Post-exercise glycogen repletion in the absence of food intake
One extreme dietary condition that would be expected to impair the synthesis of muscle glycogen during recovery from exercise is the absence of food. Is it possible for our muscles to re-build at least part of their glycogen stores after exercise if food is not available? This is a situation likely to have had a major impact on the survival of our ancestors who, as a result of their hunter- gatherers life-style, were at increased risks of experiencing regular episodes of prolonged fast. This notion that skeletal muscles might have the capacity to replenish their glycogen independently of food intake is not a novel one as it was central to the work of the Nobel Laureat, Otto Meyerhof, who, nearly a hundred years ago, provided evidence, based on the use of isolated frog muscle preparations, that skeletal muscles have such a capacity (Fournier et al., 2002). It is only over the past 30 years, however, that experiments have been performed in humans and a wide range of animal species to establish if this is also the case in intact animals. The general consensus is that, after exercise, skeletal muscles in humans have the capacity to replenish at least part of their glycogen stores without food intake, irrespective of whether they are recovering from prolonged aerobic exercise (Hultman and Bergström, 1967; Maehlum et al., 1978) or from high intensity exercise (Hermansen and Vaage, 1977; Peters-Futre et al., 1987; Astrand et al., 1986; Bangsbo et al., 1991, 1997; Fairchild et al., 2003). Moreover, we have also shown that this resynthesis occurs across all muscle fiber types (Fairchild et al., 2003).
Muscle glycogen repletion during active recovery from intense exercise
In support of the view that active recovery inhibits glycogen resynthesis is the observation that glycogen repletion in individuals fed carbohydrate post-exercise is impaired during active recovery (Bonen et al., 1985). Moreover, a more recent study also supports indirectly the view that glycogen synthesis is inhibited during active recovery (Choi et al., 1996), with a combination of active and passive recovery being accompanied by a lower extent of glycogen synthesis than with passive recovery alone in overnight fasted individuals (Choi et al., 1996). Unfortunately, the impact of active recovery per se on glycogen synthesis was not examined in this study because no muscle sampling was performed at the end of the active recovery period (Choi et al., 1996). Also, since all the muscle biopsies were obtained through the same incision site in this study, and that this has been shown to impair glycogen synthesis (Costill et al., 1988), the extent of glycogen accumulation post-exercise might have been underestimated (Choi et al., 1996).
Metabolic pathways responsible for the conversion of lactate into muscle glycogen
Given the evidence that lactate is likely to be the major carbon source mobilised for the synthesis of muscle glycogen during passive, and maybe, active recovery, this raises the question of the metabolic pathway responsible for its conversion into muscle glycogen. In theory, the synthesis of muscle glycogen from lactate could occur via two metabolic pathways, muscle lactate glyconeogenesis and the Cori cycle. These pathways have already been the object of numerous reviews (McDermott and Bonen, 1992; Pascoe and Gladden, 1996; Palmer and Fournier, 1997; Donovan and Pagliassotti, 2000; Fournier et al., 2002), and for this reason will be discussed only briefly here. The former pathway involves only the participation of skeletal muscles, and it differs from hepatic gluconeogenesis in that there is no intra-mitochondrial step involved, and the most recent evidence point to the reversal of the reaction normally catalysed by pyruvate kinase as being responsible for the conversion of pyruvate into PEP (Palmer and Fournier, 1997; Dobson et al., 2002). The Cori cycle, on the other hand, differs in many respects from lactate glyconeogenesis in that more than one organ are involved. Indeed, following its release from skeletal muscle, lactate is removed by the liver or kidneys where it is converted via gluconeogenesis into glucose. Once produced, glucose is released into the blood before being taken up and stored as glycogen in skeletal muscles. Although, there is a general agreement that the former pathway plays the major role in glycogen synthesis from lactate in fish, amphibians and reptiles (reviewed in Gleeson, 1996; Fournier et al., 2002), the relative contributions of muscle lactate glyconeogenesis and Cori cycling to the resynthesis of muscle glycogen in humans and rats have been a controversial issue. Earlier studies in humans and rats have identified muscle lactate glyconeogenesis as the primary route responsible for lactate conversion into muscle glycogen (Hermansen and Vaage, 1977; Astrand et al., 1986; Nikolovski et al., 1996), but those findings have been subsequently challenged (Gaesser and Brooks, 1984; Bangsbo et al., 1991; Palmer and Fournier, 1997), with more recent evidence indicating that the Cori cycle plays also an important role (Bangsbo et al., 1991, 1997). What is still unclear, is the relative contributions of both pathways to the recycling of lactate into muscle glycogen (reviewed in Fournier et al., 2002).
Regulation of post-exercise glycogen repletion in the absence of food intake
It is noteworthy that under conditions expected to be highly unfavourable to glycogen synthesis following high intensity exercise, such as food absence or active recovery, the rates of muscle glycogen synthesis in humans and rats are among the highest reported in the literature (Pascoe and Gladden, 1996; Nikolovski et al, 1996; Fairchild et al., 2003).
In conclusion, during recovery from exercise, it is possible for skeletal muscles to replenish their glycogen stores under conditions expected to be highly unfavourable to glycogen synthesis such as fasting or active recovery. The rates of muscle glycogen synthesis can be very high under these conditions, most probably because of the acute activation of glucose transport and glycogen synthase and inhibition of glycogen phosphorylase. This capacity of skeletal muscles to replenish their glycogen stores under extreme conditions is clearly advantageous as it allows muscles to maintain adequate levels of glycogen stores for fight or flight responses.