Prior exercise alters the difference between arterialised and venous glycaemia: implications for blood sampling procedures

Continuous glucose monitor overestimates glycemia, with the magnitude of bias varying by postprandial test and individual - a randomized crossover trial

BACKGROUND

Continuous glucose monitors (CGM) are used to characterize postprandial glycemia, yet no study has directly tested how different test foods/beverages alter CGM accuracy.

OBJECTIVES

Assess glycemic responses to test foods/drinks using CGM compared with capillary sampling (criterion).

METHODS

Fifteen healthy females (n = 9) and males (n = 6) completed 7 laboratory visits in a randomized crossover design with ≥48 h washout between visits. During each visit, participants consumed an oral carbohydrate challenge comprising either 50 g glucose or equivalent 50 g carbohydrate as whole fruits, 50 g carbohydrate as blended fruit, 50 g carbohydrate as commercially available fruit smoothie, 50 g carbohydrate as commercially available fruit smoothie ingested over 30 ± 4 min, 50 g carbohydrate as commercially available fruit smoothie with 5 g inulin, 30 g carbohydrate as commercially available fruit smoothie. The glycemia was recorded from both CGM and capillary samples every 15 min for 120 min and expressed as incremental areas under the curve. The glycemic index (GI) was calculated relative to 50 g glucose where appropriate. Exploratory analyses examined 1) interindividual heterogeneity of CGM bias compared with criterion and 2) whether CGM bias could be improved with adjustment for baseline differences.

RESULTS

CGM-estimated fasting and postprandial glucose concentrations were (mean ± standard deviation) 0.9 ± 0.6 and 0.9 ± 0.5 mmol/L higher than capillary estimates, respectively(both, P < 0.001). CGM bias varied by postprandial test such that GI for 50 g carbohydrate as commercially available fruit smoothie was higher with CGM (69; 95% confidence interval: 48, 99) compared with capillary (53; 95% confidence interval: 40, 69; P = 0.05). Furthermore, differences in CGM compared with capillary fasting glucose concentrations varied by participant (P = 0.001). Unadjusted, CGM overestimated time >7.8 mmol/L by ∼4-fold, and adjustment for baseline differences reduced this overestimate to ∼2-fold (both P < 0.01).

CONCLUSIONS

CGM overestimated glycemic responses in numerous contexts. At times, this can mischaracterize the GI. In addition, there is interindividual heterogeneity in the accuracy of CGM in estimating fasting glucose concentrations. Correction for this difference reduces, but does not eliminate, postprandial overestimate of glycemia by CGM. Caution should be applied when inferring absolute or relative glycemic responses to foods using CGM, and capillary sampling should be prioritized for accurate quantification of glycemic response. This trial was registered at clinicaltrials.gov as NCT06333184.

Availability of Open Dynamic Glycemic Data in the Field of Diabetes Research: A Scoping Review

BACKGROUND

Poor data availability and accessibility characterizing some research areas in biomedicine are still limiting potentialities for increasing knowledge and boosting technological advancement. This phenomenon also characterizes the field of diabetes research, in which glycemic data may serve as a basis for different applications. To overcome this limitation, this review aims to provide a comprehensive analysis of the publicly available data sets related to dynamic glycemic data.

METHODS

Search was performed in four different sources, namely scientific journals, Google, a comprehensive registry of clinical trials and two electronic databases. Retrieved data sets were analyzed in terms of their main characteristics and on the typology of data provided.

RESULTS

Twenty-five data sets were identified including data from challenge tests (5 of 25) or data from Continuous Glucose Monitoring (CGM, 20 of 25). As for the data sets including challenge tests, all of them were freely downloadable; most of them (80%) related only to oral glucose tolerance test (OGTT) with standard duration (2 h), but varying for timing and number of collected blood samples, and variables collected in addition to glucose levels (with insulin levels being the most common); the remaining 20% of them also included intravenous glucose tolerance test (IVGTT) data. As for the data sets related to CGM, 7 of 20 were freely downloadable, whereas the remaining 13 were downloadable upon completion of a request form.

CONCLUSIONS

This review provided an overview of the readily usable data sets, thus representing a step forward in fostering data access in diabetes field.

Are there interindividual differences in the reactive hypoglycaemia response to breakfast? A replicate crossover trial

BACKGROUND

Following consumption of a meal, circulating glucose concentrations can rise and then fall briefly below the basal/fasting concentrations. This phenomenon is known as reactive hypoglycaemia but to date no researcher has explored potential inter-individual differences in response to meal consumption.

OBJECTIVE

We conducted a secondary analysis of existing data to examine inter-individual variability of reactive hypoglycaemia in response to breakfast consumption.

METHODS

Using a replicate crossover design, 12 healthy, physically active men (age: 18-30 y, body mass index: 22.1 to 28.0 kg⋅m- 2) completed two identical control (continued overnight fasting) and two breakfast (444 kcal; 60% carbohydrate, 17% protein, 23% fat) conditions in randomised sequences. Blood glucose and lactate concentrations, serum insulin and non-esterified fatty acid concentrations, whole-body energy expenditure, carbohydrate and fat oxidation rates, and appetite ratings were determined before and 2 h after the interventions. Inter-individual differences were explored using Pearson's product-moment correlations between the first and second replicates of the fasting-adjusted breakfast response. Within-participant covariate-adjusted linear mixed models and a random-effects meta-analytical approach were used to quantify participant-by-condition interactions.

RESULTS

Breakfast consumption lowered 2-h blood glucose by 0.44 mmol/L (95%CI: 0.76 to 0.12 mmol/L) and serum NEFA concentrations, whilst increasing blood lactate and serum insulin concentrations (all p < 0.01). Large, positive correlations were observed between the first and second replicates of the fasting-adjusted insulin, lactate, hunger, and satisfaction responses to breakfast consumption (all r > 0.5, 90%CI ranged from 0.03 to 0.91). The participant-by-condition interaction response variability (SD) for serum insulin concentration was 11 pmol/L (95%CI: 5 to 16 pmol/L), which was consistent with the τ-statistic from the random-effects meta-analysis (11.7 pmol/L, 95%CI 7.0 to 22.2 pmol/L) whereas effects were unclear for other outcome variables (e.g., τ-statistic value for glucose: 0 mmol/L, 95%CI 0.0 to 0.5 mmol/L).

CONCLUSIONS

Despite observing reactive hypoglycaemia at the group level, we were unable to detect any meaningful inter-individual variability of the reactive hypoglycaemia response to breakfast. There was, however, evidence that 2-h insulin responses to breakfast display meaningful inter-individual variability, which may be explained by relative carbohydrate dose ingested and variation in insulin sensitivity of participants.

Myths and methodologies: Assessing glycaemic control and associated regulatory mechanisms in human physiology research

Accurate measurements of glycaemic control and the underpinning regulatory mechanisms are vital in human physiology research. Glycaemic control is the maintenance of blood glucose concentrations within optimal levels and is governed by physiological variables including insulin sensitivity, glucose tolerance and β-cell function. These can be measured with a plethora of methods, all with their own benefits and limitations. Deciding on the best method to use is challenging and depends on the specific research question(s). This review therefore discusses the theory and procedure, validity and reliability and any special considerations of a range common methods used to measure glycaemic control, insulin sensitivity, glucose tolerance and β-cell function. Methods reviewed include glycosylated haemoglobin, continuous glucose monitors, the oral glucose tolerance test, mixed meal tolerance test, hyperinsulinaemic euglycaemic clamp, hyperglycaemic clamp, intravenous glucose tolerance test and indices derived from both fasting concentrations and the oral glucose tolerance test. This review aims to help direct understanding, assessment and decisions regarding which method to use based on specific physiology-related research questions.

Are all sugars equal? Role of the food source in physiological responses to sugars with an emphasis on fruit and fruit juice

High (free) sugar intakes can increase self-reported energy intake and are associated with unfavourable cardiometabolic health. However, sugar source may modulate the effects of sugars due to several mechanisms including the food matrix. The aim of this review was to assess the current state of evidence in relation to food source effects on the physiological responses to dietary sugars in humans relevant to cardiometabolic health. An additional aim was to review potential mechanisms by which food sources may influence such responses. Evidence from meta-analyses of controlled intervention trials was used to establish the balance of evidence relating to the addition of sugars to the diet from sugar-sweetened beverages, fruit juice, honey and whole fruit on cardiometabolic outcomes. Subsequently, studies which have directly compared whole fruit with fruit juices, or variants of fruit juices, were discussed. In summary, the sources of sugars can impact physiological responses, with differences in glycaemic control, blood pressure, inflammation, and acute appetite. Longer-term effects and mechanisms require further work, but initial evidence implicates physical structure, energy density, fibre, potassium and polyphenol content, as explanations for some of the observed responses.

Altered glucose kinetics occurs with aging: a new outlook on metabolic flexibility

Our purpose was to determine how age affects metabolic flexibility and underlying glucose kinetics in healthy young and older adults. Therefore, glucose and lactate tracers along with pulmonary gas exchange data were used to determine glucose kinetics and respiratory exchange ratios [RER = carbon dioxide production (V̇co2)/oxygen consumption (V̇o2)] during a 2-h 75-g oral glucose tolerance test (OGTT). After an 12-h overnight fast, 28 participants, 15 young (21-35 yr; 7 men and 8 women) and 13 older (60-80 yr; 7 men and 6 women), received venous primed-continuous infusions of [6,6-2H]glucose and [3-13C]lactate with a [Formula: see text] bolus. After a 90-min metabolic stabilization and tracer equilibration period, volunteers underwent an OGTT. Arterialized glucose concentrations ([glucose]) started to rise 15 min post glucose consumption, peaked at 60 min, and remained elevated. As assessed by rates of appearance (Ra) and disposal (Rd) and metabolic clearance rate (MCR), glucose kinetics were suppressed in older compared to young individuals. As well, unlike in young individuals, fractional gluconeogenesis (fGNG) remained elevated in the older population after the oral glucose challenge. Finally, there were no differences in 12-h fasting baseline or peak RER values following an oral glucose challenge in older compared to young men and women, making RER an incomplete measure of metabolic flexibility in the volunteers we evaluated. Our study revealed that glucose kinetics are significantly altered in a healthy aged population after a glucose challenge. Furthermore, those physiological deficits are not detected from changes in RER during an OGTT.NEW & NOTEWORTHY To determine metabolic flexibility in response to an OGTT, we studied healthy young and older men and women to determine glucose kinetics and changes in RER. Compared to young subjects, glucose kinetics were suppressed in older healthy individuals during an OGTT. Surprisingly, the age-related changes in glucose flux were not reflected in RER measurements; thus, RER measurements do not give a complete view of metabolic flexibility in healthy individuals.

Acute exercise and high-glucose ingestion elicit dynamic and individualized responses in systemic markers of redox homeostasis

Background

Biomarkers of oxidation-reduction (redox) homeostasis are commonly measured in human blood to assess whether certain stimuli (e.g., high-glucose ingestion or acute exercise) lead to a state of oxidative distress (detrimental to health) or oxidative eustress (beneficial to health). Emerging research indicates that redox responses are likely to be highly individualized, yet few studies report individual responses. Furthermore, the effects of complex redox stimuli (e.g., high-glucose-ingestion after exercise) on redox homeostasis remains unclear. We investigated the effect of acute exercise (oxidative eustress), high-glucose ingestion (oxidative distress), and high-glucose ingestion after exercise (both oxidative eu/distress), on commonly measured redox biomarkers in serum/plasma.

Methods

In a randomized crossover fashion, eight healthy men (age: 28 ± 4 years; BMI: 24.5 ± 1.5 kg/m2 [mean ± SD]) completed two separate testing conditions; 1) consumption of a high-glucose mixed-nutrient meal (45% carbohydrate [1.1 g glucose.kg-1], 20% protein, and 35% fat) at rest (control trial), and 2) consumption of the same meal 3 h and 24 h after 1 h of moderate-intensity cycling exercise (exercise trial). Plasma and serum were analyzed for an array of commonly studied redox biomarkers.

Results

Oxidative stress and antioxidant defense markers (hydrogen peroxide, 8-isoprostanes, catalase, superoxide dismutase, and nitrate levels) increased immediately after exercise (p &lt; 0.05), whereas nitric oxide activity and thiobarbituric acid reactive substances (TBARS) remained similar to baseline (p &gt; 0.118). Nitric oxide activity and nitrate levels decreased at 3 h post-exercise compared to pre-exercise baseline levels. Depending on when the high-glucose mixed nutrient meal was ingested and the postprandial timepoint investigated, oxidative stress and antioxidant defense biomarkers either increased (hydrogen peroxide, TBARS, and superoxide dismutase), decreased (hydrogen peroxide, 8-isoprostanes, superoxide dismutase, nitric oxide activity, nitrate, and nitrite), or remained similar to pre-meal baseline levels (hydrogen peroxide, 8-isoprostanes, TBARS, catalase, superoxide dismutase and nitrite). Redox responses exhibited large inter-individual variability in the magnitude and/or direction of responses.

Conclusion

Findings highlight the necessity to interpret redox biomarkers in the context of the individual, biomarker measured, and stimuli observed. Individual redox responsiveness may be of physiological relevance and should be explored as a potential means to inform personalized redox intervention.

Restricting sugar or carbohydrate intake does not impact physical activity level or energy intake over 24 h despite changes in substrate use: a randomised crossover study in healthy men and women

PURPOSE

To determine the effects of dietary sugar or carbohydrate restriction on physical activity energy expenditure, energy intake, and physiological outcomes across 24 h.

METHODS

In a randomized, open-label crossover design, twenty-five healthy men (n = 10) and women (n = 15) consumed three diets over a 24-h period: moderate carbohydrate and sugar content (MODSUG = 50% carbohydrate [20% sugars], 15% protein, 35% fat); low sugar content (LOWSUG = 50% carbohydrate [< 5% sugars], 15% protein, 35% fat); and low carbohydrate content (LOWCHO = 8% carbohydrate [< 5% sugars], 15% protein, 77% fat). Postprandial metabolic responses to a prescribed breakfast (20% EI) were monitored under laboratory conditions before an ad libitum test lunch, with subsequent diet and physical activity monitoring under free-living conditions until blood sample collection the following morning.

RESULTS

The MODSUG, LOWSUG and LOWCHO diets resulted in similar mean [95%CI] rates of both physical activity energy expenditure (771 [624, 919] vs. 677 [565, 789] vs. 802 [614, 991] kcal·d-1; p = 0.29] and energy intake (2071 [1794, 2347] vs. 2195 [1918, 2473] vs. 2194 [1890, 2498] kcal·d-1; P = 0.34), respectively. The LOWCHO condition elicited the lowest glycaemic and insulinaemic responses to breakfast (P < 0.01) but the highest 24-h increase in LDL-cholesterol concentrations (P < 0.001), with no differences between the MODSUG and LOWSUG treatments. Leptin concentrations decreased over 24-h of consuming LOWCHO relative to LOWSUG (p < 0.01).

CONCLUSION

When energy density is controlled for, restricting either sugar or total dietary carbohydrate does not modulate physical activity level or energy intake over a 24-h period (~ 19-h free-living) despite substantial metabolic changes.

CLINICAL TRIALS REGISTRATION ID

NCT03509610, https://clinicaltrials.gov/show/NCT03509610.

Impact of high energy oral nutritional supplements consumed in the late afternoon on appetite, energy intake and cardio-metabolic risk factors in females with lower BMI

BACKGROUND/OBJECTIVE

Morning consumption of a single dose of high-energy oral nutritional supplement (ONS) in females with a lower BMI displaces some of the food eaten at breakfast but increases overall daily energy intake. This study investigated the effectiveness of ONS intake in the late afternoon and for longer duration.

SUBJECTS/METHODS

Twenty-one healthy females (mean ± SD, age 25 ± 5 years; BMI 18.7 ± 1.2 kg/m²) participated in a randomised, crossover study with two experimental trials. In the afternoon of days 1-5, participants consumed either ONS (2.510 MJ) or low-energy PLACEBO drink (0.377 MJ) and recorded food eaten at home. On day six, energy intake was measured during buffet meals, and energy expenditure, appetite measurements and blood samples were collected throughout the day.

RESULT

Over the 5-day period, in the ONS trial energy intake from evening meals was lower (ONS, 2.7 ± 0.25 MJ; Placebo, 3.6 ± 0.25 MJ, P = 0.01) but averaged total daily energy intake was higher (ONS, 9.2 ± 0.3 MJ; PLACEBO, 8.2 ± 0.4 MJ, P = 0.03). On day six, energy intake, appetite scores, plasma GLP-1 and PYY, and energy expenditure were not significantly different between the two trials but fasting insulin concentration and HOMA_IR, were higher (P < 0.05) and insulin sensitivity score based on fasting insulin and TAG lower (P < 0.05) in ONS trial.

CONCLUSION

Late afternoon consumption of ONS for five consecutive days by females with a lower BMI has only a partial and short-lived energy intake suppression and thus increases daily energy intake but reduces insulin sensitivity.

A Single Bout of Upper-Body Exercise Has No Effect on Postprandial Metabolism in Persons with Chronic Paraplegia

PURPOSE

The acute effects of a single bout of upper-body exercise on postprandial metabolism in persons with spinal cord injury are currently not well understood. The primary aim of this study was to evaluate the effects of a single bout of upper-body high-intensity interval exercise (HIIE) and moderate-intensity continuous exercise (MICE) in comparison with a no-exercise control (REST) condition on postprandial metabolic responses in persons with chronic paraplegia.

METHODS

Ten participants (eight males, two females; age, 49 ± 10 yr; time since injury, 22 ± 13 yr) with chronic paraplegia took part in a randomized crossover study, consisting of three trials: HIIE (8 × 60 s at 70% peak power output [PPEAK]), MICE (25 min at 45% PPEAK), and REST, at least 3 d apart. Exercise was performed in the fasted state, and participants consumed a mixed-macronutrient liquid meal 1-h postexercise. Venous blood and expired gas samples were collected at regular intervals for 6-h postmeal consumption.

RESULTS

There were no significant differences in postprandial incremental area under the curve for triglycerides (P = 0.59) or glucose (P = 0.56) between conditions. Insulin incremental area under the curve tended to be lower after MICE (135 ± 85 nmol·L-1 per 360 min) compared with REST (162 ± 93 nmol·L-1 per 360 min), but this did not reach statistical significance (P = 0.06, d = 0.30). Participants reported a greater fondness (P = 0.04) and preference for HIIE over MICE.

CONCLUSIONS

After an overnight fast, a single bout of upper-body exercise before eating has no effect on postprandial metabolism in persons with chronic paraplegia, irrespective of exercise intensity. This suggests that alternative exercise strategies may be required to stimulate postprandial substrate oxidation for this population.

The Effect of a Single Bout of Continuous Aerobic Exercise on Glucose, Insulin and Glucagon Concentrations Compared to Resting Conditions in Healthy Adults: A Systematic Review, Meta-Analysis and Meta-Regression

Background

Elevated glucose and insulin levels are major risk factors in the development of cardiometabolic disease. Aerobic exercise is widely recommended to improve glycaemic control, yet its acute effect on glycaemia and glucoregulatory hormones has not been systematically reviewed and analysed in healthy adults.

Objective

To determine the effect of a single bout of continuous aerobic exercise on circulating glucose, insulin, and glucagon concentrations in healthy adults.

Methods

CENTRAL, CINAHL, Embase, Global Health, HMIC, Medline, PubMed, PsycINFO, ScienceDirect, Scopus and Web of Science databases were searched from inception to May 2020. Papers were included if they reported a randomised, crossover study measuring glucose and/or insulin and/or glucagon concentrations before and immediately after a single bout of continuous aerobic exercise (≥ 30 min) compared to a time-matched, resting control arm in healthy adults. The risk of bias and quality of evidence were assessed using the Cochrane Risk of Bias Tool and GRADE approach, respectively. Random-effects meta-analyses were performed for glucose, insulin, and glucagon. Sub-group meta-analyses and meta-regression were performed for categorical (metabolic state [postprandial or fasted], exercise mode [cycle ergometer or treadmill]) and continuous (age, body mass index, % males, maximal aerobic capacity, exercise duration, exercise intensity) covariates, respectively.

Results

42 papers (51 studies) were considered eligible: glucose (45 studies, 391 participants), insulin (38 studies, 377 participants) and glucagon (5 studies, 47 participants). Acute aerobic exercise had no significant effect on glucose concentrations (mean difference: − 0.05 mmol/L; 95% CI, − 0.22 to 0.13 mmol/L; P = 0.589; I²: 91.08%, large heterogeneity; moderate-quality evidence). Acute aerobic exercise significantly decreased insulin concentrations (mean difference: − 18.07 pmol/L; 95% CI, − 30.47 to − 5.66 pmol/L; P = 0.004; I²: 95.39%, large heterogeneity; moderate-quality evidence) and significantly increased glucagon concentrations (mean difference: 24.60 ng/L; 95% CI, 16.25 to 32.95 ng/L; P < 0.001; I²: 79.36%, large heterogeneity; moderate-quality evidence). Sub-group meta-analyses identified that metabolic state modified glucose and insulin responses, in which aerobic exercise significantly decreased glucose (mean difference: − 0.27 mmol/L; 95% CI, − 0.55 to − 0.00 mmol/L; P = 0.049; I²: 89.72%, large heterogeneity) and insulin (mean difference: − 42.63 pmol/L; 95% CI, − 66.18 to − 19.09 pmol/L; P < 0.001; I²: 81.29%, large heterogeneity) concentrations in the postprandial but not fasted state. Meta-regression revealed that the glucose concentrations were also moderated by exercise duration and maximal aerobic capacity.

Conclusions

Acute aerobic exercise performed in the postprandial state decreases glucose and insulin concentrations in healthy adults. Acute aerobic exercise also increases glucagon concentrations irrespective of metabolic state. Therefore, aerobic exercise undertaken in the postprandial state is an effective strategy to improve acute glycaemic control in healthy adults, supporting the role of aerobic exercise in reducing cardiometabolic disease incidence.

PROSPERO registration number

CRD42020191345.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40279-021-01473-2.

Walking Initiated 20 Minutes before the Time of Individual Postprandial Glucose Peak Reduces the Glucose Response in Young Men with Overweight or Obesity: A Randomized Crossover Study

BACKGROUND

Although a single bout of postmeal exercise can lower postprandial glucose (PPG), its optimal timing remains unclear.

OBJECTIVE

This study aimed to investigate the effect of exercise timing using an individualized approach on PPG in overweight or obese young men.

METHODS

Twenty men [age: 23.0 ± 4.3 y; BMI (kg/m2): 27.4 ± 2.8] each completed three 240-min trials in a randomized order separated by 6-14 d: 1) sitting (SIT), 2) walking initiated at each participant's PPG-peak time (PPGP) (iP), and 3) walking initiated 20 min before the PPGP (20iP). For each participant, PPGP was predetermined using continuous glucose monitoring. Walking was performed at 50% maximal oxygen consumption for 30 min. Venous blood was collected at 15- and 30-min intervals for 0-120 min and 120-240 min, respectively. The primary outcome was plasma PPG. Generalized estimating equations were used for comparison between trials.

RESULTS

Compared with SIT, the 4-h incremental AUCs (iAUCs) for plasma PPG (-0.6 mmol · L-1 · h; P = 0.047) and insulin (-28.7%, P < 0.001) were reduced in 20iP only, and C-peptide concentrations were lower after iP (-14.9%, P = 0.001) and 20iP (-28.7%, P < 0.001). Plasma insulin (-11.1%, P = 0.006) and C-peptide (-8.3%, P = 0.012) were lower due to the 20iP compared with iP treatment. Finally, PPG reductions due to iP and 20iP occurred only in men with a BMI > 27.5 kg/m2 (iP, -11.2%; 20iP, -14.7%; P = 0.047) and higher glucose iAUC values during SIT (iP, -25.5%; 20iP, -25.7%; P < 0.001).

CONCLUSIONS

Walking initiated 20 min before PPGP lowered PPG and plasma insulin and C-peptide concentrations in young men with overweight or obesity, in particular in those with high BMI or glucose iAUC values during SIT; it also lowered plasma insulin and C-peptide concentrations more effectively than did exercise initiated at PPGP. This trial was registered at the Chinese Clinical Trial Registry (http://www.chictr.org.cn/index.aspx) as ChiCTR1900023175.

Carbohydrate supplementation: a critical review of recent innovations

PURPOSE

To critically examine the research on novel supplements and strategies designed to enhance carbohydrate delivery and/or availability.

METHODS

Narrative review.

RESULTS

Available data would suggest that there are varying levels of effectiveness based on the supplement/supplementation strategy in question and mechanism of action. Novel carbohydrate supplements including multiple transportable carbohydrate (MTC), modified carbohydrate (MC), and hydrogels (HGEL) have been generally effective at modifying gastric emptying and/or intestinal absorption. Moreover, these effects often correlate with altered fuel utilization patterns and/or glycogen storage. Nevertheless, performance effects differ widely based on supplement and study design. MTC consistently enhances performance, but the magnitude of the effect is yet to be fully elucidated. MC and HGEL seem unlikely to be beneficial when compared to supplementation strategies that align with current sport nutrition recommendations. Combining carbohydrate with other ergogenic substances may, in some cases, result in additive or synergistic effects on metabolism and/or performance; however, data are often lacking and results vary based on the quantity, timing, and inter-individual responses to different treatments. Altering dietary carbohydrate intake likely influences absorption, oxidation, and and/or storage of acutely ingested carbohydrate, but how this affects the ergogenicity of carbohydrate is still mostly unknown.

CONCLUSIONS

In conclusion, novel carbohydrate supplements and strategies alter carbohydrate delivery through various mechanisms. However, more research is needed to determine if/when interventions are ergogenic based on different contexts, populations, and applications.

Galactose Ingested with a High-Fat Beverage Increases Postprandial Lipemia Compared with Glucose but Not Fructose Ingestion in Healthy Men

BACKGROUND

Fructose ingestion with a high-fat beverage increases postprandial lipemia when compared with glucose. It is unknown whether other sugars, such as galactose, also increase postprandial lipemia.

OBJECTIVES

The objective was to assess whether galactose ingestion within a high-fat beverage increases postprandial lipemia relative to glucose or fructose.

METHODS

Two experiments were conducted, which contrasted different test drinks under otherwise standardized conditions. In Experiment 1, 10 nonobese men (age: 22 ± 1 y; BMI, 23.5 ± 2.2 kg/2) ingested either galactose or glucose (0.75 g supplemented carbohydrate per⋅kilogram body mass) within a high-fat test drink (0.94 g fat per kilogram body mass). In Experiment 2, a separate group of 9 nonobese men (age: 26 ± 6 y; BMI: 23.5 ± 2.6 kg/m2) ingested either galactose or fructose (identical doses as those in Experiment 1) within the same high-fat test drink. Capillary blood was sampled before and at frequent intervals after ingestion of the test drinks for a 300-min period to determine plasma triacylglycerol, glucose, lactate, nonesterified fatty acid, and insulin concentrations. Paired t tests and 2-way, repeated-measures ANOVA were used to compare conditions within each experiment.

RESULTS

The incremental AUC for triacylglycerol was greater following galactose ingestion compared with glucose (127 ± 59 compared with 80 ± 48 mmol⋅L-1 × 300 min, respectively; P = 0.04) but not compared with fructose (136 ± 74 compared with 133 ± 63 mmol⋅L-1 ×300 min, respectively; P = 0.91). Plasma lactate concentrations also increased to a greater extent with galactose compared with glucose ingestion (time-condition interaction: P < 0.001) but not fructose ingestion (time-condition interaction: P = 0.17).

CONCLUSIONS

Galactose ingestion within a high-fat beverage exacerbates postprandial lipemia and plasma lactate concentrations compared with glucose but not fructose in nonobese men. These data suggest that galactose metabolism may be more similar to fructose than to glucose, providing a rationale to reassess the metabolic fate of galactose ingestion in humans. This trial was registered at clinicaltrials.gov as NCT03439878.

Physiological responses to maximal eating in men

This study investigated metabolic, endocrine, appetite and mood responses to a maximal eating occasion in fourteen men (mean: age 28 (sd5) years, body mass 77·2 (sd6·6) kg and BMI 24·2 (sd2·2) kg/m2) who completed two trials in a randomised crossover design. On each occasion, participants ate a homogenous mixed-macronutrient meal (pizza). On one occasion, they ate until ‘comfortably full’ (ad libitum) and on the other, until they ‘could not eat another bite’ (maximal). Mean energy intake was double in the maximal (13 024 (95 % CI 10 964, 15 084) kJ; 3113 (95 % CI 2620, 3605) kcal) compared with thead libitumtrial (6627 (95 % CI 5708, 7547) kJ; 1584 (95 % CI 1364, 1804) kcal). Serum insulin incremental AUC (iAUC) increased approximately 1·5-fold in the maximal compared withad libitumtrial (mean:ad libitum43·8 (95 % CI 28·3, 59·3) nmol/l × 240 min and maximal 67·7 (95 % CI 47·0, 88·5) nmol/l × 240 min,P< 0·01), but glucose iAUC did not differ between trials (ad libitum94·3 (95 % CI 30·3, 158·2) mmol/l × 240 min and maximal 126·5 (95 % CI 76·9, 176·0) mmol/l × 240 min,P= 0·19). TAG iAUC was approximately 1·5-fold greater in the maximalv.ad libitumtrial (ad libitum98·6 (95 % CI 69·9, 127·2) mmol/l × 240 min and maximal 146·4 (95 % CI 88·6, 204·1) mmol/l × 240 min,P< 0·01). Total glucagon-like peptide-1, glucose-dependent insulinotropic peptide and peptide tyrosine–tyrosine iAUC were greater in the maximal compared withad libitumtrial (P< 0·05). Total ghrelin concentrations decreased to a similar extent, but AUC was slightly lower in the maximalv.ad libitumtrial (P= 0·02). There were marked differences on appetite and mood between trials, most notably maximal eating caused a prolonged increase in lethargy. Healthy men have the capacity to eat twice the energy content required to achieve comfortable fullness at a single meal. Postprandial glycaemia is well regulated following initial overeating, with elevated postprandial insulinaemia probably contributing.

Lipid Metabolism Links Nutrient-Exercise Timing to Insulin Sensitivity in Men Classified as Overweight or Obese

CONTEXT

Pre-exercise nutrient availability alters acute metabolic responses to exercise, which could modulate training responsiveness.

OBJECTIVE

To assess acute and chronic effects of exercise performed before versus after nutrient ingestion on whole-body and intramuscular lipid utilization and postprandial glucose metabolism.

DESIGN

(1) Acute, randomized, crossover design (Acute Study); (2) 6-week, randomized, controlled design (Training Study).

SETTING

General community.

PARTICIPANTS

Men with overweight/obesity (mean ± standard deviation, body mass index: 30.2 ± 3.5 kg⋅m-2 for Acute Study, 30.9 ± 4.5 kg⋅m-2 for Training Study).

INTERVENTIONS

Moderate-intensity cycling performed before versus after mixed-macronutrient breakfast (Acute Study) or carbohydrate (Training Study) ingestion.

RESULTS

Acute Study-exercise before versus after breakfast consumption increased net intramuscular lipid utilization in type I (net change: -3.44 ± 2.63% versus 1.44 ± 4.18% area lipid staining, P < 0.01) and type II fibers (-1.89 ± 2.48% versus 1.83 ± 1.92% area lipid staining, P < 0.05). Training Study-postprandial glycemia was not differentially affected by 6 weeks of exercise training performed before versus after carbohydrate intake (P > 0.05). However, postprandial insulinemia was reduced with exercise training performed before but not after carbohydrate ingestion (P = 0.03). This resulted in increased oral glucose insulin sensitivity (25 ± 38 vs -21 ± 32 mL⋅min-1⋅m-2; P = 0.01), associated with increased lipid utilization during exercise (r = 0.50, P = 0.02). Regular exercise before nutrient provision also augmented remodeling of skeletal muscle phospholipids and protein content of the glucose transport protein GLUT4 (P < 0.05).

CONCLUSIONS

Experiments investigating exercise training and metabolic health should consider nutrient-exercise timing, and exercise performed before versus after nutrient intake (ie, in the fasted state) may exert beneficial effects on lipid utilization and reduce postprandial insulinemia.

Co-ingestion of whey protein hydrolysate with milk minerals rich in calcium potently stimulates glucagon-like peptide-1 secretion: an RCT in healthy adults

Purpose

To examine whether calcium type and co-ingestion with protein alter gut hormone availability.

Methods

Healthy adults aged 26 ± 7 years (mean ± SD) completed three randomized, double-blind, crossover studies. In all studies, arterialized blood was sampled postprandially over 120 min to determine GLP-1, GIP and PYY responses, alongside appetite ratings, energy expenditure and blood pressure. In study 1 (n = 20), three treatments matched for total calcium content (1058 mg) were compared: calcium citrate (CALCITR); milk minerals rich in calcium (MILK MINERALS); and milk minerals rich in calcium plus co-ingestion of 50 g whey protein hydrolysate (MILK MINERALS + PROTEIN). In study 2 (n = 6), 50 g whey protein hydrolysate (PROTEIN) was compared to MILK MINERALS + PROTEIN. In study 3 (n = 6), MILK MINERALS was compared to the vehicle of ingestion (water plus sucralose; CONTROL).

Results

MILK MINERALS + PROTEIN increased GLP-1 incremental area under the curve (iAUC) by ~ ninefold (43.7 ± 11.1 pmol L⁻¹ 120 min; p < 0.001) versus both CALCITR and MILK MINERALS, with no difference detected between CALCITR (6.6 ± 3.7 pmol L⁻¹ 120 min) and MILK MINERALS (5.3 ± 3.5 pmol L⁻¹ 120 min; p > 0.999). MILK MINERALS + PROTEIN produced a GLP-1 iAUC ~ 25% greater than PROTEIN (p = 0.024; mean difference: 9.1 ± 6.9 pmol L⁻¹ 120 min), whereas the difference between MILK MINERALS versus CONTROL was small and non-significant (p = 0.098; mean difference: 4.2 ± 5.1 pmol L⁻¹ 120 min).

Conclusions

When ingested alone, milk minerals rich in calcium do not increase GLP-1 secretion compared to calcium citrate. Co-ingesting high-dose whey protein hydrolysate with milk minerals rich in calcium increases postprandial GLP-1 concentrations to some of the highest physiological levels ever reported. Registered at ClinicalTrials.gov: NCT03232034, NCT03370484, NCT03370497.

Electronic supplementary material

The online version of this article (10.1007/s00394-019-02092-4) contains supplementary material, which is available to authorized users.

Skipping Breakfast Before Exercise Creates a More Negative 24-hour Energy Balance: A Randomized Controlled Trial in Healthy Physically Active Young Men

BACKGROUND

At rest, omission of breakfast lowers daily energy intake, but also lowers energy expenditure, attenuating any effect on energy balance. The effect of breakfast omission on energy balance when exercise is prescribed is unclear.

OBJECTIVES

The aim of this study was to assess the effect on 24-h energy balance of omitting compared with consuming breakfast prior to exercise.

METHODS

Twelve healthy physically active young men (age 23 ± 3 y, body mass index 23.6 ± 2.0 kg/m2) completed 3 trials in a randomized order (separated by >1 week): a breakfast of oats and milk (431 kcal; 65 g carbohydrate, 11 g fat, 19 g protein) followed by rest (BR); breakfast before exercise (BE; 60 min cycling at 50 % peak power output); and overnight fasting before exercise (FE). The 24-h energy intake was calculated based on the food consumed for breakfast, followed by an ad libitum lunch, snacks, and dinner. Indirect calorimetry with heart-rate accelerometry was used to measure substrate utilization and 24-h energy expenditure. A [6,6-2H2]glucose infusion was used to investigate tissue-specific carbohydrate utilization.

RESULTS

The 24-h energy balance was -400 kcal (normalized 95% CI: -230, -571 kcal) for the FE trial; this was significantly lower than both the BR trial (492 kcal; normalized 95% CI: 332, 652 kcal) and the BE trial (7 kcal; normalized 95% CI: -153, 177 kcal; both P < 0.01 compared with FE). Plasma glucose utilization in FE (mainly representing liver glucose utilization) was positively correlated with energy intake compensation at lunch (r = 0.62, P = 0.03), suggesting liver carbohydrate plays a role in postexercise energy-balance regulation.

CONCLUSIONS

Neither exercise energy expenditure nor restricted energy intake via breakfast omission were completely compensated for postexercise. In healthy men, pre-exercise breakfast omission creates a more negative daily energy balance and could therefore be a useful strategy to induce a short-term energy deficit. This trial was registered at clinicaltrials.gov as NCT02258399.

Creatine is a Conditionally Essential Nutrient in Chronic Kidney Disease: A Hypothesis and Narrative Literature Review

To accommodate the loss of the plethora of functions of the kidneys, patients with chronic kidney disease require many dietary adjustments, including restrictions on the intake of protein, phosphorus, sodium and potassium. Plant-based foods are increasingly recommended as these foods contain smaller amounts of saturated fatty acids, protein and absorbable phosphorus than meat, generate less acid and are rich in fibers, polyunsaturated fatty acids, magnesium and potassium. Unfortunately, these dietary recommendations cannot prevent the occurrence of many symptoms, which typically include fatigue, impaired cognition, myalgia, muscle weakness, and muscle wasting. One threat coming with the recommendation of low-protein diets in patients with non-dialysis-dependent chronic kidney disease (CKD) and with high-protein diets in patients with dialysis-dependent CKD, particularly with current recommendations towards proteins coming from plant-based sources, is that of creatine deficiency. Creatine is an essential contributor in cellular energy homeostasis, yet on a daily basis 1.6–1.7% of the total creatine pool is degraded. As the average omnivorous diet cannot fully compensate for these losses, the endogenous synthesis of creatine is required for continuous replenishment. Endogenous creatine synthesis involves two enzymatic steps, of which the first step is a metabolic function of the kidney facilitated by the enzyme arginine:glycine amidinotransferase (AGAT). Recent findings strongly suggest that the capacity of renal AGAT, and thus endogenous creatine production, progressively decreases with the increasing degree of CKD, to become absent or virtually absent in dialysis patients. We hypothesize that with increasing degree of CKD, creatine coming from meat and dairy in food increasingly becomes an essential nutrient. This phenomenon will likely be present in patients with CKD stages 3, 4 and 5, but will likely be most pronouncedly present in patients with dialysis-dependent CKD, because of the combination of lowest endogenous production of creatine and unopposed losses of creatine into the dialysate. It is likely that these increased demands for dietary creatine are not sufficiently met. The result of which, may be a creatine deficiency with important contributions to the sarcopenia, fatigue, impaired quality of life, impaired cognition, and premature mortality seen in CKD.

Interindividual Responses of Appetite to Acute Exercise: A Replicated Crossover Study

PURPOSE

Acute exercise transiently suppresses appetite, which coincides with alterations in appetite-regulatory hormone concentrations. Individual variability in these responses is suspected, but replicated trials are needed to quantify them robustly. We examined the reproducibility of appetite and appetite-regulatory hormone responses to acute exercise and quantified the individual differences in responses.

METHODS

Fifteen healthy, recreationally active men completed two control (60-min resting) and two exercise (60-min fasted treadmill running at 70% peak oxygen uptake) conditions in randomized sequences. Perceived appetite and circulating concentrations of acylated ghrelin and total peptide YY (PYY) were measured immediately before and after the interventions. Interindividual differences were explored by correlating the two sets of response differences between exercise and control conditions. Within-participant covariate-adjusted linear mixed models were used to quantify participant-condition interactions.

RESULTS

Compared with control, exercise suppressed mean acylated ghrelin concentrations and appetite perceptions (all ES = 0.62-1.47, P < 0.001) and elevated total PYY concentrations (ES = 1.49, P < 0.001). For all variables, the standard deviation of the change scores was substantially greater in the exercise versus control conditions. Moderate-to-large positive correlations were observed between the two sets of control-adjusted exercise responses for all variables (r = 0.54-0.82, P ≤ 0.036). After adjusting for baseline measurements, participant-condition interactions were present for all variables (P ≤ 0.053).

CONCLUSIONS

Our replicated crossover study allowed, for the first time, the interaction between participant and acute exercise response in appetite parameters to be quantified. Even after adjustment for individual baseline measurements, participants demonstrated individual differences in perceived appetite and hormone responses to acute exercise bouts beyond any random within-subject variability over time.

Measurement of postprandial glucose fluxes in response to acute and chronic endurance exercise in healthy humans

The effect of endurance exercise on enhancing insulin sensitivity and glucose flux has been well established with techniques such as the hyperinsulinemic clamp. Although informative, such techniques do not emulate the physiological postprandial state, and it remains unclear how exercise improves postprandial glycaemia. Accordingly, combining mixed-meal tolerance testing and the triple-stable isotope glucose tracer approach, glucose fluxes [rates of meal glucose appearance (Ra), disposal (Rd), and endogenous glucose production (EGP)] were determined following acute endurance exercise (1 h cycling; ~70% V̇o_2max) and 4 wk of endurance training (cycling 5 days/wk). Training was associated with a modest increase in V̇o_2max (~7%, P < 0.001). Postprandial glucose and insulin responses were reduced to the same extent following acute and chronic training. Interestingly, this was not accompanied by changes to rates of meal Ra, Rd, or degree of EGP suppression. Glucose clearance (Rd relative to prevailing glucose) was, however, enhanced with acute and chronic exercise. Furthermore, the duration of EGP suppression was shorter with acute and chronic exercise, with EGP returning toward fasting levels more rapidly than pretraining conditions. These findings suggest that endurance exercise influences the efficiency of the glucoregulatory system, where pretraining rates of glucose disposal and production were achieved at lower glucose and insulin levels. Notably, there was no influence of chronic training over and above that of a single exercise bout, providing further evidence that glucoregulatory benefits of endurance exercise are largely attributed to the residual effects of the last exercise bout.

Post-Exercise Carbohydrate-Energy Replacement Attenuates Insulin Sensitivity and Glucose Tolerance the Following Morning in Healthy Adults

The carbohydrate deficit induced by exercise is thought to play a key role in increased post-exercise insulin action. However, the effects of replacing carbohydrate utilized during exercise on postprandial glycaemia and insulin sensitivity are yet to be determined. This study therefore isolated the extent to which the insulin-sensitizing effects of exercise are dependent on the carbohydrate deficit induced by exercise, relative to other exercise-mediated mechanisms. Fourteen healthy adults performed a 90-min run at 70% V˙O2max starting at 1600–1700 h before ingesting either a non-caloric artificially-sweetened placebo solution (CHO-DEFICIT) or a 15% carbohydrate solution (CHO-REPLACE; 221.4 ± 59.3 g maltodextrin) to precisely replace the measured quantity of carbohydrate oxidized during exercise. The alternate treatment was then applied one week later in a randomized, placebo-controlled, and double-blinded crossover design. A standardized low-carbohydrate evening meal was consumed in both trials before overnight recovery ahead of a two-hour oral glucose tolerance test (OGTT) the following morning to assess glycemic and insulinemic responses to feeding. Compared to the CHO-DEFICIT condition, CHO-REPLACE increased the incremental area under the plasma glucose curve by a mean difference of 68 mmol·L⁻¹ (95% CI: 4 to 132 mmol·L⁻¹; p = 0.040) and decreased the Matsuda insulin sensitivity index by a mean difference of −2 au (95% CI: −1 to −3 au; p = 0.001). This is the first study to demonstrate that post-exercise feeding to replaceme the carbohydrate expended during exercise can attenuate glucose tolerance and insulin sensitivity the following morning. The mechanism through which exercise improves insulin sensitivity is therefore (at least in part) dependent on carbohydrate availability and so the day-to-day metabolic health benefits of exercise might be best attained by maintaining a carbohydrate deficit overnight.