Consuming an onion solution inhibits glucose uptake more in lactose-intolerant people than in lactose-tolerant people

Introduction: Quercetin glucosides that are present in onions have been shown to reduce glucose uptake in cell culture models. Studies suggest that this inhibition is lost when the quercetin glucosides are hydrolysed by β-glycosidases to free quercetin. Lactase is a β-galactosidase expressed by people who are lactose tolerant. Hence, lactose tolerant people who express lactase may metabolise quercetin glucosides differently to people who do not express lactase (lactose intolerant people), and this could potentially influence glucose uptake. Objectives: To investigate if there are differences in glucose uptake between lactose tolerant people and lactose intolerant people when they consume an onion solution. Method / Design: Effects of quercetin glucosides on glucose uptake were studied in Caco-2 human colon cell culture models using 3H-glucose under Na-dependent and Na-independent conditions. For the clinical study, lactose tolerance was identified by the hydrogen breath test, and blood glucose levels were measured using an EKF glucose analyser. Results: Quercetin 4-glucoside (100 μM) (a major glucoside in onions) and onion extract (25% w/v) decreased uptake of glucose in Caco-2 cells by 48.2% (p<0.01) and 62.3% (p<0.01) respectively under Na-dependent conditions. However, neither quercetin 4-glucoside nor onion extract caused a statistically significant reduction of glucose uptake after pre-incubation with β-galactosidase. In the clinical trial, there was a statistically significant reduction in blood glucose levels after 30 min (p = 0.0005) and 60 min (p = 0.006) when glucose was consumed together with an onion solution by lactose intolerant people (n=9), compared to when only glucose was consumed. There was a smaller and statistically non-significant decrease in blood glucose levels after consumption of onion solution in lactose tolerant people (n=9). Conclusions: Glucose uptake is reduced by an onion solution to a greater extent in lactose intolerant people than in lactose tolerant people.