Impaired Metabolic Flexibility Research Articles

Exercise Makes More than an Energy Deficit: Toward Improved Protocols for the Management of Obesity?

Exercise Makes More than an Energy Deficit: Toward Improved Protocols for the Management of Obesity?

Type 2 diabetes alters metabolic and transcriptional signatures of glucose and amino acid metabolism during exercise and recovery.

The therapeutic benefit of physical activity to prevent and treat type 2 diabetes is commonly accepted. However, the impact of the disease on the acute metabolic response is less clear. To this end, we investigated the effect of type 2 diabetes on exercise-induced plasma metabolite changes and the muscular transcriptional response using a complementary metabolomics/transcriptomics approach. We analysed 139 plasma metabolites and hormones at nine time points, and whole genome expression in skeletal muscle at three time points, during a 60 min bicycle ergometer exercise and a 180 min recovery phase in type 2 diabetic patients and healthy controls matched for age, percentage body fat and maximal oxygen consumption (VO2). Pathway analysis of differentially regulated genes upon exercise revealed upregulation of regulators of GLUT4 (SLC2A4RG, FLOT1, EXOC7, RAB13, RABGAP1 and CBLB), glycolysis (HK2, PFKFB1, PFKFB3, PFKM, FBP2 and LDHA) and insulin signal mediators in diabetic participants compared with controls. Notably, diabetic participants had normalised rates of lactate and insulin levels, and of glucose appearance and disappearance, after exercise. They also showed an exercise-induced compensatory regulation of genes involved in biosynthesis and metabolism of amino acids (PSPH, GATM, NOS1 and GLDC), which responded to differences in the amino acid profile (consistently lower plasma levels of glycine, cysteine and arginine). Markers of fat oxidation (acylcarnitines) and lipolysis (glycerol) did not indicate impaired metabolic flexibility during exercise in diabetic participants. Type 2 diabetic individuals showed specific exercise-regulated gene expression. These data provide novel insight into potential mechanisms to ameliorate the disturbed glucose and amino acid metabolism associated with type 2 diabetes.

Maternal protein restriction impairs the transcriptional metabolic flexibility of skeletal muscle in adult rat offspring

Skeletal muscle exhibits a remarkable flexibility in the usage of fuel in response to the nutrient intake and energy demands of the organism. In fact, increased physical activity and fasting trigger a transcriptional programme in skeletal muscle cells leading to a switch from carbohydrate to lipid oxidation. Impaired metabolic flexibility has been reported to be associated with obesity and type 2 diabetes, but it is not known whether the disability to adapt to metabolic demands is a cause or a consequence of these pathological conditions. Inasmuch as a poor nutritional environment during early life is a predisposing factor for the development of metabolic diseases in adulthood, in the present study, we aimed to determine the long-term effects of maternal malnutrition on the metabolic flexibility of offspring skeletal muscle. To this end, the transcriptional responses of the soleus and extensor digitorum longus muscles to fasting were evaluated in adult rats born to dams fed a control (17 % protein) or a low-protein (8 % protein, protein restricted (PR)) diet throughout pregnancy and lactation. With the exception of reduced body weight and reduced plasma concentrations of TAG, PR rats exhibited a metabolic profile that was the same as that of the control rats. In the fed state, PR rats exhibited an enhanced expression of key regulatory genes of fatty acid oxidation including CPT1a, PGC-1α, UCP3 and PPARα and an impaired expression of genes that increase the capacity for fat oxidation in response to fasting. These results suggest that impaired metabolic inflexibility precedes and may contribute to the development of metabolic disorders associated with early malnutrition.

Muscle specific miRNAs are induced by testosterone and independently upregulated by age

Age dependent decline in skeletal muscle function leads to impaired metabolic flexibility in elderly individuals. Physical activity and testosterone treatment have proven efficient strategies for delaying this condition. However, a common molecular pathway has not been identified. Muscle specific miRNAs (myomiRs) regulate metabolic pathways in skeletal muscle, are regulated by physical activity, and have response elements for testosterone in their promoter region. We therefore hypothesized that myomiRs would be regulated in skeletal muscle during aging. We further investigated any potential gender-dependent regulation of these miRNAs. We found that the myomiRs miR-1, miR-133a, and miR-133b were increased in skeletal muscle of elderly men compared to younger men. In addition, miR-133a/133b expression was markedly higher in women compared to men. Elimination of circulating testosterone in men was associated with lower levels of miR-133a and miR-133b. A positive regulatory effect of testosterone on miR-133a/133b expression was confirmed in castrated male C57BL/6J mice and in a model of primary human myocytes. Yet, an improvement of fitness level in the testosterone depleted men resulted in a down-regulation of miR133a/b. In conclusion, alterations in fitness level and circulating testosterone seem to represent two independent regulatory events where testosterone is a specific regulator of miR-133a/b expression.

Metabolic Inflexibility Is a Feature of Women With Polycystic Ovary Syndrome and Is Associated With Both Insulin Resistance and Hyperandrogenism

Metabolic inflexibility, ie, the impaired ability of the body to switch from fat to carbohydrate oxidation under insulin-stimulated conditions, is associated with insulin resistance. This alteration in metabolic plasticity can lead to organ dysfunction and is considered a key issue among the abnormalities of the metabolic syndrome. It is still unknown whether this phenomenon occurs in women with polycystic ovary syndrome (PCOS). Our objective was to examine whether metabolic inflexibility is a feature of PCOS women and whether hyperandrogenism may contribute to this phenomenon. Eighty-nine Caucasian women with PCOS were submitted to hyperinsulinemic-euglycemic clamp. Respiratory exchange ratios were evaluated at baseline and during hyperinsulinemia by indirect calorimetry to quantify substrate oxidative metabolism. Total testosterone was measured by liquid chromatography mass spectrometry and free testosterone by equilibrium dialysis. Outpatients were seen in a tertiary care academic center. Metabolic flexibility was assessed by the change in respiratory quotient upon insulin stimulation. Sixty-five of the 89 PCOS women (73%) had increased serum free testosterone, 68 (76%) were insulin resistant, and 62 (70%) had an impaired metabolic flexibility. Comparison of hyperandrogenemic and normoandrogenemic women showed that the 2 subgroups were of similar age but differed in terms of several anthropometric and metabolic features. In particular, hyperandrogenemic women had greater body mass index (32.9 ± 1.0 vs 24.7 ± 0.9 kg/m(2), P < .001) and lower glucose utilization during the clamp (9.2 ± 0.4 vs 10.9 ± 0.7 mg/kg fat-free mass · min, P = .023) and metabolic flexibility (0.09 ± 0.06 vs 0.12 ± 0.01, P = .014). In univariate analysis, metabolic flexibility was associated with several anthropometric, endocrine, and metabolic features. In multivariate analysis, this feature was directly associated with baseline respiratory quotient and insulin sensitivity and inversely with free testosterone and free fatty acids concentrations under insulin suppression (R(2) = 0.634, P < .001). Metabolic inflexibility is a feature of PCOS women. Both insulin resistance and androgen excess might contribute to this abnormality.

Metabolic flexibility and obesity in children and youth

The concept of metabolic flexibility describes the ability of skeletal muscle to switch between the oxidation of lipid as a fuel during fasting periods to the oxidation of carbohydrate during insulin stimulated period. Alterations in energy metabolism in adults with obesity, insulin resistance and/or type 2 diabetes induce a state of impaired metabolic flexibility, or metabolic inflexibility. Despite the increase in the prevalence of type 2 diabetes in obese children and youth, less is known about the factors involved in the development of metabolic inflexibility in the paediatric population. Metabolic flexibility is conditioned by nutrient partitioning in response to feeding, substrate mobilization and delivery to skeletal muscle during fasting or exercising condition, and skeletal muscle oxidative capacity. Our aim in this review was to identify among these factors those making obese children at risk of metabolic inflexibility. The development of ectopic rather than peripheral fat storage appears to be a factor strongly linked with a reduced metabolic flexibility. Tissue growth and maturation are determinants of impaired energy metabolism later in life but also as a promising way to reverse metabolic inflexibility given the plasticity of many tissues in youth. Finally, we have attempted to identify perspectives for future investigations of metabolic flexibility in obese children that will improve our understanding of the genesis of metabolic diseases associated with obesity.

Metabolic inflexibility is a common feature of impaired fasting glycaemia and impaired glucose tolerance

Metabolic flexibility reflects the ability to switch from lipid to carbohydrate oxidation during insulin stimulation. Impaired metabolic flexibility is related to insulin resistance and type 2 diabetes, but whether metabolic flexibility is impaired in individuals with the pre-diabetic states isolated impaired fasting glycaemia (i-IFG) and isolated impaired glucose tolerance (i-IGT) is unknown. Using the gold standard euglycaemic hyperinsulinaemic clamp technique combined with indirect calorimetry, we measured peripheral insulin sensitivity, lipid and glucose oxidation, and thus metabolic flexibility in 66 individuals with normal glucose tolerance (NGT, n=20), i-IFG (n=18) and i-IGT (n=28). During insulin stimulation, individuals with i-IGT displayed reduced insulin sensitivity including reduced glucose oxidation. Interestingly, those with i-IFG exhibited reduced glucose oxidation and a slightly elevated lipid oxidation rate during insulin infusion despite having normal total peripheral glucose disposal. Thus, metabolic flexibility was significantly reduced in individuals with both i-IFG and i-IGT even after adjustment for BMI and insulin sensitivity. The data indicate that metabolic inflexibility may precede the development of overt peripheral insulin resistance in pre-diabetic individuals. However, prospective studies are needed to confirm this notion.

Metabolic flexibility and insulin resistance

Metabolic flexibility is the capacity for the organism to adapt fuel oxidation to fuel availability. The inability to modify fuel oxidation in response to changes in nutrient availability has been implicated in the accumulation of intramyocellular lipid and insulin resistance. The metabolic flexibility assessed by the ability to switch from fat to carbohydrate oxidation is usually impaired during a hyperinsulinemic clamp in insulin-resistant subjects; however, this "metabolic inflexibility" is mostly the consequence of impaired cellular glucose uptake. Indeed, after controlling for insulin-stimulated glucose disposal rate (amount of glucose available for oxidation), metabolic flexibility is not altered in obesity regardless of the presence of type 2 diabetes. To understand how intramyocellular lipids accumulate and cause insulin resistance, the assessment of metabolic flexibility to high-fat diets is more relevant than metabolic flexibility during a hyperinsulinemic clamp. An impaired capacity to upregulate muscle lipid oxidation in the face of high lipid supply may lead to increased muscle fat accumulation and insulin resistance. Surprisingly, very few studies have investigated the response to high-fat diets. In this review, we discuss the role of glucose disposal rate, adipose tissue lipid storage, and mitochondrial function on metabolic flexibility. Additionally, we emphasize the bias of using the change in respiratory quotient to calculate metabolic flexibility and propose novel approaches to assess metabolic flexibility. On the basis of current evidence, one cannot conclude that impaired metabolic flexibility is responsible for the accumulation of intramyocellular lipid and insulin resistance. We propose to study metabolic flexibility in response to high-fat diets in individuals having contrasting degree of insulin sensitivity and/or mitochondrial characteristics.

Impaired Skeletal Muscle Substrate Oxidation in Glucose‐intolerant Men Improves After Weight Loss

An impaired fatty acid handling in skeletal muscle may be involved in the development of insulin resistance and diabetes mellitus type 2 (DM2). We investigated muscle fatty acid metabolism in glucose-intolerant men (impaired glucose tolerance (IGT)), a prediabetic state, relative to BMI-matched control men (normal glucose tolerance (NGT)) during fasting and after a meal, because most people in the western society are in the fed state most of the day. Skeletal muscle free fatty acid (FFA) uptake and oxidation were studied using the stable isotope tracer [2,2-(2)H]-palmitate and muscle indirect calorimetry in the forearm model during fasting and after a mixed meal (33 energy % (E%) carbohydrates, 61 E% fat). Intramyocellular triglycerides (IMTGs) were monitored with 1H-magnetic resonance spectroscopy. IGT men were re-examined after weight loss (-15% of body weight (BW)). The postprandial increase in forearm muscle respiratory quotient (RQ) was blunted in IGT compared to NGT, but improved after weight loss. Weight loss also improved fasting-fat oxidation and tended to decrease IMTGs (P=0.08). No differences were found in fasting and postprandial forearm muscle fatty acid uptake between NGT and IGT, or in IGT before and after weight loss. The ability to switch from fat oxidation to carbohydrate oxidation after a meal is already impaired in the prediabetic state, suggesting this may be an early factor in the development toward DM2. This impaired ability to regulate fat oxidation during fasting and after a meal (impaired metabolic flexibility) can be (partly) reversed by weight loss.

Metabolic Flexibility in Response to Glucose Is Not Impaired in People With Type 2 Diabetes After Controlling for Glucose Disposal Rate

Compared with nondiabetic subjects, type 2 diabetic subjects are metabolically inflexible with impaired fasting fat oxidation and impaired carbohydrate oxidation during a hyperinsulinemic clamp. We hypothesized that impaired insulin-stimulated glucose oxidation is a consequence of the lower cellular glucose uptake rate in type 2 diabetes. Therefore, we compared metabolic flexibility to glucose adjusted for glucose disposal rate in nondiabetic versus type 2 diabetic subjects and in the latter group after 1 year of lifestyle intervention (the Look AHEAD [Action For Health in Diabetes] trial). Macronutrient oxidation rates under fasting and hyperinsulinemic conditions (clamp at 80 mU/m(2) per min), body composition (dual-energy X-ray absorptiometry), and relevant hormonal/metabolic blood variables were assessed in 59 type 2 diabetic and 42 nondiabetic individuals matched for obesity, sex, and race. Measures were repeated in diabetic participants after weight loss. Metabolic flexibility to glucose (change in respiratory quotient [RQ]) was mainly related to insulin-stimulated glucose disposal rate (R(2) = 0.46, P < 0.0001) with an additional 3% of variance accounted for by plasma free fatty acid concentration at the end of the clamp (P = 0.03). The impaired metabolic flexibility to glucose observed in type 2 diabetic versus nondiabetic subjects (Delta RQ 0.06 +/- 0.01 vs. 0.10 +/- 0.01, respectively, P < 0.0001) was no longer observed after adjusting for glucose disposal rate (P = 0.19). Additionally, the increase in metabolic flexibility to glucose after weight loss was accounted for by the concomitant increase in insulin-stimulated glucose disposal rate. This study suggests that metabolic inflexibility to glucose in type 2 diabetic subjects is mostly related to defective glucose transport.

Type 2 Diabetes Mellitus and Skeletal Muscle Metabolic Function

Type 2 diabetic patients are characterized by a decreased fat oxidative capacity and high levels of circulating free fatty acids (FFAs). The latter is known to cause insulin resistance, in particularly in skeletal muscle, by reducing insulin stimulated glucose uptake, most likely via accumulation of lipid inside the muscle cell. A reduced skeletal muscle oxidative capacity can exaggerate this. Furthermore, type 2 diabetes is associated with impaired metabolic flexibility, i.e. an impaired switching from fatty acid to glucose oxidation in response to insulin. Thus, a reduced fat oxidative capacity and metabolic inflexibility are important components of skeletal muscle insulin resistance. The cause of these derangements in skeletal muscle of type 2 diabetic patients remains to be elucidated. An impaired mitochondrial function is a likely candidate. Evidence from both in vivo and ex vivo studies supports the idea that an impaired skeletal muscle mitochondrial function is related to the development of insulin resistance and type 2 diabetes mellitus. A decreased mitochondrial oxidative capacity in skeletal muscle was revealed in diabetic patients, using in vivo 31-Phosphorus Magnetic Resonance Spectroscopy (31P-MRS). However, quantification of mitochondrial function using ex vivo high-resolution respirometry revealed opposite results. Future (human) studies should challenge this concept of impaired mitochondrial function underlying metabolic defects and prove if mitochondria are truly functional impaired in insulin resistance, or low in number, and whether it represents the primary starting point of pathogenesis of insulin resistance, or is just an other feature of the insulin resistant state.

Overexpression of pyruvate dehydrogenase kinase 4 in heart perturbs metabolism and exacerbates calcineurin-induced cardiomyopathy

The heart adapts to changes in nutritional status and energy demands by adjusting its relative metabolism of carbohydrates and fatty acids. Loss of this metabolic flexibility such as occurs in diabetes mellitus is associated with cardiovascular disease and heart failure. To study the long-term consequences of impaired metabolic flexibility, we have generated mice that overexpress pyruvate dehydrogenase kinase (PDK)4 selectively in the heart. Hearts from PDK4 transgenic mice have a marked decrease in glucose oxidation and a corresponding increase in fatty acid catabolism. Although no overt cardiomyopathy was observed in the PDK4 transgenic mice, introduction of the PDK4 transgene into mice expressing a constitutively active form of the phosphatase calcineurin, which causes cardiac hypertrophy, caused cardiomyocyte fibrosis and a striking increase in mortality. These results demonstrate that cardiac-specific overexpression of PDK4 is sufficient to cause a loss of metabolic flexibility that exacerbates cardiomyopathy caused by the calcineurin stress-activated pathway.