Extracts and flavonoids from onion inhibit the intestinal sodium-coupled glucose transporter 1 (SGLT1) in vitro but show no anti-hyperglycaemic effects in vivo in normoglycaemic mice and human volunteers

Extracts and flavonoids from onion are described as having anti-diabetic activities. We here demonstrate that an onion extract and individual flavonoids thereof diminish glucose uptake mediated by the intestinal glucose transporter SGLT1 when expressed in oocytes and studied in mouse intestinal segments in vitro. Strongest inhibition of SGLT1 was observed for quercetin-4'-O-glucoside (Q4'glc) in oocytes but with only moderate inhibition in jejunal segments of mice. An oral glucose tolerance test (OGTT) performed in obese/hyperglycaemic mice revealed that the onion extract to reduce blood glucose increases significantly. However, an OGTT performed in healthy volunteers after administration of the onion extract failed to reveal an effect on glucose and insulin levels. Despite its capability to inhibit intestinal glucose uptake via SGLT1 in vitro and in mice in vivo, the onion extract did not alter blood glucose levels during an OGTT in human volunteers and this may predominantly be due to a dosing effect.