Muscle Insulin Resistance in Youth with Obesity and Normoglycemia is Associated with Altered Fat Metabolism

High-fat diet during pregnancy promotes fetal skeletal muscle fatty acid oxidation and insulin resistance in an ovine model

Maternal diet during pregnancy is associated with offspring metabolic risk trajectory in humans and animal models, but the prenatal origins of these effects are less clear. We examined the effects of a high-fat diet (HFD) during pregnancy on fetal skeletal muscle metabolism and metabolic risk parameters using an ovine model. White-faced ewes were fed a standardized diet containing 5% fat wt/wt (CON), or the same diet supplemented with 6% rumen-protected fats (11% total fat wt/wt; HFD) beginning 2 wk before mating until midgestation (GD75). Maternal HFD increased maternal weight gain, fetal body weight, and low-density lipoprotein levels in the uterine and umbilical circulation but had no significant effects on circulating glucose, triglycerides, or placental fatty acid transporters. Fatty acid (palmitoylcarnitine) oxidation capacity of permeabilized hindlimb muscle fibers was >50% higher in fetuses from HFD pregnancies, whereas pyruvate and maximal (mixed substrate) oxidation capacities were similar to CON. This corresponded to greater triacylglycerol content and protein expression of fatty acid transport and oxidation enzymes in fetal muscle but no significant effect on respiratory chain complexes or pyruvate dehydrogenase expression. However, serine-308 phosphorylation of insulin receptor substrate-1 was greater in fetal muscle from HFD pregnancies along with c-jun-NH2 terminal kinase activation, consistent with prenatal inhibition of skeletal muscle insulin signaling. These results indicate that maternal high-fat feeding shifts fetal skeletal muscle metabolism toward a greater capacity for fatty acid over glucose utilization and favors prenatal development of insulin resistance, which may predispose offspring to metabolic syndrome later in life.NEW & NOTEWORTHY Maternal diet during pregnancy is associated with offspring metabolic risk trajectory in humans and animal models, but the prenatal origins of these effects are less clear. This study examined the effects of a high-fat diet during pregnancy on metabolic risk parameters using a new sheep model. Results align with findings previously reported in nonhuman primates, demonstrating changes in fetal skeletal muscle metabolism that may predispose offspring to metabolic syndrome later in life.

Effect of free fatty acids on insulin secretion, insulin sensitivity and incretin effect - a narrative review

Deleterious effects of free fatty acids, FFAs, on insulin sensitivity are observed in vivo studies in humans. Mechanisms include impaired insulin signaling, oxidative stress, inflammation, and mitochondrial dysfunction, but the effects on insulin secretion are less well known. Our aim was to review the relationship of increased FFAs with insulin resistance, secretion and mainly with the incretin effect in humans. Narrative review. Increased endogenous or administered FFAs induce insulin resistance. FFAs effects on insulin secretion are debatable; inhibition and stimulation have been reported, depending on the type and duration of lipids exposition and the study subjects. Chronically elevated FFAs seem to decrease insulin biosynthesis, glucose-stimulated insulin secretion and β-cell glucose sensitivity. Lipids infusion decreases the response to incretins with unchanged incretin levels in volunteers with normal glucose tolerance. In contrast, FFAs reduction by acipimox did not restore the incretin effect in type-2 diabetes, probably due to the dysfunctional β-cell. Possible mechanisms of FFAs excess on incretin effect include reduction of the expression and levels of GLP-1 (glucagon like peptide-1) receptor, reduction of connexin-36 expression thus the coordinated secretory activity in response to GLP-1, and GIP (glucose-dependent insulinotropic polypeptide) receptors downregulation in islets cells. Increased circulating FFAs impair insulin sensitivity. Effects on insulin secretion are complex and controversial. Deleterious effects on the incretin-induced potentiation of insulin secretion were reported. More investigation is needed to better understand the extent and mechanisms of β-cell impairment and insulin resistance induced by increased FFAs and how to prevent them.

The Role of Mitochondrial Adaptation and Metabolic Flexibility in the Pathophysiology of Obesity and Insulin Resistance: an Updated Overview

PURPOSE OF REVIEW

The term "metabolic flexibility" denotes the dynamic responses of the cellular oxidative machinery in order to adapt to changes in energy substrate availability. A progressive loss of this adaptive capacity has been implicated in the development of obesity-related comorbidities. Mitochondria are dynamic intracellular organelles which play a fundamental role in energy metabolism, and the mitochondrial adaptation to environmental challenges may be viewed as the functional component of metabolic flexibility. Herein, we attempt to comprehensively review the available evidence regarding the role of mitochondrial adaptation and metabolic flexibility in the pathogenesis of obesity and related morbidities, namely insulin resistance states and non-alcoholic fatty liver disease (NAFLD).

RECENT FINDINGS

Overall, there is a concrete body of evidence to support the presence of impaired mitochondrial adaptation as a principal component of systemic metabolic inflexibility in conditions related to obesity. There are still many unresolved questions regarding the relationship between the gradual loss of mitochondrial adaptability and the progression of obesity-related complications, such as causality issues, the timely appearance and reversibility of the described disturbances, and the generalizability of the findings to the mitochondrial content of every affected tissue or organ. The evidence regarding the causality between the observed associations remains inconclusive, although most of the available data points towards a bidirectional, potentially mutually amplifying relationship. The spectrum of NAFLD is of particular interest, since functional and pathological changes in the course of its development closely mirror the progression of dysmetabolism, if not constituting a dynamic component of the latter.