Dietary impact on fasting and stimulated GLP-1 secretion in different metabolic conditions - a narrative review

Glucagon-like peptide 1 (GLP-1), a gastrointestinal peptide and central mediator of glucose metabolism, is secreted by L cells in the intestine in response to food intake. Postprandial secretion of GLP-1 is triggered by nutrient-sensing via transporters and G-protein-coupled receptors (GPCRs). GLP-1 secretion may be lower in adults with obesity/overweight (OW) or type 2 diabetes mellitus (T2DM) than in those with normal glucose tolerance (NGT), but these findings are inconsistent. Because of the actions of GLP-1 on stimulating insulin secretion and promoting weight loss, GLP-1 and its analogs are used in pharmacologic preparations for the treatment of T2DM. However, physiologically stimulated GLP-1 secretion through the diet might be a preventive or synergistic method for improving glucose metabolism in individuals who are OW, or have impaired glucose tolerance (IGT) or T2DM. This narrative review focuses on fasting and postprandial GLP-1 secretion in individuals with different metabolic conditions and degrees of glucose intolerance. Further, the influence of relevant diet-related factors (e.g., specific diets, meal composition, and size, phytochemical content, and gut microbiome) that could affect fasting and postprandial GLP-1 secretion are discussed. Some studies showed diminished glucose- or meal-stimulated GLP-1 response in participants with T2DM, IGT, or OW compared with those with NGT, whereas other studies have reported an elevated or unchanged GLP-1 response in T2DM or IGT. Meal composition, especially the relationship between macronutrients and interventions targeting the microbiome can impact postprandial GLP-1 secretion, although it is not clear which macronutrients are strong stimulants of GLP-1. Moreover, glucose tolerance, antidiabetic treatment, grade of overweight/obesity, and sex were important factors influencing GLP-1 secretion. The results presented in this review highlight the potential of nutritional and physiologic stimulation of GLP-1 secretion. Further research on fasting and postprandial GLP-1 concentrations and the resulting metabolic consequences under different metabolic conditions is needed.

Measuring Postprandial Metabolic Flexibility to Assess Metabolic Health and Disease

Metabolic abnormalities substantially increase the risk of noncommunicable diseases, which are among the leading causes of mortality globally. Mitigating and preventing these adverse consequences remains challenging due to a limited understanding of metabolic health. Metabolic flexibility, a key tenet of metabolic health, encompasses the responsiveness of interrelated pathways to maintain energy homeostasis throughout daily physiologic challenges, such as the response to meal challenges. One critical underlying research gap concerns the measurement of postprandial metabolic flexibility, which remains incompletely understood. We concisely review the methodology for assessment of postprandial metabolic flexibility in recent human studies. We identify 3 commonalities of study design, specifically the nature of the challenge, nature of the response measured, and approach to data analysis. Primary interventions were acute short-term nutrition challenges, including single- and multiple-macronutrient tolerance tests. Postmeal challenge responses were measured via laboratory assays and instrumentation, based on a diverse set of metabolic flexibility indicators [e.g., energy expenditure (whole-body indirect calorimetry), glucose and insulin kinetics, metabolomics, transcriptomics]. Common standard approaches have been diabetes-centric with single-macronutrient challenges (oral-glucose-tolerance test) to characterize the postprandial response based on glucose and insulin metabolism; or broad measurements of energy expenditure with calculated macronutrient oxidation via indirect calorimetry. Recent methodological advances have included the use of multiple-macronutrient meal challenges that are more representative of physiologic meals consumed by free-living humans, combinatorial approaches for assays and instruments, evaluation of other metabolic flexibility indicators via precision health, systems biology, and temporal perspectives. Omics studies have identified potential novel indicators of metabolic flexibility, which provide greater granularity to prior evidence from canonical approaches. In summary, recent findings indicate the potential for an expanded understanding of postprandial metabolic flexibility, based on nonclassical measurements and methodology, which could represent novel dynamic indicators of metabolic diseases.

Metabolomic Profiling Demonstrates Postprandial Changes in Fatty Acids and Glycerophospholipids Are Associated with Fasting Inflammation in Guatemalan Adults

BACKGROUND

Metabolic flexibility is the responsiveness to heterogeneous physiological conditions, such as food ingestion. A key unresolved question is how inflammation affects metabolic flexibility.

OBJECTIVES

Our study objective was to compare metabolic flexibility, specifically the metabolomic response to a standardized meal, by fasting inflammation status.

METHODS

Participants in Guatemala (n = 302, median age 44 y, 43.7% men) received a standardized, mixed-macronutrient liquid meal. Plasma samples (fasting, 2 h postmeal) were assayed by dual-column LC [reverse phase (C18) and hydrophilic interaction LC (HILIC)] with ultra-high-resolution MS, for concentrations of 6 inflammation biomarkers: high-sensitivity C-reactive protein (hsCRP), leptin, resistin, IL-10, adiponectin, and soluble TNF receptor II (TNFsR). We summed the individual inflammation biomarker z-scores, after reverse-coding of anti-inflammation biomarkers. We identified features with peak areas that differed between fasting and postmeal (false discovery rate-adjusted q <0.05) and compared median log2 postprandial/fasting peak area ratios by inflammation indicators.

RESULTS

We found 1397 C18 and 974 HILIC features with significant postprandial/fasting feature ratios (q <0.05). Overall inflammation z-score was directly associated with the postprandial/fasting feature ratios of arachidic acid, and inversely associated with the feature ratio of lysophosphatidic acid (LPA), adjusting for age and sex (all P < 0.05). The postprandial/fasting ratio of arachidic acid was negatively correlated with resistin, IL-10, adiponectin, and TNFsR concentrations (all P < 0.05). Feature ratios of several fatty acids-myristic acid [m/z 227.2018, retention time (RT) 229], heptadecanoic acid (m/z 269.2491, RT 276), linoleic acid (m/z 280.2358, RT 236)-were negatively correlated with fasting plasma concentrations of leptin (nanograms per milliliter) and adiponectin (micrograms per milliliter), respectively (all P < 0.05). The postprandial/fasting ratio of LPA was positively correlated with IL-10 and adiponectin (both P < 0.05); and the ratio of phosphatidylinositol was positively correlated with hsCRP (P < 0.05).

CONCLUSIONS

Postprandial responses of fatty acids and glycerophospholipids are associated with fasting inflammation status in adults in Guatemala.

Metabolomic Profiling After a Meal Shows Greater Changes and Lower Metabolic Flexibility in Cardiometabolic Diseases

Context

Metabolic flexibility is the physiologic acclimatization to differing energy availability and requirement states. Effectively maintaining metabolic flexibility remains challenging, particularly since metabolic dysregulations in meal consumption during cardiometabolic disease (CMD) pathophysiology are incompletely understood.

Objective

We compared metabolic flexibility following consumption of a standardized meal challenge among adults with or without CMDs.

Design, Setting, and Participants

Study participants (n = 349; age 37-54 years, 55% female) received a standardized meal challenge (520 kcal, 67.4 g carbohydrates, 24.3 g fat, 8.0 g protein; 259 mL). Blood samples were collected at baseline and 2 hours postchallenge. Plasma samples were assayed by high-resolution, nontargeted metabolomics with dual-column liquid chromatography and ultrahigh-resolution mass spectrometry. Metabolome-wide associations between features and meal challenge timepoint were assessed in multivariable linear regression models.

Results

Sixty-five percent of participants had ≥1 of 4 CMDs: 33% were obese, 6% had diabetes, 39% had hypertension, and 50% had metabolic syndrome. Log₂-normalized ratios of feature peak areas (postprandial:fasting) clustered separately among participants with versus without any CMDs. Among participants with CMDs, the meal challenge altered 1756 feature peak areas (1063 reversed-phase [C18], 693 hydrophilic interaction liquid chromatography [HILIC]; all q < 0.05). In individuals without CMDs, the meal challenge changed 1383 feature peak areas (875 C18; 508 HILIC; all q < 0.05). There were 108 features (60 C18; 48 HILIC) that differed by the meal challenge and CMD status, including dipeptides, carnitines, glycerophospholipids, and a bile acid metabolite (all P < 0.05).

Conclusions

Among adults with CMDs, more metabolomic features differed after a meal challenge, which reflected lower metabolic flexibility relative to individuals without CMDs.

Macronutrient, Energy, and Bile Acid Metabolism Pathways Altered Following a Physiological Meal Challenge, Relative to Fasting, among Guatemalan Adults

BACKGROUND

The healthy human metabolome, including its physiological responses after meal consumption, remains incompletely understood. One major research gap is the limited literature assessing how human metabolomic profiles differ between fasting and postprandial states after physiological challenges.

OBJECTIVES

Our study objective was to evaluate alterations in high-resolution metabolomic profiles following a standardized meal challenge, relative to fasting, in Guatemalan adults.

METHODS

We studied 123 Guatemalan adults without obesity, hypertension, diabetes, metabolic syndrome, or comorbidities. Every participant received a standardized meal challenge (520 kcal, 67.4 g carbohydrates, 24.3 g fat, 8.0 g protein) and provided blood samples while fasting and at 2 h postprandial. Plasma samples were assayed by high-resolution metabolomics with dual-column LC [C18 (negative electrospray ionization), hydrophilic interaction LC (HILIC, positive electrospray ionization)] coupled to ultra-high-resolution MS. Associations between metabolomic features and the meal challenge timepoint were assessed in feature-by-feature multivariable linear mixed regression models. Two algorithms (mummichog, gene set enrichment analysis) were used for pathway analysis, and P values were combined by the Fisher method.

RESULTS

Among participants (62.6% male, median age 43.0 y), 1130 features (C18: 777; HILIC: 353) differed between fasting and postprandial states (all false discovery rate-adjusted q < 0.05). Based on differing C18 features, top pathways included: tricarboxylic acid cycle (TCA), primary bile acid biosynthesis, and linoleic acid metabolism (all Pcombined < 0.05). Mass spectral features included: taurine and cholic acid in primary bile acid biosynthesis; and fumaric acid, malic acid, and citric acid in the TCA. HILIC features that differed in the meal challenge reflected linoleic acid metabolism (Pcombined < 0.05).

CONCLUSIONS

Energy, macronutrient, and bile acid metabolism pathways were responsive to a standardized meal challenge in adults without cardiometabolic diseases. Our findings reflect metabolic flexibility in disease-free individuals.

Two-Hour Postprandial Lipoprotein Particle Concentration Differs Between Lean and Obese Individuals

The concentrations of lipoprotein particles [high-density lipoproteins (HDLs), low-density lipoproteins (LDLs), very-low-density lipoproteins (VLDLs), and chylomicrons] are associated with the risk of cardiovascular diseases. Most studies have examined these associations in the fasting state. Previous studies have shown lipoprotein particle concentration change following meal, and these changes are different in individuals with obesity. In this study, we aimed to assess whether various meal compositions lead to adverse short-term (2-h) postprandial lipoproteinemia in obese insulin resistant (obese-IR) subjects as compared to lean insulin sensitive (lean-IS) subjects. In a randomized crossover trial, nine lean-IS and nine obese-IR Chinese men aged 22–35 years were challenged with isoenergetic and isovolumic meals rich in protein (HP), fat (HF), or carbohydrate (HC). Plasma samples were collected after a 10-h fast, as well as 1-h and 2-h post-meal and analyzed using nuclear magnetic resonance. Plasma concentration of large VLDLs and chylomicron particles was higher and increased more after all meals in obese-IR compared to lean-IS subjects. The HP meal decreased small LDL particle concentration in obese-IR subjects, and increased small HDL particle concentration in all subjects. The HF meal led to a decrease in small HDL concentration in all subjects. In conclusion, obese-IR subjects revealed a detrimental response to meal challenges even as early as 2-h after meal intake.

Prolonged monitoring of postprandial lipid metabolism after a western meal rich in linoleic acid and carbohydrates

Today, awareness has been raised regarding high consumption of n-6 polyunsaturated fatty acids (PUFA) in western diets. A comprehensive analysis of total and individual postprandial fatty acids profiles would provide insights into metabolic turnover and related health effects. After an overnight fast, 9 healthy adults consumed a mixed meal comprising 97 g carbohydrate and 45 g fat, of which 26.4 g was linoleic acid (LA). Nonesterified fatty acids (NEFA), phospholipid fatty acids (PL-FA) and triacylglycerol fatty acids (TG-FA) were monitored in plasma samples, at baseline and hourly over a 7-h postprandial period. Total TG-FA concentration peaked at 2 h after the meal and steadily decreased thereafter. LA from TG18:2n-6 and behenic acid from TG22:0 showed the highest response among TG-FA, with a biphasic response detected for the former. PL-FA exhibited no change. Total NEFA initially decreased to nadir at 1 h, then increased to peak at 7 h. The individual NEFA showed the same response curve except LA and some very-long-chain saturated fatty acids (VLCSFA, ≥20 carbon chain length) that markedly increased shortly after the meal intake. The similarities and dissimilarities in lipid profiles between study subjects at different time points were visualized using nonmetric multi-dimensional scaling. Overall, the results indicate that postprandial levels of LA and VLCSFA, either as NEFA or TG, were most affected by the test meal, which might provide an explanation for the health effects of this dietary lifestyle characterized by high intake of mixed meals rich in n-6 PUFA.

Using MRI to study the alterations in liver blood flow, perfusion, and oxygenation in response to physiological stress challenges: Meal, hyperoxia, and hypercapnia

BACKGROUND

Noninvasive assessment of dynamic changes in liver blood flow, perfusion, and oxygenation using MRI may allow detection of subtle hemodynamic alterations in cirrhosis.

PURPOSE

To assess the feasibility of measuring dynamic liver blood flow, perfusion, and T₂ * alterations in response to meal, hypercapnia, and hyperoxia challenges.

STUDY TYPE

Prospective.

SUBJECTS

Ten healthy volunteers (HV) and 10 patients with compensated cirrhosis (CC).

FIELD STRENGTH/SEQUENCE

3T; phase contrast, arterial spin labeling, and T 2 * mapping.

ASSESSMENT

Dynamic changes in portal vein and hepatic artery blood flow (using phase contrast MRI), liver perfusion (using arterial spin labeling), and blood oxygenation ( T 2 * mapping) following a meal challenge (660 kcal), hyperoxia (target P_ET O₂ of 500 mmHg), and hypercapnia (target increase P_ET CO₂ of ∼6 mmHg).

STATISTICAL TESTS

Tests between baseline and each challenge were performed using a paired two-tailed t-test (parametric) or Wilcoxon-signed-ranks test (nonparametric). Repeatability and reproducibility were determined by the coefficient of variation (CoV).

RESULTS

Portal vein velocity increased following the meal (70 ± 9%, P < 0.001) and hypercapnic (7 (5-11)%, P = 0.029) challenge, while hepatic artery flow decreased (-30 ± 18%, P = 0.005) following the meal challenge in HV. In CC patients, portal vein velocity increased (37 ± 13%, P = 0.012) without the decrease in hepatic artery flow following the meal. In both groups, the meal increased liver perfusion (HV: 82 ± 50%, P < 0.0001; CC: 27 (16-42)%, P = 0.011) with faster arrival time of blood (HV: -54 (-56-30)%, P = 0.074; CC: -42 ± 32%, P = 0.005). In HVs, T 2 * increased after the meal and in response to hyperoxia, with a decrease in hypercapnia (6 ± 8% P = 0.052; 3 ± 5%, P = 0.075; -5 ± 6%, P = 0.073, respectively), but no change in CC patients. Baseline between-session CoV <15% for blood flow and <10% for T 2 * measures.

DATA CONCLUSION

Dynamic changes in liver perfusion, blood flow, and oxygenation following a meal, hyperoxic, and hypercapnic challenges can be measured using noninvasive MRI and potentially be used to stratify patients with cirrhosis.

LEVEL OF EVIDENCE

2 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2019;49:1577-1586.

A Clear Difference Emerges in Hormone Patterns Following a Standard Midday Meal in Young Women Who Regularly Eat or Skip Breakfast

BACKGROUND

Multiple hormones are involved in the regulation of food intake and glucose metabolism. Past intervention studies showed a benefit of eating breakfast on satiety, but this was possibly confounded by the disruption of habitual meal patterns.

OBJECTIVE

The objective of this study was to compare hormonal responses, including insulin, leptin, glucagon-like peptide-1, ghrelin, peptide YY (PYY3-36), and cholecystokinin (CCK), between habitual breakfast eaters (Br-Es) and habitual skippers (Br-Ss) to a standard midday meal.

METHODS

Thirty-two women [mean ± SD age: 22.6 ± 3.3 y; body mass index (in kg/m2): 21.8 ± 2.0] participated in a cross-sectional study that consisted of a 3-h test protocol that included a standard test meal served at 1230 with pre- and postmeal blood sampling. The protocol required that Br-Es eat a typical breakfast between 0700 and 1000, whereas Br-Ss had no breakfast meal and had fasted for 12 h. Blood was drawn 35 and 5 min prelunch and 5, 20, 35, 50, and 110 min postlunch.

RESULTS

Repeated-measures ANOVA revealed a group difference for PYY3-36 (P = 0.001), with the Br-E group exhibiting 50-90% higher concentrations throughout the test period. Leptin tended to be different (P = 0.08) between groups, with higher mean ± SD values for the Br-S group (27.6 ± 29.6 ng/mL) compared with the Br-E group (11.5 ± 9.8 ng/mL). Partial least squares regression analysis confirmed that these 2 hormones were important contributors to the patterns of the hormones, anthropometric, clinical, and behavioral variables that differed between groups; insulin and CCK were important as well.

CONCLUSION

We found differences between the Br-E and Br-S groups in circulating gut and adipose-derived hormones measured midday, indicating that the breakfast habit is associated with the hormonal milieu before and after a midday meal. The different patterns may be short-lived or may impact metabolism later in the day. This report is a secondary analysis of a trial registered at clinicaltrials.gov as NCT01427556.

Insulin secretory effect of sitagliptin: assessment with a hyperglycemic clamp combined with a meal challenge

Sitagliptin, a dipeptidyl peptidase-IV inhibitor (DPP-4), sustains activity of the incretin hormones GLP-1 and GIP and improves hyperglycemia in Type 2 diabetes mellitus (T2DM). It has however proven challenging to quantify the effect of sitagliptin on rates of insulin secretion (ISR) during a prandial challenge. The tight feedback governance of ISR by plasma glucose means that in the face of treatment-related lowering of postprandial glycemia, corresponding stimulation of ISR is lessened. We postulated that sustaining a stable level of moderate hyperglycemia before and during a meal challenge (MC) would be a platform that enables greater clarity to assess the effect of sitagliptin on ISR and an approach that could be valuable to evaluate novel targets that increase insulin secretion directly and by augmenting incretins. A hyperglycemic clamp (HGC) at 160 mg/dl was conducted in 12 healthy volunteers (without diabetes) for 6 h; 3 h into the HGC, MC was administered while maintaining stable hyperglycemia of the HGC for an additional 3 h. Modeling of C-peptide response was used to calculate ISR. In crossover design of three periods (sitagliptin twice and placebo once), the effect of sitagliptin vs. placebo on ISR and the reproducibility of the response to sitagliptin were assessed. Sitagliptin increased ISR compared with placebo by 50% and 20% during the HGC alone and the HGC-MC phases, respectively ( P < 0.001 for both). There was an associated significant treatment-based increase in circulating insulin, as well as active levels of GLP-1. Robust reproducibility of the sitagliptin-mediated ISR response was observed; the intraclass correlation value was 0.94. The findings delineate the effect of sitagliptin to stimulate insulin secretion, and these benchmark data also demonstrate that an HGC-MC can be a useful platform for interrogating therapeutic targets that could potentially modulate ISR via direct action on beta-cells as well as by augmenting release or action of incretins.

Emerging Applications of Abdominal 4D Flow MRI

OBJECTIVE

Comprehensive assessment of abdominal hemodynamics is crucial for many clinical diagnoses but is challenged by a tremendous complexity of anatomy, normal physiology, and a wide variety of pathologic abnormalities. This article introduces 4D flow MRI as a powerful technique for noninvasive assessment of the hemodynamics of abdominal vascular territories.

CONCLUSION

Four-dimensional flow MRI provides clinicians with a more extensive and straightforward approach to evaluate disorders that affect blood flow in the abdomen. This review presents a series of clinical cases to illustrate the utility of 4D flow MRI in the comprehensive assessment of the abdominal circulation.

Impaired regulation of portal venous flow in response to a meal challenge as quantified by 4D flow MRI

PURPOSE

Portal and mesenteric hemodynamics is greatly altered in portal hypertension patients. This study utilizes 4D flow magnetic resonance imaging (MRI) to visualize and quantify changes in abdominal hemodynamics in patients with portal hypertension undergoing meal challenge.

MATERIALS AND METHODS

Twelve portal hypertension patients and six healthy subjects participated in the study. Baseline MRI was acquired after 5 hours of fasting. Postmeal MRI was obtained 20 minutes after subjects ingested EnSure Plus (574 mL). Imaging was performed at 3T using 4D flow MRI with an undersampled radial acquisition. Flow measurements were performed blinded to subject status (fasting/meal). Flow values for each vessel were compared before and after the meal challenge using paired Student's t-tests (P < 0.05).

RESULTS

After meal challenge, significant increases in blood flow were observed in supraceliac aorta, portal vein, superior mesenteric vein, and artery in both groups (P < 0.05). In patients, hepatic artery (P = 0.001) and splenic vein (P = 0.045) flow decreased while azygos vein flow (P = 0.002) increased.

CONCLUSION

Portal venous flow regulation to adjust the increasing mesenteric venous flow after a meal challenge may be impaired in patients with cirrhosis. The ability to comprehensively quantify the hemodynamic response of the abdominal vasculature to a meal challenge using 4D flow MRI reveals the potential of this technique to noninvasively characterize portal hypertension hemodynamics.

Acute and chronic effects of sprint interval exercise on postprandial lipemia in women at-risk for the metabolic syndrome

Individuals diagnosed with the metabolic syndrome (MetS) exhibit elevated postprandial lipemia (PPL). The aims of this investigation were to determine 1) if an acute bout of sprint interval training (SIT) attenuates PPL; and 2) if the attenuation of PPL following 6 wk of SIT is magnified compared with a single session of SIT prior to training in women at-risk for MetS (n = 45; 30-65 yr). Women were randomized to SIT (n = 22) or a nonexercise control (n = 23; CON) for 6 wk. Postprandial responses to a high-fat meal challenge (HFMC) were assessed in the CON group before (B-HFMC) and after (Post-HFMC) without prior exercise and in the SIT group at baseline (B-HFMC) without prior exercise, after an acute bout of SIT (four 30-s all-out sprints with 4-min recovery) prior to (Pre-HFMC), and after the 6-wk intervention (Post-HFMC). Responses to the HFMC were assessed by collecting venous blood samples in the fasted state and at 0, 30, 60, 120, and 180 min postprandial. Compared with baseline, an acute bout of SIT before (Pre-HFMC) and after the 6-wk intervention (Post-HFMC) significantly attenuated fasted TG (P < 0.05; 16.6% and 12.3%, respectively) and postprandial area under the curve (13.1% and 9.7%, respectively; tAUC) TG responses. There was no difference in fasted or tAUC TG responses between Pre-HFMC and Post-HFMC. SIT is an effective mode of exercise to reduce fasted and postprandial TG concentrations in women at-risk for MetS. Six weeks of SIT does not magnify the attenuation of PPL in response to a single session of SIT.