Yacon diet (Smallanthus sonchifolius, Asteraceae) improves hepatic insulin resistance via reducing Trb3 expression in Zucker fa/fa rats

OBJECTIVE: Yacon is a perennial plant forming a clump of > 20 big, edible underground tubers. Yacon, which originates from South America, has become increasingly popular in the Japanese diet for tubers have a lower caloric value and a high fiber content. Recent studies have suggested that yacon feeding ameliorates diabetes as indicated by reduced blood glucose. METHODS: We fed male Zucker fa/fa rats for 5 weeks with isocaloric normal chow diet containing from 6.5% control aroid or 6.5% yacon. Insulin sensitivity was evaluated by euglycemic-hyperinsulinemic clamp study. RESULTS: Body weight was comparable between yacon- and aroid-fed rats. In the basal state, yacon feeding had an effect to lower fasting glucose levels from 184.1 +/- 4.1 to 167.8 +/- 2.7 mg dl(-1) (P < 0.01), as well as basal hepatic glucose output (HGO) from 9.9 +/- 0.4 to 7.4 +/- 0.2 mg kg(-1) per min (P < 0.01). During the clamp studies, the glucose infusion rate required to maintain euglycemia was increased by 12.3% in yacon-fed rat. The insulin suppression of HGO was also increased in yacon-fed rats compared with control rats (85.3 +/- 2.4% vs 77.0 +/- 3.0%; P < 0.05), whereas the glucose disposal rate was not different between the two groups. Consistent with the clamp data, the insulin-stimulated phosphorylation of Akt was significantly enhanced in liver but not in skeletal muscle. Furthermore, tribbles 3 (Trb3) expression, which is a negative regulator of Akt activity, was markedly reduced in the liver of yacon-fed rats compared with control rats. CONCLUSION: These results indicate that the effect of yacon feeding to reduce blood glucose is likely due to its beneficial effects on hepatic insulin sensitivity in the insulin resistant state.