LAUSR

Abstract Seaweeds are gaining importance due to their antidiabetic characteristics. This study investigated the inhibitory effects of aqueous Ascophyllum nodosum extracts, obtained by ultrasound‐assisted extraction with different sonication powers (70–90 W/cm2) and subjected to resin purification, against α‐amylase and α‐glucosidase enzymes. Different inhibition methodologies were carried out, preincubating the extract either with the enzyme or the substrate. Chemical characterization, in terms of proximate analysis, antioxidant capacity (2,2‐diphenyl‐1‐picryl‐hydrazyl‐hydrate [DPPH] and FRAP), and polyphenols characteristics (reversed‐phase high‐performance liquid chromatography [RP‐HPLC] and 1H‐NMR) were carried out to explain inhibitory activities of extracts. Sonication power did not influence the proximal composition nor antiradical activity of extracts, but increasing sonication power increased inhibition capacity (>15%) against both starch digestive enzymes. The extract purification largely improved the inhibition efficiency decreasing the IC50 of α‐amylase and α‐glucosidase by 3.0 and 6.1 times, respectively. Seaweed extracts showed greater inhibition effect when they were preincubated with the enzyme instead of the substrate. RP‐HPLC together with 1H‐NMR spectra allowed relating the presence of uronic acids–polyphenols complexes and quinones in the extracts with the different inhibitory capacities of samples. Practical Application The study confirms that ultrasound‐assisted extracts from Ascophyllum nodosum can be used to inhibit digestive enzymes. This opens the alternative to be used in foods for modulating glycemic index.