Maarten R. Soeters

Impact of oral vancomycin on gut microbiota, bile acid metabolism, and insulin sensitivity

BACKGROUND & AIMS Obesity has been associated with changes in the composition and function of the intestinal microbiota. Modulation of the microbiota by antibiotics also alters bile acid and glucose metabolism in mice. Hence, we hypothesized that short term administration of oral antibiotics in humans would affect fecal microbiota composition and subsequently bile acid and glucose metabolism. METHODS In this single blinded randomized controlled trial, 20 male obese subjects with metabolic syndrome were randomized to 7 days of amoxicillin 500 mg t.i.d. or 7 days of vancomycin 500 mg t.i.d. At baseline and after 1 week of therapy, fecal microbiota composition (Human Intestinal Tract Chip phylogenetic microarray), fecal and plasma bile acid concentrations as well as insulin sensitivity (hyperinsulinemic euglycemic clamp using [6,6-(2)H2]-glucose tracer) were measured. RESULTS Vancomycin reduced fecal microbial diversity with a decrease of gram-positive bacteria (mainly Firmicutes) and a compensatory increase in gram-negative bacteria (mainly Proteobacteria). Concomitantly, vancomycin decreased fecal secondary bile acids with a simultaneous postprandial increase in primary bile acids in plasma (p<0.05). Moreover, changes in fecal bile acid concentrations were predominantly associated with altered Firmicutes. Finally, administration of vancomycin decreased peripheral insulin sensitivity (p<0.05). Amoxicillin did not affect any of these parameters. CONCLUSIONS Oral administration of vancomycin significantly impacts host physiology by decreasing intestinal microbiota diversity, bile acid dehydroxylation and peripheral insulin sensitivity in subjects with metabolic syndrome. These data show that intestinal microbiota, particularly of the Firmicutes phylum contributes to bile acid and glucose metabolism in humans. This trial is registered at the Dutch Trial Register (NTR2566).

Acylcarnitines: Reflecting or Inflicting Insulin Resistance?

The incidence of obesity and insulin resistance is growing, and the increase in type 2 diabetes mellitus (DM2) constitutes one of the biggest challenges for our healthcare systems. Many theories are proposed for the induction of insulin resistance in glucose and lipid metabolism and its metabolic sequelae. One of these mechanisms is lipotoxicity (1–4): excess lipid supply and subsequent lipid accumulation in insulin-sensitive tissues such as skeletal muscle interfere with insulin-responsive metabolic pathways. Various lipid intermediates, like ceramides, gangliosides, diacylglycerol, and other metabolites, have been held responsible for insulin resistance (2,3,5–10). These intermediates can exert such effects because they are signaling molecules and building blocks of cellular membranes, which harbor the insulin receptor. In addition, lipids play an important role in energy homeostasis. Fatty acids (FA) can be metabolized via mitochondrial FA oxidation (FAO), which yields energy (11). As such, FAO competes with glucose oxidation in a process known as the glucose-FA, or Randle, cycle (12). Muoio and colleagues (1,13,14) proposed an alternative mechanism in which FAO rate outpaces the tricarboxylic acid cycle (TCA), thereby leading to the accumulation of intermediary metabolites such as acylcarnitines that may interfere with insulin sensitivity. This accumulation of acylcarnitines corroborates with some human studies showing that acylcarnitines are associated with insulin resistance (15–17). In addition, acylcarnitines have a long history in the diagnosis and neonatal screening of FAO defects and other inborn errors of metabolism (18). This knowledge may aid to understand the interaction between FAO and insulin resistance and fuel future research. In this review, we discuss the role of acylcarnitines in FAO and insulin resistance as emerging from animal and human studies. ### Carnitine biosynthesis and regulation of tissue carnitine content. To guarantee continuous energy supply, the human body oxidizes considerable amounts of fat besides glucose. …

The evolutionary benefit of insulin resistance

Insulin resistance is perceived as deleterious, associated with conditions as the metabolic syndrome, type 2 diabetes mellitus and critical illness. However, insulin resistance is evolutionarily well preserved and its persistence suggests that it benefits survival. Insulin resistance is important in various states such as starvation, immune activation, growth and cancer, to spare glucose for different biosynthetic purposes such as the production of NADPH, nucleotides in the pentose phosphate pathway and oxaloacetate for anaplerosis. In these conditions, total glucose oxidation by the tricarboxylic acid cycle is actually low and energy demands are largely met by fatty acid and ketone body oxidation. This beneficial role of insulin resistance has consequences for treatment and research. Insulin resistance should be investigated at the cellular, tissue and whole organism level. The metabolic pathways discussed here, should be integrated in the accepted and valid mechanistic events of insulin resistance before interfering with them to promote insulin sensitivity at any cost.

Type 2 iodothyronine deiodinase in skeletal muscle: Effects of hypothyroidism and fasting

CONTEXT The iodothyronine deiodinases D1, D2, and D3 enable tissue-specific adaptation of thyroid hormone levels in response to various conditions, such as hypothyroidism or fasting. The possible expression of D2 mRNA in skeletal muscle is intriguing because this enzyme could play a role in systemic as well as local T3 production. OBJECTIVE We determined D2 activity and D2 mRNA expression in human skeletal muscle biopsies under control conditions and during hypothyroidism, fasting, and hyperinsulinemia. DESIGN This was a prospective study. SETTING The study was conducted at a university hospital. PATIENTS We studied 11 thyroidectomized patients with differentiated thyroid carcinoma (DTC) on and after 4 wk off T4( replacement and six healthy lean subjects in the fasting state and during hyperinsulinemia after both 14 and 62 h of fasting. MEAN OUTCOME MEASURES D2 activity and D2 mRNA levels were measured in skeletal muscle samples. RESULTS No differences were observed in muscle D2 mRNA levels in DTC patients on and off T4 replacement therapy. In healthy subjects, muscle D2 mRNA levels were lower after 62 h compared to 14 h of fasting. Insulin increased mRNA expression after 62 h, but not after 14 h of fasting. Skeletal muscle D2 activities were very low and not influenced by hypothyroidism and fasting. CONCLUSION Human skeletal muscle D2 mRNA expression is modulated by fasting and insulin, but not by hypothyroidism. The lack of a clear effect of D2 mRNA modulation on the observed low D2 activities questions the physiological relevance of D2 activity in human skeletal muscle.

Muscle acylcarnitines during short-term fasting in lean healthy men

The transition from the fed to the fasted resting state is characterized by, among other things, changes in lipid metabolism and peripheral insulin resistance. Acylcarnitines have been suggested to play a role in insulin resistance, as well as other long-chain fatty acid metabolites. Plasma levels of long-chain acylcarnitines increase during fasting, but this is unknown for muscle long-chain acylcarnitines. In the present study we investigated whether muscle long-chain acylcarnitines increase during fasting and we investigated their relationship with glucose/fat oxidation and insulin sensitivity in lean healthy humans. After 14 h and 62 h of fasting, glucose fluxes, substrate oxidation, and plasma and muscle acylcarnitines were measured before and during a hyperinsulinaemic-euglycaemic clamp. Hyperinsulinaemia decreased long-chain muscle acylcarnitines after 14 h of fasting, but not after 62 h of fasting. In both the basal state and during the clamp, glucose oxidation was lower and fatty acid oxidation was higher after 62 h compared with 14 h of fasting. Absolute changes in glucose and fatty acid oxidation in the basal compared with hyperinsulinaemic state were not different. Muscle long-chain acylcarnitines did not correlate with glucose oxidation, fatty acid oxidation or insulin-mediated peripheral glucose uptake. After 62 h of fasting, the suppression of muscle long-chain acylcarnitines by insulin was attenuated compared with 14 h of fasting. Muscle long-chain acylcarnitines do not unconditionally reflect fatty acid oxidation. The higher fatty acid oxidation during hyperinsulinaemia after 62 h compared with 14 h of fasting, although the absolute decrease in fatty acid oxidation was not different, suggests a different set point.

Adaptive reciprocity of lipid and glucose metabolism in human short-term starvation

The human organism has tools to cope with metabolic challenges like starvation that are crucial for survival. Lipolysis, lipid oxidation, ketone body synthesis, tailored endogenous glucose production and uptake, and decreased glucose oxidation serve to protect against excessive erosion of protein mass, which is the predominant supplier of carbon chains for synthesis of newly formed glucose. The starvation response shows that the adaptation to energy deficit is very effective and coordinated with different adaptations in different organs. From an evolutionary perspective, this lipid-induced effect on glucose oxidation and uptake is very strong and may therefore help to understand why insulin resistance in obesity and type 2 diabetes mellitus is difficult to treat. The importance of reciprocity in lipid and glucose metabolism during human starvation should be taken into account when studying lipid and glucose metabolism in general and in pathophysiological conditions in particular.

Intermittent fasting does not affect whole-body glucose, lipid, or protein metabolism

BACKGROUND Intermittent fasting (IF) was shown to increase whole-body insulin sensitivity, but it is uncertain whether IF selectively influences intermediary metabolism. Such selectivity might be advantageous when adapting to periods of food abundance and food shortage. OBJECTIVE The objective was to assess effects of IF on intermediary metabolism and energy expenditure. DESIGN Glucose, glycerol, and valine fluxes were measured after 2 wk of IF and a standard diet (SD) in 8 lean healthy volunteers in a crossover design, in the basal state and during a 2-step hyperinsulinemic euglycemic clamp, with assessment of energy expenditure and phosphorylation of muscle protein kinase B (AKT), glycogen synthase kinase (GSK), and mammalian target of rapamycine (mTOR). We hypothesized that IF selectively increases peripheral glucose uptake and lowers proteolysis, thereby protecting protein stores. RESULTS No differences in body weight were observed between the IF and SD groups. Peripheral glucose uptake and hepatic insulin sensitivity during the clamp did not significantly differ between the IF and SD groups. Likewise, lipolysis and proteolysis were not different between the IF and SD groups. IF decreased resting energy expenditure. IF had no effect on the phosphorylation of AKT but significantly increased the phosphorylation of glycogen synthase kinase. Phosphorylation of mTOR was significantly lower after IF than after the SD. CONCLUSIONS IF does not affect whole-body glucose, lipid, or protein metabolism in healthy lean men despite changes in muscle phosphorylation of GSK and mTOR. The decrease in resting energy expenditure after IF indicates the possibility of an increase in weight during IF when caloric intake is not adjusted. This study was registered at www.trialregister.nl as NTR1841.

Combining 123I-Metaiodobenzylguanidine SPECT/CT and 18F-FDG PET/CT for the Assessment of Brown Adipose Tissue Activity in Humans During Cold Exposure

Brown adipose tissue (BAT) has become a focus of research in the hope of finding a new target to fight obesity. Metabolic BAT activity can be visualized with 18F-FDG PET/CT. Furthermore, the sympathetic innervation of BAT can be visualized with the radiolabeled norepinephrine analog 123I-metaiodobenzylguanidine (123I-MIBG). We aimed to determine whether 123I-MIBG SPECT/CT and 18F-FDG PET/CT identify the same anatomic regions as active BAT in adult humans. Furthermore, we investigated whether the magnitude of BAT activity measured by these techniques correlated. Finally, we tried to establish the optimal time interval between 123I-MIBG administration and subsequent SPECT/CT acquisition to visualize sympathetic stimulation of BAT. Methods: Ten lean (body mass index, 19–25 kg/m2), healthy Caucasian men (age, 18–32 y) underwent one 18F-FDG PET/CT and two 123I-MIBG-SPECT/CT scans within a 2-wk interval. On 2 separate occasions, the subjects were exposed to mild cold (17°C) for 2 h after an overnight fast. After 1 h of cold exposure, 18F-FDG (one occasion) or 123I-MIBG (other occasion) was administered. 18F-FDG PET/CT was performed at 1 h after 18F-FDG administration, and 123I-MIBG-SPECT/CT was performed at 4 and 24 h after 123I-MIBG injection. Results: 18F-FDG uptake in BAT was observed in 8 of 10 subjects, whereas 123I-MIBG uptake was observed in 7 of 10 subjects in both the SPECT/CT scans acquired at 4 h after 123I-MIBG administration and the SPECT/CT scans acquired at 24 h after 123I-MIBG administration. All subjects who showed 123I-MIBG uptake in BAT also showed 18F-FDG uptake in BAT. There was no statistically significant correlation between maximal standardized uptake value of 18F-FDG and semiquantitative uptake of 123I-MIBG at 4 h after administration. However, a positive correlation was found between the maximal standardized uptake value of 18F-FDG and semiquantitative uptake of 123I-MIBG at 24 h after administration (r = 0.64, P = 0.04). Conclusion: 123I-MIBG SPECT/CT, as a marker of sympathetic activity, and 18F-FDG PET/CT, as a marker of metabolic activity, identified the same anatomic regions as active BAT. Moreover, when 123I-MIBG SPECT/CT was performed at 24 h after 123I-MIBG administration, the magnitude of BAT activity measured with these techniques correlated strongly. This finding not only supports that BAT activity in humans is sympathetically influenced but also identifies 123I-MIBG SPECT/CT, when performed 24 h after 123I-MIBG injection, as a method to visualize and quantify sympathetic stimulation of BAT.

Fasting and Postprandial Activity of Brown Adipose Tissue in Healthy Men

The role of brown adipose tissue (BAT) in adult metabolism is poorly understood. This study aimed to examine the differential effects of an overnight fast and the postprandial state on BAT activity. Methods: We included 10 healthy, lean male volunteers. BAT uptake of glucose was visualized using 18F-FDG PET/CT during mild cold exposure. Each subject underwent PET/CT twice. The first scan was obtained after an overnight fast; the second after a standardized meal. Results: 18F-FDG uptake in BAT was observed in 6 of 10 volunteers. These subjects were found to have a higher maximal standardized uptake value when fasting (median, 13.1 g/mL; range, 6.1–27.6 g/mL) than when in the postprandial state (median, 6.8 g/mL; range, 2.1–13.4 g/mL) (P = 0.03). Conclusion: Cold-stimulated 18F-FDG uptake by BAT in humans is more pronounced during fasting. The lower maximal standardized uptake value in the postprandial state may be explained by increased insulin-stimulated glucose uptake in muscle.

Insulin resistance in obesity can be reliably identified from fasting plasma insulin

Background/Objectives:Insulin resistance is the major contributor to cardiometabolic complications of obesity. We aimed to (1) establish cutoff points for insulin resistance from euglycemic hyperinsulinemic clamps (EHCs), (2) identify insulin-resistant obese subjects and (3) predict insulin resistance from routinely measured variables.Subjects/Methods:We assembled data from non-obese (n=112) and obese (n=100) men who underwent two-step EHCs using [6,6-2H2]glucose as tracer (insulin infusion dose 20 and 60 mU m−2 min−1, respectively). Reference ranges for hepatic and peripheral insulin sensitivity were calculated from healthy non-obese men. Based on these reference values, obese men with preserved insulin sensitivity or insulin resistance were identified.Results:Cutoff points for insulin-mediated suppression of endogenous glucose production (EGP) and insulin-stimulated glucose disappearance rate (Rd) were 46.5% and 37.3 μmol kg−1 min−1, respectively. Most obese men (78%) had EGP suppression within the reference range, whereas only 12% of obese men had Rd within the reference range. Obese men with Rd <37.3 μmol kg−1 min−1 did not differ from insulin-sensitive obese men in age, body mass index (BMI), body composition, fasting glucose or cholesterol, but did have higher fasting insulin (110±49 vs 63±29 pmol l−1, P<0.001) and homeostasis model assessment of insulin resistance (HOMA-IR) (4.5±2.2 vs 2.7±1.4, P=0.004). Insulin-resistant obese men could be identified with good sensitivity (80%) and specificity (75%) from fasting insulin >74 pmol l−1.Conclusions:Most obese men have hepatic insulin sensitivity within the range of non-obese controls, but below-normal peripheral insulin sensitivity, that is, insulin resistance. Fasting insulin (>74 pmol l−1 with current insulin immunoassay) may be used for identification of insulin-resistant (or metabolically unhealthy) obese men in research and clinical settings.