In vitro antioxidant activity and antidiabetic potential of five spiral ginger (Costus speciosus (J. Koenig) Sm.) populations available in Sri Lanka

Diabetes mellitus has caused millions of deaths worldwide. Costus speciosus leaves are consumed as a green leafy vegetable in Sri Lanka due to their claimed medicinal properties including hypoglycemic effect. This work aimed mainly to evaluate the antioxidant and antidiabetic potential of the five distinct C. speciosus leaves populations (P1, P2, P3, P4, and P5) available in Sri Lanka for future therapeutic food applications. Methanol extracts and hot water infusions of C. speciosus leaves were obtained and the total phenolic content, flavonoid content, total antioxidant capacity, and DPPH radical scavenging activity were determined by colorimetric assays. Moreover, in vitro antidiabetic activity was assessed using alpha-glucosidase and alpha-amylase enzyme inhibition assays. There was a remarkable variation in the functional properties among the five populations of C. speciosus as well as among the five different leaf positions from the apical to the basal part of the twig. The highest total phenolic content, DPPH radical scavenging activity, and total antioxidant capacity were observed in methanol leaf extracts of P3 as 7.49 ± 0.21 mg GAE g-1, 91.57 ± 0.08%, and 10.39 ± 0.50 mg AAE g-1 respectively. Leaf bud contained significantly (p < 0.05) the highest total phenolic content and DPPH radical scavenging activity than the other leaf positions. C. speciosus population 1 exhibited significantly (p < 0.05) the highest antidiabetic activity in terms of alpha-glucosidase enzyme inhibition, with an IC50 value of 2.57 mg mL−1 compared to the standard acarbose (12.85 mg mL−1). In the conclusion of the study, C. specious variety available in Sri Lanka can be considered as a potential candidate for therapeutic food applications.