In addition to its essential role in permitting mitochondrial import and oxidation of long chain fatty acids, carnitine also functions as an acyl group acceptor that facilitates mitochondrial export of excess carbons in the form of acylcarnitines. Recent evidence suggests carnitine requirements increase under conditions of sustained metabolic stress. Accordingly, we hypothesized that carnitine insufficiency might contribute to mitochondrial dysfunction and obesity-related impairments in glucose tolerance. Consistent with this prediction whole body carnitine dimunition was identified as a common feature of insulin-resistant states such as advanced age, genetic diabetes, and diet-induced obesity. In rodents fed a lifelong (12 month) high fat diet, compromised carnitine status corresponded with increased skeletal muscle accumulation of acylcarnitine esters and diminished hepatic expression of carnitine biosynthetic genes. Diminished carnitine reserves in muscle of obese rats was accompanied by marked perturbations in mitochondrial fuel metabolism, including low rates of complete fatty acid oxidation, elevated incomplete ␤-oxidation, and impaired substrate switching from fatty acid to pyruvate. These mitochondrial abnormalities were reversed by 8 weeks of oral carnitine supplementation, in concert with increased tissue efflux and urinary excretion of acetylcarnitine and improvement of whole body glucose tolerance. Acetylcarnitine is produced by the mitochondrial matrix enzyme, carnitine acetyltransferase (CrAT). A role for this enzyme in combating glucose intolerance was further supported by the finding that CrAT overexpression in primary human skeletal myocytes increased glucose uptake and attenuated lipid-induced suppression of glucose oxidation. These results implicate carnitine insufficiency and reduced CrAT activity as reversible components of the metabolic syndrome.Disturbances in mitochondrial genesis, morphology, and function are increasingly recognized as components of insulin resistance and the metabolic syndrome (1-3). Still unclear is whether poor mitochondrial performance is a predisposing factor or a consequence of the disease process. The latter view is supported by recent animal studies linking diet-induced insulin resistance to a dysregulated mitochondrial phenotype in skeletal muscle, marked by excessive ␤-oxidation, impaired substrate switching during the fasted to fed transition, and coincident reduction of organic acid intermediates of the tricarboxylic acid cycle (4, 5). In these studies, both diet-induced and genetic forms of insulin resistance were specifically linked to high rates of incomplete fat oxidation and intramuscular accumulation of fatty acylcarnitines, byproducts of lipid catabolism that are produced under conditions of metabolic stress (5, 6). Most compelling, we showed that genetically engineered inhibition of fat oxidation lowered intramuscular acylcarnitine levels and preserved glucose tolerance in mice fed a high fat diet (5, 7). In aggregate, the findings established a stron...

Higher acylcarnitine factor scores were associated with lower levels of objectively measured physical performance in this group of older, largely overweight men. Metabolic profiles of rodents exhibiting lipid-induced mitochondrial dysfunction show a similar phenotypic predominance of medium- and long-chain acylcarnitines.

Background/Objective To determine whether a home-based multi-component physical activity counseling (PAC) intervention is effective in reducing glycemic measures in older prediabetic outpatients. Design, Setting, and Participants Controlled clinical trial of 302 overweight (body mass index 25–45 kg/m2), older (ages 60–89) outpatients with impaired glucose tolerance (fasting blood glucose 100–125 mg/dL, HbA1c <7%), randomly assigned to a PAC intervention group (n=180), compared to a Usual Care (UC) control group (n=122) and recruited through primary care clinics of the Durham VA Medical Center between September 29, 2008 and March 25, 2010. Intervention A 12 month, home-based multi-component PAC program including one in-person baseline counseling session, regular telephone counseling, physician endorsement in clinic with monthly automated encouragement, and tailored mailed materials. All study participants, including UC, received a consult to a VA weight management program. Measurements The primary outcome was HOMA-IR, calculated from fasting insulin and glucose levels at baseline, 3 and 12 months. Hemoglobin A1C was the secondary indicator of glycemic control. Other secondary outcomes included anthropometric measures, and self-reported physical activity, health-related quality of life, and physical function. Results There were no significant differences between the PAC or Usual Care groups over time for any of the glycemic indicators. Both groups had small declines over time of approximately 6% in fasting blood glucose, p< 0.001, while other glycemic indicators remained stable. The declines in glucose were not sufficient to affect the change in HOMA-IR scores due to fluctuations in insulin over time. Endurance physical activity increased significantly in PAC group, P<0.001 compared to UC. Conclusion Home-based telephone counseling increased physical activity levels but was insufficient for improving glycemic indicators among older prediabetic outpatients.

Aims/hypothesis Insulin-sensitive tissues (muscle, liver) of individuals with obesity and type 2 diabetes mellitus are in a state of low-grade inflammation, characterised by increased Toll-like receptor (TLR) expression and TLR-driven signalling. However, the cause of this mild inflammatory state is unclear. We tested the hypothesis that a prolonged mild increase in plasma NEFA will increase TLR expression and TLR-driven signalling (nuclear factor κB [NFκB] and mitogen-activated kinase [MAPK]) and impair insulin action in muscle of lean healthy individuals. Methods Twelve lean, normal-glucose-tolerant participants were randomised to receive a 48 h infusion (30 ml/h) of saline or Intralipid followed by a euglycaemic–hyperinsulinaemic clamp. Vastus lateralis muscle biopsies were performed before and during the clamp. Results Lipid infusion impaired insulin-stimulated IRS-1 tyrosine phosphorylation and reduced peripheral insulin sensitivity (p < 0.01). The elevation in circulating NEFA increased expression of TLR3, TLR4 and TLR5, and several MAPK (MAPK8, MAP4K4, MAP2K3) and inhibitor of κB kinase-NFκB (CHUK [IKKA], c-REL [REL] and p65 [RELA, NFKB3,p65]) signalling genes (p < 0.05). The lipid infusion also increased extracellular signal-regulated kinase (ERK) phosphorylation (p < 0.05) and tended to reduce the content of nuclear factor of light polypeptide gene enhancer in B cells inhibitor α (p = 0.09). The muscle content of most diacyglycerol, ceramide and acylcarnitine species was unaffected. In summary, insulin resistance induced by prolonged low-dose lipid infusion occurs together with increased TLR-driven inflammatory signalling and impaired insulin-stimulated IRS-1 tyrosine phosphorylation. Conclusions/interpretation A sustained, mild elevation in plasma NEFA is sufficient to increase TLR expression and TLR-driven signalling (NFκB and MAPK) in lean individuals. The activation of this pathway by NEFA may be involved in the pathogenesis of insulin resistance in humans.