In vitro α-glucosidase inhibition by honeybush ( Cyclopia genistoides ) food ingredient extract—potential for dose reduction of acarbose through synergism

Extracts of Cyclopia species are used as food ingredients. In vitro α-glucosidase (AG) inhibition by ultrafiltered C. genistoides extract, fractions enriched in xanthones (XEF) and benzophenones (BEF), as well as mangiferin, isomangiferin, 3-β- d -glucopyranosyliriflophenone (I3G) and 3-β- d -glucopyranosyl-4- O -β- d -glucopyranosyliriflophenone (IDG) was determined with acarbose as positive control. XEF was more potent than the extract and BEF (IC 50 = 43.3, 95.5 and 205.7 μg mL −1 , respectively). Compounds demonstrated potency in the descending order: acarbose (IC 50 = 44.3 μM) > mangiferin (102.2 μM) > isomangiferin (119.8 μM) > I3G (237.5 μM) > IDG (299.4 μM). The combination index (CI) was used to determine synergism (CI < 0.7) as demonstrated for combinations of acarbose with XEF, BEF or the respective compounds at 50% and 75% effect levels. The greatest potential acarbose dose reductions (>six-fold) across all effect levels were calculated for combinations of acarbose with mangiferin or isomangiferin, explaining the greater acarbose dose reduction potential of XEF vs. BEF. The effect of batch-to-batch variation ( n = 10) of raw plant material on AG inhibition was quantified at a fixed concentration (160 μg mL −1 ). XEFs (xanthone content = 223–481 g kg −1 ) achieved AG inhibition of 63–72%, whereas BEFs (benzophenone content = 114–251 g kg −1 ) achieved AG inhibition of 26–34%, with weak linear correlation ( R 2 < 0.43) between target compound content of the fractions and their achieved AG inhibition. Thus, extract fractions of C. genistoides , enriched in xanthones and benzophenones, show potential in reducing the effective dose of acarbose required to prevent postprandial hyperglycaemia.