The pyruvate dehydrogenase complex (PDC) controls the rate-limiting step in carbohydrate (CHO) oxidation, the oxidative decarboxylation of pyruvate to acetyl-CoA, and occupies a central role in skeletal muscle intermediary metabolism. The activation of PDC is controlled by a phosphatase (PDP), which dephosphorylates and hence activates PDC and a kinase (PDK), which reduces PDC activity by phosphorylating the E1 component of the complex. The identification of 4 different PDK isoforms (PDK1-4) has intensified the research on their role as potential molecular regulators of glucose oxidation under conditions of altered glucose homeostasis. Two of those isoforms (PDK2 and PDK4) are abundant in human skeletal muscle, whereas PDK1 and 3 isoforms are expressed at very low levels. Physiological hyperinsulinaemia stimulates skeletal muscle PDC activity, and hence glucose oxidation, but this effect is impaired in metabolic states characterised by elevated lipid availability and resistance to insulin’s actions, such as starvation (Tsintzas et al. 2006), high fat diet (Chokkalingam et al. 2007) and lipid infusion (Tsintzas et al. 2007). An increase in muscle PDK4, but not PDK2, protein content is responsible for the long-term control of muscle PDC activity and the consequent reduction in glucose oxidation in response to starvation and high fat diet. Interestingly, this shift in substrate utilization from CHO to fat appears to precede changes in overall glucose uptake, which highlights a key role for PDK4 through its regulation of PDC in substrate metabolism and insulin action in human skeletal muscle (Chokkalingam et al. 2007). Acute lipid overload mediates both allosteric (via acetyl-CoA accumulation) and transcriptional modulations of skeletal muscle PDC activity (via changes in PDK4 content). In healthy humans, insulin infusion can rapidly suppress PDK4, but not PDK2, gene expression in skeletal muscle, whereas elevated levels of plasma free fatty acids (FFA) are positive regulators of PDK4 expression. Increasing the circulating FFA levels through lipid infusion during short-term physiological hyperinsulinaemia attenuates the insulin-stimulated suppression of PDK4 expression and results in accumulation of intracellular long-chain Acyl-CoA, suggesting that intramuscular lipids may play an important role in the regulation of PDK4 expression in a manner independent from the Akt/FOXO3 pathway (Tsintzas et al. 2007). Although PDK4 is a target gene for a number of lipid-regulated transcription factors including PPARα and PPARδ, studies using primary human muscle culture revealed that the stimulatory effect of FFAs on PDK4 is largely mediated through activation of PPARδ. However, transient changes in intramuscular substrate flux might not be important for the transcriptional regulation of PDK4. Indeed, in healthy humans short-term infusions of lipid and adrenaline can induce skeletal muscle PDK4 gene expression to a similar degree despite differential effects on metabolic fuel utilisation. Hyperglycaemia increases the activity of PDC in human skeletal muscle and recent evidence suggests that it results in rapid downregulation of PDK4 gene expression in that tissue. Although the mechanism underlying the suppressive effect of hyperglycaemia on PDK4 is unknown, it appears that factors other than activation of the IRS1/Akt axis mediate this effect in skeletal muscle. The activity of PDC also increases during exercise in a calcium-dependent manner resulting in an increase in pyruvate flux, the formation of acetyl-CoA and a concomitant increase in CHO oxidation. Carbohydrate ingestion immediately before exercise (resulting in increased blood glucose and insulin concentrations) augments the exercise-induced activation of PDC in human skeletal muscle, which facilitates the increase in insulin-stimulated glucose oxidation under those conditions (Tsintzas et al. 2000). However, neither PDK4 protein content nor basal or insulin-stimulated PDC activity are affected by a single bout of exercise performed the day before (Stephens et al. 2010). These findings do not support the notion that a PDK4-mediated inhibition of PDC activation, and thus glucose oxidation, may facilitate the increase in insulin-mediated skeletal muscle glycogen synthesis during recovery from exercise. In summary, in healthy humans, the impairment of insulin action in metabolic states characterised by elevated lipid metabolism is associated with, and often preceded by, impaired basal and insulin-stimulated activation of skeletal muscle PDC, and hence glucose oxidation, as a result of a selective upregulation of PDK4 protein content. Therefore, by virtue of its key role in the regulation of glucose oxidation, PDK4 is a potential molecular target for future strategies aiming to maintain optimum insulin action.