GLYCEMIC INDICES OF SOME NIGERIAN FRUIT JUICES AND THEIR INHIBITION OF α-AMYLASE AND α-GLUCOSIDASE ACTIVITES

Abstract

This study sought to investigate the starch composition, antioxidant potential, phenolic phytoconstituents, glycemic indices, α-amylase and α-glucosidase inhibitory activities of some Nigerian fruits. Thirteen fresh fruits were sourced for and the fruits juices were extracted, freeze dried and then reconstituted for analysis. The sugar, starch, amylose and amylopectin contents as well as glycemic indices, antioxidant properties and the ability of the fruits to inhibit starch hydrolyzing enzymes were determined. Also, the phenolic constituents of the fruits were characterized using HPLC-DAD. The starch, sugar, amylose and amylopectin contents ranged between 3.01-3.89g/100g, 35.34-60.91g/100g, 0.84-1.46g/100g and 1.68-2.86g/100g respectively while the glycemic indices range from 28.01-68.34 with African star apple (28.01) having the least glycemic index while Watermelon (68.34) had the highest. Furthermore, the fruits exhibited high antioxidant properties as exemplified by their DPPH, ABTS + , • OH and NO radical scavenging abilities. Likewise, the HPLC analysis revealed the presence of quercetin, gallic acid, catechin, epicatechin, rutin and chlorogenic acid in most of the fruits with resveratrol found only in breadfruit, isoquercitrin in carrot, orientin in guava, cyanidin-3-O glycoside and cyanidin in African star apple. The fruits also demonstrated α- amylase and α-glucosidase inhibitory property with Soursop (IC 50 =18.52μg/mL), guava (IC 50 =19.77μg/mL) and african star apple (IC 50 =20.86μg/mL) showing the highest inhibitory potential among the thirteen fruits. Similarly, the same trend was followed for α-glucosidase inhibitory activity. The fruits’ low glycemic indices, antioxidant properties and inhibition of α-amylase and α-glucosidase activities could be possible mechanisms for their use in the management and prevention of type-2 diabetes and as well justify their recommendation for the diabetics.