Profiling the activity of edible European macroalgae towards pharmacological targets for type 2 diabetes mellitus

In traditional medicine marine extracts are extensively used as therapies for diabetes. With the increasing rate of incidence of type 2 diabetes mellitus and rising cost of treatments, we investigated the anti-diabetic properties of extracts of common edible seaweeds in Europe including their ability to inhibit alpha-glucosidase and dipeptidyl peptidase 4 (DPP-4) enzymatic activity, block sodium glucose transporter-2 (SGLT-2) activity and stimulate glucagon-like peptide-1 (GLP-1) secretion and synthesis. The most promising seaweed extracts were tested for anti-hyperglycaemic activity in vivo. Some brown seaweed (Phaeophyceae) extracts had inhibitory effects on alpha-glucosidase ranging from 13.9 ± 0.2% to 89.5 ± 0.4% (p < 0.001). However, none of the seaweed extracts was able to block SGLT-2 activity. Ethanol extracts of the kelp Alaria esculenta and water extracts of Laminaria digitata strongly inhibited DPP-4 activity by 91.3 ± 0.1% and 90.0 ± 0.2%, respectively (p < 0.001), while ethanol extracts of Ulva rigida (Chlorophyta) had the greatest potential to stimulate GLP-1 secretion and GLP-1 synthesis (p < 0.001). A water extract of Porphyra linearis (Rhodophyta) significantly reduced the overall glycaemic excursion during an oral glucose tolerance test in normal mice (p < 0.05). These results demonstrate future potential for common edible seaweeds to be used as medicinal foods or bio-therapeutics to tackle type 2 diabetes mellitus by targeting GLP-1 secretion, DPP-4 activity or alpha-glucosidase activity.