Coffee is of interest to nutritional scientists because it has been shown to contain a number of chemicals with biological action in humans. The most widely known of these is caffeine, which has been shown to activate the sympathetic nervous system. Caffeine is one of the main reasons for the consumption because of its associated effects on alertness and motivation. Coffee also contains hydroxycinnamic acids, which are an important group of phenols derived from plants. The major hydroxycinnamic acid found in coffee is called chlorogenic acid (5‑caffeoylquinic acid; CGA), which is chemically caffeic acid (a hydroxycinnamic acid) as an ester bound to quinic acid. Research shows that CGA may have cardioprotective effects because it is a know in vitro antioxidant. In addition to this, evidence is starting to accumulate that CGA and other phenols are able to alter intestinal glucose uptake.
To assess the effects of CGA in coffee on the plasma concentrations of glucose, insulin, glucose-dependent insulinotrophic polypeptide (GIP) and glucagons-like peptid-1 (GLP-1), researchers1 fed 9 healthy subjects 25 g of glucose with either water, decaffeinated coffee, or normal coffee. The amount of coffee was carefully calculated to contain 2.5 mmol/L chlorogenic acid). Blood samples taken over the subsequent 3h showed that coffee increased both glucose and insulin levels for the first 30 min after consumption when compared to decaffeinated coffee or water. These results are consistent with the effects of caffeine on glucose metabolism. Caffeine increases glycogenolysis via its inhibitory action on phosphodiesterase, the enzyme responsible for the catabolism of cyclic adenosine monophosphate. Cyclic AMP is known to increase glycogenolysis. Caffeine is also an adenosine receptor antagonsist and may prevent the uptake of glucose to muscle cells, even in the presence of insulin.
By comparing the caffeinated and decaffeinated coffee with the control (water), the researchers could assess the possible effects caused by the phenols in the coffee. The gastrointestinal hormone profiles of GIP and GLP-1 were both consistent with delayed intestinal glucose absorption, which cannot have been due to the caffeine. The researcher demonstrated that coffee consumption results in a decrease in GIP and an increase in GLP-1 when compared with water. The researchers concluded that the dietary phenols in coffee may be responsible for attenuation of the intestinal glucose absorption rates by shifting the site of glucose absorption to more distal parts of the intestine. This supported previous work published on the effects of apple juice (also high in CQG) on glucose absorption. The effects may be due to the effect of phenols on the Na+ electrochemical gradient necessary for glucose absorption.
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