[Effect of caffeine on various metabolic parameters in vivo]

Caffeine can decrease insulin sensitivity in humans

OBJECTIVE

Caffeine is a central stimulant that increases the release of catecholamines. As a component of popular beverages, caffeine is widely used around the world. Its pharmacological effects are predominantly due to adenosine receptor antagonism and include release of catecholamines. We hypothesized that caffeine reduces insulin sensitivity, either due to catecholamines and/or as a result of blocking adenosine-mediated stimulation of peripheral glucose uptake.

RESEARCH DESIGN AND METHODS

Hyperinsulinemic-euglycemic glucose clamps were used to assess insulin sensitivity. Caffeine or placebo was administered intravenously to 12 healthy volunteers in a randomized, double-blind, crossover design. Measurements included plasma levels of insulin, catecholamines, free fatty acids (FFAs), and hemodynamic parameters. Insulin sensitivity was calculated as whole-body glucose uptake corrected for the insulin concentration. In a second study, the adenosine reuptake inhibitor dipyridamole was tested using an identical protocol in 10 healthy subjects.

RESULTS

Caffeine decreased insulin sensitivity by 15% (P < 0.05 vs. placebo). After caffeine administration, plasma FFAs increased (P < 0.05) and remained higher than during placebo. Plasma epinephrine increased fivefold (P < 0.0005), and smaller increases were recorded in plasma norepinephrine (P < 0.02) and blood pressure (P < 0.001). Dipyridamole did not alter insulin sensitivity and only increased plasma norepinephrine (P < 0.01).

CONCLUSIONS

Caffeine can decrease insulin sensitivity in healthy humans, possibly as a result of elevated plasma epinephrine levels. Because dipyridamole did not affect glucose uptake, peripheral adenosine receptor antagonism does not appear to contribute to this effect.

Physiologic hyperinsulinemia enhances human skeletal muscle perfusion by capillary recruitment

Despite intensive study, the relation between insulin's action on blood flow and glucose metabolism remains unclear. Insulin-induced changes in microvascular perfusion, independent from effects on total blood flow, could be an important variable contributing to insulin's metabolic action. We hypothesized that modest, physiologic increments in plasma insulin concentration alter microvascular perfusion in human skeletal muscle and that these changes can be assessed using contrast-enhanced ultrasound (CEU), a validated method for quantifying flow by measurement of microvascular blood volume (MBV) and microvascular flow velocity (MFV). In the first protocol, 10 healthy, fasting adults received insulin (0.05 mU. kg(-1). min(-1)) via a brachial artery for 4 h under euglycemic conditions. At baseline and after insulin infusion, MBV and MFV were measured by CEU during continuous intravenous infusion of albumin microbubbles with intermittent harmonic ultrasound imaging of the forearm deep flexor muscles. In the second protocol, 17 healthy, fasting adults received a 4-h infusion of either insulin (0.1 mU. kg(-1). min(-1), n = 9) or saline (n = 8) via a brachial artery. Microvascular volume was assessed in these subjects by an alternate CEU technique using an intra-arterial bolus injection of albumin microbubbles at baseline and after the 4-h infusion. With both protocols, muscle glucose uptake, plasma insulin concentration, and total blood flow to the forearm were measured at each stage. In protocol 2 subjects, tissue extraction of 1-methylxanthine (1-MX) was measured as an index of perfused capillary volume. Caffeine, which produces 1-MX as a metabolite, was administered to these subjects before the study to raise plasma 1-MX levels. In protocol 1 subjects, insulin increased muscle glucose uptake (180%, P < 0.05) and MBV (54%, P < 0.01) and decreased MFV (-42%, P = 0.07) in the absence of significant changes in total forearm blood flow. In protocol 2 subjects, insulin increased glucose uptake (220%, P < 0.01) and microvascular volume (45%, P < 0.05) with an associated moderate increase in total forearm blood flow (P < 0.05). Using forearm 1-MX extraction, we observed a trend, though not significant, toward increasing capillary volume in the insulin-treated subjects. In conclusion, modest physiologic increments in plasma insulin concentration increased microvascular blood volume, indicating altered microvascular perfusion consistent with a mechanism of capillary recruitment. The increases in microvascular (capillary) volume (despite unchanged total blood flow) indicate that the relation between insulin's vascular and metabolic actions cannot be fully understood using measurements of bulk blood flow alone.

New aspects on the chemistry of tea and coffee. III

The previous review for 1983/1984 is now continued with references to recent publications (1984/1985) and as before covers the chemistry and technology of tea (leaf, green and black) and coffee (raw, roasted and instant soluble), with particular reference to composition and changes during manufacture, extraction and storage. The effects of composition on the quality of the beverage, with mention of particular physiological properties, are included (210 references).

[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).