Disclosing caffeine action on insulin sensitivity: effects on rat skeletal muscle

Caffeine and sport

Caffeine is a trimethylxanthine found in coffee and several other foods and beverages. Its stimulatory effects make it an interesting strategy to boost performance for athletic populations. Scientific evidence supports its efficacy to improve high-intensity endurance exercise, explosive and high-intensity efforts, resistance exercise, team sports and combat sports, though individual variation in the ergogenic response to caffeine exists. Supplementation can be taken in many forms including dissolved in water, via capsules, coffee, energy drinks and caffeinated gum; ingestion via capsules, dissolved in water or in caffeinated gum appear to be most effective. Variability in the exercise response following caffeine supplementation may be explained by genetic factors or habitual caffeine consumption. Caffeine is an excellent supplement for athletes looking to improve their exercise performance, though some consideration of side-effects and impact on sleep are warranted.

Identifying Glucose Metabolism Status in Nondiabetic Japanese Adults Using Machine Learning Model with Simple Questionnaire

We aimed to identify the glucose metabolism statuses of nondiabetic Japanese adults using a machine learning model with a questionnaire. In this cross-sectional study, Japanese adults (aged 20–64 years) from Tokyo and surrounding areas were recruited. Participants underwent an oral glucose tolerance test (OGTT) and completed a questionnaire regarding lifestyle and physical characteristics. They were classified into four glycometabolic categories based on the OGTT results: category 1: best glucose metabolism, category 2: low insulin sensitivity, category 3: low insulin secretion, and category 4: combined characteristics of categories 2 and 3. A total of 977 individuals were included; the ratios of participants in categories 1, 2, 3, and 4 were 46%, 21%, 14%, and 19%, respectively. Machine learning models (decision tree, support vector machine, random forest, and XGBoost) were developed for identifying the glycometabolic category using questionnaire responses. Then, the top 10 most important variables in the random forest model were selected, and another random forest model was developed using these variables. Its areas under the receiver operating characteristic curve (AUCs) to classify category 1 and the others, category 2 and the others, category 3 and the others, and category 4 and the others were 0.68 (95% confidence intervals: 0.62–0.75), 0.66 (0.58–0.73), 0.61 (0.51–0.70), and 0.70 (0.62–0.77). For external validation of the model, the same dataset of 452 Japanese adults in Hokkaido was obtained. The AUCs to classify categories 1, 2, 3, and 4 and the others were 0.66 (0.61–0.71), 0.57 (0.51–0.62), 0.60 (0.50–0.69), and 0.64 (0.57–0.71). In conclusion, our model could identify the glucose metabolism status using only 10 factors of lifestyle and physical characteristics. This model may help the larger general population without diabetes to understand their glucose metabolism status and encourage lifestyle improvement to prevent diabetes.

Experimental Evidence of A2A-D2 Receptor-Receptor Interactions in the Rat and Human Carotid Body

Adenosine A_2A receptors (A_2AR) and dopamine D₂ receptors (D₂R) are known to be involved in the physiological response to hypoxia, and their expression/activity may be modulated by chronic sustained or intermittent hypoxia. To date, A_2AR and D₂R can form transient physical receptor–receptor interactions (RRIs) giving rise to a dynamic equilibrium able to influence ligand binding and signaling, as demonstrated in different native tissues and transfected mammalian cell systems. Given the presence of A_2AR and D₂R in type I cells, type II cells, and afferent nerve terminals of the carotid body (CB), the aim of this work was to demonstrate here, for the first time, the existence of A_2AR–D₂R heterodimers by in situ proximity ligation assay (PLA). Our data by PLA analysis and tyrosine hydroxylase/S100 colocalization indicated the formation of A_2AR–D₂R heterodimers in type I and II cells of the CB; the presence of A_2AR–D₂R heterodimers also in afferent terminals is also suggested by PLA signal distribution. RRIs could play a role in CB dynamic modifications and plasticity in response to development/aging and environmental stimuli, including chronic intermittent/sustained hypoxia. Exploring other RRIs will allow for a broad comprehension of the regulative mechanisms these interactions preside over, with also possible clinical implications.

Novel insights on caffeine supplementation, CYP1A2 genotype, physiological responses and exercise performance

Caffeine is a popular ergogenic aid due to its primary physiological effects that occur through antagonism of adenosine receptors in the central nervous system. This leads to a cascade of physiological reactions which increases focus and volition, and reduces perception of effort and pain, contributing to improved exercise performance. Substantial variability in the physiological and performance response to acute caffeine consumption is apparent, and a growing number of studies are implicating a single-nucleotide polymorphism in the CYP1A2 gene, responsible for caffeine metabolism, as a key factor that influences the acute responses to caffeine ingestion. However, existing literature regarding the influence of this polymorphism on the ergogenic effects of caffeine is controversial. Fast caffeine metabolisers (AA homozygotes) appear most likely to benefit from caffeine supplementation, although over half of studies showed no differences in the responses to caffeine between CYP1A2 genotypes, while others even showed either a possible advantage or disadvantage for C-allele carriers. Contrasting data are limited by weak study designs and small samples sizes, which did not allow separation of C-allele carriers into their sub-groups (AC and CC), and insufficient mechanistic evidence to elucidate findings. Mixed results prevent practical recommendations based upon genotype while genetic testing for CYP1A2 is also currently unwarranted. More mechanistic and applied research is required to elucidate how the CYP1A2 polymorphism might alter caffeine's ergogenic effect and the magnitude thereof, and whether CYP1A2 genotyping prior to caffeine supplementation is necessary.

Caffeine and Caffeine Metabolites in Relation to Insulin Resistance and Beta Cell Function in U.S. Adults

The relationship between caffeine and insulin resistance (IR) has been assessed only in terms of caffeine intake, and the association between caffeine and beta cell function (BCF) remains unclear. This study examines the association between urinary caffeine and its metabolites, IR, and BCF in nondiabetic, noninstitutionalized US adults in order to account for the inter-individual differences in caffeine metabolism. Data on urinary caffeine and its metabolites, IR and BCF from adults aged 20 years and older who participated in the 2009–2010 and 2011–2012 National Health and Nutrition Examination Surveys were analyzed (n for caffeine = 994). IR and BCF were assessed using homeostatic model assessment (HOMA) and urinary caffeine and its metabolites were measured using high-performance liquid chromatography-electrospray ionization-tandem quadrupole mass spectrometry. After adjusting for all covariates, increases in urinary 1,3-DMU, 1,7-DMU, 1,3,7-TMU, theophylline, paraxanthine, caffeine, and AAMU were significantly associated with increased HOMA-IR and HOMA-β (HOMA of insulin resistance and beta cell function). Compared with individuals in the lowest quartile of urinary 1,3-DMU, 1,7-DMU, 1,3,7-TMU, theophylline, paraxanthine, caffeine, and AAMU, the regression coefficients for HOMA-IR and HOMA-β were significantly higher among those in the highest quartile. After stratification by prediabetes status, HOMA-IR and HOMA-β showed significant positive associations with urinary caffeine and its metabolites among subjects with normal fasting plasma glucose levels. Our cross-sectional study showed that caffeine and its metabolites were positively related to IR and BCF.

Beneficial Impact of Semicarbazide-Sensitive Amine Oxidase Inhibition on the Potential Cytotoxicity of Creatine Supplementation in Type 2 Diabetes Mellitus

Creatine supplementation of the population with type 2 diabetes mellitus (T2DM) combined with an exercise program is known to be a possible therapy adjuvant with hypoglycemic effects. However, excessive administration of creatine leads to the production of methylamine which is deaminated by the enzyme semicarbazide-sensitive amine oxidase (SSAO) and as a result, cytotoxic compounds are produced. SSAO activity and reaction products are increased in the serum of T2DM patients. Creatine supplementation by diabetics will further augment the activity of SSAO. The current review aims to find a feasible way to ameliorate T2DM for patients who exercise and desire to consume creatine. Several natural agents present in food which are involved in the regulation of SSAO activity directly or indirectly are reviewed. Particularly, zinc-α2-glycoprotein (ZAG), zinc (Zn), copper (Cu), histamine/histidine, caffeine, iron (Fe), and vitamin D are discussed. Inhibiting SSAO activity by natural agents might reduce the potential adverse effects of creatine metabolism in population of T2DM.

A high-throughput chemical-genetics screen in murine adipocytes identifies insulin-regulatory pathways

Pathways linking activation of the insulin receptor to downstream targets of insulin have traditionally been studied using a candidate gene approach. To elucidate additional pathways regulating insulin activity, we performed a forward chemical-genetics screen based on translocation of a glucose transporter 4 (Glut4) reporter expressed in murine 3T3-L1 adipocytes. To identify compounds with known targets, we screened drug-repurposing and natural product libraries. We identified, confirmed, and validated 64 activators and 65 inhibitors that acutely increase or rapidly decrease cell-surface Glut4 in adipocytes stimulated with submaximal insulin concentrations. These agents were grouped by target, chemical class, and mechanism of action. All groups contained multiple hits from a single drug class, and several comprised multiple structurally unrelated hits for a single target. Targets include the β-adrenergic and adenosine receptors. Agonists of these receptors increased and inverse agonists/antagonists decreased cell-surface Glut4 independently of insulin. Additional activators include insulin sensitizers (thiazolidinediones), insulin mimetics, dis-inhibitors (the mTORC1 inhibitor rapamycin), cardiotonic steroids (the Na⁺/K⁺-ATPase inhibitor ouabain), and corticosteroids (dexamethasone). Inhibitors include heterocyclic amines (tricyclic antidepressants) and 21 natural product supplements and herbal extracts. Mechanisms of action include effects on Glut4 trafficking, signal transduction, inhibition of protein synthesis, and dissipation of proton gradients. Two pathways that acutely regulate Glut4 translocation were discovered: de novo protein synthesis and endocytic acidification. The mechanism of action of additional classes of activators (tanshinones, dalbergiones, and coumarins) and inhibitors (flavonoids and resveratrol) remains to be determined. These tools are among the most sensitive, responsive, and reproducible insulin-activity assays described to date.

Caffeine and cardiovascular diseases: critical review of current research

Caffeine is a most widely consumed physiological stimulant worldwide, which is consumed via natural sources, such as coffee and tea, and now marketed sources such as energy drinks and other dietary supplements. This wide use has led to concerns regarding the safety of caffeine and its proposed beneficial role in alertness, performance and energy expenditure and side effects in the cardiovascular system. The question remains "Which dose is safe?", as the population does not appear to adhere to the strict guidelines listed on caffeine consumption. Studies in humans and animal models yield controversial results, which can be explained by population, type and dose of caffeine and low statistical power. This review will focus on comprehensive and critical review of the current literature and provide an avenue for further study.

Consumption of caffeinated and artificially sweetened soft drinks is associated with risk of early menarche

BACKGROUND

Early menarche has been linked to risk of several chronic diseases. Prospective research on whether the intake of soft drinks containing caffeine, a modulator of the female reproductive axis, is associated with risk of early menarche is sparse.

OBJECTIVE

We examined the hypothesis that consumption of caffeinated soft drinks in childhood is associated with higher risk of early menarche.

DESIGN

The National Heart, Lung, and Blood Institute Growth and Health Study recruited and enrolled 2379 (1213 African American, 1166 Caucasian) girls aged 9-10 y (from Richmond, CA; Cincinnati, OH; and Washington, DC) and followed them for 10 y. After exclusions were made, there were 1988 girls in whom we examined prospective associations between consumption of caffeinated and noncaffeinated sugar- and artificially sweetened soft drinks and early menarche (defined as menarche age <11 y). We also examined associations between intakes of caffeine, sucrose, fructose, and aspartame and early menarche.

RESULTS

Incident early menarche occurred in 165 (8.3%) of the girls. After adjustment for confounders and premenarcheal percentage body fat, greater consumption of caffeinated soft drinks was associated with a higher risk of early menarche (RR for 1 serving/d increment: 1.47; 95% CI: 1.22, 1.79). Consumption of artificially sweetened soft drinks was also positively associated with risk of early menarche (RR for 1 serving/d increment: 1.43; 95% CI: 1.08, 1.88). Consumption of noncaffeinated soft drinks was not significantly associated with early menarche (RR for 1 serving/d increment: 0.88; 95% CI: 0.62, 1.25); nor was consumption of sugar-sweetened soft drinks (RR for 1 serving/d increment: 1.15; 95% CI: 0.95, 1.39). Consistent with the beverage findings, intakes of caffeine (RR for 1-SD increment: 1.22; 95% CI: 1.08, 1.37) and aspartame (RR for 1-SD increment: 1.20; 95% CI: 1.10, 1.31) were positively associated with risk of early menarche.

CONCLUSION

Consumption of caffeinated and artificially sweetened soft drinks was positively associated with risk of early menarche in a US cohort of African American and Caucasian girls.