Interrelationships of zinc with glucose and insulin metabolism in humans

Hyperzincemia has been reported to cause alterations in the homeostasis of glycid metabolism. To determine this effect on plasma glucose and insulin levels, we studied 36 normal individuals of both sexes aged 22–26 y after a 12-h fast. The tests were initiated at 7:00am when an antecubital vein was punctured and a device for infusion was installed and maintained with physiological saline. Zinc was administered orally at 8:00am. Subjects were divided into an experimental group of 22 individuals who received doses of 25, 37.5, and 50 mg of zinc and a control group of 14 individuals. Blood samples were collected over a period of 240 min after the basal samples (−30 and 0 min). We did not detect any change in plasma glucose or insulin levels, a fact that we attribute either to the ineffectiveness of the 50 mg dose of zinc or to the lack of human response to the acute action of this trace element. The individuals who ingested zinc showed a significant fall in plasma cortisol, probably caused by the action of this trace element.     

Effects of fructose and glucose overfeeding on hepatic insulin sensitivity and intrahepatic lipids in healthy humans

Objective:

To assess how intrahepatic fat and insulin resistance relate to daily fructose and energy intake during short‐term overfeeding in healthy subjects.

Design and methods:

The analysis of the data collected in several studies in which fasting hepatic glucose production (HGP), hepatic insulin sensitivity index (HISI), and intrahepatocellular lipids (IHCL) had been measured after both 6‐7 days on a weight‐maintenance diet (control, C; n = 55) and 6‐7 days of overfeeding with 1.5 (F1.5, n = 7), 3 (F3, n = 17), or 4 g fructose/kg/day (F4, n = 10), with 3 g glucose/kg/day (G3, n = 11), or with 30% excess energy as saturated fat (fat30%, n = 10).

Results:

F3, F4, G3, and fat30% all significantly increased IHCL, respectively by 113 ± 86, 102 ± 115, 59 ± 92, and 90 ± 74% as compared to C (all P < 0.05). F4 and G3 increased HGP by 16 ± 10 and 8 ± 11% (both P < 0.05), and F3 and F4 significantly decreased HISI by 20 ± 22 and 19 ± 14% (both P < 0.01). In contrast, there was no significant effect of fat30% on HGP or HISI.

Conclusions:

Short‐term overfeeding with fructose or glucose decreases hepatic insulin sensitivity and increases hepatic fat content. This indicates short‐term regulation of hepatic glucose metabolism by simple carbohydrates.     

Plasma glucose and insulin responses to oral and intravenous glucose in cold-exposed humans

Although glucose tolerance and skeletal muscle glucose uptake are markedly improved by cold exposure in animals, little is known about such responses in humans. This study used two variations of a glucose tolerance test (GTT) to investigate changes in carbohydrate metabolism in healthy males during nude exposure to cold. In experiment 1, an oral GTT was performed in the cold and in the warm (3 h at 10 or 29 degrees C). To bypass the gastrointestinal tract, and to suppress hepatic glucose output, a second experiment was carried out as described above, using an intravenous GTT. Even though cold exposure raised metabolic rate greater than 2.5 times, plasma glucose and insulin responses to an oral GTT remained unaltered. In contrast, cold exposure reduced the entire plasma glucose profile as a function of time during the intravenous GTT (P less than 0.05), as plasma glucose was returned to basal levels within 1 h in comparison to a full 2 h in the warm, despite low insulin levels. The results of the intravenous GTT demonstrate that even with low insulin levels, carbohydrate metabolism is increased in cold-exposed males. This effect could be masked in the oral GTT by gastrointestinal factors and a high hepatic glucose output. Cold exposure may enhance insulin sensitivity and/or responsiveness for glucose uptake, mainly in shivering skeletal muscles.     

Hyperbolic relationship between insulin secretion and sensitivity on oral glucose tolerance test

The utility of the disposition index as a measure of beta-cell compensatory capacity rests on the established hyperbolic relationship between its component insulin secretion and sensitivity measures as derived from the intravenous glucose tolerance test (IVGTT). If one is to derive an analogous measure of beta-cell compensation from the oral glucose tolerance test (OGTT), it is thus necessary to first establish the existence of this hyperbolic relationship between OGTT-based measures of insulin secretion and insulin sensitivity. In this context, we tested five OGTT-based measures of secretion (insulinogenic index, Stumvoll first phase, Stumvoll second phase, ratio of total area-under-the-insulin-curve to area-under-the-glucose-curve (AUC(ins/gluc)), and incremental AUC(ins/gluc)) with two measures of sensitivity (Matsuda index and 1/Homeostasis Model of Assessment for insulin resistance (HOMA-IR)). Using a model of log(secretion measure) = constant + beta x log(sensitivity measure), a hyperbolic relationship can be established if beta is approximately equal to -1, with 95% confidence interval (CI) excluding 0. In 277 women with normal glucose tolerance (NGT), the pairing of total AUC(ins/gluc) and Matsuda index was the only combination that satisfied these criteria (beta = -0.99, 95% CI (-1.66, -0.33)). This pairing also satisfied hyperbolic criteria in 53 women with impaired glucose tolerance (IGT) (beta = -1.02, (-1.72, -0.32)). In a separate data set, this pairing yielded distinct hyperbolae for NGT (n = 245) (beta = -0.99, (-1.67, -0.32)), IGT (n = 116) (beta = -1.18, (-1.84, -0.53)), and diabetes (n = 43) (beta = -1.37, (-2.46, -0.29)). Moreover, the product of AUC(ins/gluc) and Matsuda index progressively decreased from NGT (212) to IGT (193) to diabetes (104) (P < 0.001), consistent with declining beta-cell function. In summary, a hyperbolic relationship can be demonstrated between OGTT-derived AUC(ins/gluc) and Matsuda index across a range of glucose tolerance. Based on these findings, the product of these two indices emerges as a potential OGTT-based measure of beta-cell function.     

Absorption and metabolism of oral zinc gluconate in humans in fasting state,during, and after a meal

The absorption and metabolism of zinc in a commercial form for oral use (Rubozinc^®, 15 mg zinc as gluconate) were investigated in 10 subjects by a kinetic study of the serum zinc profile after administration of 45 mg zinc under three conditions: after an overnight fast, during a standardized breakfast, and 2 h after this meal. The pharmacokinetic parameters were calculated by a method suitable to the characterization of rebound effects (recycling of the element in the gastrointestinal tract). In fasting state, the parameters were comparable to those previously collected in the same subjects with oral 45 mg zinc as sulfate, except with very significantly higherC _max and area under curve (AUC), showing a better bioavailability for zinc in the commercial form. The light meal perturbed the absorption process as evidenced by the significant increases in the lag time (+180%), thet _max (+57%), and the lag times for the first two cycles during the meal. However, the parameters returned to normal values 2 h after the meal. TheC _max only moderately decreased during the meal (31%) as did the AUC (?28%). An important delay in the absorption of zinc in the commercial form when taken during a meal was therefore demonstrated, but the effect on zinc bioavailability was only moderate.     

Assessment of beta-cell function in humans, simultaneously with insulin sensitivity and hepatic extraction, from intravenous and oral glucose tests

Assessment of insulin secretion in humans under physiological conditions has been a challenge because of its complex interplay with insulin action and hepatic insulin extraction. The possibility of simultaneously assessing beta-cell function, insulin sensitivity, and hepatic insulin extraction under physiological conditions using a simple protocol is appealing, since it has the potential to provide novel insights regarding the regulation of fasting and postprandial glucose metabolism in diabetic and nondiabetic humans. In this Perspective, we review data indicating that an oral glucose tolerance test (OGTT) or a meal test is able to accomplish this goal when interpreted with the oral beta-cell minimal model. We begin by using the well-established intravenous minimal model to highlight how the oral minimal model was developed and how the oral assessment parallels that of an intravenous glucose tolerance test (IVGTT). We also point out the unique aspects of both approaches in relation to their ability to assess different aspects of the beta-cell secretory cascade. We review the ability of the oral model to concurrently measure insulin sensitivity and hepatic insulin extraction, thereby enabling it to quantitatively portray the complex relationship among beta-cell function, hepatic insulin extraction, and insulin action. In addition, data from 204 individuals (54 young and 159 elderly) who underwent both IVGTT and meal tolerance tests are used to illustrate how these different approaches provide complementary but differing insights regarding the regulation of beta-cell function in humans.     

Effects of acetazolamide on insulin release, serum glucose and insulin, glucose tolerance, and alloxan sensitivity of mice

Isolated pancreatic islets exposed to 100 mM acetazolamide (AZM) and low glucose concentration exhibited increased insulin release, whereas those subjected to AZM and high glucose concentration exhibited decreased secretion of insulin. A slight transient hyperglycaemia was found 24 h after administration of 1.5 g/kg b.wt. of AZM to fed mice, whereas no such response was seen in starved mice. The serum insulin concentration was increased in the 24 h after AZM injection. Pretreatment with AZM caused decreased glucose tolerance and protection against alloxan toxicity. Inhibited carbonic anhydrase activity and ionic alterations might have played a role in the development of these effects of AZM in mice.     

Effects of oral L-carnitine supplementation on insulin sensitivity indices in response to glucose feeding in lean and overweight/obese males

Infusion of carnitine has been observed to increase non-oxidative glucose disposal in several studies, but the effect of oral carnitine on glucose disposal in non-diabetic lean versus overweight/obese humans has not been examined. This study examined the effects of 14 days of l-carnitine l-tartrate oral supplementation (LC) on blood glucose, insulin, NEFA and GLP-1 responses to an oral glucose tolerance test (OGTT). Sixteen male participants were recruited [lean (n = 8) and overweight/obese (n = 8)]. After completing a submaximal predictive exercise test, participants were asked to attend three experimental sessions. These three visits were conducted in the morning to obtain fasting blood samples and to conduct 2 h OGTTs. The first visit was a familiarisation trial and the final two visits were conducted 2 weeks apart following 14 days of ingestion of placebo (PL, 3 g glucose/day) and then LC (3 g LC/day) ingested as two capsules 3×/day with meals. On each visit, blood was drawn at rest, at intervals during the OGTT for analysis of glucose, insulin, non-esterified fatty acids (NEFA) and total glucagon-like peptide-1 (GLP-1). Data obtained were used for determination of usual insulin sensitivity indices (HOMA-IR, AUC glucose, AUC insulin, 1st phase and 2nd phase β-cell function, estimated insulin sensitivity index and estimated metabolic clearance rate). Data were analysed using RMANOVA and post hoc comparisons where appropriate. There was a significant difference between groups for body mass, % fat and BMI with no significant difference in age and height. Mean (SEM) plasma glucose concentration at 30 min was significantly lower (p < 0.05) in the lean group on the LC trial compared with PL [8.71(0.70) PL; 7.32(0.36) LC; mmol/L]. Conversely, plasma glucose concentration was not different at 30 min, but was significantly higher at 90 min (p < 0.05) in the overweight/obese group on the LC trial [5.09(0.41) PL; 7.11(0.59) LC; mmol/L]. Estimated first phase and second phase β-cell function both tended to be greater following LC in the lean group only. No effects of LC were observed on NEFA or total GLP-1 response to OGTT. It is concluded that LC supplementation induces changes in blood glucose handling/disposal during an OGTT, which is not influenced by GLP-1. The glucose handling/disposal response to oral LC is different between lean and overweight/obese suggesting that further investigation is required. LC effects on gastric emptying and/or direct ‘insulin-like’ actions on tissues should be examined in larger samples of overweight/obese and lean participants, respectively.     

Caffeine ingestion elevates plasma insulin response in humans during an oral glucose tolerance test

We tested the hypothesis that caffeine ingestion results in an exaggerated response in blood glucose and (or) insulin during an oral glucose tolerance test (OGTT). Young, fit adult males (n = 18) underwent 2 OGTT. The subjects ingested caffeine (5 mg/kg) or placebo (double blind) and 1 h later ingested 75 g of dextrose. There were no differences between the fasted levels of serum insulin, C peptide, blood glucose, or lactate and there were no differences within or between trials in these measures prior to the OGTT. Following the OGTT, all of these parameters increased (P < or = 0.05) for the duration of the OGTT. Caffeine ingestion resulted in an increase (P < or = 0.05) in serum fatty acids, glycerol, and plasma epinephrine prior to the OGTT. During the OGTT, these parameters decreased to match those of the placebo trial. In the caffeine trial the serum insulin and C peptide concentrations were significantly greater (P < or = 0.001) than for placebo for the last 90 min of the OGTT and the area under the curve (AUC) for both measures were 60 and 37% greater (P < or = 0.001), respectively. This prolonged, increased elevation in insulin did not result in a lower blood glucose level; in fact, the AUC for blood glucose was 24% greater (P = 0.20) in the caffeine treatment group. The data support our hypothesis that caffeine ingestion results in a greater increase in insulin concentration during an OGTT. This, together with a trend towards a greater rather than a more modest response in blood glucose, suggests that caffeine ingestion may have resulted in insulin resistance.     

Effects of age and anesthetic on plasma glucose and insulin levels and insulin sensitivity in spontaneously hypertensive and Wistar rats

We examined the effects of anesthetic, age, and strain on oral glucose tolerance tests (OGTT, 1 g/kg body weight) and intraperitoneal glucose tolerance tests (IPGTT, 2 g/kg body weight) in spontaneously hypertensive (SH) and Wistar rats. Pentobarbital anesthesia caused an elevation in basal glucose and insulin levels in Wistar rats at 9 and 16 weeks of age and in SH rats at 9 weeks. Anesthesia increased the insulin output during an OGTT in both strains of rats while glucose was unchanged. Anesthesia reduced the insulin sensitivity index calculated from the OGTT but not from the IPGTT data. The age of the rats (9-11 vs. 16-18 weeks) had no effect on the basal glucose or insulin levels, but older Wistar rats had a greater insulin output following oral glucose and older SH rats had a greater insulin output following intraperitoneal glucose. On the basis of the insulin sensitivity index, SH rats were clearly more insulin resistant than age-matched Wistar rats. The SH rats also had higher basal insulin levels, as well as higher insulin output, following both glucose challenges. In summary, SH rats are more insulin resistant than Wistar rats, and anesthesia, which elevated basal glucose and insulin levels and increased the insulin output in response to a glucose challenge, may increase insulin resistance.     

Effects of biotin deficiency on pancreatic islet morphology, insulin sensitivity and glucose homeostasis

Several studies have revealed that physiological concentrations of biotin are required for the normal expression of critical carbohydrate metabolism genes and for glucose homeostasis. However, the different experimental models used in these studies make it difficult to integrate the effects of biotin deficiency on glucose metabolism. To further investigate the effects of biotin deficiency on glucose metabolism, we presently analyzed the effect of biotin deprivation on glucose homeostasis and on pancreatic islet morphology. Three-week-old male BALB/cAnN Hsd mice were fed a biotin-deficient or a biotin-control diet (0 or 7.2 μmol of free biotin/kg diet, respectively) over a period of 8 weeks. We found that biotin deprivation caused reduced concentrations of blood glucose and serum insulin concentrations, but increased plasma glucagon levels. Biotin-deficient mice also presented impaired glucose and insulin tolerance tests, indicating defects in insulin sensitivity. Altered insulin signaling was linked to a decrease in phosphorylated Akt/PKB but induced no change in insulin receptor abundance. Islet morphology studies revealed disruption of islet architecture due to biotin deficiency, and an increase in the number of α-cells in the islet core. Morphometric analyses found increased islet size, number of islets and glucagon-positive area, but a decreased insulin-positive area, in the biotin-deficient group. Glucagon secretion and gene expression increased in islets isolated from biotin-deficient mice. Our results suggest that biotin deficiency promotes hyperglycemic mechanisms such as increased glucagon concentration and decreased insulin secretion and sensitivity to compensate for reduced blood glucose concentrations. Variations in glucose homeostasis may participate in the changes observed in pancreatic islets.     

Modified minimal model for effect of physical exercise on insulin sensitivity and glucose effectiveness in type 2 diabetes and healthy human

The Bergman’s minimal model of glucose and insulin plasma levels is commonly used to analyse the results of glucose tolerance tests in humans. In this paper, we present the modified minimal model with plasma insulin compartment under the assumption that if the plasma glucose compartment drops below the basal glucose levels, the rate of insulin entering the plasma glucose compartment is zero. Insulin is cleared from the plasma insulin compartment at a rate proportional to the amount of insulin in the plasma insulin compartment. The modified minimal model was used to study the effect of physical exercise via parameters of a mathematical model to qualitative the magnitude of changes in insulin sensitivity (S _I) and glucose effectiveness (S _G) in response to exercise in type 2 diabetes and healthy human. The short-term effects of physical exercise in type 2 diabetes did not improve S _G, but markedly improved the low S _I values found in type 2 diabetes, indicating that the effects of exercise on S _I are quantitatively important in the interpretation of training-related S _I changes and may even be therapeutically useful in type 2 diabetes patients. Physical exercise is indicated either to prevent or delay the onset of type 2 diabetes or to assure a good control of type 2 diabetes by increasing insulin sensitivity.     

Ras inhibition induces insulin sensitivity and glucose uptake

Background

Reduced glucose uptake due to insulin resistance is a pivotal mechanism in the pathogenesis of type 2 diabetes. It is also associated with increased inflammation. Ras inhibition downregulates inflammation in various experimental models. The aim of this study was to examine the effect of Ras inhibition on insulin sensitivity and glucose uptake, as well as its influence on type 2 diabetes development.

Methods and Findings

The effect of Ras inhibition on glucose uptake was examined both in vitro and in vivo. Ras was inhibited in cells transfected with a dominant-negative form of Ras or by 5-fluoro-farnesylthiosalicylic acid (F-FTS), a small-molecule Ras inhibitor. The involvement of IκB and NF-κB in Ras-inhibited glucose uptake was investigated by immunoblotting. High fat (HF)-induced diabetic mice were treated with F-FTS to test the effect of Ras inhibition on induction of hyperglycemia. Each of the Ras-inhibitory modes resulted in increased glucose uptake, whether in insulin-resistant C2C12 myotubes in vitro or in HF-induced diabetic mice in vivo. Ras inhibition also caused increased IκB expression accompanied by decreased expression of NF-κB . In fat-induced diabetic mice treated daily with F-FTS, both the incidence of hyperglycemia and the levels of serum insulin were significantly decreased.

Conclusions

Inhibition of Ras apparently induces a state of heightened insulin sensitization both in vitro and in vivo. Ras inhibition should therefore be considered as an approach worth testing for the treatment of type 2 diabetes.     

Effects of creatine supplementation on glucose tolerance and insulin sensitivity in sedentary healthy males undergoing aerobic training

Recent findings have indicated that creatine supplementation may affect glucose metabolism. This study aimed to examine the effects of creatine supplementation, combined with aerobic training, on glucose tolerance in sedentary healthy male. Subjects (n = 22) were randomly divided in two groups and were allocated to receive treatment with either creatine (CT) ( approximately 10 g . day over three months) or placebo (PT) (dextrose). Administration of treatments was double blind. Both groups underwent moderate aerobic training. An oral glucose tolerance test (OGTT) was performed and both fasting plasma insulin and the homeostasis model assessment (HOMA) index were assessed at the start, and after four, eight and twelve weeks. CT demonstrated significant decrease in OGTT area under the curve compared to PT (P = 0.034). There were no differences between groups or over time in fasting insulin or HOMA. The results suggest that creatine supplementation, combined with aerobic training, can improve glucose tolerance but does not affect insulin sensitivity, and may warrant further investigation with diabetic subjects.     

Depot-specific regulation of glucose uptake and insulin sensitivity in HIV-lipodystrophy

Altered fat distribution is associated with insulin resistance in HIV, but little is known about regional glucose metabolism in fat and muscle depots in this patient population. The aim of the present study was to quantify regional fat, muscle, and whole body glucose disposal in HIV-infected men with lipoatrophy. Whole body glucose disposal was determined by hyperinsulinemic clamp technique (80 mU x m(-2) x min(-1)) in 6 HIV-infected men and 5 age/weight-matched healthy volunteers. Regional glucose uptake in muscle and subcutaneous (SAT) and visceral adipose tissue (VAT) was quantified in fasting and insulin-stimulated states using 2-deoxy-[18F]fluoro-D-glucose positron emission tomography. HIV-infected subjects with lipoatrophy had significantly increased glucose uptake into SAT (3.8 +/- 0.4 vs. 2.3 +/- 0.5 micromol x kg tissue(-1) x min(-1), P < 0.05) in the fasted state. Glucose uptake into VAT did not differ between groups. VAT area was inversely related with whole body glucose disposal, insulin sensitivity, and muscle glucose uptake during insulin stimulation. VAT area was highly predictive of whole body glucose disposal (r2 = 0.94, P < 0.0001). This may be mediated by adiponectin, which was significantly associated with VAT area (r = -0.75, P = 0.008), and whole body glucose disposal (r = 0.80, P = 0.003). This is the first study to directly demonstrate increased glucose uptake in subcutaneous fat of lipoatrophic patients, which may partially compensate for loss of SAT. Furthermore, we demonstrate a clear relationship between VAT and glucose metabolism in multiple fat and muscle depots, suggesting the critical importance of this depot in the regulation of glucose and highlighting the significant potential role of adiponectin in this process.     

Serum dihydroceramides correlate with insulin sensitivity in humans and decrease insulin sensitivity in vitro

Serum ceramides, especially C16:0 and C18:0 species, are linked to CVD risk and insulin resistance, but details of this association are not well understood. We performed this study to quantify a broad range of serum sphingolipids in individuals spanning the physiologic range of insulin sensitivity and to determine if dihydroceramides cause insulin resistance in vitro. As expected, we found that serum triglycerides were significantly greater in individuals with obesity and T2D compared with athletes and lean individuals. Serum ceramides were not significantly different within groups but, using all ceramide data relative to insulin sensitivity as a continuous variable, we observed significant inverse relationships between C18:0, C20:0, and C22:0 species and insulin sensitivity. Interestingly, we found that total serum dihydroceramides and individual species were significantly greater in individuals with obesity and T2D compared with athletes and lean individuals, with C18:0 species showing the strongest inverse relationship to insulin sensitivity. Finally, we administered a physiological mix of dihydroceramides to primary myotubes and found decreased insulin sensitivity in vitro without changing the overall intracellular sphingolipid content, suggesting a direct effect on insulin resistance. These data extend what is known regarding serum sphingolipids and insulin resistance and show the importance of serum dihydroceramides to predict and promote insulin resistance in humans.Supplementary key words: sphingolipids, circulating ceramides, serum, insulin resistance, lipidomics, CVD, T2D, obesity, myotube

Circulating ceramides, especially specific saturated ceramide species, and other sphingolipids are linked to CVD risk and insulin resistance (1, 2, 3, 4, 5, 6, 7, 8, 9, 10). In fact, circulating ceramide and sphingolipid contents predict development of CVD better than some common risk factors such as plasma cholesterol, LDLs, and triglycerides (6, 9, 11, 12). As a result, it was recently proposed that plasma ceramide could be the new cholesterol for assessing risk of CVD (11). Beyond the cross-sectional studies referenced above, there are several lines of evidence supporting the link between ceramides, CVD, and insulin resistance. Plasma ceramide content decreases after insulin-sensitizing gastric bypass surgery and weight loss interventions (13, 14, 15). Animal studies show that ceramides accumulate in atherosclerotic lesions, which may explain the increased risk associated with plasma content (16). However, the relationship of circulating sphingolipids to insulin resistance is not absolute, as insulin-sensitizing treatments do not always change plasma sphingolipid content (17). Combined, most data from epidemiology studies, as well as human interventions and animal models, support the concept that circulating ceramides and sphingolipids are related to insulin resistance and CVD risk.Ceramides circulate primarily bound to lipoproteins and are secreted predominately by the liver. Circulating ceramides are mainly increased in LDL in individuals with obesity (15). Obese rodents have increased hepatic ceramide secretion, which may explain increased plasma ceramide content in individuals with obesity (15). In one mechanistic study, an LDL-ceramide mixture was infused in mice to recapitulate increased plasma ceramide content in obesity, which caused membrane ceramide accumulation, decreased insulin signaling, and a decrease in insulin sensitivity specifically in skeletal muscle, providing evidence for a direct effect of circulating ceramides on tissues (15). Similarly, LDL-ceramide administration to myotubes caused ceramide accumulation, decreased insulin sensitivity, and signaling independent of inflammation. These data indicate that plasma ceramides are not simply markers of insulin resistance but play mechanistic roles in decreasing insulin sensitivity.Ceramides are only one member of the sphingolipid family, and other sphingolipids may also be related to insulin resistance and CVD risk. Lactosylceramides and glucosylceramides are sphingolipids that also accumulate in atherosclerotic plaques and therefore may be involved in the CVD process (18). Sphingomyelins are the most abundant sphingolipids circulating in lipoproteins and, while they are positively related to obesity and waist circumference, they are not correlated to insulin sensitivity in cross-sectional human studies (5, 19). Dihydroceramides are immediate precursors to ceramide synthesis and are negatively related to insulin sensitivity (20, 21) and insulin secretion (21), are positively related to waist circumference (22), are elevated in plasma of individuals with prediabetes and T2D compared with controls (23), and predict development of diabetes 9 years before onset (21). Despite strong evidence linking plasma dihydroceramides to decreased insulin sensitivity, mechanistic studies to determine if circulating dihydroceramides cause insulin resistance are lacking.To address this knowledge gap, we performed the current study to assess serum sphingolipids in humans across the metabolic spectrum as well as determine if dihydroceramides induce insulin resistance in vitro.