Caffeine-induced impairment of insulin action but not insulin signaling in human skeletal muscle is reduced by exercise

Impact of caffeine and protein on postexercise muscle glycogen synthesis

BACKGROUND

Both protein and caffeine coingestion with CHO have been suggested to represent effective dietary strategies to further accelerate postexercise muscle glycogen synthesis in athletes.

PURPOSE

This study aimed to assess the effect of protein or caffeine coingestion on postexercise muscle glycogen synthesis rates when optimal amounts of CHO are ingested.

METHODS

Fourteen male cyclists were studied on three different test days. Each test day started with a glycogen-depleting exercise session. This was followed by a 6-h recovery period, during which subjects received 1.2 g·kg⁻¹·h⁻¹ CHO, the same amount of CHO with 0.3 g·kg⁻¹·h⁻¹ of a protein plus leucine mixture (CHO + PRO), or 1.7 mg·kg⁻¹·h⁻¹ caffeine (CHO + CAF). All drinks were enriched with [U-¹³C₆]-labeled glucose to assess potential differences in the appearance rate of ingested glucose from the gut. Muscle biopsies were collected immediately after cessation of exercise and after 6 h of postexercise recovery.

RESULTS

The plasma insulin response was higher in CHO + PRO compared with CHO and CHO + CAF (P < 0.01). Plasma glucose responses and glucose appearance rates did not differ between experiments. Muscle glycogen synthesis rates averaged 31 ± 4, 34 ± 4, and 31 ± 4 mmol·kg⁻¹ dry weight·h⁻¹ in CHO, CHO + PRO, and CHO + CAF, respectively (P = NS). In accordance, histochemical analyses did not show any differences between net changes in Type I and Type II muscle fiber glycogen content between experiments.

CONCLUSIONS

Coingestion of protein or caffeine does not further accelerate postexercise muscle glycogen synthesis when ample amounts of CHO (1.2 g·kg⁻¹·h⁻¹) are ingested.

Role of coffee in modulation of diabetes risk

Coffee consumption has been associated with a lower risk of type 2 diabetes. This association does not depend on race, gender, geographic distribution of the study populations, or the type of coffee consumed (i.e., caffeinated or decaffeinated). This review discusses the strength of this relationship, examines the possibility that the pattern of coffee consumption could influence the association, and evaluates the possible relationship between coffee consumption and other risk factors associated with diabetes. Particular attention is paid to the identification, on the basis of the scientific evidence, of the possible mechanisms by which coffee components might affect diabetes development, especially in light of the paradoxical effect of caffeine on glucose metabolism. In addition to the role of coffee in reducing the risk of developing type 2 diabetes, the possible role of coffee in the course of the illness is explored. Finally, the possibility that coffee can also affect the risk of other forms of diabetes (e.g., type 1 diabetes and gestational diabetes) is examined.

Chlorogenic acid stimulates glucose transport in skeletal muscle via AMPK activation: a contributor to the beneficial effects of coffee on diabetes

Chlorogenic acid (CGA) has been shown to delay intestinal glucose absorption and inhibit gluconeogenesis. Our aim was to investigate the role of CGA in the regulation of glucose transport in skeletal muscle isolated from db/db mice and L6 skeletal muscle cells. Oral glucose tolerance test was performed on db/db mice treated with CGA and soleus muscle was isolated for 2-deoxyglucose transport study. 2DG transport was also examined in L6 myotubes with or without inhibitors such as wortmannin or compound c. AMPK was knocked down with AMPKα1/2 siRNA to study its effect on CGA-stimulated glucose transport. GLUT 4 translocation, phosphorylation of AMPK and Akt, AMPK activity, and association of IRS-1 and PI3K were investigated in the presence of CGA. In db/db mice, a significant decrease in fasting blood sugar was observed 10 minutes after the intraperitoneal administration of 250 mg/kg CGA and the effect persisted for another 30 minutes after the glucose challenge. Besides, CGA stimulated and enhanced both basal and insulin-mediated 2DG transports in soleus muscle. In L6 myotubes, CGA caused a dose- and time-dependent increase in glucose transport. Compound c and AMPKα1/2 siRNA abrogated the CGA-stimulated glucose transport. Consistent with these results, CGA was found to phosphorylate AMPK and ACC, consistent with the result of increased AMPK activities. CGA did not appear to enhance association of IRS-1 with p85. However, we observed activation of Akt by CGA. These parallel activations in turn increased translocation of GLUT 4 to plasma membrane. At 2 mmol/l, CGA did not cause any significant changes in viability or proliferation of L6 myotubes. Our data demonstrated for the first time that CGA stimulates glucose transport in skeletal muscle via the activation of AMPK. It appears that CGA may contribute to the beneficial effects of coffee on Type 2 diabetes mellitus.

New perspectives on nutritional interventions to augment lipid utilisation during exercise

The enhancement of fat oxidation during exercise is an aim for both recreational exercising individuals and endurance athletes. Nutritional status may explain a large part of the variation in maximal rates of fat oxidation during exercise. This review reveals novel insights into nutritional manipulation of substrate selection during exercise, explaining putative mechanisms of action and evaluating the current evidence. Lowering the glycaemic index of the pre-exercise meal can enhance lipid utilisation by up to 100 % through reduced insulin concentrations, although its application may be restricted to specific training sessions rather than competition. Chronic effects of dietary glycaemic index are less clear and warrant future study before firm recommendations can be made. A flurry of recent advances has overthrown the conventional view of l-carnitine supplementation, with skeletal muscle uptake possible under certain dietary conditions and providing a strategy to influence energy metabolism in an exercise intensity-dependent manner. Use of non-carbohydrate nutrients to stimulate muscle l-carnitine uptake may prove more beneficial for optimising lipid utilisation, but this requires more research. Studies investigating fish oil supplementation on fat oxidation during exercise are conflicting. In spite of some strong putative mechanisms, the only crossover trial showed no significant effect on lipid use during exercise. Ca may increase NEFA availability although it is not clear whether these effects occur. Ca and caffeine can increase NEFA availability under certain circumstances which could theoretically enhance fat oxidation, yet strong experimental evidence for this effect during exercise is lacking. Co-administration of nutrients to maximise their effectiveness needs further investigation.

Caffeine modulates phosphorylation of insulin receptor substrate-1 and impairs insulin signal transduction in rat skeletal muscle

Caffeine decreases insulin sensitivity and insulin-stimulated glucose transport in skeletal muscle; however, the precise mechanism responsible for this deleterious effect is not understood fully. We investigated the effects of incubation with caffeine on insulin signaling in rat epitrochlearis muscle. Caffeine (≥1 mM, ≥15 min) suppressed insulin-stimulated insulin receptor substrate (IRS)-1 Tyr(612) phosphorylation in a dose- and time-dependent manner. These responses were associated with inhibition of the insulin-stimulated phosphorylation of phosphatidylinositol 3-kinase (PI3K) Tyr(458), Akt Ser(473), and glycogen synthase kinase-3β Ser(9) and with inhibition of insulin-stimulated 3-O-methyl-d-glucose (3MG) transport but not with inhibition of the phosphorylation of insulin receptor-β Tyr(1158/62/63). Furthermore, caffeine enhanced phosphorylation of IRS-1 Ser(307) and an IRS-1 Ser(307) kinase, inhibitor-κB kinase (IKK)-α/β Ser(176/180). Blockade of IKK/IRS-1 Ser(307) by caffeic acid ameliorated the caffeine-induced downregulation of IRS-1 Tyr(612) phosphorylation and 3MG transport. Caffeine also increased the phosphorylation of IRS-1 Ser(789) and an IRS-1 Ser(789) kinase, 5'-AMP-activated protein kinase (AMPK). However, inhibition of IRS-1 Ser(789) and AMPK phosphorylation by dantrolene did not rescue the caffeine-induced downregulation of IRS-1 Tyr(612) phosphorylation or 3MG transport. In addition, caffeine suppressed the phosphorylation of insulin-stimulated IRS-1 Ser(636/639) and upstream kinases, including the mammalian target of rapamycin and p70S6 kinase. Intravenous injection of caffeine at a physiological dose (5 mg/kg) in rats inhibited the phosphorylation of insulin-stimulated IRS-1 Tyr(612) and Akt Ser(473) in epitrochlearis muscle. Our results indicate that caffeine inhibits insulin signaling partly through the IKK/IRS-1 Ser(307) pathway, via a Ca(2+)- and AMPK-independent mechanism in skeletal muscle.

Effects of caffeinated and decaffeinated coffee on biological risk factors for type 2 diabetes: a randomized controlled trial

Background

Coffee consumption has been associated with a lower risk of type 2 diabetes in prospective cohort studies, but the underlying mechanisms remain unclear. The aim of this study was to evaluate the effects of regular and decaffeinated coffee on biological risk factors for type 2 diabetes.

Methods

Randomized parallel-arm intervention conducted in 45 healthy overweight volunteers who were nonsmokers and regular coffee consumers. Participants were assigned to consumption of 5 cups (177 mL each) per day of instant caffeinated coffee, decaffeinated coffee, or no coffee (i.e., water) for 8 weeks.

Results

Average age was 40 years and body mass index was 29.5 kg/m². Compared with consuming no coffee, consumption of caffeinated coffee increased adiponectin (difference in change from baseline 1.4 μg/mL; 95% CI: 0.2, 2.7) and interleukin-6 (difference: 60%; 95% CI: 8, 138) concentrations and consumption of decaffeinated coffee decreased fetuin-A concentrations (difference: -20%; 95% CI: -35, -1). For measures of glucose tolerance, insulin sensitivity, and insulin secretion, no significant differences were found between treatment groups.

Conclusions

Although no changes in glycemia and/or insulin sensitivity were observed after 8 weeks of coffee consumption, improvements in adipocyte and liver function as indicated by changes in adiponectin and fetuin-A concentrations may contribute to beneficial metabolic effects of long-term coffee consumption.

Trial Registration

clinicaltrials.gov NCT00305097

Caffeine and insulin resistance in pregnancy

Outside pregnancy, acute caffeine consumption is associated with insulin resistance. We investigated if during pregnancy plasma concentrations of caffeine and its metabolite, paraxanthine, were associated with insulin resistance. Caffeine, paraxanthine, glucose, and insulin were measured and insulin resistance estimated by homeostasis model assessment (HOMA) in banked samples from 251 fasting subjects at mean gestational age of 20.3 ± 2.0 weeks. Analysis of covariance and adjusted logistic regression were performed. Most (96.4%) women had caffeine and/or paraxanthine present. Caffeine concentrations in the upper two quartiles (>266 ng/mL) were associated with threefold higher odds of having higher insulin resistance estimated by log HOMA ≥75th percentile (third quartile odds ratio [OR], 3.02; 95% confidence interval [CI]: 1.21 to 7.54 and fourth quartile OR, 2.95; 95% CI: 1.19 to 7.31). Paraxanthine concentrations in the upper quartile (>392 ng/mL) were also associated with threefold higher odds of having higher insulin resistance (OR, 3.04; 95% CI: 1.28 to 7.25). Adjusted mean HOMA in the first caffeine-to-paraxanthine ratio quartile was 1.5 ± 2.2 versus 1.3 ± 2.3 in the fourth quartile ( P < 0.01). Both high caffeine and paraxanthine concentrations were associated with insulin resistance, but slow versus fast metabolism did not make an important difference.

A new method to study changes in microvascular blood volume in muscle and adipose tissue: real-time imaging in humans and rat

We employed and evaluated a new application of contrast-enhanced ultrasound for real-time imaging of changes in microvascular blood volume (MBV) in tissues in females, males, and rat. Continuous real-time imaging was performed using contrast-enhanced ultrasound to quantify infused gas-filled microbubbles in the microcirculation. It was necessary to infuse microbubbles for a minimum of 5-7 min to obtain steady-state bubble concentration, a prerequisite for making comparisons between different physiological states. Insulin clamped at a submaximal concentration (∼75 μU/ml) increased MBV by 27 and 39% in females and males, respectively, and by 30% in female subcutaneous adipose tissue. There was no difference in the ability of insulin to increase muscle MBV in females and males, and microvascular perfusion rate was not increased significantly by insulin. However, perfusion rate of the microvascular space was higher in females compared with males. In rats, insulin clamped at a maximal concentration increased muscle MBV by 60%. Large increases in microvascular volume and perfusion rate were detected during electrical stimulation of muscle in rats and immediately after exercise in humans. We have demonstrated that real-time imaging of changes in MBV is possible in human and rat muscle and in subcutaneous adipose tissue and that the method is sensitive enough to pick up relatively small changes in MBV when performed with due consideration of steady-state microbubble concentration. Because of real-time imaging, the method has wide applications for determining MBV in different organs during various physiological or pathophysiological conditions.

An oral lipid challenge and acute intake of caffeinated coffee additively decrease glucose tolerance in healthy men

Lipid-induced insulin resistance has been investigated primarily with i.v. infusions, and caffeine-induced insulin resistance, with alkaloid caffeine. The effects of orally consumed lipids and coffee have not been established and to our knowledge have never been simultaneously investigated. The goals of this study were to determine whether an oral lipid challenge and caffeinated coffee would disrupt glucose homeostasis and to characterize their respective incretin responses. It was hypothesized that oral ingestion of saturated lipids would impair glucose tolerance and that caffeinated coffee would further hinder glucose management. Ten young, healthy males participated in 5 trials in a randomized, cross-over design. At time 0 h, they underwent an oral fat tolerance test (OFTT: 1 g lipid/kg body weight) or consumed water, followed 5 h later by caffeinated (5 mg/kg) coffee, decaffeinated coffee, or water. At 6 h, volunteers underwent an oral glucose tolerance test (OGTT). Consumption of the OFTT increased glucose concentrations (P < 0.05) after a subsequent OGTT. At 7 h, caffeinated coffee produced the highest glucose concentrations (P < 0.05). Glucagon-like peptide-1 active (GLP-1a) and glucose-dependent insulinotropic polypeptide (GIP) were both increased for up to 6 h in all OFTT trials (P < 0.05). Compared to all other treatments, caffeinated and decaffeinated coffee produced higher GLP-1a response at 6.25 h (P < 0.05), whereas only caffeinated coffee increased GIP secretion (P < 0.05). These results show that oral consumption of lipids and caffeinated coffee can independently and additively decrease glucose tolerance. Incretin hormones could explain at least in part this impaired glucose homeostasis.

Dietary supplements and team-sport performance

A well designed diet is the foundation upon which optimal training and performance can be developed. However, as long as competitive sports have existed, athletes have attempted to improve their performance by ingesting a variety of substances. This practice has given rise to a multi-billion-dollar industry that aggressively markets its products as performance enhancing, often without objective, scientific evidence to support such claims. While a number of excellent reviews have evaluated the performance-enhancing effects of most dietary supplements, less attention has been paid to the performance-enhancing claims of dietary supplements in the context of team-sport performance. Dietary supplements that enhance some types of athletic performance may not necessarily enhance team-sport performance (and vice versa). Thus, the first aim of this review is to critically evaluate the ergogenic value of the most common dietary supplements used by team-sport athletes. The term dietary supplements will be used in this review and is defined as any product taken by the mouth, in addition to common foods, that has been proposed to have a performance-enhancing effect; this review will only discuss substances that are not currently banned by the World Anti-Doping Agency. Evidence is emerging to support the performance-enhancing claims of some, but not all, dietary supplements that have been proposed to improve team-sport-related performance. For example, there is good evidence that caffeine can improve single-sprint performance, while caffeine, creatine and sodium bicarbonate ingestion have all been demonstrated to improve multiple-sprint performance. The evidence is not so strong for the performance-enhancing benefits of β-alanine or colostrum. Current evidence does not support the ingestion of ribose, branched-chain amino acids or β-hydroxy-β-methylbutyrate, especially in well trained athletes. More research on the performance-enhancing effects of the dietary supplements highlighted in this review needs to be conducted using team-sport athletes and using team-sport-relevant testing (e.g. single- and multiple-sprint performance). It should also be considered that there is no guarantee that dietary supplements that improve isolated performance (i.e. single-sprint or jump performance) will remain effective in the context of a team-sport match. Thus, more research is also required to investigate the effects of dietary supplements on simulated or actual team-sport performance. A second aim of this review was to investigate any health issues associated with the ingestion of the more commonly promoted dietary supplements. While most of the supplements described in the review appear safe when using the recommended dose, the effects of higher doses (as often taken by athletes) on indices of health remain unknown, and further research is warranted. Finally, anecdotal reports suggest that team-sport athletes often ingest more than one dietary supplement and very little is known about the potential adverse effects of ingesting multiple supplements. Supplements that have been demonstrated to be safe and efficacious when ingested on their own may have adverse effects when combined with other supplements. More research is required to investigate the effects of ingesting multiple supplements (both on performance and health).

Methylxanthines and human health: epidemiological and experimental evidence

When considering methylxanthines and human health, it must be recognized that in many countries most caffeine is consumed as coffee. This is further confounded by the fact that coffee contains many bioactive substances in addition to caffeine; it is rich in phenols (quinides, chlorogenic acid, and lactones) and also has diterpenes (fatty acid esters), potassium, niacin, magnesium, and the vitamin B(3) precursor trigonelline. There is a paradox as consumption of either caffeine or caffeinated coffee results in a marked insulin resistance and yet habitual coffee consumption has repeatedly been reported to markedly reduce the risk for type 2 diabetes. There is strong evidence that caffeine reduces insulin sensitivity in skeletal muscle and this may be due to a combination of direct antagonism of A(1) receptors and indirectly β-adrenergic stimulation as a result of increased sympathetic activity. Caffeine may also induce reduced hepatic glucose output. With the exception of bone mineral, there is little evidence that caffeine impacts negatively on other health issues. Coffee does not increase the risk of cardiovascular diseases or cancers and there is some evidence suggesting a positive relationship for the former and for some cancers, particularly hepatic cancer.

Coffee consumption and risk of incident gout in women: the Nurses' Health Study

BACKGROUND

Coffee is one of the most widely consumed beverages in the world and may affect the risk of gout via various mechanisms, but prospective data on the relation between coffee intake and the risk of incident gout are limited.

DESIGN

Over a 26-y period, we prospectively examined the relation between coffee intake and risk of incident gout in 89,433 female participants in the Nurses' Health Study. We assessed the consumption of coffee, decaffeinated coffee, tea, and total caffeine in participants every 2-4 y through validated questionnaires. We used a supplementary questionnaire to ascertain whether participants met the survey criteria of the American College of Rheumatology for gout.

RESULTS

During the 26 y of follow-up, we documented 896 confirmed incident cases of gout. There was an inverse association between higher coffee intake and the risk of gout. The multivariate relative risks (RRs) for incident gout according to coffee-consumption categories [ie, 0, 1-237, 238-947, and ≥948 mL coffee/d (237 mL = one 8-ounce cup)] were 1.00, 0.97, 0.78 (95% CI: 0.64, 0.95), and 0.43 (95% CI: 0.30, 0.61; P for trend < 0.0001), respectively. For decaffeinated coffee, the multivariate RRs according to consumption categories (0, 1-237, and ≥237 mL decaffeinated coffee/d) were 1.00, 1.02, and 0.77 (95% CI: 0.63, 0.95; P for trend = 0.02), respectively. There was an inverse association between total caffeine from all sources and the risk of gout; the multivariate RR of the highest quintile compared with the lowest quintile was 0.52 (95% CI: 0.41, 0.68; P for trend <0.0001).

CONCLUSION

These prospective data suggest that long-term coffee consumption is associated with a lower risk of incident gout in women.

Caffeine and theophylline block insulin-stimulated glucose uptake and PKB phosphorylation in rat skeletal muscles

AIM

Caffeine and theophylline inhibit phosphatidylinositol 3-kinase (PI3-kinase) activity and insulin-stimulated protein kinase B (PKB) phosphorylation. Insulin-stimulated glucose uptake involves PI3-kinase/PKB, and the aim of the present study was to test the hypothesis that caffeine and theophylline inhibit insulin-stimulated glucose uptake in skeletal muscles.

METHODS

Rat epitrochlearis muscles and soleus strips were incubated with insulin and different concentrations of caffeine and theophylline for measurement of glucose uptake, force development and PKB phosphorylation. The effect of caffeine was also investigated in muscles stimulated electrically.

RESULTS

Caffeine and theophylline completely blocked insulin-stimulated glucose uptake in both soleus and epitrochlearis muscles at 10 mm. Furthermore, insulin-stimulated PKB Ser(473) and Thr(308) and GSK-3beta Ser(9) phosphorylation were blocked by caffeine and theophylline. Caffeine reduced and theophylline blocked insulin-stimulated glycogen synthase activation. Caffeine stimulates Ca(2+) release and force development increased rapidly to 10-20% of maximal tetanic contraction. Dantrolene (25 microm), a well-known inhibitor of Ca(2+)-release, prevented caffeine-induced force development, but caffeine inhibited insulin-stimulated glucose uptake in the presence of dantrolene. Contraction, like insulin, stimulates glucose uptake via translocation of glucose transporter-4 (GLUT4). Caffeine and theophylline reduced contraction-stimulated glucose uptake by about 50%, whereas contraction-stimulated glycogen breakdown was normal.

CONCLUSION

Caffeine and theophylline block insulin-stimulated glucose uptake independently of Ca(2+) release, and the likely mechanism is via blockade of insulin-stimulated PI3-kinase/PKB activation. Caffeine and theophylline also reduced contraction-stimulated glucose uptake, which occurs independently of PI3-kinase/PKB, and we hypothesize that caffeine and theophylline also inhibit glucose uptake in skeletal muscles via an additional and hitherto unknown molecule involved in GLUT4 translocation.

Coffee intake can promote activity of antioxidant enzymes with increasing MDA level and decreasing HDL-cholesterol in physically trained rats

This study investigated the effect of coffee intake and exercise on the antioxidative activity and plasma cholesterol profile of physically trained rats while they were exercising. Forty eight rats were under either the control diet with water (C) or control diet with coffee (CF) and at the same time they were given physical training for 4 weeks. In terms of physical training, the rats were exercised on a treadmill for 30 minutes everyday. At the end of 4 weeks, animals in each dietary group were subdivided into 3 groups: before-exercise (BE); during-exercise (DE); after-exercise (AE). Animals in the DE group were exercised on a treadmill for one hour, immediately before being sacrificed. Animals in the AE group were allowed to take a rest for one hour after exercise. TG levels were significantly high in coffee intake group than in control group. Also TG level of AE group was significantly higher than that of BE group. Exercise and coffee-exercise interaction effects were significant in total cholesterol (P = 0.0004, 0.0170). The AE of coffee intake group showed highest total cholesterol levels. HDL-cholesterol was significantly lower in coffee intake group than in control group. Coffee, exercise, and coffee-exercise interaction effects were significant in SOD (P = 0.0001, 0.0001, and 0.0001). The AE and BE of coffee intake group showed higher SOD levels than the other four groups. Catalase activities were significantly higher in coffee intake group than control group. No significant main effect was found in GSH/GSSG. Coffee, exercise, and coffee-exercise interaction effects were significant in MDA levels (P = 0.0464, 0.0016, and 0.0353). The DE and AE of coffee intake group and the DE of control group showed higher MDA levels than the BE of control group. Therefore, coffee intake can promote activities of antioxidant enzyme but it also increases MDA and decreases HDL-cholesterol in physically trained rats.

Induction of neutral endopeptidase (NEP) activity of SK-N-SH cells by natural compounds from green tea

Deposition of amyloid β-peptide as senile plaques in the brain is one of the neuropathological hallmarks of Alzheimer's disease, which is the most prevalent progressive neurodegenerative disease leading to dementia. Neutral endopeptidase is one of the major β-amyloid-degrading enzymes in the brain. To examine the influence of different polyphenols and other natural products from green tea extract (from Camellia sinensis, Theaceae), we used the neuroblastoma cell line SK-N-SH and studied the changes in the specific cellular neutral endopeptidase activity after long-term treatment with these substances. We have shown that caffeine leads to an increase in specific cellular neutral endopeptidase activity more than theophylline, theobromine or theanine. We have also shown that the combination of epicatechin, epigallocatechin and epigallocatechingallate with caffeine, theobromine or theophylline induced cellular neutral endopeptidase activity. It is suggested that the enhancement of cellular neutral endopeptidase activity by green tea extract and its natural products might be correlated with an elevated level of intracellular cyclic adenosine monophosphate.

Integration of microRNA changes in vivo identifies novel molecular features of muscle insulin resistance in type 2 diabetes

Background

Skeletal muscle insulin resistance (IR) is considered a critical component of type II diabetes, yet to date IR has evaded characterization at the global gene expression level in humans. MicroRNAs (miRNAs) are considered fine-scale rheostats of protein-coding gene product abundance. The relative importance and mode of action of miRNAs in human complex diseases remains to be fully elucidated. We produce a global map of coding and non-coding RNAs in human muscle IR with the aim of identifying novel disease biomarkers.

Methods

We profiled >47,000 mRNA sequences and >500 human miRNAs using gene-chips and 118 subjects (n = 71 patients versus n = 47 controls). A tissue-specific gene-ranking system was developed to stratify thousands of miRNA target-genes, removing false positives, yielding a weighted inhibitor score, which integrated the net impact of both up- and down-regulated miRNAs. Both informatic and protein detection validation was used to verify the predictions of in vivo changes.

Results

The muscle mRNA transcriptome is invariant with respect to insulin or glucose homeostasis. In contrast, a third of miRNAs detected in muscle were altered in disease (n = 62), many changing prior to the onset of clinical diabetes. The novel ranking metric identified six canonical pathways with proven links to metabolic disease while the control data demonstrated no enrichment. The Benjamini-Hochberg adjusted Gene Ontology profile of the highest ranked targets was metabolic (P < 7.4 × 10^-8), post-translational modification (P < 9.7 × 10^-5) and developmental (P < 1.3 × 10^-6) processes. Protein profiling of six development-related genes validated the predictions. Brain-derived neurotrophic factor protein was detectable only in muscle satellite cells and was increased in diabetes patients compared with controls, consistent with the observation that global miRNA changes were opposite from those found during myogenic differentiation.

Conclusions

We provide evidence that IR in humans may be related to coordinated changes in multiple microRNAs, which act to target relevant signaling pathways. It would appear that miRNAs can produce marked changes in target protein abundance in vivo by working in a combinatorial manner. Thus, miRNA detection represents a new molecular biomarker strategy for insulin resistance, where micrograms of patient material is needed to monitor efficacy during drug or life-style interventions.

Consumption of caffeinated coffee and a high carbohydrate meal affects postprandial metabolism of a subsequent oral glucose tolerance test in young, healthy males

Caffeine and caffeinated coffee (CC) elicit acute insulin insensitivity when ingested before a carbohydrate load. The effects of CC on glucose tolerance and insulin sensitivity when co-ingested with a high carbohydrate meal and on postprandial metabolism of a subsequent (second) carbohydrate load have not been studied. In a randomised, crossover design, ten healthy males ingested either CC (5 mg caffeine/kg body weight), decaffeinated coffee (DC) or water (W; equal volume) co-ingested with a high glycaemic index cereal followed 3 h later by a 75 g oral glucose tolerance test. After the initial meal, insulin area under the curve (AUC) and insulin sensitivity index did not differ between treatments, although glucose AUC for CC (107 (sem 18) mmol/l × 3 h) and DC (74 (sem 15) mmol/l × 3 h) was greater than W ( − 0·2 (sem 29) mmol/l × 3 h, P < 0·05). After the second carbohydrate load, insulin AUC for CC was 49 % and 57 % greater (P < 0·01) than for DC and W, respectively. Despite the greater insulin response, glucose AUC for CC (217 (sem 24) mmol/l × 2 h) was greater than both DC (126 (sem 11) mmol/l × 2 h, P = 0·01) and W (55 (sem 34) mmol/l × 2 h, P < 0·001). Insulin sensitivity index after the second meal was lower after CC (8·2 (sem 0·9)) compared with both DC (12·4 (sem 1·2), P < 0·01) and W (13·4 (sem 1·4), P < 0·001). Co-ingestion of CC with one meal resulted in insulin insensitivity during the postprandial phase of a second meal in the absence of further CC ingestion. Thus, CC may play a role in daily glycaemic management.

Increased AS160 phosphorylation, but not TBC1D1 phosphorylation, with increased postexercise insulin sensitivity in rat skeletal muscle

A single exercise bout can increase insulin-independent glucose transport immediately postexercise and insulin-dependent glucose transport (GT) for several hours postexercise. Akt substrate of 160 kDa (AS160) and TBC1D1 are paralog Rab GTPase-activating proteins that have been proposed to contribute to these exercise effects. Previous research demonstrated greater AS160 and Akt threonine phosphorylation in rat skeletal muscle at 3-4 h postexercise concomitant with enhanced insulin-stimulated GT. To further probe whether these signaling events or TBC1D1 phosphorylation were important for the enhanced postexercise insulin-stimulated GT, male Wistar rats were studied using four experimental protocols (2-h swim exercise, differing with regard to timing of muscle sampling and whether food was provided postexercise) that were known to vary in their influence of insulin-independent and insulin-dependent GT postexercise. The results indicated that, in isolated rat epitrochlearis muscle, 1) elevated phosphorylation of AS160 (measured using anti-phospho-Akt substrate, PAS-AS160, and phosphospecific anti-Thr(642)-AS160, pThr(642)-AS160) consistently tracked with elevated insulin-stimulated GT; 2) PAS-TBC1D1 was not different from sedentary values at 3 or 27 h postexercise, when insulin sensitivity was increased; 3) insulin-stimulated Akt activity was not increased postexercise in muscles with increased insulin sensitivity; 4) PAS-TBC1D1 was increased immediately postexercise, when insulin-independent GT was elevated, and reversed at 3 and 27 h postexercise, when insulin-independent GT was also reversed; and 5) there was no significant effect of exercise or insulin on total abundance of AS160, TBC1D1, Akt, or GLUT4 protein with any of the protocols. The results are consistent with increased AS160 phosphorylation (PAS-AS160 or pThr(642)-AS160) but not increased PAS-TBC1D1 or Akt activity, which is important for increased postexercise insulin-stimulated GT in rat skeletal muscle. They also support the idea that increased TBC1D1 phosphorylation may play a role in the insulin-independent increase in GT postexercise.

Does caffeine alter muscle carbohydrate and fat metabolism during exercise?

Caffeine, an adenosine receptor antagonist, has been studied for decades as a putative ergogenic aid. In the past 2 decades, the information has overwhelmingly demonstrated that it indeed is a powerful ergogenic aid, and frequently theories have been proposed that this is due to alterations in fat and carbohydrate metabolism. While caffeine certainly mobilizes fatty acids from adipose tissue, rarely have measures of the respiratory exchange ratio indicated an increase in fat oxidation. However, this is a difficult measure to perform accurately during exercise, and small changes could be physiologically important. The few studies examining human muscle metabolism directly have also supported the fact that there is no change in fat or carbohydrate metabolism, but these usually have had a small sample size. We combined the data from muscle biopsy analyses of several similar studies to generate a sample size of 16-44, depending on the measure. We examined muscle glycogen, citrate, acetyl-CoA, glucose-6-phosphate, and cyclic adenosine monophosphate (cAMP) in resting samples and in those obtained after 10-15 min of exercise at 70%-85% maximal oxygen consumption. Exercise decreased (p < 0.05) glycogen and increased (p < 0.05) citrate, acetyl-CoA, and glucose-6-phosphate. The only effects of caffeine were to increase (p < 0.05) citrate in resting muscle and cAMP in exercise. There is very little evidence to support the hypothesis that caffeine has ergogenic effects as a result of enhanced fat oxidation. Individuals may, however, respond differently to the effects of caffeine, and there is growing evidence that this could be explained by common genetic variations.

Coffee, glucose homeostasis, and insulin resistance: physiological mechanisms and mediators

Epidemiological studies show coffee consumption to be correlated to large risk reductions in the prevalence of type 2 diabetes (T2D). Such correlations are seen with decaffeinated and caffeinated coffee, and occur regardless of gender, method of brewing, or geography. They also exist despite clear evidence showing that caffeine causes acute postprandial hyperglycemia and lower whole-body insulin sensitivity. As the beneficial effects of coffee consumption exist for both decaffeinated and caffeinated coffee, a component of coffee other than caffeine must be responsible. This review examines the specific coffee compounds responsible for coffee's effects on T2D, and their potential physiological mechanisms of action. Being plant-derived, coffee contains many beneficial compounds found in fruits and vegetables, including antioxidants. In fact, coffee is the largest source of dietary antioxidants in industrialized nations. When green coffee is roasted at high temperatures, Maillard reactions create a number of unique compounds. Roasting causes a portion of the antioxidant, chlorogenic acid, to be transformed into quinides, compounds known to alter blood glucose levels. Coffee consumption may also mediate levels of gut peptides (glucose-dependent insulinotropic polypeptide and glucagon-like peptide-1), hormones intimately involved in the regulation of satiety and insulin secretion. Finally, coffee may have prebiotic-like properties, altering gut flora and ultimately digestion. In summary, it is evident that a better understanding of the role of coffee in the development and prevention of T2D has the potential to uncover novel therapeutic targets and nutraceutical formulations for the disease.

Coffee consumption and risk of coronary heart diseases: a meta-analysis of 21 prospective cohort studies

BACKGROUND

A large amount of cohort studies addressed coffee consumption and risk of coronary heart disease (CHD) and yielded inconsistent results. We conducted a meta-analysis to estimate the pooling effects.

METHODS

We searched for all published English articles indexed in MEDLINE or PubMed from January 1966 to January 2008. Twenty-one independent prospective cohort studies, which tested CHD risk by coffee consumption, were identified. A general variance-based method was used to pool the relative risks (RR). 15,599 cases from 407,806 participants were included in pooling the overall effects.

RESULTS

As compared to the light coffee consumption (<1 cup/d in US or <or=2 cups/d in Europe), under the random-effects model, the pooled CHD RRs (95% CI) for all studies combined were 0.96 (0.87-1.06), 1.04 (0.92-1.17) and 1.07 (0.87-1.32) for the moderate (1-3 or 3-4 cups/d), heavy (4-5 or 5-6 cups/d) and very heavy (>or=6 or >or=7 cups/d) categories of coffee consumption (all p>0.05); Moderate coffee consumption showed significantly lower CHD RR (95% CI) of 0.82 (0.73-0.92) (p<0.001) in women, and of 0.87 (0.80-0.86) (p=0.001) in men and women followed <or=10 years.

CONCLUSION

Our findings do not support the hypothesis that coffee consumption increases the long-term risk of coronary heart disease. Habitual moderate coffee drinking was associated with a lower risk of CHD in women.

High rates of muscle glycogen resynthesis after exhaustive exercise when carbohydrate is coingested with caffeine

We determined the effect of coingestion of caffeine (Caff) with carbohydrate (CHO) on rates of muscle glycogen resynthesis during recovery from exhaustive exercise in seven trained subjects who completed two experimental trials in a randomized, double-blind crossover design. The evening before an experiment subjects performed intermittent exhaustive cycling and then consumed a low-CHO meal. The next morning subjects rode until volitional fatigue. On completion of this ride subjects consumed either CHO [4 g/kg body mass (BM)] or the same amount of CHO + Caff (8 mg/kg BM) during 4 h of passive recovery. Muscle biopsies and blood samples were taken at regular intervals throughout recovery. Muscle glycogen levels were similar at exhaustion [ approximately 75 mmol/kg dry wt (dw)] and increased by a similar amount ( approximately 80%) after 1 h of recovery (133 +/- 37.8 vs. 149 +/- 48 mmol/kg dw for CHO and Caff, respectively). After 4 h of recovery Caff resulted in higher glycogen accumulation (313 +/- 69 vs. 234 +/- 50 mmol/kg dw, P < 0.001). Accordingly, the overall rate of resynthesis for the 4-h recovery period was 66% higher in Caff compared with CHO (57.7 +/- 18.5 vs. 38.0 +/- 7.7 mmol x kg dw(-1) x h(-1), P < 0.05). After 1 h of recovery plasma Caff levels had increased to 31 +/- 11 microM (P < 0.001) and at the end of the recovery reached 77 +/- 11 microM (P < 0.001) with Caff. Phosphorylation of CaMK(Thr286) was similar after exercise and after 1 h of recovery, but after 4 h CaMK(Thr286) phosphorylation was higher in Caff than CHO (P < 0.05). Phosphorylation of AMP-activated protein kinase (AMPK)(Thr172) and Akt(Ser473) was similar for both treatments at all time points. We provide the first evidence that in trained subjects coingestion of large amounts of Caff (8 mg/kg BM) with CHO has an additive effect on rates of postexercise muscle glycogen accumulation compared with consumption of CHO alone.

Caffeinated coffee consumption impairs blood glucose homeostasis in response to high and low glycemic index meals in healthy men

BACKGROUND

The ingestion of caffeine (5 mg/kg body weight) and a 75-g oral glucose load has been shown to elicit an acute insulin-insensitive environment in healthy and obese individuals and in those with type 2 diabetes.

OBJECTIVE

In this study we investigated whether a similar impairment in blood glucose management exists when coffee and foods typical of a Western diet were used in a similar protocol.

DESIGN

Ten healthy men underwent 4 trials in a randomized order. They ingested caffeinated (5 mg/kg) coffee (CC) or the same volume of decaffeinated coffee (DC) followed 1 h later by either a high or low glycemic index (GI) cereal (providing 75 g of carbohydrate) mixed meal tolerance test.

RESULTS

CC with the high GI meal resulted in 147%, 29%, and 40% greater areas under the curve for glucose (P < 0.001), insulin (NS), and C-peptide (P < 0.001), respectively, compared with the values for DC. Similarly, with the low GI treatment, CC elicited 216%, 44%, and 36% greater areas under the curve for glucose (P < 0.001), insulin (P < 0.01), and C-peptide (P < 0.01), respectively. Insulin sensitivity was significantly reduced (40%) with the high GI treatment after CC was ingested compared with DC; with the low GI treatment, CC ingestion resulted in a 29% decrease in insulin sensitivity, although this difference was not significant.

CONCLUSION

The ingestion of CC with either a high or low GI meal significantly impairs acute blood glucose management and insulin sensitivity compared with ingestion of DC. Future investigations are warranted to determine whether CC is a risk factor for insulin resistance.

A1 receptor deficiency causes increased insulin and glucagon secretion in mice

Adenosine influences metabolism and the adenosine receptor antagonist caffeine decreases the risk of type 2 diabetes. In this study the metabolic role of one adenosine receptor subtype, the adenosine A(1)R, was evaluated in mice lacking this receptor [A(1)R (-/-)]. The HbA1c levels and body weight were not significantly different between wild type [A(1)R (+/+)] and A(1)R (-/-) mice (3-4 months) fed normal lab chow. At rest, plasma levels of glucose, insulin and glucagon were similar in both genotypes. Following glucose injection, glucose tolerance was not appreciably altered in A(1)R (-/-) mice. Glucose injection induced sustained increases in plasma insulin and glucagon levels in A(1)R (-/-) mice, whereas A(1)R (+/+) control mice reacted with the expected transient increase in insulin and decrease in glucagon levels. Pancreas perfusion experiments showed that A(1)R (-/-) mice had a slightly higher basal insulin secretion than A(1)R (+/+) mice. The first phase insulin secretion (initiated with 16.7 mM glucose) was of the same magnitude in both genotypes, but the second phase was significantly enhanced in the A(1)R (-/-) pancreata compared with A(1)R (+/+). Insulin- and contraction-mediated glucose uptake in skeletal muscle were not significantly different between in A(1)R (-/-) and A(1)R (+/+) mice. All adenosine receptors were expressed at mRNA level in skeletal muscle in A(1)R (+/+) mice and the mRNA A(2A)R, A(2B)R and A(3)R levels were similar in A(1)R (-/-) and A(1)R (+/+) mice. In conclusion, the A(1)R minimally affects muscle glucose uptake, but is important in regulating pancreatic islet function.

Metabolic and hormonal effects of caffeine: randomized, double-blind, placebo-controlled crossover trial

In short-term studies, caffeine has been shown to increase insulin levels, reduce insulin sensitivity, and increase cortisol levels. However, epidemiological studies have indicated that long-term consumption of beverages containing caffeine such as coffee and green tea is associated with a reduced risk of type 2 diabetes mellitus. There is a paucity of randomized studies addressing the metabolic and hormonal effects of consuming caffeine over periods of more than 1 day. We evaluated the effect of oral intake of 200 mg of caffeine taken twice a day for 7 days on glucose metabolism, as well as on serum cortisol, dehydroepiandrosterone (DHEA), and androstenedione, and on nighttime salivary melatonin. A double-blind, randomized, placebo-controlled crossover study with periods of 7 days and washouts of 5 days comparing caffeine with placebo capsules was conducted. Participants were 16 healthy adults aged 18 to 22 years with a history of caffeine consumption. Blood samples from each subject were assayed for glucose, insulin, serum cortisol, DHEA, and androstenedione on the eighth day of each period after an overnight fast. Nighttime salivary melatonin was also measured. Insulin levels were significantly higher (by 1.80 microU/mL; 95% confidence interval, 0.33-3.28) after caffeine intake than after placebo. The homeostasis model assessment index of insulin sensitivity was reduced by 35% (95% confidence interval, 7%-62%) by caffeine. There were no differences in glucose, DHEA, androstenedione, and melatonin between treatment periods. This study provides evidence that daily caffeine intake reduces insulin sensitivity; the effect persists for at least a week and is evident up to 12 hours after administration.

Glucose homeostasis remains altered by acute caffeine ingestion following 2 weeks of daily caffeine consumption in previously non-caffeine-consuming males

Acute caffeine ingestion increases serum NEFA and plasma adrenaline and decreases insulin sensitivity. Although frequently suggested, it is not known if a tolerance to these alterations in glucose homeostasis is developed in habitual caffeine consumers. Our objective was to determine whether acute caffeine ingestion continued to alter insulin, glucose, NEFA and adrenaline during an oral glucose tolerance test (OGTT) following 14 d of caffeine consumption. Twelve caffeine-naive young males underwent four OGTTs over a 4-week period. Subjects ingested a gelatin-filled placebo (PLA) capsule on the first trial day and 5 mg caffeine/kg body weight on the remaining three trial days (day 0, day 7, day 14) before a 2 h OGTT. Following day 0 and day 7, subjects were given six dosages of 5 mg caffeine/kg to consume per d between trials. Serum insulin and blood glucose area under the curve (AUC) were significantly elevated (P < 0·05) v. PLA on day 0 (36 and 103 %, respectively) and were not different from PLA on day 7. On day 14, insulin AUC was 29 % greater than PLA (P < 0·05), and glucose was greater (P < 0·05) during the first hour, although the 50 % elevation in glucose AUC was not different from PLA. Before the OGTT, caffeine resulted in greater (P < 0·05) serum NEFA and plasma adrenaline concentrations in all three caffeine trials, but both NEFA and adrenaline concentrations were decreased (P < 0·05) on day 14 v. day 0. Although 14 d of caffeine consumption by previously caffeine-naive subjects reduced its impact on glucose homeostasis, carbohydrate metabolism remained disrupted.

Role of Akt substrate of 160 kDa in insulin-stimulated and contraction-stimulated glucose transport

Insulin and exercise, the most important physiological stimuli to increase glucose transport in skeletal muscle, trigger a redistribution of GLUT4 glucose transporter proteins from the cell interior to the cell surface, thereby increasing glucose transport capacity. The most distal insulin signaling protein that has been linked to GLUT4 translocation, Akt substrate of 160 kDa (AS160), becomes phosphorylated in insulin-stimulated 3T3-L1 adipocytes; this is important for insulin-stimulated GLUT4 translocation and glucose transport. Insulin also induces a rapid and dose-dependent increase in AS160 phosphorylation in skeletal muscle. Available data from skeletal muscle support the concepts developed in adipocytes with regard to the role AS160 plays in the regulation of insulin-stimulated glucose transport. In vivo exercise, in vitro contractions, or in situ contractions can also stimulate AS160 phosphorylation. AMP-activated protein kinase (AMPK) is likely important for phosphorylating AS160 in response to exercise/contractile activity, whereas Akt2 appears to be important for insulin-stimulated AS160 phosphorylation in muscle. Evidence of a role for AS160 in exercise/contraction-stimulated glucose uptake is currently inconclusive. The distinct signaling pathways that are stimulated by insulin and exercise/contraction converge at AS160. Although AS160 phosphorylation is apparently important for insulin-stimulated GLUT4 translocation and glucose transport, it is uncertain whether elevated AS160 phosphorylation plays a similar role with exercise/contraction.

Activation of the A1adenosine receptor increases insulin-stimulated glucose transport in isolated rat soleus muscle

The A1adenosine receptor (A1AR) has been suggested to participate in insulin- and contraction-stimulated glucose transport in skeletal muscle, but the qualitative and quantitative nature of the effect are controversial. We sought to determine if A1AR is expressed in rat soleus muscle and then characterize its role in glucose transport in this muscle. A1AR mRNA and protein expression were determined by RT-PCR and Western blotting, respectively. To examine the role of adenosine in 3-O-methylglucose transport, isolated muscles were exposed to adenosine deaminase and α,β-methylene adenosine diphosphate to remove endogenous adenosine and were left unstimulated (basal) or stimulated with insulin. To assess the functional participation of A1AR in 3-O-methylglucose transport, muscles were incubated with A1-selective agonist and (or) antagonist in the absence of endogenous adenosine and with or without insulin. A1AR mRNA was expressed in soleus muscle and A1AR was present at the plasma membrane. Removal of endogenous adenosine reduced glucose transport in response to 100 μU/mL insulin (~50%). The A1-selective agonist, N6-cyclopentyladenosine, increased submaximal (100 μU/mL) insulin-stimulated glucose transport in a dose-dependent manner (0.001–1.0 μmol/L). This stimulatory effect was inhibited by the A1-selective receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine in a concentration-dependent manner (0.001–1.0 μmol/L). However, neither activation nor inhibition of A1AR altered basal or maximal (10 mU/mL) insulin-stimulated glucose transport. Our results suggest that adenosine contributes ~50% to insulin-stimulated muscle glucose transport by activating the A1AR. This effect is limited to increasing insulin sensitivity, but not to either basal or maximal insulin-stimulated glucose uptake in rat soleus muscle.

Effects of chronic coffee consumption on glucose kinetics in the conscious rat

Epidemiological studies indicate that regular coffee consumption reduces the risk of developing type 2 diabetes. Despite these findings, the biological mechanisms by which coffee consumption exerts these effects are unknown. The aim of this study was twofold: to develop a rat model that would further delineate the effects of regular coffee consumption on glucose kinetics, and to determine whether coffee, with or without caffeine, alters the actions of insulin on glucose kinetics in vivo. Male Sprague–Dawley rats were fed a high-fat diet for 4 weeks in combination with one of the following: (i) drinking water as placebo (PL), (ii) decaffeinated coffee (2 g/100 mL) (DC), or (iii) alkaloid caffeine (20 mg/100 mL) added to decaffeinated coffee (2 g/100 mL) (CAF). Catheters were chronically implanted in a carotid artery and jugular vein for sampling and infusions, respectively. Recovered animals (5 days postoperative) were fasted for 5 h before hyperinsulinemic-euglycemic clamps (2 mU·kg–1·min–1). Glucose was clamped at 6 mmol/L and isotopes (2-deoxy-[14C]glucose and [3-3H]glucose) were administered to obtain indices of whole-body and tissue-specific glucose kinetics. Glucose infusion rates and measures of whole-body metabolic clearance were greater in DC than in PL or CAF, indicating increased whole-body insulin sensitivity. As the only difference between DC and CAF was the addition of alkaloid caffeine, it can be concluded that caffeine antagonizes the beneficial effects of DC. Given these findings, decaffeinated coffee may represent a nutritional means of combating insulin resistance.

Caffeine's impairment of insulin-mediated glucose disposal cannot be solely attributed to adrenaline in humans

Caffeine (CAF) impedes insulin-mediated glucose disposal (IMGD) and increases plasma adrenaline concentrations ([ADR]; 0.6 nm). While the antagonism of ADR abolishes the CAF effect, infusion of ADR (0.75 nm) has no effect on IMGD. We have now examined CAF and ADR in concert to determine whether or not they elicit an additive response on IMGD. We hypothesized that CAF + ADR would elicit a greater effect than either CAF or ADR alone (i.e. that CAF effects would not be solely attributed to ADR). Subjects (n = 8) completed four trials in a randomized manner. An isoglycaemic-hyperinsulinaemic clamp was performed 30 min after the following treatments were administered: (1) placebo capsules and saline infusion ([ADR] = 0.29 nm) (PL trial), (2) CAF capsules (dose = 5 mg kg(-1)) and saline infusion ([ADR] = 0.62 nm) (CAF trial), (3) PL capsules and ADR infusion ([ADR] = 1.19 nm) (ADR trial), and (4) CAF capsules (dose = 5 mg kg(-1)) and ADR infusion ([ADR] = 0.93 nm) (CAF + ADR trial). As expected, CAF, ADR and CAF + ADR decreased (P <or= 0.05) IMGD compared to PL. CAF + ADR resulted in a more pronounced decrease in IMGD versus PL (42%) compared to CAF (26%) or ADR (24%) alone; however, the effect was not fully additive (P = 0.08). Furthermore, CAF decreased IMGD to a similar magnitude as ADR despite a 50% lower [ADR]. In summary, while ADR contributes to the CAF-induced impairment in IMGD, it is not solely responsible for caffeine's effects.

Coffee consumption and risk of incident gout in men: a prospective study

OBJECTIVE

Coffee is one of the most widely consumed beverages in the world and may affect the risk of gout via various mechanisms. We prospectively evaluated the relationship between coffee intake and the risk of incident gout in a large cohort of men.

METHODS

Over a 12-year period, we studied 45,869 men with no history of gout at baseline. Intake of coffee, decaffeinated coffee, tea, and total caffeine was assessed every 4 years through validated questionnaires. We used a supplementary questionnaire to ascertain whether participants met the American College of Rheumatology survey criteria for gout.

RESULTS

We documented 757 confirmed incident cases of gout. Increasing coffee intake was inversely associated with the risk of gout. The multivariate relative risks (RRs) for incident gout according to coffee consumption categories (0, <1, 1-3, 4-5, and > or = 6 cups per day) were 1.00, 0.97, 0.92, 0.60 (95% confidence interval [95% CI] 0.41-0.87), and 0.41 (95% CI 0.19-0.88), respectively (P for trend = 0.009). For decaffeinated coffee, the multivariate RRs according to consumption categories (0, <1, 1-3, and > or = 4 cups per day) were 1.00, 0.83, 0.67 (95% CI 0.54-0.82), and 0.73 (95% CI 0.46-1.17), respectively (P for trend = 0.002). Total caffeine from all sources and tea intake were not associated with the risk of gout.

CONCLUSION

These prospective data suggest that long-term coffee consumption is associated with a lower risk of incident gout.

Coffee, tea, and caffeine consumption and serum uric acid level: the third national health and nutrition examination survey

OBJECTIVE

Coffee is one of the most widely consumed beverages in the world and may affect serum uric acid levels and risk of gout via various mechanisms. Our objective was to evaluate the relationship between coffee, tea, and caffeine intake and serum uric acid level in a nationally representative sample of men and women.

METHODS

Using data from 14,758 participants ages >/=20 years in the Third National Health and Nutrition Examination Survey (1988-1994), we examined the relationship between coffee, tea, and caffeine intake and serum uric acid level using linear regression. Additionally, we examined the relationship with hyperuricemia (serum uric acid >7.0 mg/dl among men and >5.7 mg/dl among women) using logistic regression. Intake was assessed by a food frequency questionnaire.

RESULTS

Serum uric acid level decreased with increasing coffee intake. After adjusting for age and sex, serum uric acid level associated with coffee intake of 4 to 5 and >/=6 cups daily was lower than that associated with no intake by 0.26 mg/dl (95% confidence interval [95% CI] 0.11, 0.41) and 0.43 mg/dl (95% CI 0.23, 0.65; P for trend < 0.001), respectively. After adjusting for other covariates, the differences remained significant (P for trend < 0.001). Similarly, there was a modest inverse association between decaffeinated coffee intake and serum uric acid levels (multivariate P for trend 0.035). Total caffeine from coffee and other beverages and tea intake were not associated with serum uric acid levels (multivariate P for trend 0.15). The multivariate odds ratio for hyperuricemia in individuals with coffee intake >/=6 cups daily compared with those with no coffee use was 0.57 (95% CI 0.35, 0.94; P for trend 0.001).

CONCLUSION

These findings from a nationally representative sample of US adults suggest that coffee consumption is associated with lower serum uric acid level and hyperuricemia frequency, but tea consumption is not. The inverse association with coffee appears to be via components of coffee other than caffeine.

Effects of caffeine and coffee consumption on cardiovascular disease and risk factors

Clinical and observational studies have recently reported that coffee consumption is associated with cardiac arrhythmia, with increased serum cholesterol, blood pressure and affected heart rate variability, leading to an increased cardiovascular risk. Analysis of these papers shows that these data are controversial and strongly depend on methodology. Recent reports demonstrate the beneficial effects of coffee consumption due to anti-inflammatory actions mediated by antioxidant compound of the beverages.

Increased insulin-stimulated Akt pSer473 and cytosolic SHP2 protein abundance in human skeletal muscle following acute exercise and short-term training

The purpose of the present study was to determine in human skeletal muscle whether a single exercise bout and 7 days of consecutive endurance (cycling) training 1) increased insulin-stimulated Akt pSer(473) and 2) altered the abundance of the protein tyrosine phosphatases (PTPases), PTP1B and SHP2. In healthy, untrained men (n = 8; 24 +/- 1 yr), glucose infusion rate during a hyperinsulinemic euglycemic clamp, when compared with untrained values, was not improved 24 h following a single 60-min bout of endurance cycling but was significantly increased ( approximately 30%; P < 0.05) 24 h following completion of 7 days of exercise training. Insulin-stimulated Akt pSer(473) was approximately 50% higher (P < 0.05) 24 h following the acute bout of exercise, with this effect remaining after 7 days of training (P < 0.05). Insulin-stimulated insulin receptor and insulin receptor substrate-1 tyrosine phosphorylation were not altered 24 h after acute exercise and short-term training. Insulin did not acutely regulate the localization of the PTPases, PTP1B or SHP2, although cytosolic protein abundance of SHP2 was increased (P < 0.05; main effect) 24 h following acute exercise and short-term training. In conclusion, insulin-sensitive Akt pSer(473) and cytosolic SHP2 protein abundance are higher after acute exercise and short-term training, and this effect appears largely due to the residual effects of the last bout of prior exercise. The significance of exercise-induced alterations in cytosolic SHP2 and insulin-stimulated Akt pSer(473) on the improvement in insulin sensitivity requires further elucidation.

Acute caffeine ingestion does not impair glucose tolerance in persons with tetraplegia

Acute caffeine (Caf) ingestion impairs glucose tolerance in able-bodied humans during an oral glucose tolerance test (OGTT). The mechanism responsible for this effect remains unclear, however, it is suggested to be due to the accompanying increase in epinephrine concentration. We examined whether or not Caf would elicit a glucose intolerance in persons with tetraplegia (TP) who do not exhibit an increased epinephrine response following Caf ingestion. All TP [n = 14; 9 incomplete (Inc) lesion, 5 complete (Com) lesion] completed two OGTT 1 h after consuming either gelatin (Pl) or Caf capsules (dose = 4 mg/kg). Blood samples were collected at baseline (time = 0 min), 1 h after capsule ingestion (time = 60 min), and every 30 min during the OGTT (time = 90-180 min). Glucose, insulin, proinsulin, and C-peptide responses were similar (P > 0.05) between treatments, demonstrating no effect of Caf on glucose tolerance. This lack of a Caf effect may be due to the low epinephrine concentration that remained unchanged (P > 0.05) throughout all experiments. Interestingly, the Com exhibited a 50% higher glucose response (P <or= 0.05) and a 46% (P > 0.05) lower insulin response (vs. Inc), suggesting a more pronounced glucose intolerance within this subgroup. Furthermore, nine TP (5 Com, 4 Inc) had glucose levels of >or= 7.8 mM at the end of the OGTT (time = 180 min), classifying them as glucose intolerant. In summary, acute Caf ingestion does not increase epinephrine concentration or impair glucose tolerance in TP.

Coffee, diabetes, and weight control

Several prospective epidemiologic studies over the past 4 y concluded that ingestion of caffeinated and decaffeinated coffee can reduce the risk of diabetes. This finding is at odds with the results of trials in humans showing that glucose tolerance is reduced shortly after ingestion of caffeine or caffeinated coffee and suggesting that coffee consumption could increase the risk of diabetes. This review discusses epidemiologic and laboratory studies of the effects of coffee and its constituents, with a focus on diabetes risk. Weight loss may be an explanatory factor, because one prospective epidemiologic study found that consumption of coffee was followed by lower diabetes risk but only in participants who had lost weight. A second such study found that both caffeine and coffee intakes were modestly and inversely associated with weight gain. It is possible that caffeine and other constituents of coffee, such as chlorogenic acid and quinides, are involved in causing weight loss. Caffeine and caffeinated coffee have been shown to acutely increase blood pressure and thereby to pose a health threat to persons with cardiovascular disease risk. One short-term study found that ground decaffeinated coffee did not increase blood pressure. Decaffeinated coffee, therefore, may be the type of coffee that can safely help persons decrease diabetes risk. However, the ability of decaffeinated coffee to achieve these effects is based on a limited number of studies, and the underlying biological mechanisms have yet to be elucidated.

Influence of caffeine on perception of effort, metabolism and exercise performance following a high-fat meal

This study examined the effects of caffeine, co-ingested with a high fat meal, on perceptual and metabolic responses during incremental (Experiment 1) and endurance (Experiment 2) exercise performance. Trained participants performed three constant-load cycling tests at approximately 73% of maximal oxygen uptake (VO2max) for 30 min at 20 degrees C (Experiment 1, n = 8) and to the limit of tolerance at 10 degrees C (Experiment 2, n = 10). The 30 min constant-load exercise in Experiment 1 was followed by incremental exercise (15 W . min-1) to fatigue. Four hours before the first test, the participants consumed a 90% carbohydrate meal (control trial); in the remaining two tests, the participants consumed a 90% fat meal with (fat + caffeine trial) and without (fat-only trial) caffeine. Caffeine and placebo were randomly assigned and ingested 1 h before exercise. In both experiments, ratings of perceived leg exertion were significantly lower during the fat + caffeine than fat-only trial (Experiment 1: P < 0.001; Experiment 2: P < 0.01). Ratings of perceived breathlessness were significantly lower in Experiment 1 (P < 0.01) and heart rate higher in Experiment 2 (P < 0.001) on the fat + caffeine than fat-only trial. In the two experiments, oxygen uptake, ventilation, blood [glucose], [lactate] and plasma [glycerol] were significantly higher on the fat + caffeine than fat-only trial. In Experiment 2, plasma [free fatty acids], blood [pyruvate] and the [lactate]:[pyruvate] ratio were significantly higher on the fat + caffeine than fat-only trial. Time to exhaustion during incremental exercise (Experiment 1: control: 4.9, s = 1.8 min; fat-only: 5.0, s = 2.2 min; fat + caffeine: 5.0, s = 2.2 min; P > 0.05) and constant-load exercise (Experiment 2: control: 116 (88 - 145) min; fat-only: 122 (96 - 144) min; fat + caffeine: 127 (107 - 176) min; P > 0.05) was not different between the fat-only and fat + caffeine trials. In conclusion, while a number of metabolic responses were increased during exercise after caffeine ingestion, perception of effort was reduced and this may be attributed to the direct stimulatory effect of caffeine on the central nervous system. However, this caffeine-induced reduction in effort perception did not improve exercise performance.

Increased submaximal insulin-stimulated glucose uptake in mouse skeletal muscle after treadmill exercise

The primary purpose of this study was to determine the effect of prior exercise on insulin-stimulated glucose uptake with physiological insulin in isolated muscles of mice. Male C57BL/6 mice completed a 60-min treadmill exercise protocol or were sedentary. Paired epitrochlearis, soleus, and extensor digitorum longus (EDL) muscles were incubated with [3H]-2-deoxyglucose without or with insulin (60 microU/ml) to measure glucose uptake. Insulin-stimulated glucose uptake for paired muscles was calculated by subtracting glucose uptake without insulin from glucose uptake with insulin. Muscles from other mice were assessed for glycogen and AMPK Thr172 phosphorylation. Exercised vs. sedentary mice had decreased glycogen in epitrochlearis (48%, P < 0.001), soleus (51%, P < 0.001), and EDL (41%, P < 0.01) and increased AMPK Thr172 phosphorylation (P < 0.05) in epitrochlearis (1.7-fold), soleus (2.0-fold), and EDL (1.4-fold). Insulin-independent glucose uptake was increased 30 min postexercise vs. sedentary in the epitrochlearis (1.2-fold, P < 0.001), soleus (1.4-fold, P < 0.05), and EDL (1.3-fold, P < 0.01). Insulin-stimulated glucose uptake was increased (P < 0.05) approximately 85 min after exercise in the epitrochlearis (sedentary: 0.266 +/- 0.045 micromol x g(-1) x 15 min(-1); exercised: 0.414 +/- 0.051) and soleus (sedentary: 0.102 +/- 0.049; exercised: 0.347 +/- 0.098) but not in the EDL. Akt Ser473 and Akt Thr308 phosphorylation for insulin-stimulated muscles did not differ in exercised vs. sedentary. These results demonstrate enhanced submaximal insulin-stimulated glucose uptake in the epitrochlearis and soleus of mice 85 min postexercise and suggest that it will be feasible to probe the mechanism of enhanced postexercise insulin sensitivity by using genetically modified mice.

The glucose intolerance induced by caffeinated coffee ingestion is less pronounced than that due to alkaloid caffeine in men

Although acute alkaloid caffeine (CAF) ingestion results in an impaired glucose tolerance, chronic coffee (RCOF) ingestion decreases the risk of developing type 2 diabetes. This study examines the hypothesis that CAF ingestion impairs glucose tolerance to a greater extent than RCOF and that the ingestion of decaffeinated coffee (DECAF) results in a positive effect. Eleven healthy males underwent 4 double-blinded randomized trials. Each trial included the ingestion of either: 1) CAF in capsule form (4.45 mg/kg body weight), 2) RCOF (4.45 mg/kg body weight caffeine), 3) dextrose (placebo, PL) in capsule form, or 4) DECAF (equal in volume to the RCOF trial), followed 1-h later by a 2-h oral glucose tolerance test. Blood samples were collected at baseline (-30), 0 (time of treatment ingestion), 60 (initiation of oral glucose tolerance test), 75, 90, 120, 150, and 180 min. Area under the curve for glucose and insulin were higher (P < or = 0.05) following CAF than both PL and DECAF and, although a similar trend (P = 0.07) was observed following RCOF compared with DECAF, the effect was less pronounced. Interestingly, DECAF resulted in a 50% lower glucose response (P < or = 0.05) than PL, suggesting that the effects of PL and DECAF on glucose tolerance are not the same. These findings suggest that the effects of CAF and RCOF are not identical and may provide a partial explanation as to why acute CAF ingestion impairs glucose tolerance while chronic RCOF ingestion protects against type 2 diabetes.

Possibility of autocrine beta-adrenergic signaling in C2C12 myotubes

Levodopa reportedly inhibits insulin action in skeletal muscle. Here we show that C2C12 myotubes produce levodopa and that insulin-stimulated glucose transport is enhanced when endogenous levodopa is depleted. Exogenous levodopa prevented the stimulation of glucose transport by insulin (P < 0.05) and increased cAMP concentrations (P < 0.05). The decrease in insulin-stimulated glucose transport caused by levodopa was attenuated by propranolol (a beta-adrenergic antagonist) and prevented by NSD-1015 (NSD), an inhibitor of DOPA decarboxylase (DDC; converts levodopa to dopamine). Propranolol and NSD both prevented levodopa-related increases in [cAMP]. However, the effects of levodopa were unlikely to be dependent on the conversion of levodopa to catecholamines because we could detect neither DDC in myotubes nor catecholamines in media after incubation of myotubes with levodopa. The data suggest the possibility of novel autocrine beta-adrenergic action in C2C12 myotubes in which levodopa, produced by myotubes, could have hormone-like effects that impinge on glucose metabolism.

Coffee, caffeine, and risk of type 2 diabetes: a prospective cohort study in younger and middle-aged U.S. women

OBJECTIVE

High habitual coffee consumption has been associated with a lower risk of type 2 diabetes, but data on lower levels of consumption and on different types of coffee are sparse.

RESEARCH DESIGN AND METHODS

This is a prospective cohort study including 88,259 U.S. women of the Nurses' Health Study II aged 26-46 years without history of diabetes at baseline. Consumption of coffee and other caffeine-containing foods and drinks was assessed in 1991, 1995, and 1999. We documented 1,263 incident cases of confirmed type 2 diabetes between 1991 and 2001.

RESULTS

After adjustment for potential confounders, the relative risk of type 2 diabetes was 0.87 (95% CI 0.73-1.03) for one cup per day, 0.58 (0.49-0.68) for two to three cups per day, and 0.53 (0.41-0.68) for four or more cups per day compared with nondrinkers (P for trend <0.0001). Associations were similar for caffeinated (0.87 [0.83-0.91] for a one-cup increment per day) and decaffeinated (0.81 [0.73-0.90]) coffee and for filtered (0.86 [0.82-0.90]) and instant (0.83 [0.74-0.93]) coffee. Tea consumption was not substantially associated with risk of type 2 diabetes (0.88 [0.64-1.23] for four or more versus no cups per day; P for trend = 0.81).

CONCLUSIONS

These results suggest that moderate consumption of both caffeinated and decaffeinated coffee may lower risk of type 2 diabetes in younger and middle-aged women. Coffee constituents other than caffeine may affect the development of type 2 diabetes.

Coffee and type 2 diabetes: from beans to beta-cells

Coffee consumption has been associated with improved glucose tolerance and a lower risk of type 2 diabetes in diverse populations in the U.S., Europe, and Japan. This review discusses the strength of the evidence, relevant mechanisms, possible implications, and directions for further research. The finding that higher consumption of decaffeinated coffee was associated with a lower risk of type 2 diabetes suggests that coffee constituents other than caffeine play a role. Coffee is a source of several compounds that improved glucose metabolism in animal studies, including the chlorogenic acids and lignans. Further research on phytochemicals in coffee may lead to the identification of novel mechanisms for effects of diet on the development of type 2 diabetes. In addition, knowledge on effects of coffee components may aid in the development or selection of types of coffee with improved health effects. Longer-term randomized intervention studies that test the effects of coffee consumption on glucose tolerance are warranted. Physical activity and weight management should be the mainstay of public health strategies to prevent type 2 diabetes. For individual choices regarding coffee consumption, potential effects of coffee on various health outcomes should be considered.

A chlorogenic acid-induced increase in GLP-1 production may mediate the impact of heavy coffee consumption on diabetes risk

Recent prospective epidemiology links heavy coffee consumption to a substantial reduction in risk for type 2 diabetes. Yet there is no evidence that coffee improves insulin sensitivity and, at least in acute studies, caffeine has a negative impact in this regard. Thus, it is reasonable to suspect that coffee influences the risk for beta cell "failure" that precipitates diabetes in subjects who are already insulin resistant. Indeed, there is recent evidence that coffee increases production of the incretin hormone glucagon-like peptide-1 (GLP-1), possibly owing to an inhibitory effect of chlorogenic acid (CGA -- the chief polyphenol in coffee) on glucose absorption. GLP-1 acts on beta cells, via cAMP-dependent mechanisms, to promote the synthesis and activity of the transcription factor IDX-1, crucial for maintaining the responsiveness of beta cells to an increase in plasma glucose. Conversely, the "glucolipotoxicity" thought to initiate and sustain beta cell dysfunction in diabetics can suppress expression of this transcription factor. The increased production of GLP-1 associated with frequent coffee consumption could thus be expected to counteract the adverse impact of chronic free fatty acid overexposure on beta cell function in overweight insulin resistant subjects. CGA's putative impact on glucose absorption may reflect the ability of this compound to inhibit glucose-6-phosphate translocase 1, now known to play a role in intestinal glucose transport. Delayed glucose absorption may itself protect beta cells by limiting postprandial hyperglycemia -- though, owing to countervailing effects of caffeine on plasma glucose, and a paucity of relevant research studies, it is still unclear whether coffee ingestion blunts the postprandial rise in plasma glucose. More generally, diets high in "lente carbohydrate", or administration of nutraceuticals/pharmaceuticals which slow the absorption of dietary carbohydrate, should help preserve efficient beta cell function by boosting GLP-1 production, as well as by blunting the glucotoxic impact of postprandial hyperglycemia on beta cell function.

The effect of caffeine on glucose kinetics in humans--influence of adrenaline

While caffeine impedes insulin-mediated glucose disposal in humans, its effect on endo-genous glucose production (EGP) remains unknown. In addition, the mechanism involved in these effects is unclear, but may be due to the accompanying increase in adrenaline concentration. We studied the effect of caffeine on EGP and glucose infusion rates (GIR), and whether or not adrenaline can account for all of caffeine's effects. Subjects completed three isoglycaemic-hyperinsulinaemic clamps (with 3-[(3)H]glucose infusion) 30 min after ingesting: (1) placebo capsules (n= 12); (2) caffeine capsules (5 mg kg(-1)) (n= 12); and either (3) placebo plus a high-dose adrenaline infusion (HAdr; adrenaline concentration, 1.2 nM; n= 8) or (4) placebo plus a low-dose adrenaline infusion (LAdr; adrenaline concentration, 0.75 nM; n= 6). With caffeine, adrenaline increased to 0.6 nM but no effect on EGP was observed. While caffeine and HAdr decreased GIR by 13 (P < 0.05) and 34% (P < 0.05) versus the placebo, respectively, LAdr did not result in a significant reduction (5%) in GIR versus the placebo. Due to the fact that both caffeine and LAdr resulted in similar adrenaline concentrations, but resulted in different decreases in GIR, it is concluded that adrenaline alone does not account for the effects of caffeine and additional mechanisms must be involved.

Caffeine increases exogenous carbohydrate oxidation during exercise

Both carbohydrate (CHO) and caffeine have been used as ergogenic aids during exercise. It has been suggested that caffeine increases intestinal glucose absorption, but there are also suggestions that it may decrease muscle glucose uptake. The purpose of the study was to investigate the effect of caffeine on exogenous CHO oxidation. In a randomized crossover design, eight male cyclists (age 27 +/- 2 yr, body mass 71.2 +/- 2.3 kg, maximal oxygen uptake 65.7 +/- 2.2 ml x kg(-1) x min(-1)) exercised at 64 +/- 3% of maximal oxygen uptake for 120 min on three occasions. During exercise subjects ingested either a 5.8% glucose solution (Glu; 48 g/h), glucose with caffeine (Glu+Caf, 48 g/h + 5 mg x kg(-1) x h(-1)), or plain water (Wat). The glucose solution contained trace amounts of [U-13C]glucose so that exogenous CHO oxidation could be calculated. CHO and fat oxidation were measured by indirect calorimetry, and 13C appearance in the expired gases was measured by continuous-flow IRMS. Average exogenous CHO oxidation over the 90- to 120-min period was 26% higher (P < 0.05) in Glu+Caf (0.72 +/- 0.04 g/min) compared with Glu (0.57 +/- 0.04 g/min). Total CHO oxidation rates were higher (P < 0.05) in the CHO ingestion trials compared with Wat, but they were highest when Glu+Caf was ingested (1.21 +/- 0.37, 1.84 +/- 0.14, and 2.47 +/- 0.23 g/min for Wat, Glu, and Glu+Caf, respectively; P < 0.05). There was also a trend (P = 0.082) toward an increased endogenous CHO oxidation with Glu+Caf (1.81 +/- 0.22 g/min vs. 1.27 +/- 0.13 g/min for Glu and 1.12 +/- 0.37 g/min for Wat). In conclusion, compared with glucose alone, 5 mg x kg(-1) x h(-1) of caffeine coingested with glucose increases exogenous CHO oxidation, possibly as a result of an enhanced intestinal absorption.

Caffeine ingestion is associated with reductions in glucose uptake independent of obesity and type 2 diabetes before and after exercise training

OBJECTIVE

We investigated the effect of caffeine ingestion on insulin sensitivity in sedentary lean men (n = 8) and obese men with (n = 7) and without (n = 8) type 2 diabetes. We also examined whether chronic exercise influences the relationship between caffeine and insulin sensitivity in these individuals.

RESEARCH DESIGN AND METHODS

Subjects underwent two hyperinsulinemic-euglycemic clamp procedures, caffeine (5 mg/kg body wt) and placebo, in a double-blind, randomized manner before and after a 3-month aerobic exercise program. Body composition was measured by magnetic resonance imaging.

RESULTS

At baseline, caffeine ingestion was associated with a significant reduction (P < 0.05) in insulin sensitivity by a similar magnitude in the lean (33%), obese (33%), and type 2 diabetic (37%) groups in comparison with placebo. After exercise training, caffeine ingestion was still associated with a reduction (P < 0.05) in insulin sensitivity by a similar magnitude in the lean (23%), obese (26%), and type 2 diabetic (36%) groups in comparison with placebo. Exercise was not associated with a significant increase in insulin sensitivity in either the caffeine or placebo trials, independent of group (P > 0.10).

CONCLUSIONS

Caffeine consumption is associated with a substantial reduction in insulin-mediated glucose uptake independent of obesity, type 2 diabetes, and chronic exercise.