Effects of coffee ingestion on oral glucose tolerance curves in normal human subjects

Higher intakes of dietary caffeine are associated with 25-hydroxyvitamin D deficiency

Low serum 25-hydroxyvitamin D [25(OH)D] levels remain a challenge worldwide. While some in vitro studies show a caffeine-induced decrease in vitamin D receptor expression, there is a paucity of research to define the extent of caffeine intake on 25(OH)D levels. Therefore, we aimed to associate dietary caffeine intake with 25(OH)D deficiency through a recognized dataset. Using data collected from the 2005-2006 National Health and Nutrition Examination Survey (NHANES), 25(OH)D levels and dietary caffeine intake were extracted from 13134 individuals (30-47 years, interquartile range). We used one-way ANOVA and chi-square tests for quantitative and qualitative variables, respectively, and performed multivariate logistic regression for four models to assess the odds ratio (OR) of 25(OH)D deficiency (<20 ng/ml or <50 nmol/L) based on quartiles of dietary caffeine intake. Both crude and multivariable models detected higher OR for 25(OH)D deficiency according to the highest intakes of caffeine (15.8±9.5, 51.9±11.9, and 177±156 mg/d) when compared to the reference category (2.19±1.04 mg/d), in which the OR in the highest category of caffeine intake was 1.24 (95% CI: 1.12 to 1.37) and 1.48 (95% CI: 1.16 to 1.78) for the crude model and the most complete multivariable analysis (adjustment for age, sex, race, body mass index, smoking, physical activity, occupation, energy intake, protein intake, and fat intake), respectively. In conclusion, higher dietary intakes of caffeine were associated with 25(OH)D deficiency in a representative sample of the American population, but further investigation is warranted to determine causation.

Potential contributions of the methodology to the variability of glycaemic index of foods

Glycaemic index (GI) testing provides a useful point of comparison between carbohydrate sources. For this comparison to be meaningful, the methods used to determine GI values need to be rigorous and consistent between testing events. This requirement has led to increasing standardization of the GI methodology, with an international standard developed in joint consultation with FAO/WHO (ISO 26642:2010) currently the most up to date document. The purpose of this review is to compare the international standard to methods of published studies claiming to have performed a GI test. This analysis revealed that the international standard permits a wide range of choices for researchers when designing a GI testing plan, rather than a single standardized protocol. It has also been revealed that the literature contains significant variation, both between studies and from the international standard for critical aspects of GI testing methodology. The primary areas of variation include; what glucose specification is used, which reference food is used, how much reference food is given, what drink is given during testing, the blood sampling site chosen and what assay and equipment is used to measure blood glucose concentration. For each of these aspects we have explored some of the methodological and physiological implications of these variations. These insights suggest that whilst the international standard has assisted with framing the general parameters of GI testing, further stan-dardization to testing procedures is still required to ensure the continued relevance of the GI to clinical nutrition.

Acute effects of light and dark roasted coffee on glucose tolerance: a randomized, controlled crossover trial in healthy volunteers

PURPOSE

Epidemiological evidence suggests that coffee consumption is associated with a lower risk of type 2 diabetes. Coffee contains caffeine and several other components that may modulate glucose regulation. The chlorogenic acids (CGA) in coffee have been indicated as constituents that may help to normalize the acute glucose response after a carbohydrate challenge. The aim of this study was to investigate whether two coffee beverages that differ in CGA content due to different roasting degrees will differentially affect glucose regulation.

METHODS

In a controlled crossover trial, 11 healthy fasted volunteers consumed 300 mL of either light (LIR) or dark (DAR) roasted coffee, or water, followed 30 min later by a 75-g oral glucose tolerance test (OGTT). Blood samples were drawn at baseline, 30, 60, and 120 min. Differences in glucose and insulin responses and insulin sensitivity index (ISI) were analyzed. The CGA and caffeine contents in the coffees were analyzed using UPLC-MS/MS.

RESULTS

No differences in glucose area under the curve (AUC) were found between treatments. Glucose concentrations were higher at 60 min after ingestion of DAR compared with water, while ingestion of LIR showed similar glucose concentrations as ingestion of water. Insulin AUC was higher after ingestion of DAR compared with water, and both coffees raised insulin concentrations and reduced ISI compared with water, with no difference between the two coffees.

CONCLUSION

Two coffees with different CGA contents did not differentially affect glucose or insulin responses during an OGTT, but both increased the insulin response compared with water.

Effect of chronic coffee consumption on weight gain and glycaemia in a mouse model of obesity and type 2 diabetes

Objective:

Epidemiological evidence shows that chronic coffee consumption in humans is correlated with a lower incidence of type 2 diabetes mellitus. For the experimental exploration of the underlying mechanisms, this effect needs to be replicated in an animal model of type 2 diabetes with a short lifespan.

Design:

Male C57BL/6 mice consumed regular coffee or water ad libitum and the development of obesity and diabetes caused by high-fat diet (55% lipids, HFD) was observed from week 10 on for 35 weeks in comparison with mice feeding on a defined normal diet (9% lipids, ND).

Results:

The massive weight gain in HFD mice was dose-dependently retarded (P=0.034), the moderate weight gain in ND mice was abolished (P<0.001) by coffee consumption, probably because of a lower feeding efficiency. The consumption of fluid (water or coffee) was significantly diminished by HFD (P<0.001), resulting in a higher coffee exposure of ND mice. On week 21 intraperitoneal glucose tolerance tests (IPGTT) showed a dose-dependent faster decline of elevated glucose levels in coffee-consuming HFD mice (P=0.016), but not in ND mice. Remarkably, a spontaneous decrease in non-fasting glycaemia occurred after week 21 in all treatment groups (P<0.001). On week 39 the IPGTT showed diminished peak of glucose levels in coffee-consuming HFD mice (P<0.05). HFD mice were hyperinsulinaemic and had significantly (P<0.001) enlarged islets. Coffee consumption did not affect islet size or parameters of beta-cell apoptosis, proliferation and insulin granule content.

Conclusion:

Coffee consumption retarded weight gain and improved glucose tolerance in a mouse model of type 2 diabetes and corresponding controls. This gives rise to the expectation that further insight into the mechanism of the diabetes-preventive effect of coffee consumption in humans may be gained by this approach.

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.

High oolong tea consumption predicts future risk of diabetes among Japanese male workers: a prospective cohort study

AIMS

Acute administration of oolong tea decreases blood glucose levels. We investigated the association between long-term oolong tea intake and subsequent risk of developing diabetes among men of working age.

METHODS

Data were analysed from a cohort of participants in the High-risk and Population Strategy for Occupational Health Promotion Study (HIPOP-OHP), conducted in Japan from 1999 to 2004. Oolong tea intake at baseline and subsequent risk of diabetes was evaluated using a Cox proportional hazards model.

RESULTS

Of 4975 male workers, a total of 201 cases of diabetes were reported over a median of 3.4 years of follow-up. Mean age and BMI of all participants at baseline were 38.3 years and 22.9 kg/m(2) , respectively. Compared with those not consuming oolong tea, multivariable adjusted hazard ratios for developing diabetes were 1.00 (95% CI 0.67-1.49) for those who drank one cup of oolong tea per day and 1.64 (95% CI 1.11-2.40) for those drinking two or more cups per day. Fasting blood glucose increment per year was 0.11 mmol/l (95% CI 0.09-0.12 mmol/l), 0.12 mmol/l (95% CI 0.09-0.15 mmol/l) and 0.15 mmol/l (95% CI 0.11-0.18 mmol/l), respectively, for oolong tea consumption of 0, 1 and ≥ 2 cups/day, with a significant linear trend (P < 0.0001).

CONCLUSIONS

Long-term consumption of oolong tea may be a predictive factor for new onset diabetes. Further studies are necessary to elucidate the role of oolong tea in the risk of developing diabetes.

Effect of coffee and tea on the glycaemic index of foods: no effect on mean but reduced variability

Coffee and tea may influence glycaemic responses but it is not clear whether they affect the glycaemic index (GI) value of foods. Therefore, to see if coffee and tea affected the mean and SEM of GI values, the GI of fruit leather (FL) and cheese puffs (CP) were determined twice in ten subjects using the FAO/WHO protocol with white bread as the reference food. In one series subjects chose to drink 250 ml of either coffee or tea with all test meals, while in the other series they drank 250 ml water. The tests for both series were conducted as a single experiment with the order of all tests being randomised. Coffee and tea increased the overall mean peak blood glucose increment compared with water by 0.25 (SEM 0.09) mmol/l (P=0.02), but did not significantly affect the incremental area under the glucose response curve. Mean GI values were not affected by coffee or tea but the SEM was reduced by about 30% (FL: 31 (SEM 4) v. 35 (SEM 7) and CP: 76 (SEM 6) v. 75 (SEM 8) for coffee or tea v. water, respectively). The error mean square term from the ANOVA of the GI values was significantly smaller for coffee or tea v. water (F(18, 18) = 2.31; P=0.04). We conclude that drinking coffee or tea with test meals does not affect the mean GI value obtained, but may reduce variability and, hence, improve precision.

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.

Nutraceutical resources for diabetes prevention--an update

There is considerable need for safe agents that can reduce risk for diabetes in at-risk subjects. Although certain drugs--including metformin, acarbose, and orlistat--have shown diabetes-preventive activity in large randomized studies, nutraceuticals have potential in this regard as well. Natural agents which slow carbohydrate absorption may mimic the protective effect of acarbose; these include: soluble fiber--most notably glucomannan; chlorogenic acid--likely responsible for reduction in diabetes risk associated with heavy coffee intake; and legume-derived alpha-amylase inhibitors. There does not appear to be a natural lipase inhibitor functionally equivalent to orlistat, although there are poorly documented claims for Cassia nomame extracts. Metformin's efficacy reflects activation of AMP-activated kinase; there is preliminary evidence that certain compounds in barley malt have similar activity, without the side effects associated with metformin. In supraphysiological concentrations, biotin directly activates soluble guanylate cyclase; this implies that, at some sufficient intake, biotin should exert effects on beta cells, the liver, and skeletal muscle that favor good glucose tolerance and maintenance of effective beta cell function. Good magnesium status is associated with reduced diabetes risk and superior insulin sensitivity in recent epidemiology; ample intakes of chromium picolinate appear to promote insulin sensitivity in many individuals and improve glycemic control in some diabetics; calcium/vitamin D may help preserve insulin sensitivity by preventing secondary hyperparathyroidism. Although conjugated linoleic acid--like thiazolidinediones, a PPAR-gamma agonist--has not aided insulin sensitivity in clinical trials, the natural rexinoid phytanic acid exerts thiazolidinedione-like effect in animals and cell cultures, and merits clinical examination. Other natural agents with the potential to treat and possibly prevent diabetes include extracts of bitter melon and of cinnamon. Nutraceuticals featuring meaningful doses of combinations of these agents would likely have substantial diabetes-preventive efficacy, and presumably could be marketed legally as aids to good glucose tolerance and insulin sensitivity.

Coffee and incidence of diabetes in Swedish women: a prospective 18-year follow-up study

OBJECTIVES

To examine the long-term incidence of diabetes in relation to coffee consumption in Swedish women.

DESIGN

Prospective longitudinal cohort study.

SETTING

City of Göteborg, Sweden.

SUBJECTS

A random population sample of 1361 women, aged 39-65 years, without prior diabetes or cardiovascular disease took part in a screening study in 1979-1981 with questionnaires, physical examination and blood sampling.

MAIN OUTCOME MEASURES

The development of diabetes until 1999 was identified by questionnaires in a second screening and the Swedish hospital discharge register.

RESULTS

Altogether, there were 74 new cases of diabetes. The risk of developing diabetes was 475 per 100 000 person-years in women who consumed two cups of coffee or less per day, 271 in women who consumed three to four cups per day, 202 with a consumption of five to six cups per day, and 267 in drinkers of seven cups or more per day. Associated hazard ratios, after adjustment for age, smoking, low physical activity, education and body mass index were 0.55 (0.32-0.95), 0.39 (0.20-0.77) and 0.48 (0.22-1.06) for daily consumption of three to four, five to six and seven cups or more, respectively, with a consumption of less than two per day as reference. Additional adjustment for serum cholesterol and triglycerides attenuated the relation between coffee and diabetes slightly, indicating a possible mediating effect on the effect of coffee by serum lipids.

CONCLUSIONS

The findings of the present study support the hypothesis that coffee consumption protects from the development of diabetes in women.

Caffeine ingestion decreases glucose disposal during a hyperinsulinemic-euglycemic clamp in sedentary humans

The purpose of this investigation was to examine the effect of caffeine (an adenosine receptor antagonist) on whole-body insulin-mediated glucose disposal in resting humans. We hypothesized that glucose disposal would be lower after the administration of caffeine compared with placebo. Healthy, lean, sedentary (n = 9) men underwent two trial sessions, one after caffeine administration (5 mg/kg body wt) and one after placebo administration (dextrose) in a double-blind randomized design. Glucose disposal was assessed using a hyperinsulinemic-euglycemic clamp. Before the clamp, there were no differences in circulating levels of methylxanthines, catecholamines, or glucose. Euglycemia was maintained throughout the clamp with no difference in plasma glucose concentrations between trials. The insulin concentrations were also similar in the caffeine and placebo trials. After caffeine administration, glucose disposal was 6.38 +/- 0.76 mg/kg body wt compared with 8.42 +/- 0.63 mg/kg body wt after the placebo trial. This represents a significant (P < 0.05) decrease (24%) in glucose disposal after caffeine ingestion. In addition, carbohydrate storage was 35% lower (P < 0.05) in the caffeine trial than in the placebo trial. Furthermore, even when the difference in glucose disposal was normalized between the trials, there was a 23% difference in the amount of carbohydrate stored after caffeine administration compared with placebo administration. Caffeine ingestion also resulted in higher plasma epinephrine levels than placebo ingestion (P < 0.05). These data support our hypothesis that caffeine ingestion decreases glucose disposal and suggests that adenosine plays a role in regulating glucose disposal in resting humans.

Synthesis and availability of niacin in roasted coffee

The coffee bean contains about 1% of trigonelline that is demethylated at temperatures approaching 200 degrees C; it is partially converted into nicotinic acid. This operation is mainly proportional to the severity of dry heat treatment; various other physico-chemical factors also influence the synthesis of niacin during the roasting. The niacin content of weakly roasted commercial coffee is about 10 mg/100 g (American coffee) and it reaches 40 mg in heavy roasted coffees, i.e. Italian coffee. Caffeine-free coffee is lower in niacin than the corresponding raw coffee. The drinking retains 85% of the niacin formed during roasting; it is totally available for the organism and can constitute a noticeable part of the daily supply in niacin.

Stimulation of glycogenolysis in isolated hepatocytes by adenosine and one of its analogues is inhibited by caffeine

The adenosine analogues 5'-(N-ethyl)carboxamidoadenosine (NECA) and N6-(phenylisopropyl)adenosine (PIA) activate glycogen phosphorylase 5-fold and 4.2-fold respectively in rat hepatocytes incubated in the absence of endogenous adenosine. Half-maximally effective concentrations are 0.5 microM for NECA and 20 microM for PIA, demonstrating the presence of A2-adenosine receptors. Exogenous adenosine activates phosphorylase 4.6-fold, but high rates of adenosine disappearance from the medium render estimates of its half-maximally effective concentration unreliable. These effects of NECA and adenosine are inhibited by 0.1 mM-caffeine. Activation of phosphorylase by a physiological concentration of adenosine (3.3 microM) was 50% inhibited by a physiological concentration of caffeine (35 microM).

[Effect of caffeine on various metabolic parameters in vivo]

The metabolic actions of caffeine were investigated in the rat (90 mg/kg bw caffeine intravenously during 3 hours) and in human volunteers (35 mg/kg bw caffeine orally) in the fasting state. Additionally, the effects of caffeine were measured during simultaneous intravenous glucose infusion (0.25 mg/kg bw/h during 6 hours in humans and 1.8 mg/kg bw/h during 3 hours in the rat). In the fasting rat, intravenous caffeine caused an increase in the serum concentrations of glucose, urea, insulin, and free fatty acids, whereas a decrease in glucoplastic amino acids was found. As the liver glycogen concentration was not altered, the increase in blood glucose should be due to an increase in glyconeogenesis. During simultaneous application of carbohydrates and caffeine, the increases in the concentration of blood glucose and serum insulin were intensified, whereas the serum concentrations of lactate and urea as well as hepatic glycogen were not altered. In fasting male volunteers caffeine caused an increase in the concentrations of blood glucose, cortisol, insulin, free fatty acids, free glycerol and ketone bodies. During intravenous glucose infusion, caffeine intensified the decrease in serum phosphate induced by carbohydrates. Neither in volunteers nor in the experimental animal, an alteration in the concentrations of cholesterol or serum triglycerides or serum uric acid was effected by caffeine. It is concluded that high dosed caffeine causes peripheral insulin resistance in the human being as well as in the experimental animal. This peripheral insulin resistance is shown by the simultaneous large increases in concentrations of serum insulin, blood glucose and concentration of free fatty acids. In this situation insulin obviously is not able to inhibit lipolysis or gluconeogenesis nor to increase peripheral glucose utilisation. These metabolic effects of caffeine show some similarities to the metabolic situation in diabetes mellitus type 2 (Non Insulin Dependent Diabetes mellitus).

Effects of chronic ethanol ingestion on male and female rat liver glycogen phosphorylase phosphatase

The effects of chronic ethanol ingestion on the interconversion of the active to the inactive form of glycogen phosphorylase by phosphorylase phosphatase was studied. Male and female rats also were compared. Chronic ethanol feeding decreased phosphorylase a and total phosphorylase activity in male rats. In females, no change was observed in phosphorylase a, whereas total phosphorylase activity was increased 73%. This was found to correlate with the relative activities of phosphorylase phosphatase. The data show differences between the two sexes with regard to the AMP inhibition of phosphorylase phosphatase and the caffeine stimulation of the phosphatase. Ethanol markedly enhanced the AMP inhibition of the phosphatase in males but had no effect in females. Further studies in females showed that ethanol completely obliterated the well documented stimulation of the phosphatase by caffeine; however, it did not alter the caffeine effect in males. These data suggest possible alterations in the tertiary structure of phosphorylase a.

[The effect of caffeine on the serum insulin level during intravenous glucose tolerance test in patients with chemical diabetes]

Nine mild male diabetic patients were studied by intravenous glucose tolerance tests with and without pretreatment with caffeine. The dose of caffeine was 90 mg given as 195 mg of caffeine citrate. The parameters measured were blood glucose and serum insulin. Caffeine given 2 hours previously did not affect the mean glucose disappearance rate of the group, however, those with the highest rates did show a reduced rate after caffeine. Insulin levels showed a decrease particularly during the first five minutes after glucose. This finding was present particularly in those patients who exhibited the larger first peak. It is possible that a relationship exists between the magnitude of the caffeine effect upon early insulin secretion and the glucose disappearance rate.