Preliminary findings from a project to explore the efficacy and mechanisms for blackcurrants to reduce risk of type 2 diabetes
Authors: Lawrence A. Jones, Daniel J. Owens, Juliette A. Strauss and Sam O. Shepherd
Elevated postprandial glucose is a primary risk factors for developing insulin resistance in individuals with overweight or obesity. Both acute ingestion and short-term supplementation of anthocyanins improves postprandial glucose responses to a high-carbohydrate meal. However, the optimal anthocyanin dose is not yet known. In vitro studies suggest that the glucose-lowering effects of anthocyanins may act by enhancing the expression of proteins associated with glucose uptake and mimicking the action of insulin. The precise mechanism by which anthocyanins exert their beneficial effect on glucose metabolism in skeletal muscle remains to be determined.
Purpose. This project has two aims: 1) to examine the dose-response effects of short-term supplementation with anthocyanin-rich New Zealand blackcurrant (NZBC) extract on postprandial glucose responses to a high carbohydrate challenge, 2) and to investigate the potential mechanism by which anthocyanins to improve skeletal muscle glucose uptake.
Methods. In a randomised, double-blind, counterbalanced Latin-square design, five overweight (BMI > 25 kg m2) sedentary individuals completed four separate oral glucose tolerance tests after ingesting no dose (PLA), or one of three doses (300, 600, or 900 mg day−1) of NZBC extract (CurraNZTM) for 7 days. Parallel experimentation using murine C2C12 myotubes enabled insights into the biochemical mechanisms by which anthocyanins may improve glucose uptake. Cells were either untreated (CON) or treated with NZBC, insulin (INS) or NZBC + INS and lysed for the isolation of RNA and subsequent determination of mRNA transcripts by RT-qPCR.
Results. The data demonstrate a pattern whereby PLA > 300 > 600 > 900 mg for the area-under-the-curve for plasma glucose (PLA: 678 257, 300mg: 623 203, 600mg: 583 169, 900mg: 527 128 mmol L−1120 min−1) and were 8.1%, 14.0% and 22.3% lower than the placebo condition, respectively. At this stage the data is too underpowered to perform any statistical analysis. In vitro, the presence of the NZBC extract enhanced the mRNA expression of GLUT4 (10-fold), Hexokinase 2 (6-fold), and Myocyte Enhancer Factor (2-fold), thereby suggesting an adaptive remodelling of the apparatus within skeletal muscle responsible for glucose uptake.
Conclusion. Together, these preliminary data indicate that repeated intake of NZBC extract increases glucose tolerance. Our findings suggest a role for anthocyanins in skeletal muscle adaptative remodelling, although others have proposed that anthocyanins interact with key targets involved in insulin signalling pathways. Further, in-vitro studies are warranted to delineate the precise mode of action by which anthocyanins improve glucose uptake.
