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King's College London, School of Biomedical and Health Sciences, Nutrition Health and Food Research Centre, Biopolymers Group, London (J.A.B., P.R.E.)
University of Central Lancashire, Lancashire School of Health and Postgraduate Medicine, Preston (P.A.J.), UNITED KINGDOM
Address reprint requests to: Dr Peter R. Ellis, King's College London, School of Biomedical and Health Sciences, Nutritional Sciences Division, Biopolymers Group, Franklin-Wilkins Building, 150 Stamford Street, London SE1 9NN, UNITED KINGDOM. E-mail: p.ellis{at}kcl.ac.uk
Objective: To determine the effects of black tea on postprandial plasma glucose and insulin concentrations in healthy humans in response to an oral glucose load.
Methods: A four-way randomised, crossover trial was designed in which 16 healthy fasted subjects would consume 75g of glucose in either 250ml of water (control), 250ml of water plus 0.052g of caffeine (positive control) or 250 ml of water plus 1.0g or 3.0g of instant black tea. Blood samples were collected at fasting and at 30min intervals for 150min from commencement of drink ingestion. Glucose and insulin concentrations were measured using standard methodology. The tea was chemically characterised using colorimetric and HPLC methods.
Results: Chemical analysis showed that the tea was rich in polyphenolic compounds (total, 350mg/g). Results from only 3 treatment arms are reported because the 3.0g tea drink caused gastrointestinal symptoms. Plasma glucose concentrations <60min in response to the drinks were similar, but were significantly reduced at 120min (P<0.01), following ingestion of the 1.0g tea drink, relative to the control and caffeine drinks. Tea consumption resulted in elevated insulin concentrations compared with the control and caffeine drinks at 90min (P<0.01) and compared with caffeine drink alone at 150min (P<0.01).
Conclusions: The 1.0g tea drink reduced the late phase plasma glucose response in healthy humans with a corresponding increase in insulin. This may indicate that the attenuation in postprandial glycemia was achieved as a result of an elevated insulin response following stimulation of pancreatic ß-cells. This effect may be attributable to the presence of phenolic compounds in the tea
Key words: tea, polyphenols, postprandial glycemia, plasma insulin concentrations
Abbreviations: AUC = area under the curve • CGA = chlorogenic acids • EC = epicatechin • ECG = epicatechin gallate • EDTA = ethylendiaminetetraacetic acid • EGC = epigallocatechin • EGCG = epigalocatechin gallate • EIA = enteroinsular axis • GIP = glucose-dependent insulinotropic-polypeptide • GLP-1 = glucagon-like peptide-1 • SGLT-1 = sodium glucose co-transporter-1
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