Results Both TZDs and metformin  enhance insulin suppression of endogenous glucose production and fasting plasma glucose clearance .
TZDs , but not metformin , also improve insulin-mediated glucose uptake at all insulin levels .
Conclusions /interpretationIn patients with type 2 diabetes , metformin improves fasting hepatic insulin sensitivity and glucose clearance ; TZDs improve fasting hepatic insulin sensitivity and glucose clearance , and potentiate glucose disposal under insulinised conditions
Aims /hypothesisInsulin resistance , which manifests itself as endogenous glucose overproduction and reduced insulin-mediated glucose uptake , is a core defect in type 2 diabetes .
Metformin and the peroxisome proliferator-activated receptor-γ agonists , the thiazolidinediones ( TZDs ) , both lower glucose , although their mechanism of action is still subject to debate .
This review analyses the evidence relevant to these mechanisms in vivo .
Mechanisms of early insulin-sensitizing effects of thiazolidinediones in type 2 diabetes.
Whereas thiazolidinediones ( TZDs ) are known to rapidly improve insulin action in animals , short duration s of TZD therapy have never been studied in humans . Among the many known actions of TZDs , increased circulating levels of the high molecular weight ( HMW ) multimer of adiponectin may be an important insulin-sensitizing mechanism . We examined the effects of only 21 days of 45 mg of pioglitazone ( P+ ) versus placebo ( P- ) in nine subjects with type 2 diabetes ( HbA(1c ) , 10.9 + /- 0.6 % ; BMI , 31.9 + /- 1.5 kg/m(2 ) ) .  Total adiponectin levels increased by approximately twofold in P+ in association with increased adipose tissue gene expression . However , plasma free fatty acid and glucose levels were unchanged , and there were only minimal changes in other " adipokines . " Glucose fluxes ( [3-(3)H]glucose infusion ) were measured during 6-h euglycemic ( 5 mmol/l ) " pancreatic clamp " studies ( somatostatin/glucagon/growth hormone ) with stepped insulin levels . Pioglitazone induced marked decreases in endogenous glucose production ( P+ = 0.9 + /- 0.1 vs. P- = 1.7 + /- 0.3 mg . kg(-1 ) . min(-1 ) ; P < 0.05 ) at physiologic hyperinsulinemia ( approximately 50 microU/ml ) , which was highly correlated with an increased ratio of HMW adiponectin/total levels ( r(2 ) = 0.90 ) . Maximal insulin stimulation ( approximately 400 microU/ml ) revealed pioglitazone-associated increases in glucose uptake ( P+ = 10.5 + /- 0.9 vs. P- = 8.9 + /- 0.8 mg . kg(-1 ) . min(-1 ) ; P < 0.05 ) , which did not correlate with HMW or total adiponectin levels . Thus , only 21 days of pioglitazone therapy improved insulin action in humans with type 2 diabetes . Increased abundance of the HMW adiponectin multimer may contribute to the hepatic insulin-sensitizing effects of these agents
Results Both TZDs and metformin  enhance insulin suppression of endogenous glucose production and fasting plasma glucose clearance .
TZDs , but not metformin , also improve insulin-mediated glucose uptake at all insulin levels .
Conclusions /interpretationIn patients with type 2 diabetes , metformin improves fasting hepatic insulin sensitivity and glucose clearance ; TZDs improve fasting hepatic insulin sensitivity and glucose clearance , and potentiate glucose disposal under insulinised conditions
Aims /hypothesisInsulin resistance , which manifests itself as endogenous glucose overproduction and reduced insulin-mediated glucose uptake , is a core defect in type 2 diabetes .
Metformin and the peroxisome proliferator-activated receptor-γ agonists , the thiazolidinediones ( TZDs ) , both lower glucose , although their mechanism of action is still subject to debate .
This review analyses the evidence relevant to these mechanisms in vivo .