Direct Relationship Between Metabolic Flexibility Measured During Glucose Clamp and Prolonged Fast in Men

Validation of a novel approach to assess metabolic flexibility to a high-fat meal in a whole-body room calorimeter

OBJECTIVE

Metabolic inflexibility to Western high-fat diets may contribute to the obesity epidemic. However, validated methods for assessing metabolic flexibility (MetFlex) to high-fat meals are currently lacking. The purpose of this study was to evaluate the reliability of a novel approach for determining MetFlex to a high-fat meal and to compare it with the gold standard for measuring MetFlex to high-carbohydrate loads.

METHODS

Eight healthy adults were enrolled in our study, which consisted of the following two assessments of MetFlex: 1) MetFlex to fat, via two overnight stays in a metabolic chamber separated by 5 to 7 days; and 2) Metflex to carbohydrates, via a two-step hyperinsulinemic-euglycemic clamp measured >5 days later.

RESULTS

Participants were predominantly White and male, with mean (SD) age of 29.4 (6.3) years and BMI of 25.4 (4.1) kg/m2. MetFlex to fat displayed satisfactory test-retest reliability (intraclass correlation coefficient > 0.70) for several outcomes but showed no correlation to MetFlex measured during the clamp.

CONCLUSIONS

Overnight changes in substrate oxidation following a high-fat dinner meal represent a unique aspect of MetFlex that cannot be captured using more conventional methods. Our findings warrant prospective studies to determine whether these parameters are predictive of the development of obesity or metabolic dysfunction.

Metabolic flexibility to lipid during exercise is not associated with metabolic health outcomes in individuals without obesity

A low metabolic flexibility to lipid (MetF-lip) in skeletal muscle may promote ectopic lipid accumulation, thus inducing metabolic disturbances. We aimed to determine the association between MetF-lip in skeletal muscle and metabolic health outcomes in individuals without obesity. We also explored the association between MetF-lip and the inflammatory signaling pathway in skeletal muscle. This was a cross-sectional study in 17 individuals aged (median [IQR]) 55.4 [48.6, 58.5] years, with a BMI of 24.4 [22.6, 26.0] kg/m2. MetF-lip was assessed as the increase in relative lipid oxidation during a single exercise session (~ 50% VO2max, 2 hours), quantified as the drop in whole-body respiratory exchange ratio (ΔRER = RER at 2 hours - maximum RER attained). HOMA-IR, metabolic syndrome z-score, fat percentage, trunk-to-appendicular fat, and VO2max were included as metabolic health outcomes. The abundance of proteins of the inflammatory pathway was analyzed in resting muscle. Acute exercise progressively increased relative lipid oxidation (ΔRER = -0.04 [-0.08, -0.02]). MetF-lip was not associated with any metabolic health outcome but correlated inversely with p-p38Thr180/Tyr182 in muscle. A low MetF-lip in skeletal muscle does not seem a major determinant of metabolic disturbances but associates with a partial activation of the inflammatory signaling in individuals without obesity.

Assessing metabolic flexibility response to a multifibre diet: a randomised-controlled trial

INTRODUCTION

Metabolic flexibility (MetF), defined as the ability to switch between fat and glucose oxidation, is increasingly recognised as a critical marker for assessing responses to dietary interventions. Previously, we showed that the consumption of multifibre bread improved insulin sensitivity and reduced low-density lipoprotein cholesterol (LDLc) levels in overweight and obese individuals. As a secondary objective, we aimed to explore whether our intervention could also improve MetF.

METHODS

In this study, 39 subjects at cardiometabolic risk participated in a double-blind, randomised, crossover trial lasting 8 weeks, repeated twice. During each phase, participants consumed either 150 g of standard bread daily or bread enriched with a mixture of seven dietary fibres. MetF response was assessed using a mixed-meal tolerance test (MMTT), analysing changes in respiratory quotient (∆RQ) measured using indirect calorimetry.

RESULTS

Although there were no significant differences in ∆RQ changes induced by dietary fibre between the two diets, these changes were positively correlated with postprandial triglyceride excursion (∆TG) at baseline. Subgroup analysis of baseline fasting and postprandial plasma metabolites was conducted to characterise MetF responders. These responders exhibited higher baseline fasting LDLc levels and greater post-MMTT ∆TG.

CONCLUSION

In conclusion, although dietary fibres did not directly impact MetF in this study, our findings highlight potential determinants of MetF response, warranting further investigation in dedicated future interventions.

Effects of ketone bodies on energy expenditure, substrate utilization, and energy intake in humans

The potential of ketogenic approaches to regulate energy balance has recently gained attention since ketones may influence both energy expenditure and energy intake. In this narrative review, we summarized the most relevant evidence about the role of ketosis on energy expenditure, substrate utilization, and energy intake in humans. We considered different strategies to induce ketosis, such as fasting, dietary manipulation, and exogenous ketone sources. In general, ketosis does not have a major influence on energy expenditure but promotes a shift in substrate utilization towards ketone body oxidation. The strategies to induce ketosis by reduction of dietary carbohydrate availability (e.g., ketogenic diets) do not independently influence energy intake, being thus equally effective for weight loss as diets with higher carbohydrate content. In contrast, the intake of medium-chain triglycerides and ketone esters induces ketosis and appears to increase energy expenditure and reduce energy intake in the context of high carbohydrate availability. These latter strategies lead to slightly enhanced weight loss. Unfortunately, distinguishing the effects of the various ketogenic strategies per se from the effects of other physiological responses is not possible with the available human data. Highly controlled, inpatient studies using targeted strategies to isolate the independent effects of ketones are required to adequately address this knowledge gap.

Mitochondria-SR interaction and mitochondrial fusion/fission in the regulation of skeletal muscle metabolism

Mitochondria-endoplasmic/sarcoplasmic reticulum (ER/SR) interaction and mitochondrial fusion/fission are critical processes that influence substrate oxidation. This narrative review summarizes the evidence on the effects of substrate availability on mitochondrial-SR interaction and mitochondria fusion/fission dynamics to modulate substrate oxidation in human skeletal muscle. Evidence shows that an increase in mitochondria-SR interaction and mitochondrial fusion are associated with elevated fatty acid oxidation. In contrast, a decrease in mitochondria-SR interaction and an increase in mitochondrial fission are associated with an elevated glycolytic activity. Based on the evidence reviewed, we postulate two hypotheses for the link between mitochondrial dynamics and insulin resistance in human skeletal muscle. First, glucose and fatty acid availability modifies mitochondria-SR interaction and mitochondrial fusion/fission to help the cell to adapt substrate oxidation appropriately. Individuals with an impaired response to these substrate challenges will accumulate lipid species and develop insulin resistance in skeletal muscle. Second, a chronically elevated substrate availability (e.g. overfeeding) increases mitochondrial production of reactive oxygen species and induced mitochondrial fission. This decreases fatty acid oxidation, thus leading to the accumulation of lipid species and insulin resistance in skeletal muscle. Altogether, we propose mitochondrial dynamics as a potential target for disturbances associated with low fatty acid oxidation.

Closed-loop control of air supply to whole-room indirect calorimeters to improve accuracy and standardize measurements during 24-hour dynamic metabolic studies

OBJECTIVE

The aim of this study was to test proportional-integral-derivative (PID) control of air inflow rate in a whole-room indirect calorimeter to improve accuracy in measuring oxygen (O₂ ) consumption ( V ̇ O 2 ) and carbon dioxide (CO₂ ) production ( V ̇ CO 2 ).

METHODS

A precision gas blender infused nitrogen (N₂ ) and CO₂ into the calorimeter over 24 hours based on static and dynamic infusion profiles mimicking V ̇ O 2 and V ̇ CO 2 patterns during resting and non-resting conditions. Constant (60 L/min) versus time-variant flow set by a PID controller based on the CO₂ concentration was compared based on errors between measured versus expected values for V ̇ O 2 , V ̇ CO 2 , respiratory exchange ratio, and metabolic rate.

RESULTS

Compared with constant inflow, the PID controller allowed both a faster rise time and long-term maintenance of a stable CO₂ concentration inside the calorimeter, resulting in more accurate V ̇ CO 2 estimates (mean hourly error, PID: -0.9%, 60 L/min = -2.3%, p < 0.05) during static infusions. During dynamic infusions mimicking exercise sessions, the PID controller achieved smaller errors for V ̇ CO 2 (mean: -0.6% vs. -2.7%, p = 0.02) and respiratory exchange ratio (mean: 0.5% vs. -3.1%, p = 0.02) compared with constant inflow conditions, with similar V ̇ O 2 (p = 0.97) and metabolic rate (p = 0.76) errors.

CONCLUSIONS

PID control in a whole-room indirect calorimeter system leads to more accurate measurements of substrate oxidation during dynamic metabolic studies.

The association between gut hormones and diet-induced metabolic flexibility in metabolically healthy adults

OBJECTIVE

This study investigated whether interindividual variance in diet-induced metabolic flexibility is explained by differences in gut hormone concentrations.

METHODS

A total of 69 healthy volunteers with normal glucose regulation underwent 24-hour assessments of respiratory quotient (RQ) in a whole-room indirect calorimeter during eucaloric feeding (EBL; 50% carbohydrate, 30% fat) and then, in a crossover design, during 24-hour fasting and three normal-protein (20%) overfeeding diets (200% energy requirements). Metabolic flexibility was defined as the change in 24-hour RQ from EBL during standard (50% carbohydrate), high-fat (60%), and high-carbohydrate (75%) overfeeding diets. Plasma concentrations of glucagon-like peptide-1 (GLP-1) and peptide YY (PYY) after an overnight fast were measured prior to and after each diet.

RESULTS

Compared with EBL, on average, 24-hour RQ decreased by ~4% during high-fat overfeeding, whereas it increased by ~4% during standard overfeeding and by ~9% during high-carbohydrate overfeeding. During high-carbohydrate overfeeding, but not during any other overfeeding diet or fasting, increased GLP-1 concentration was associated with increased RQ (r = 0.44, p < 0.001), higher/lower carbohydrate/lipid oxidation rates (r = 0.34 and r = -0.51, both p < 0.01), respectively, and increased plasma insulin concentration (r = 0.38, p = 0.02).

CONCLUSIONS

Increased GLP-1 concentration following high-carbohydrate overfeeding associated with a greater shift to carbohydrate oxidation, suggesting that GLP-1 may be implicated in diet-induced metabolic flexibility to carbohydrate overload.

Similar Metabolic Health in Overweight/Obese Individuals With Contrasting Metabolic Flexibility to an Oral Glucose Tolerance Test

Background: Low metabolic flexibility (MetF) may be an underlying factor for metabolic health impairment. Individuals with low MetF are thus expected to have worse metabolic health than subjects with high MetF. Therefore, we aimed to compare metabolic health in individuals with contrasting MetF to an oral glucose tolerance test (OGTT).

Methods: In individuals with excess body weight, we measured MetF as the change in respiratory quotient (RQ) from fasting to 1 h after ingestion of a 75-g glucose load (i.e., OGTT). Individuals were then grouped into low and high MetF (Low-MetF n = 12; High-MetF n = 13). The groups had similar body mass index, body fat, sex, age, and maximum oxygen uptake. Metabolic health markers (clinical markers, insulin sensitivity/resistance, abdominal fat, and intrahepatic fat) were compared between groups.

Results: Fasting glucose, triglycerides (TG), and high-density lipoprotein (HDL) were similar between groups. So were insulin sensitivity/resistance, visceral, and intrahepatic fat. Nevertheless, High-MetF individuals had higher diastolic blood pressure, a larger drop in TG concentration during the OGTT, and a borderline significant (P = 0.05) higher Subcutaneous Adipose Tissue (SAT). Further, compared to Low-MetF, High-MetF individuals had an about 2-fold steeper slope for the relationship between SAT and fat mass index.

Conclusion: Individuals with contrasting MetF to an OGTT had similar metabolic health. Yet High-MetF appears related to enhanced circulating TG clearance and enlarged subcutaneous fat.

Association Between Adipose Tissue Characteristics and Metabolic Flexibility in Humans: A Systematic Review

Adipose tissue total amount, distribution, and phenotype influence metabolic health. This may be partially mediated by the metabolic effects that these adipose tissue characteristics exert on the nearby and distant tissues. Thus, adipose tissue may influence the capacity of cells, tissues, and the organism to adapt fuel oxidation to fuel availability, i.e., their metabolic flexibility (MetF). Our aim was to systematically review the evidence for an association between adipose tissue characteristics and MetF in response to metabolic challenges in human adults. We searched in PubMed (last search on September 4, 2021) for reports that measured adipose tissue characteristics (total amount, distribution, and phenotype) and MetF in response to metabolic challenges (as a change in respiratory quotient) in humans aged 18 to <65 years. Any study design was considered, and the risk of bias was assessed with a checklist for randomized and non-randomized studies. From 880 records identified, 22 remained for the analysis, 10 of them measured MetF in response to glucose plus insulin stimulation, nine in response to dietary challenges, and four in response to other challenges. Our main findings were that: (a) MetF to glucose plus insulin stimulation seems inversely associated with adipose tissue total amount, waist circumference, and visceral adipose tissue; and (b) MetF to dietary challenges does not seem associated with adipose tissue total amount or distribution. In conclusion, evidence suggests that adipose tissue may directly or indirectly influence MetF to glucose plus insulin stimulation, an effect probably explained by skeletal muscle insulin sensitivity. Systematic Review Registration: PROSPERO [CRD42020167810].

The fasting-feeding metabolic transition regulates mitochondrial dynamics

In humans, insulin resistance has been linked to an impaired metabolic transition from fasting to feeding (metabolic flexibility; MetFlex). Previous studies suggest that mitochondrial dynamics response is a putative determinant of MetFlex; however, this has not been studied in humans. Thus, the aim of this study was to investigate the mitochondrial dynamics response in the metabolic transition from fasting to feeding in human peripheral blood mononuclear cells (PBMCs). Six male subjects fasted for 16 h (fasting), immediately after which they consumed a 75-g oral glucose load (glucose). In both fasting and glucose conditions, blood samples were taken to obtain PBMCs. Mitochondrial dynamics were assessed by electron microscopy images. We exposed in vitro acetoacetate-treated PBMCs to the specific IP3R inhibitor Xestospongin B (XeB) to reduce IP3R-mediated mitochondrial Ca²⁺ accumulation. This allowed us to evaluate the role of ER-mitochondria Ca²⁺ exchange in the mitochondrial dynamic response to substrate availability. To determine whether PBMCs could be used in obesity context (low MetFlex), we measured mitochondrial dynamics in mouse spleen-derived lymphocytes from WT and ob/ob mice. We demonstrated that the transition from fasting to feeding reduces mitochondria-ER interactions, induces mitochondrial fission and reduces mitochondrial cristae density in human PBMCs. In addition, we demonstrated that IP3R activity is key in the mitochondrial dynamics response when PBMCs are treated with a fasting-substrate in vitro. In murine mononuclear-cells, we confirmed that mitochondria-ER interactions are regulated in the fasted-fed transition and we further highlight mitochondria-ER miscommunication in PBMCs of diabetic mice. In conclusion, our results demonstrate that the fasting/feeding transition reduces mitochondria-ER interactions, induces mitochondrial fission and reduces mitochondrial cristae density in human PBMCs, and that IP3R activity may potentially play a central role.

Metabolic flexibility during sleep

Known as metabolic flexibility, oxidized substrate is selected in response to changes in the nutritional state. Sleep imposes an extended duration of fasting, and oxidized substrates during sleep were assumed to progressively shift from carbohydrate to fat, thereby gradually decreasing the respiratory quotient (RQ). Contrary to this assumption, whole-room indirect calorimetry with improved time resolution revealed that RQ re-ascended prior to awakening, and nadir of RQ in non-obese young adults occurred earlier in women than men after bedtime. The transient decrease in RQ during sleep was blunted in metabolically inflexible men with smaller amplitude of diurnal rhythm in RQ. Similarly, the effect of 10 years difference in age on RQ became significant during sleep; the decrease in RQ during sleep was blunted in older subjects. Inter-individual difference in RQ become apparent during sleep, and it might serve as a window to gain insight into the early-stage pathogenesis of metabolic inflexibility.

Low abundance of Mfn2 protein correlates with reduced mitochondria-SR juxtaposition and mitochondrial cristae density in human men skeletal muscle: Examining organelle measurements from TEM images

The role of mitofusin 2 (Mfn2) in the regulation of skeletal muscle (SM) mitochondria-sarcoplasmic (SR) juxtaposition, mitochondrial morphology, mitochondrial cristae density (MCD), and SM quality has not been studied in humans. In in vitro studies, whether Mfn2 increases or decreases mitochondria-SR juxtaposition remains controversial. Transmission electron microscopy (TEM) images are commonly used to measure the organelle juxtaposition, but the measurements are performed "by-hand," thus potentially leading to between-rater differences. The purposes of this study were to: (1) examine the repeatability and reproducibility of mitochondrial-SR juxtaposition measurement from TEM images of human SM between three raters with different experience and (2) compare the mitochondrial-SR juxtaposition, mitochondrial morphology, MCD (stereological-method), and SM quality (cross-sectional area [CSA] and the maximum voluntary contraction [MVC]) between subjects with high abundance (Mfn2-HA; n = 6) and low abundance (Mfn2-LA; n = 6) of Mfn2 protein. The mitochondria-SR juxtaposition had moderate repeatability and reproducibility, with the most experienced raters showing the best values. There were no differences between Mfn2-HA and Mfn2-LA groups in mitochondrial size, distance from mitochondria to SR, CSA, or MVC. Nevertheless, the Mfn2-LA group showed lower mitochondria-SR interaction, MCD, and VO_2max . In conclusion, mitochondrial-SR juxtaposition measurement depends on the experience of the rater, and Mfn2 protein seems to play a role in the metabolic control of human men SM, by regulating the mitochondria-SR interaction.

Pathophysiological role of metabolic flexibility on metabolic health

Glucose, fatty acids, and amino acids among others are oxidized to generate adenosine triphosphate (ATP). These fuels are supplied from the environment (through food intake) and internal depots (through lipolysis, glycogenolysis, and proteolysis) at different rates throughout the day. Complex adaptive systems permit to accommodate fuel oxidation according to fuel availability. This capacity of a cell, tissue, or organism to adapt fuel oxidation to fuel availability is defined as metabolic flexibility (MetF). There are conditions, such as insulin resistance, diabetes, and obesity, in which MetF seems to be impaired. The observation that those conditions are accompanied by mitochondrial dysfunction has set the basis to propose a link between mitochondrial dysfunction, metabolic inflexibility, and metabolic health. We here highlight the evidence about the notion that MetF influences metabolic health.