exercise as compared with all other age groups (all p < 0.05) and less CGM data in the hypoglycemic range (<70 mg/dL) as compared with those aged 20-39 yrs (p < 0.05). Those who were underweight, based on body mass index (BMI: <18.5 kg/m2), had higher pre-exercise glycemia than the healthy BMI group (104 ± 20 vs. 97 ± 17 mg/dL, p = 0.02) but similar glucose levels after exercise. Resistance exercise was associated with less of a drop in glycemia as compared with aerobic or mixed forms of exercise (p = 0.008) and resulted in a lower percent of time in the hypoglycemic (p = 0.04) or hyperglycemic (glucose > 140 mg/dL) (p = 0.03) ranges. In summary, various factors such as age, sex and exercise type appear to have subtle but potentially important influence on CGM measurements during exercise in healthy individuals.

Estimation of cardiorespiratory fitness using heart rate and step count data

Predicting cardiorespiratory fitness levels can be useful for measuring progress in an exercise program as well as for stratifying cardiovascular risk in asymptomatic adults. This study proposes a model to predict fitness level in terms of maximal oxygen uptake using anthropometric, heart rate, and step count data. The model was trained on a diverse cohort of 3115 healthy subjects (1035 women and 2080 men) aged 42 ± 10.6 years and tested on a cohort of 779 healthy subjects (260 women and 519 men) aged 42 ± 10.18 years. The developed model is capable of making accurate and reliable predictions with the average test set error of 3.946 ml/kg/min. The maximal oxygen uptake labels were obtained using wearable devices (Apple Watch and Garmin) during recorded workout sessions. Additionally, the model was validated on a sample of 10 subjects with maximal oxygen uptake determined directly using a treadmill protocol in a laboratory setting and showed an error of 4.982 ml/kg/min. Unlike most other models, which use accelerometer readings as additional input data, the proposed model relies solely on heart rate and step counts-data readily available on the majority of fitness trackers. The proposed model provides a point estimation and a probabilistic prediction of cardiorespiratory fitness level, thus it can estimate the prediction's uncertainty and construct confidence intervals.

The impact of natural menstrual cycle and oral contraceptive pill phase on substrate oxidation during rest and acute submaximal aerobic exercise

Previous research has identified sex differences in substrate oxidation during submaximal aerobic exercise including a lower respiratory exchange ratio (RER) in females compared with males. These differences may be related to differences in sex hormones. Our purpose was to examine the impact of the natural menstrual cycle (NAT) and second- and third-generation oral contraceptive pill (OCP2 and OCP3) cycle phases on substrate oxidation during rest and submaximal aerobic exercise. Fifty female participants (18 NAT, 17 OCP2, and 15 OCP3) performed two experimental trials that coincided with the low (i.e., nonactive pill/early follicular) and the high hormone (i.e., active pill/midluteal) phase of their cycle. RER and carbohydrate and lipid oxidation rates were determined from gas exchange measurements performed during 10 min of supine rest, 5 min of seated rest, and two 8-min bouts of submaximal cycling exercise at ∼40% and ∼65% of peak oxygen uptake (V̇o2peak). For all groups, there were no differences in RER between the low and high hormone phases during supine rest (0.73 ± 0.05 vs. 0.74 ± 0.05), seated rest (0.72 ± 0.04 vs. 0.72 ± 0.04), exercise at 40% (0.77 ± 0.04 vs. 0.78 ± 0.04), and 65% V̇o2peak (0.85 ± 0.04 vs. 0.86 ± 0.03; P > 0.19 for all). Similarly, carbohydrate and lipid oxidation rates remained largely unchanged across phases during both rest and exercise, apart from higher carbohydrate oxidation in NAT vs. OCP2 at 40% V̇o2peak (P = 0.019) and 65% V̇o2peak (P = 0.001). NAT and OCPs do not appear to largely influence substrate oxidation at rest and during acute submaximal aerobic exercise.NEW & NOTEWORTHY This study was the first to examine the influence of NAT and two generations of OCPs on substrate oxidation during rest and acute submaximal aerobic exercise. We reported no differences across cycle phases or groups on RER, and minimal impact on carbohydrate or lipid oxidation apart from an increase in carbohydrate oxidation in NAT compared with OCP2 during exercise. Based on these findings, NAT/OCP phase controls may not be necessary in studies investigating substrate oxidation.

Circulating levels of all proglucagon-derived peptides are differentially regulated postprandially by obesity status and in response to high-fat meals vs. high-carbohydrate meals

BACKGROUND & AIMS

We measured all proglucagon-derived peptides (PGDPs) levels in response to administration of three mixed meal tolerance tests (MMTs), examining differences in postprandial PGDP responses in subjects with leanness and obesity or between high-fat vs. high carbohydrate meals.

METHODS

We designed three physiology interventional studies, administering MMTs over a 180-min period to individuals without diabetes after an overnight fast. In Study 1, a 450 kcal MMT was administered to n = 4 normal weight and n = 9 individuals with obesity. In Study 2, a 600 kcal high-fat MMT was administered to n = 15 normal-weight and n = 15 individuals with obesity. In Study 3, n = 32 participants with obesity were assigned to receive a 600-kcal high-fat (n = 15) or an isocaloric high-carbohydrate MMT (n = 17). Fasting and postprandial levels of c-peptide and PGDPs (proglucagon, GLP-1, GLP-2, glicentin, oxyntomodulin, glucagon, major proglucagon fragment [MPGF]) were assessed.

RESULTS

In study 1, individuals with normal weight displayed elevated glicentin postprandial secretion compared with people with obesity (p = 0.002). Following a high-fat MMT with 33% higher energy content in study 2, all postprandial PGDPs levels were elevated (p-time<0.001), irrespective of weight status. In study 3, a prolonged postprandial upregulation of PGDPs during the high-fat MMT was observed in contrast with the acute, short-term (max 60 min) PGDP responses to a high-carbohydrate MMT (p-time∗meal<0.001). Across both studies 2 and 3, the postprandial responses of glucagon and MPGF were higher in subjects with male sex whereas glicentin was higher in subjects with female sex.

CONCLUSIONS

Fat and carbohydrate content of a meal can substantially affect the postprandial levels of PGDPs. Circulating levels of PGDPs are influenced by the energy content of the meal, and additionally, the presence of leanness or obesity affects circulating levels of select PGDPs. These results, which are to be confirmed by additional studies, expand our understanding of PGDP physiology in leanness and obesity.

CLINICALTRIALS

GOV REGISTRATION NUMBERS: (NCT04170010, NCT04430946, NCT04575194).

Continuous Glucose Monitoring in Healthy Adults—Possible Applications in Health Care, Wellness, and Sports

Introduction: Continuous glucose monitoring (CGM) systems were primarily developed for patients with diabetes mellitus. However, these systems are increasingly being used by individuals who do not have diabetes mellitus. This mini review describes possible applications of CGM systems in healthy adults in health care, wellness, and sports. Results: CGM systems can be used for early detection of abnormal glucose regulation. Learning from CGM data how the intake of foods with different glycemic loads and physical activity affect glucose responses can be helpful in improving nutritional and/or physical activity behavior. Furthermore, states of stress that affect glucose dynamics could be made visible. Physical performance and/or regeneration can be improved as CGM systems can provide information on glucose values and dynamics that may help optimize nutritional strategies pre-, during, and post-exercise. Conclusions: CGM has a high potential for health benefits and self-optimization. More scientific studies are needed to improve the interpretation of CGM data. The interaction with other wearables and combined data collection and analysis in one single device would contribute to developing more precise recommendations for users.

Analysis of sex-based differences in energy substrate utilization during moderate-intensity aerobic exercise

PURPOSE

To explore sex-based differences in energy substrate utilization during moderate-intensity aerobic exercise; to identify the underpinning candidate physiological mechanisms.

METHODS

Three databases were searched from inception to August 2020. Pertinent studies quantifying the utilization of substrates during moderate aerobic exercise in healthy men and reproductive-age women were considered. Studies conducted on sedentary/recreationally active and athletic populations were included and analyzed separately.

RESULTS

Thirty-five studies entered the meta-analysis (21 in sedentary/recreationally active, 14 in athletic populations). Compared to women, the respiratory exchange ratio was significantly higher both in sedentary (mean difference, MD: + 0.03; p < 0.00001) and athletic men (MD: + 0.02; p < 0.0001). Greater carbohydrate oxidation was observed both in sedentary (standardized MD, SMD: 0.53; p = 0.006) and athletic men (SMD: 1.24; p < 0.00001). Regarding lipid substrates, sedentary men oxidized less fat than women (SMD:  - 0.77; p = 0.0002), while no sex-based differences in fat oxidation were observed in athletes (SMD: 0.06; p = 0.77). Paucity of data prevented robust meta-analyses for protein sources. Sex hormones and different adrenergic activation were the most cited mechanisms to discuss sex-based differences.

CONCLUSIONS

Meta-analyses confirmed that men display greater reliance on carbohydrates while women rely more on lipids to sustain moderate aerobic exercise. The latter finding was not confirmed in athletes, a novel aspect of the present study. Mechanistically driven research is needed to further dissect the physiological underpinnings of sex differences in substrate utilization during aerobic exercise, especially for proteins, which are still less investigated than other substrates.

Wearable Glucose Monitoring and Implantable Drug Delivery Systems for Diabetes Management

The global cost of diabetes care exceeds $1 trillion each year with more than $327 billion being spent in the United States alone. Despite some of the advances in diabetes care including continuous glucose monitoring systems and insulin pumps, the technology associated with managing diabetes has largely remained unchanged over the past several decades. With the rise of wearable electronics and novel functional materials, the field is well-poised for the next generation of closed-loop diabetes care. Wearable glucose sensors implanted within diverse platforms including skin or on-tooth tattoos, skin-mounted patches, eyeglasses, contact lenses, fabrics, mouthguards, and pacifiers have enabled noninvasive, unobtrusive, and real-time analysis of glucose excursions in ambulatory care settings. These wearable glucose sensors can be integrated with implantable drug delivery systems, including an insulin pump, glucose responsive insulin release implant, and islets transplantation, to form self-regulating closed-loop systems. This review article encompasses the emerging trends and latest innovations of wearable glucose monitoring and implantable insulin delivery technologies for diabetes management with a focus on their advanced materials and construction. Perspectives on the current unmet challenges of these strategies are also discussed to motivate future technological development toward improved patient care in diabetes management.

Correlations between Basal Trace Minerals and Hormones in Middle and Long-Distance High-Level Male Runners

Several essential trace minerals play an important role in the endocrine system; however, toxic trace minerals have a disruptive effect. The aim of this research was to determine basal concentrations and the possible correlations between trace minerals in plasma and several plasma hormones in runners. Sixty high-level male endurance runners (21 ± 3 years; 1.77 ± 0.05 m; 64.97 ± 7.36 kg) participated in the present study. Plasma hormones were analyzed using an enzyme-linked immunosorbent assay (ELISA) and plasma trace minerals were analyzed with inductively coupled plasma mass spectrometry (ICP-MS). Correlations and simple linear regression were used to assess the association between trace minerals and hormones. Plasma testosterone concentrations were inversely correlated with manganese (r = -0.543; β = -0.410; p < 0.01), selenium (r = -0.292; β = -0.024; p < 0.05), vanadium (r = -0.406; β = -1.278; p < 0.01), arsenic (r = -0.336; β = -0.142; p < 0.05), and lead (r = -0.385; β = -0.418; p < 0.01). Plasma luteinizing hormone (LH) levels were positively correlated with arsenic (r = 0.298; β = 0.327; p < 0.05) and cesium (r = 0.305; β = 2.272; p < 0.05), and negatively correlated with vanadium (r = -0.303; β = -2.467; p < 0.05). Moreover, cortisol concentrations showed significant positive correlations with cadmium (r = 0.291; β = 209.01; p < 0.05). Finally, insulin concentrations were inversely related to vanadium (r = -0.359; β = -3.982; p < 0.05). In conclusion, endurance runners living in areas with high environmental levels of toxic minerals should check their concentrations of anabolic hormones.

Hormonal Changes in High-Level Aerobic Male Athletes during a Sports Season

The aim of this study was to determine the possible changes in plasma of several hormones such as Luteinizing Hormone, Testosterone, Cortisol and Insulin in endurance runners during the sports season. Twenty-one high-level male endurance runners (22 ± 3.2 years, 1.77 ± 0.05 m) participated in the study. Basal plasma hormones were measured at four moments during the season (initial, 3, 6 and 9 months), and were analyzed using ELISA (enzyme-linked immunosorbent assay). Testosterone and Luteinizing Hormone (LH) suffered very significant decreases (p < 0.01) at 3 months compared with the beginning and an increase (p < 0.05) at 6 and 9 months compared with 3 months. Insulin level was significantly lower (p < 0.05) at 3, 6 and 9 months compared with the initial test. Insulin and cortisol were associated inversely (r = 0.363; β = -0.577; p = 0.017) and positively (r = 0.202; β = 0.310; p = 0.043), respectively, with the amount of km per week performed by the runners. There was a significant association between km covered at a higher intensity than the anaerobic threshold and I (r = 0.580; β = -0.442; p = 0.000). Our findings indicate that testosterone, LH and insulin were more sensitive to changes in training volume and intensity than cortisol in high-level endurance runners. Basal testosterone and LH concentrations decrease in athletes who perform a high volume of aerobic km in situations of low energy availability.

Assessment of the Effects of Aerobic Fitness on Cerebrovascular Function in Young Adults Using Multiple Inversion Time Arterial Spin Labeling MRI

This cross-sectional study investigated the effects of aerobic fitness on cerebrovascular function in the healthy brain. Gray matter cerebral blood flow (CBF) and cerebrovascular reactivity (CVR) were quantified in a sample of young adults within a normal fitness range. Based on existing Transcranial Doppler ultrasound and fMRI evidence, we predicted a positive relationship between fitness and resting gray matter CBF and CVR. Exploratory hypotheses that higher V . O2peak would be associated with higher GM volume and cognitive performance were also investigated. 20 adults underwent a V . O2peak test and a battery of cognitive tests. All subjects also underwent an MRI scan where multiple inversion time (MTI) pulsed arterial spin labeling (PASL) was used to quantify resting CBF and CVR to 5% CO2. Region of interest analysis showed a non-significant inverse correlation between whole-brain gray matter CBF and V . O2peak; r = -0.4, p = 0.08, corrected p (p') = 0.16 and a significant positive correlation between V . O2peak and whole-brain averaged gray matter CVR; r = 0.62, p = 0.003, p' = 0.006. Voxel-wise analysis revealed a significant inverse association between V . O2peak and resting CBF in the left and right thalamus, brainstem, right lateral occipital cortex, left intra-calcarine cortex and cerebellum. The results of this study suggest that aerobic fitness is associated with lower baseline CBF and greater CVR in young adults.

Association between physical activity and abnormal glucose metabolism-A population-based cross-sectional study in China

BACKGROUND

Association between physical activity and abnormal glucose metabolism (AGM) remains unclear.

METHODS

Data were collected from a random sample of the Chinese population aged 26-77 years. Participants were classified into an AGM group or a normal glucose tolerance (NGT) group. Total physical activity energy expenditure (PAEE), sedentary time, and ratios of PAEE in different domains to the total PAEE were calculated according to the International Physical Activity Questionnaire. Logistic regression was used to analyze the associations between total PAEE, ratios of PAEE under different domains to total PAEE, sedentary time, and AGM.

RESULTS

Data from 3510 participants aged 50 ± 12 years were analyzed; 1390 (39.6%) were diagnosed with AGM. In the multivariate logistic regression analysis, total PAEE (1000 MET-min/week) was significantly associated with AGM [OR (95% CI) 0.96 (0.94, 0.99)]. After stratification by sex and BMI, total PAEE was negatively associated with AGM in men and overweight people. The ratios of PAEE in different domains to total PAEE or sedentary time were not associated with AGM.

CONCLUSIONS

Total PAEE was negatively associated with AGM. Ratios of PAEE in different domains to total PAEE and sedentary time were not associated with AGM in this Chinese population.

Fasting hepatic glucose uptake is higher in men than women

Differences in glucose metabolism between men and women have previously been reported. Our purpose was to determine if there is a gender difference in fasting hepatic glucose uptake (MRglu). Fifty‐five patients (44 men, 11 women) referred for routine PET/CT using the glucose tracer 2‐deoxy‐2‐[F‐18]fluoro‐D‐glucose (FDG), mainly for cancer, had dynamic imaging for 30 min immediately following injection. Hepatic FDG clearance (mL/min/100 mL) was measured as gradient divided by intercept from Patlak–Rutland graphical analysis using a volume of interest over the abdominal aorta to record input function. Hepatic MRglu was obtained by multiplication of clearance by blood glucose concentration. Hepatic steatosis was diagnosed as CT density ≤40 HU. Mean (standard deviation) hepatic MRglu in 44 men was 2.30 (1.14) μmol/min/100 mL, significantly higher than in 11 women in whom it was 1.07 (1.35) μmol/min/100 mL (P = 0.003). CT density was 52 (12) HU in women compared with 45 (9) HU in men (P = 0.04), but there was no significant difference in blood glucose, BMI, or prevalence of recent chemotherapy (within 6 months preceding PET/CT). When patients were subdivided into those without hepatic steatosis (31 men/9 women), those without evidence of FDG‐avid malignancy on PET/CT (15/6), and those without either (11/5), gender differences in hepatic MRglu remained highly significant, but there were no significant differences in CT density, blood glucose, BMI, or recent chemotherapy history. Despite this being a population of clinically referred patients, the results strongly suggest that fasting hepatic MRglu is higher in men than in women.

High-intensity interval exercise and glycemic control in adolescents with type one diabetes mellitus: a case study

Current physical activity guidelines for youth with type 1 diabetes (T1D) are poorly supported by empirical evidence and the optimal dose of physical activity to improve glycemic control is unknown. This case report documents the effect of acute high‐intensity interval exercise (HIIE) and moderate‐intensity exercise (MIE) on 24‐h glycemic control in three adolescents with T1D using continuous glucose monitoring. Results highlight varied individual response to exercise across the participants. In two participants both MIE and HIIE resulted in a drop in blood glucose during exercise (−38 to −42% for MIE and −21–46% in HIIE) and in one participant both MIE and HIIE resulted in increased blood glucose (+19% and + 36%, respectively). Over the 24‐h period average blood glucose was lower for all participants in the HIIE condition, and for two for the MIE condition, compared to no exercise. All three participants reported HIIE to be more enjoyable than MIE. These data show both HIIE and MIE have the potential to improve short‐term glycemic control in youth with T1D but HIIE was more enjoyable. Future work with a larger sample size is required to explore the potential for HIIE to improve health markers in youth with T1D.

Sex-related differences in fuel utilization and hormonal response to exercise: implications for individuals with type 1 diabetes

Sex-related differences in metabolic and neuroendocrine response to exercise in individuals without diabetes have been well established. Men and women differ in fuel selection during exercise, in which women rely to a greater extent on fat oxidation, whereas males rely mostly on carbohydrate oxidation for energy production. The difference in fuel selection appears to be mediated by sex-related differences in hormonal (including catecholamines, growth hormone, and estrogen) response to different types and intensities of exercise. In general, men exhibit an amplified counter-regulatory response to exercise, with elevated levels of catecholamines compared with women. However, women exhibit greater sensitivity to the lipolytic action of the catecholamines and deplete less of their glycogen stores than men during exercise, which suggests that women may experience a greater defense in blood glucose control after exercise than men. Conversely, little is known about sex-related differences in response to exercise in individuals with type 1 diabetes (T1D). A single study investigating sex-related differences in response to moderate aerobic exercise in individuals with T1D found sex-related differences in catecholamine response and fuel selection, but changes in blood glucose were not measured. To our knowledge, there are no studies investigating sex-related differences in blood glucose responses to different types and intensities of exercise in individuals with T1D. This review summarizes sex-related differences in exercise responses that could potentially impact blood glucose levels during exercise in individuals with T1D and highlights the need for further research.

Timing of Exercise Affects Glycemic Control in Type 2 Diabetes Patients Treated with Metformin

OBJECTIVE

The purpose of the study was to examine the acute effects of the timing of exercise on the glycemic control during and after exercise in T2D.

METHODS

This study included 26 T2D patients (14 women and 12 men) who were treated with metformin. All patients were tested on four occasions: metformin administration alone (Metf), high-intensity interval training (HIIT) performed at 30 minutes (EX30), 60 minutes (EX60), and 90 minutes (EX90) postbreakfast, respectively. Glucose, insulin, and superoxide dismutase (SOD) activity were examined.

RESULTS

Glucose decreased significantly after the exercise in EX30, EX60, and EX90. Compared with Metf, the decline in glucose immediately after the exercise was larger in EX30 (-2.58 mmol/L; 95% CI, -3.36 to -1.79 mmol/L; p < 0.001), EX60 (-2.13 mmol/L; 95% CI, -2.91 to -1.34 mmol/L; p < 0.001), and EX90 (-1.87 mmol/L; 95% CI, -2.65 to -1.08 mmol/L; p < 0.001), respectively. Compared with Metf, the decrease in insulin was larger in EX30 and EX60 (both p < 0.001).

CONCLUSIONS

Timing of exercise is a factor to consider when prescribing exercise for T2D patients treated with metformin. This trial is registered with ChiCTR-IOR-16008469 on 13 May 2016.

Correlation Analysis of Exercise-Induced Changes in Plasma Trace Element and Hormone Levels During Incremental Exercise in Well-Trained Athletes

This study analyzes the relationship between hormonal changes induced by exercise and variations in trace elements associated with oxidative stress during incremental exercise. Nineteen well-trained endurance athletes performed a cycle ergometer test: after a warm-up of 10 min at 2.0 W kg(-1), workload increased by 0.5 W kg(-1) every 10 min until exhaustion. The analysis was controlled for prior diet and activity patterns, levels of exercise training, and time of day (circadian rhythms). Whole blood lactate concentration and plasma concentrations of ions (Zn, Se, Mn, and Co), insulin, glucagon, aldosterone, thyroid stimulating hormone (TSH), calcitonin, and parathyroid hormone (PTH) were measured at rest; at the end of each stage; and 3, 5, and 7 min post-exercise. The statistical analysis involved paired non-parametric tests and correlation coefficients. No significant differences were found in Mn or Co levels as a function of exercise intensity. Zn and Se levels at the end of the exercise protocol and over the recovery time were significantly different to baseline. Further, Zn levels were significantly correlated with aldosterone, calcitonin, and PTH levels, while Se levels were associated with aldosterone, calcitonin, and TSH levels. Our results indicate several different patterns of association between acute changes in hormone concentrations and variations in trace element concentrations related to oxidative stress during submaximal exercise.

A new insulin-glucose metabolic model of type 1 diabetes mellitus: An in silico study

Diabetes mellitus is a serious metabolic disease that threatens people's health. The artificial pancreas system (APS) has been generally considered as the ultimate cure of type 1 diabetes mellitus (T1DM). The simulation model of insulin-glucose metabolism is an essential part of an APS as it processes the measured glucose level and generates control signal to the insulin infusion system. This paper presents a new insulin-glucose metabolic model using model reduction methods applied to the popular but complex Cobelli's model. The performances of three different model reduction methods, namely Padé approximation, Routh approximation and system identification, are compared. The results of in silico simulation based on 30 virtual patients of three groups for adults, adolescents, and children show that the approximation error between this new model and the original Cobelli's model is so small that can be neglected. It can be concluded that the proposed simplified model can describe the insulin-glucose metabolism process rather accurately as well as can be easily implemented and integrated into an APS to make the APS technology more mature and closer to clinical use. The FPGA implementation, testing and further simplification possibility will be explored in the next stage of research.

Carbohydrate electrolyte solutions enhance endurance capacity in active females

The purpose of the present study was to investigate the effects of supplementation with a carbohydrate-electrolyte solution (CES) in active females during a prolonged session of submaximal running to exhaustion. Eight healthy active females volunteered to perform a session of open-ended running to exhaustion at 70% of their maximal oxygen consumption on a treadmill during the follicular phase of their menstrual cycle on two occasions. During each run, the subjects consumed either 3mL·kg⁻¹ body mass of a 6% CES or a placebo drink (PL) every 20 min during exercise. The trials were administered in a randomized double-blind, cross-over design. During the run, the subjects ingested similar volumes of fluid in two trials (CES: 644 ± 75 mL vs. PL: 593 ± 66 mL, p > 0.05). The time to exhaustion was 16% longer during the CES trial (106.2 ± 9.4 min) than during the PL trial (91.6 ± 5.9 min) (p < 0.05). At 45 min during exercise, the plasma glucose concentration in the CES trial was higher than that in PL trial. No differences were observed in the plasma lactate level, respiratory exchange ratio, heart rate, perceived rate of exertion, sensation of thirst, or abdominal discomfort between the two trials (p > 0.05). The results of the present study confirm that CES supplementation improves the moderate intensity endurance capacity of active females during the follicular phases of the menstrual cycle. However, the exogenous oxidation of carbohydrate does not seem to explain the improved capacity after CES supplementation.

Risk factors for adverse symptoms during dipeptidyl peptidase-IV inhibitor therapy: a questionnaire-based study carried out by the Japan Pharmaceutical Association Drug Event Monitoring project in Kumamoto Prefecture

BACKGROUND

Meta-analyses of randomized clinical trials have reported that dipeptidyl peptidase IV (DPP-4) inhibitors are well tolerated and that the incidence of hypoglycemia with the use of DPP-4 inhibitors is similar to that observed with placebos. However, in general, provider-oriented methods using medical record reviews offer lower rates of non-serious, symptomatic adverse drug reactions (ADRs) than patient-oriented methods. Moreover, severe hypoglycemia occurred in three clinical trials using sitagliptin, but in two of these trials this phenomenon has been previously described only in the drug application data in the US.

OBJECTIVE

The aim of this study was to assess the profile of patient-reported symptomatic ADRs under DPP-4 inhibitor therapy and to detect risk factors for hypoglycemic and non-hypoglycemic adverse symptoms in daily clinical practice.

METHODS

We analyzed a subpopulation of participants in the Drug Event Monitoring (DEM) project of the Japan Pharmaceutical Association. An anonymous survey was conducted in February 2012 to assess the self-perception of adverse symptoms during a median 28 (4-88) days after the last prescription of DPP-4 inhibitors by means of interviews of pharmacists using structured questionnaires.

RESULTS

A total of 864 males and 686 females were included. The prescribed DPP-4 inhibitors included sitagliptin (75.4 %), alogliptin (15.5 %), vildagliptin (8.8 %) and linagliptin (0.3 %). Mild hypoglycemic symptoms were reported by 34 individuals (2.2 %) receiving monotherapy of sitagliptin (10/402) or alogliptin (3/65), or combination therapy of sitagliptin (15/767) or alogliptin (6/176) with other hypoglycemic agents. In the multiple regression model, hypoglycemic symptoms were found to be significantly associated with liver disease, female sex and alcohol consumption more than three times per week. Non-hypoglycemic symptoms were reported by 57 individuals (3.7 %), the most common symptoms of which were gastrointestinal symptoms (2.1 %). Combination therapy was only found to be associated with nonhypoglycemic symptoms.

CONCLUSIONS

The present study suggested that hypoglycemic symptoms under therapy with sitagliptin or alogliptin may be associated with liver disease, female sex and alcohol consumption, all of which are potentially capable of leading to poor gluconeogenesis because they decrease the counter-regulatory hormonal responses to hypoglycemia.

Postprandial glucose fluxes and insulin sensitivity during exercise: a study in healthy individuals

Quantifying the effect size of acute exercise on insulin sensitivity (SI(exercise)) and simultaneous measurement of glucose disappearance (R(d)), endogenous glucose production (EGP), and meal glucose appearance in the postprandial state has not been developed in humans. To do so, we studied 12 healthy subjects [5 men, age 37.1 ± 3.1 yr, body mass index 24.1 ± 1.1 kg/m², fat-free mass (FFM) 50.9 ± 3.9 kg] during moderate exercise at 50% V(O₂max) for 75 min, 120-195 min after a triple-tracer mixed meal consumed at time 0. Tracer infusion rates were adjusted to achieve constant tracer-to-tracee ratio and minimize non-steady-state errors. Glucose turnover was estimated by accounting for the nonstationary kinetics introduced by exercise. Insulin sensitivity index was calculated in each subject both in the absence [time (t) = 0-120 min, SI(rest)] and presence (t = 0-360 min, SI(exercise)) of physical activity. EGP at t = 0 min (13.4 ± 1.1 μM·kg FFM⁻¹·min⁻¹) fell at t = 120 min (2.4 ± 0.4 μM·kg FFM⁻¹·min⁻¹) and then rapidly rose almost eightfold at t = 180 min (18.2 ± 2.6 μM·kg FFM⁻¹·min⁻¹) before gradually falling at t = 360 min (10.6 ± 0.9 μM·kg FFM⁻¹·min⁻¹). R(d) rapidly peaked at t = 120 min at the start of exercise (89.5 ± 11.6 μM·kg FFM⁻¹·min⁻¹) and then gradually declined at t = 195 min (26.4 ± 3.3 μM·kg FFM⁻¹·min⁻¹) before returning to baseline at t = 360 min. SI(exercise) was significantly higher than SI(rest) (21.6 ± 3.7 vs. 12.5 ± 2.0 10⁻⁴ dl·kg⁻¹·min⁻¹ per μU/ml, P < 0.0005). Glucose turnover was estimated for the first time during exercise with the triple-tracer technique. Our results, applying state-of-the-art techniques, show that moderate exercise almost doubles postprandial insulin sensitivity index in healthy subjects.

Whole-body glucose oxidation rate during prolonged exercise in type 1 diabetic patients under usual life conditions

OBJECTIVE

Fuel oxidation during exercise was studied in type 1 insulin-dependent (T1DM) patients mainly under quite constant insulin and glycemia; these protocols, however, likely do not reflect patients' usual metabolic conditions. The glucose oxidation rate (GLUox) in T1DM patients under usual life conditions was thus investigated during prolonged exercise (3-h) and its behavior was described mathematically.

MATERIALS/METHODS

Whole-body GLUox was determined in eight T1DM patients (4/8 M; aged 35-59 years) and eight well-matched healthy subjects. Venous blood was drawn prior to and every 30 min until the end of exercise; glycemia, insulin, cortisol, and growth hormone concentrations were determined. Oxygen consumption, carbon dioxide production, and ventilation were measured at rest and thereafter every 30 min of the exercise. To prevent hypoglycemia, patients were given fruit fudge (93% sucrose) prior to / during exercise.

RESULTS

Insulin concentration and glycemia were significantly higher in patients across the entire exercise period (group effect, p<0.001 for both). GLUox decreased significantly with increasing exercise duration (time effect, p<0.001), but no significant difference was detected between the two groups (group effect, p=NS). GLUox, expressed as the percentage of the starting value, was described by an exponential function showing a time constant of 90 min (n=96; mean corrected R(2)=0.666).

CONCLUSIONS

GLUox in T1DM patients was not significantly different from the rate observed in the control subjects. The function describing the time course of GLUox may be useful to correct an estimated GLUox for the duration of exercise and help T1DM patients avoiding exercise-induced glycemic imbalances.

Prevalence and clinical characteristics of mental stress-induced myocardial ischemia in patients with coronary heart disease

OBJECTIVES

The goal of this study was to evaluate the prevalence and clinical characteristics of mental stress-induced myocardial ischemia.

BACKGROUND

Mental stress-induced myocardial ischemia is prevalent and a risk factor for poor prognosis in patients with coronary heart disease, but past studies mainly studied patients with exercise-induced myocardial ischemia.

METHODS

Eligible patients with clinically stable coronary heart disease, regardless of exercise stress testing status, underwent a battery of 3 mental stress tests followed by a treadmill test. Stress-induced ischemia, assessed by echocardiography and electrocardiography, was defined as: 1) development or worsening of regional wall motion abnormality; 2) left ventricular ejection fraction reduction ≥ 8%; and/or 3) horizontal or downsloping ST-segment depression ≥ 1 mm in 2 or more leads lasting for ≥ 3 consecutive beats during at least 1 mental test or during the exercise test.

RESULTS

Mental stress-induced ischemia occurred in 43.45%, whereas exercise-induced ischemia occurred in 33.79% (p = 0.002) of the study population (N = 310). Women (odds ratio [OR]: 1.88), patients who were not married (OR: 1.99), and patients who lived alone (OR: 2.24) were more likely to have mental stress-induced ischemia (all p < 0.05). Multivariate analysis showed that compared with married men or men living with someone, unmarried men (OR: 2.57) and married women (OR: 3.18), or living alone (male OR: 2.25 and female OR: 2.72, respectively) had higher risk for mental stress-induced ischemia (all p < 0.05).

CONCLUSIONS

Mental stress-induced ischemia is more common than exercise-induced ischemia in patients with clinically stable coronary heart disease. Women, unmarried men, and individuals living alone are at higher risk for mental stress-induced ischemia. (Responses of Myocardial Ischemia to Escitalopram Treatment [REMIT]; NCT00574847).

Metabolic responses to acute physical exercise in young rats recovered from fetal protein malnutrition with a fructose-rich diet

BACKGROUND

Malnutrition in utero can "program" the fetal tissues, making them more vulnerable to metabolic disturbances. Also there is association between excessive consumption of fructose and the development of metabolic syndrome. However, there is little information regarding the acute effect of physical exercise on subjects recovered from malnutrition and/or fed with a fructose-rich diet. The objective of this study was to evaluate the metabolic aspects and the response to acute physical exercise in rats recovered from fetal protein malnutrition with a fructose-rich diet.

METHODS

Pregnant Wistar rats were fed with a balanced (B) diet or a low-protein (L) diet. After birth and until 60 days of age, the offspring were distributed into four groups according to the diet received: B: B diet during the whole experiment; balanced/fructose (BF): B diet until birth and fructose-rich (F) diet afterwards; low protein/balanced (LB): L diet until birth and B diet afterwards; low protein/fructose (LF): L diet until birth and F diet afterwards.

RESULTS

The excess fructose intake reduced the body weight gain, especially in the BF group. Furthermore, the serum total cholesterol and the LDL cholesterol were elevated in this group. In the LF group, the serum total cholesterol and the muscle glycogen increased. Acute physical exercise increased the serum concentrations of glucose, triglycerides, HDL cholesterol and liver lipids and reduced the concentrations of muscle glycogen in all groups.

CONCLUSION

An excess fructose intake induced some signs of metabolic syndrome. However, protein malnutrition appeared to protect against the short term effects of fructose. In other hand, most responses to acute physical exercise were not influenced by early malnutrition and/or by the fructose overload.

Greater systemic lipolysis in women compared with men during moderate-dose infusion of epinephrine and/or norepinephrine

Women have lower circulating catecholamine levels during metabolic perturbations, such as exercise or hypoglycemia, but similar rates of systemic lipolysis. This suggests women may be more sensitive to the lipolytic action of catecholamines, while maintaining similar glucoregulatory effects. The aim of the present study, therefore, was to determine whether women have higher rates of systemic lipolysis compared with men in response to matched peripheral infusion of catecholamines, but similar rates of glucose turnover. Healthy, nonobese women (n = 11) and men (n = 10) were recruited and studied on 3 separate days with the following infusions: epinephrine (Epi), norepinephrine (NE), or the two combined. Tracer infusions of glycerol and glucose were used to determine systemic lipolysis and glucose turnover, respectively. Following basal measurements of substrate kinetics, the catecholamine infusion commenced, and measures of substrate kinetics continued for 60 min. Catecholamine concentrations were similarly elevated in women and men during each infusion: Epi, 182-197 pg/ml and NE, 417-507 pg/ml. There was a significant sex difference in glycerol rate of appearance and rate of disappearance with the catecholamine infusions (P < 0.0001), mainly due to a significantly greater glycerol turnover during the first 30 min of each infusion: glycerol rate of appearance during Epi was only 268 +/- 18 vs. 206 +/- 21 micromol/min in women and men, respectively; during NE, only 173 +/- 13 vs. 153 +/- 17 micromol/min, and during Epi+NE, 303 +/- 24 vs. 257 +/- 21 micromol/min. No sex differences were observed in glucose kinetics under any condition. In conclusion, these data suggest that women are more sensitive to the lipolytic action of catecholamines, but have no difference in their glucoregulatory response. Thus the lower catcholamine levels observed in women vs. men during exercise and other metabolic perturbations may allow women to maintain a similar or greater level of lipid mobilization while minimizing changes in glucose turnover.

Sex differences in exercise metabolism and the role of 17-beta estradiol

Women oxidize more lipid and less carbohydrate and protein compared with men during endurance exercise. The increase in fat oxidation is associated with higher intramyocellular lipid content and use as well as greater adipocyte lipolysis. Glucose rates of appearance and disappearance are lower for women than for men, with no change in basal muscle glycogen, and some evidence for muscle glycogen sparing during endurance exercise. Women oxidize less protein compared with men and show lower leucine oxidation during exercise. The consistent and robust finding of higher mRNA abundance for most components of fat-oxidation pathways in women compared with men is directionally consistent with the substrate-oxidation data. A lack of directional consistency between mRNA species involved in carbohydrate and protein metabolism and the known sex differences during exercise implies that fat oxidation is regulated and that carbohydrate and protein oxidation follow by metabolic demand. Administration of 17-beta-estradiol to men recapitulates most of the described sex differences in metabolism and mRNA content. The greater fat oxidation for women during submaximal endurance exercise compared with men seems to occur partly through a sex hormone-mediated enhancement of lipid-oxidation pathways.

Glucoregulation is more precise in women than in men during postexercise recovery

BACKGROUND

The regulation of glycemia is challenged in healthy men and women after exercise bouts of substantial energy expenditure.

OBJECTIVE

We determined rates of glucose appearance (Ra), disappearance (Rd), and metabolic clearance (MCR) before, during, and after isoenergetic moderate and hard-intensity exercise.

DESIGN

Ten men and 8 women received primed-continuous infusion of [6,6-(2)H(2)]glucose tracer to measure glucose kinetics. Participants were studied under 3 different conditions with diet unchanged between trials: 1) before, during, and 3 h after 90 min of exercise at 45% of peak oxygen consumption (VO(2)peak; E45); 2) before, during, and 3 h after 60 min of exercise at 65% VO(2)peak (E65), and 3) in a time-matched sedentary control trial.

RESULTS

In men and women, Ra, Rd, and MCR increased above the control trial during exercise and were higher in E65 than in E45 (P < 0.05). Average Ra, Rd, and MCR remained elevated above the control over 3 h of postexercise recovery in men after exercise in E45 and E65 (P < 0.05), and blood glucose concentrations were depressed below the control during recovery (P < 0.05). Glucose concentrations were not depressed in women during 3 h of postexercise recovery, and in contrast with that in men, average Ra and Rd did not remain significantly elevated during postexercise recovery in women, although MCR did remain elevated in E65 (P < 0.05).

CONCLUSIONS

After exercise bouts, women are better able to maintain glucose concentrations at sedentary control levels, thus not requiring the counter-regulation of glucose production that is seen in men and requiring less accentuation of lipid metabolism.

Exogenous carbohydrate spares muscle glycogen in men and women during 10 h of exercise

PURPOSE

The purpose of the study was to evaluate the effects of carbohydrate (CHO) supplementation on whole-body and net muscle substrate use during 10 h of discontinuous exercise, simulating occupational settings in men and women.

METHODOLOGY

Recreationally trained subjects (N = 7 males, N = 6 females) performed a graded exercise test on a treadmill (TM) and cycle ergometer (CE) to determine ventilatory threshold (VT) and V O2peak. In a double-blind, randomized crossover design, subjects received either CHO [20% maltodextrin (0.6 g.kg FFM.h)] or flavored placebo (PLA) drink each hour across 10 h of exercise. Exercise intensity was 71.3 +/- 3% and 72.4 +/- 4% VT for TM and CE, respectively. Hourly exercise included 9 min of upper-body ergometery, 19 min of cycling, and 20 min of treadmill walking, with a 1-min transition between modes, followed by a 10-min rest and feeding period. The protocol was selected to simulate arduous occupational settings. Vastus lateralis biopsies were obtained before and after exercise. Expired gases were collected every other hour to establish average rates of whole-body CHO and fat oxidation. Blood glucose (BG) was measured continuously.

RESULTS

Whole-body CHO oxidation was maintained during CHO trial compared with the PLA trial. Net muscle glycogen use was 52% higher for the PLA trial (176.0 +/- 16.7, 117.0 +/- 20.9 and 164.5 +/- 11.0, 133.8 +/- 10.9 mmol.kg w.w. for PLA and CHO, respectively, P < 0.05). There were no significant sex-specific differences in glycogen use, whole-body substrate oxidation, or BG values.

CONCLUSION

The ingestion of CHO during long-duration exercise decreases net muscle glycogen use while better maintaining whole-body carbohydrate oxidation, and potentially increasing performance in field settings. There are limited differences in sex-specific substrate oxidation.

Validity of criteria for establishing maximal O2 uptake during ramp exercise tests

The incremental or ramp exercise test to the limit of tolerance has become a popular test for determination of maximal O(2) uptake (VO(2max)). However, many subjects do not evidence a definitive plateau of the VO(2) -work rate relationship on this test and secondary criteria based upon respiratory exchange ratio (RER), maximal heart rate (HR(max)) or blood [lactate] have been adopted to provide confidence in the measured VO(2max). We hypothesized that verification of VO(2max) using these variables is fundamentally flawed in that their use could either allow underestimation of VO(2max) (if, for any reason, a test were ended at a sub-maximal [Formula: see text]), or alternatively preclude subjects from recording a valid VO(2max). Eight healthy male subjects completed a ramp exercise test (at 20 W/min) to the limit of tolerance on an electrically braked cycle ergometer during which pulmonary gas exchange was measured breath-by-breath and blood [lactate] was determined every 90 s. Using the most widely used criterion values of RER (1.10 and 1.15), VO(2max) as determined during the ramp test (4.03 +/- 0.10 l/min) could be undermeasured by 27% (2.97 +/- 0.24 l/min) and 16% (3.41 +/- 0.15 l/min), respectively (both P < 0.05). The criteria of HR(max) (age predicted HR(max) +/- 10 b/min) and blood [lactate] (> or = 8 mM) were untenable because they resulted in rejection of 3/8 and 6/8 of the subjects, most of whom (5/8) had demonstrated a plateau of VO(2max) at VO(2max). These findings provide a clear mandate for rejecting these secondary criteria as a means of validating VO(2max) on ramp exercise tests.

Metabolismo energético em atletas de endurance é diferente entre os sexos

Atletas de endurance têm suas necessidades energéticas aumentadas devido ao seu alto gasto energético durante o exercício. Contudo, ainda não estão claros quais são as diferenças do metabolismo energético entre os sexos e se essas diferenças vão implicar em mudanças em suas dietas. Sendo assim, o objetivo deste trabalho foi reunir informações da literatura sobre as diferenças entre os sexos em relação ao metabolismo energético em atletas de endurance. Alguns estudos demonstram que, durante exercício de longa duração, as mulheres utilizam como fonte de energia maior quantidade de lipídeos e menor de carboidratos e proteínas, quando comparadas aos homens. Já outros autores não encontraram diferença entre os sexos na proporção dos substratos metabolizados, mas sim nos tipos de lipídeos utilizados. Essa diferença na utilização de substratos vem sendo relacionada a distintas concentrações de alguns hormônios entre homens e mulheres, como: 17beta-estradiol, progesterona, testosterona, epinefrina, norepinefrina, hormônio de crescimento, insulina e glucagon. Também foi observado que os atletas aumentam seus estoques de glicogênio quando consomem dieta com sobrecarga glicídica, enquanto que as atletas apenas têm seus estoques de glicogênio aumentados quando ingerem dieta com sobrecarga glicídica e hiperenergética. Esse achado é bastante relevante, pois a concentração de glicogênio está diretamente ligada ao rendimento do atleta durante o exercício de endurance. Portanto, a literatura sugere que há diferenças entre os sexos quanto à utilização de substratos em atletas. Porém, novos estudos são necessários para melhor esclarecimento do metabolismo energético dos atletas de endurance possibilitando, assim, a adequação de suas recomendações nutricionais.

Quantitative Circulatory Physiology: an integrative mathematical model of human physiology for medical education

We have developed Quantitative Circulatory Physiology (QCP), a mathematical model of integrative human physiology containing over 4,000 variables of biological interactions. This model provides a teaching environment that mimics clinical problems encountered in the practice of medicine. The model structure is based on documented physiological responses within peer-reviewed literature and serves as a dynamic compendium of physiological knowledge. The model is solved using a desktop, Windows-based program, allowing students to calculate time-dependent solutions and interactively alter over 750 parameters that modify physiological function. The model can be used to understand proposed mechanisms of physiological function and the interactions among physiological variables that may not be otherwise intuitively evident. In addition to open-ended or unstructured simulations, we have developed 30 physiological simulations, including heart failure, anemia, diabetes, and hemorrhage. Additional stimulations include 29 patients in which students are challenged to diagnose the pathophysiology based on their understanding of integrative physiology. In summary, QCP allows students to examine, integrate, and understand a host of physiological factors without causing harm to patients. This model is available as a free download for Windows computers at http://physiology.umc.edu/themodelingworkshop.

Muscle glycogen oxidation during prolonged exercise measured with oral [13C]glucose: comparison with changes in muscle glycogen content

Plasma glucose and muscle glycogen oxidation during prolonged exercise [75-min at 48 and 76% maximal O2 uptake (V̇o2 max)] were measured in eight well-trained male subjects [V̇o2 max = 4.50 l/min (SD 0.63)] using a simplified tracer technique in which a small amount of glucose highly enriched in 13C was ingested: plasma glucose oxidation was computed from 13C/12C in plasma glucose (which was stable beginning at minute 30 and minute 15 during exercise at 48 and 76% V̇o2 max, respectively) and 13CO2 production, and muscle glycogen oxidation was estimated by subtracting plasma glucose oxidation from total carbohydrate oxidation. Consistent data from the literature suggest that this small dose of exogenous glucose does not modify muscle glycogen oxidation and has little effect, if any, on plasma glucose oxidation. The percent contributions of plasma glucose and muscle glycogen oxidation to the energy yield at 48% V̇o2 max [15.1% (SD 3.8) and 45.9% (SD 5.8)] and at 76% V̇o2 max [15.4% (SD 3.6) and 59.8% (SD 9.2)] were well in line with data previously reported for similar work loads and exercise durations using conventional tracer techniques. The significant reduction in glycogen concentration measured from pre- and postexercise vastus lateralis muscle biopsies paralleled muscle glycogen oxidation calculated using the tracer technique and was larger at 76% than at 48% V̇o2 max. However, the correlation coefficients between these two estimates of muscle glycogen utilization were not different from zero at each of the two work loads. The simplified tracer technique used in the present experiment appears to be a valid alternative approach to the traditional tracer techniques for computing plasma glucose and muscle glycogen oxidation during prolonged exercise.

Influence of the glycaemic index of an evening meal on substrate oxidation following breakfast and during exercise the next day in healthy women

OBJECTIVE

To investigate whether the 'overnight second-meal effect' results in altered substrate oxidation during the postprandial period following breakfast and subsequent sub-maximal exercise in women.

SUBJECTS/METHODS

Seven recreationally active women were recruited for the study. In each trial, participants were provided with their evening meal on day 1, which was composed of either high glycaemic index (HGI) or low glycaemic index (LGI) carbohydrates (CHO). On day 2, participants were provided with a standard HGI breakfast and then performed a 60 min run at 65% \[V.]O(2 max) 3 h later.

RESULTS

The incremental area under the curve (IAUC) for plasma glucose concentrations during the postprandial period following breakfast was greater in the HGI trial compared to the LGI trial (P<0.01). Similarly, the IAUC for serum insulin concentrations was greater in the HGI trial than the LGI trial (P<0.05). No differences in plasma free-fatty acids (FFA) or plasma glycerol concentrations were found between trials during the postprandial period. During subsequent exercise, there were no significant differences in substrate metabolism.

CONCLUSION

The glycaemic index of an evening meal does not alter substrate oxidation at rest following breakfast or during subsequent submaximal exercise in women. This study provides further evidence for the overnight second-meal effect on glycaemic responses following a LGI mixed evening meal.

Sex differences in left ventricular function and beta-receptor responsiveness following prolonged strenuous exercise

Sex differences in neuroendocrine and metabolic responses to prolonged strenuous exercise (PSE) have been well documented. The aim of this investigation was to examine sex differences in left ventricular function and cardiac beta-receptor responsiveness following a single bout of PSE. Nine male and eight female triathletes were examined during three separate sessions: before, immediately after, and 24 h following a half-ironman triathlon using dobutamine stress echocardiography. Steady-state graded infusions of dobutamine were used to assess beta-receptor responsiveness. Slopes calculated from linear regressions between dobutamine doses and changes in heart rate and contractility for each participant were used as an index of beta-receptor responsiveness. Despite no change in preload, fractional area change decreased from baseline after the race in both men and women, with a greater decrease in men [men: 54.1% (SD 2.1) to 50.7% (SD 3.4) vs. women: 55.4% (SD 2.7) to 53.3% (SD 2.5); P < 0.05]. The amount of dobutamine necessary to increase heart rate by 25 beats/min [men: 29.6 microg x kg(-1) x min(-1) (SD 6.6) to 42.7 microg x kg(-1) x min(-1) (SD 12.9) vs. women: 23.5 microg x kg(-1) x min(-1) (SD 4.0) to 30.0 microg x kg(-1) x min(-1) (SD 7.8); P < 0.05] and contractility by 10 mmHg/cm2 [men: 20.9 microg x kg(-1) x min(-1) (SD 5.1) to 37.0 microg x kg(-1) x min(-1) (SD 11.5) vs. women: 22.6 microg x kg(-1) x min(-1) (SD 6.4) to 30.7 microg x kg(-1) x min(-1) (SD 7.2); P < 0.05] was greater in both men and women postrace. However, the amount of dobutamine required to induce these changes was greater in men, reflecting larger beta-receptor alterations in male triathletes following PSE relative to women. These data suggest that following an acute bout of PSE, male triathletes demonstrate an attenuated chronotropic and inotropic response to beta-adrenergic stimulation compared with female triathletes.