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Glycogen is a highly branched glucose polymer that constitutes the principal storage form of carbohydrate in animals and is functionally analogous, though structurally distinct, from plant starch. In mammals, glycogen is predominantly localized in the liver and skeletal muscle. Hepatic glycogen, present at the highest concentration per gram of tissue, plays a central role in maintaining systemic glucose homeostasis, particularly during fasting. In contrast, skeletal muscle contains the largest total glycogen reserve due to greater mass and utilizes these stores to meet the energetic demands of contraction. Smaller glycogen pools are present in additional tissues, including the kidney, heart, brain, and glial cells, where localized metabolic requirements are supported. Structurally, glycogen is composed of α-D-glucose residues linked by α(1→4) glycosidic bonds with α(1→6) linkages at branch points occurring approximately every 8 to 12 residues. This extensive branching enhances solubility and generates multiple nonreducing ends, facilitating rapid glucose mobilization through the coordinated action of glycogen phosphorylase and debranching enzymes. Glycogen metabolism is tightly regulated by integrated hormonal and allosteric mechanisms. Insulin promotes glycogenesis primarily through activation of glycogen synthase and inhibition of glycogen phosphorylase, whereas glucagon in the liver and epinephrine in both liver and muscle stimulate glycogenolysis via protein kinase A (PKA) signaling pathways activated by cyclic adenosine monophosphate (cAMP). Disruptions in enzymes governing glycogen synthesis, degradation, or regulation give rise to glycogen storage diseases (GSDs), a heterogeneous group of metabolic disorders characterized by tissue-specific manifestations such as hypoglycemia, hepatomegaly, myopathy, and cardiomyopathy.…

PMID: 30969624

The interaction between insulin and exercise is an example of balancing and modifying the effects of two opposing metabolic regulatory forces under varying conditions. While insulin is secreted after food intake and is the primary hormone increasing glucose storage as glycogen and fatty acid storage as triglycerides, exercise is a condition where fuel stores need to be mobilized and oxidized. Thus, during physical activity the fuel storage effects of insulin need to be suppressed. This is done primarily by inhibiting insulin secretion during exercise as well as activating local and systemic fuel mobilizing processes. In contrast, following exercise there is a need for refilling the fuel depots mobilized during exercise, particularly the glycogen stores in muscle. This process is facilitated by an increase in insulin sensitivity of the muscles previously engaged in physical activity which directs glucose to glycogen resynthesis. In physically trained individuals, insulin sensitivity is also higher than in untrained individuals due to adaptations in the vasculature, skeletal muscle and adipose tissue. In this paper, we review the interactions between insulin and exercise during and after exercise, as well as the effects of regular exercise training on insulin action.…

PMID: 34751700

Since the introduction of the muscle biopsy technique in the late 1960s, our understanding of the regulation of muscle glycogen storage and metabolism has advanced considerably. Muscle glycogenolysis and rates of carbohydrate (CHO) oxidation are affected by factors such as exercise intensity, duration, training status and substrate availability. Such changes to the global exercise stimulus exert regulatory effects on key enzymes and transport proteins via both hormonal control and local allosteric regulation. Given the well-documented effects of high CHO availability on promoting exercise performance, elite endurance athletes are typically advised to ensure high CHO availability before, during and after high-intensity training sessions or competition. Nonetheless, in recognition that the glycogen granule is more than a simple fuel store, it is now also accepted that glycogen is a potent regulator of the molecular cell signaling pathways that regulate the oxidative phenotype. Accordingly, the concept of deliberately training with low CHO availability has now gained increased popularity amongst athletic circles. In this review, we present an overview of the regulatory control of CHO metabolism during exercise (with a specific emphasis on muscle glycogen utilization) in order to discuss the effects of both high and low CHO availability on modulating exercise performance and training adaptations, respectively.…

PMID: 29498691

Short Term Body Mass Manipulation (SBM) is frequently used in powerlifting by athletes to qualify for lower weight classes and improve relative competitiveness. The three primary physiological pathways that SBM leverages are gastrointestinal content reduction, glycogen storage and body water manipulation, in addition to post-weigh-in refueling. Despite its high prevalence among athletes competing in the International Powerlifting Federation (IPF), the scientific literature on SBM remains limited, and sport-specific guidelines are currently lacking. This narrative review summarizes the current evidence on SBM in powerlifting, with a focus on physiological mechanisms, practical implementation, and associated risks. The specific demands of IPF competition, consisting of maximal strength performance after only a two-hour window between weigh-in and competition, necessitate uniquely tailored SBM strategies. SBM should not be regarded as a standard preparation method. Instead, it should be seen as a targeted intervention to be applied with caution and strategic intent. The decision to implement SBM must be based on individual assessment, physiological plausibility, and a well-considered cost-benefit rationale. Ensuring effective rehydration and refueling between weigh-in and competition is critical to support both safety and performance. This review provides sport specific, evidence-based recommendations to assist practitioners in applying SBM responsibly within the context of powerlifting.…

PMID: 41320256

Short Term Body Mass Manipulation (SBM) is frequently used in powerlifting by athletes to qualify for lower weight classes and improve relative competitiveness. The three primary physiological pathways that SBM leverages are gastrointestinal content reduction, glycogen storage and body water manipulation, in addition to post-weigh-in refueling. Despite its high prevalence among athletes competing in the International Powerlifting Federation (IPF), the scientific literature on SBM remains limited, and sport-specific guidelines are currently lacking. This narrative review summarizes the current evidence on SBM in powerlifting, with a focus on physiological mechanisms, practical implementation, and associated risks. The specific demands of IPF competition, consisting of maximal strength performance after only a two-hour window between weigh-in and competition, necessitate uniquely tailored SBM strategies. SBM should not be regarded as a standard preparation method. Instead, it should be seen as a targeted intervention to be applied with caution and strategic intent. The decision to implement SBM must be based on individual assessment, physiological plausibility, and a well-considered cost-benefit rationale. Ensuring effective rehydration and refueling between weigh-in and competition is critical to support both safety and performance. This review provides sport specific, evidence-based recommendations to assist practitioners in applying SBM responsibly within the context of powerlifting.…

PMID: 41320256