We investigated the effect of an aqueous extract of onion skin on pancreatic lipase activity in vitro. The aqueous extract from onion skin significantly inhibited the pancreatic lipase activity in vitro. Based on this result, we examined the effect of the aqueous extract from onion skin on the blood triacylglycerol level after the oral administration of a lipid emulsion to rats. The aqueous extract from onion skin at a dose of 125, 500mg/kg inhibited the elevation of blood triacylglycerol level compared with the control. In a longer-term experiment (25 days), we examined the effect of the aqueous extract from onion skin on the fat storage induced in mice by feeding a high-fat diet for 25 days. Intake of the aqueous extract from onion skin (125mg/ml) reduced the blood triacylglycerol level compared with the control. To identify the active substance in the aqueous extract from onion skin, we examined the effects of protocatechuic acid and quercetin on the pancreatic lipase activity in vitro. Protocatechuic acid inhibited the pancreatic lipase activity in vitro, but quercetin did not. Based on these results, we examined the effect of protocatechuic acid on the blood triacylglycerol elevation in rats orally administered with an oral lipid emulsion. Protocatechuic acid at a dose of 25, 250mg/kg inhibited the elevation of blood triacylglycerol compared with the control. These findings suggest that the intake of an aqueous extract of onion skin would be helpful to prevent postprandial triacylglycerol elevation.
