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Introduction: Challenging Conventional Carbohydrate Wisdom

For decades, endurance athletes have been advised to consume substantial carbohydrate-rich breakfasts before training or competition to “top up” their glycogen stores. This practice, deeply embedded in sports nutrition culture, is based on the principle that more carbohydrate equals more stored energy, which translates to better performance.

However, a groundbreaking study has challenged this long-held belief, revealing that in well-trained endurance athletes, a high-carbohydrate breakfast does not increase either liver or muscle glycogen levels. This finding has profound implications for how we approach pre-exercise nutrition in athletic populations.

In this comprehensive article, we’ll explore chat this research means for athletes, coaches, and sports nutrition professionals, examining the science behind glycogen metabolism and practical applications for optimising performance.

Understanding Glycogen: The Body’s Carbohydrate Bank Account

What Is Glycogen?

Glycogen is the storage form of glucose in the human body, functioning as a readily accessible energy reserve during physical activity. Unlike fat stores, which require more time to mobilise, glycogen can be rapidly broken down to provide immediate fuel for working muscles and maintain blood glucose homeostasis.

The body stores glycogen in three primary locations:

• Muscle glycogen: Approximately 400-500 grammes in the average individual, though this can increase to 600-800 mmol/kg dry weight in highly trained endurance athletes
• Liver glycogen: About 80-120 grammes (varying with body size and recent carbohydrate intake)
• Blood glucose: A small but critical reserve of approximately 5 grammes

The Three Glycogen Compartments in Muscle

Recent advances in transmission electron microscopy have revealed that muscle glycogen isn’t simply stored as a homogenous pool. Rather, it exists in three distinct subcellular compartments:

1. Intra-myofibrillar glycogen (5-15% of total): Stored within the contractile apparatus
2. Inter-myofibrillar glycogen (75% of total): Located between muscle fibres
3. Sub-sarcolemmal glycogen (5-15% of total): Positioned beneath the cell membrane

This compartmentalisation has important implications for both energy metabolism and cellular signalling during exercise.

The Study That Changed Our Understanding

Study Design and Methodology

The recent investigation involved 16 well-trained male cyclists who consumed a substantial carbohydrate breakfast (3 grammes per kilogramme of body weight) after an overnight fast. For a 70-kilogramme athlete, this equates to 210 grammes of carbohydrate—a considerable intake by any measure.

The researchers employed sophisticated measurement techniques:

• Liver glycogen assessment: Non-invasive magnetic resonance spectroscopy allowed repeated measurements without tissue sampling
• Muscle glycogen measurement: Traditional muscle biopsies taken at baseline and post-meal
• Observation period: Three hours following breakfast consumption

The Surprising Results

The findings contradicted conventional expectations:

• Muscle glycogen remained unchanged despite the substantial carbohydrate intake
• Liver glycogen showed no significant increase across the three-hour observation window
• Baseline glycogen levels were already high, even after overnight fasting

The researchers concluded that these well-trained cyclists arrived at the laboratory essentially glycogen-replete due to their consistent training regimen and adequate carbohydrate intake on preceding days.

The Glycogen Ceiling: Why More Isn’t Always Better

Training Adaptations Increase Storage Capacity

Endurance exercise training enhances the capacity of human skeletal muscle to accumulate glycogen after glycogen-depleting exercise. One landmark study demonstrated that muscle glycogen accumulation rate was twofold higher in trained versus untrained states, with GLUT-4 content (the primary glucose transporter) increasing by more than 100%.

However, this increased capacity doesn’t mean that storage is unlimited. The body has evolved specific regulatory mechanisms that prevent excessive glycogen accumulation, which would be metabolically disadvantageous due to the water-binding properties of glycogen (approximately 3 grammes of water per gramme of glycogen stored).

Overnight Fasting and Glycogen Dynamics

A common misconception is that overnight fasting substantially depletes muscle glycogen stores. Research demonstrates that after a 12-14 hour overnight fast, liver glycogen concentration drops by approximately one-third (33-40%), whilst muscle glycogen stores remain virtually unchanged.

This differential depletion occurs because:

• The liver actively supplies glucose to the brain and other glucose-dependent tissues during sleep
• Skeletal muscle cannot release glucose back into the bloodstream, as only liver cells contain the enzyme glucose-6-phosphatase necessary for glucose secretion
• Resting muscle has minimal energy requirements during sleep

For the average 70-kilogramme runner, this translates to a loss of approximately 20-26 grammes of carbohydrate from liver stores overnight—roughly equivalent to one energy gel or banana.

The Glycogen Threshold for Performance

An average glycogen repletion rate of 5-6 mmol/kg wet weight per hour is usually required for complete glycogen restoration within 24 hours. When athletes train consistently and consume adequate carbohydrate (typically 6-10 grammes per kilogramme body weight daily), their glycogen stores remain persistently high.

Studies examining short-duration, high-intensity exercise (1-15 minutes) have found that even with glycogen utilisation rates of approximately 100 mmol/kg dry weight per minute, post-exercise glycogen values remain well above the threshold where performance becomes compromised.

Practical Implications for Athletes and Coaches

When Does Pre-Exercise Carbohydrate Loading Matter?

The recent research doesn’t suggest that pre-exercise carbohydrate intake is universally unnecessary. Rather, it highlights the importance of context:

Scenarios where acute carbohydrate loading remains beneficial:

1. Following glycogen-depleting sessions: When previous training has substantially reduced stores
2. Multi-day competitions: Where complete restoration between events may be challenging
3. After periods of restricted carbohydrate intake: Such as intentional training-low strategies or inadequate dietary intake
4. For untrained or recreationally active individuals: Who haven’t developed the enhanced glycogen storage capacity of elite athletes

Scenarios where additional pre-exercise carbohydrate may be less critical:

1. Well-trained athletes with consistent high-carbohydrate intake: Who likely start most days glycogen-replete
2. Moderate-intensity training sessions: Where glycogen depletion is minimal
3. When total daily carbohydrate intake is adequate: Making acute timing less crucial

Optimising Daily Carbohydrate Intake

A glycogen storage threshold appears to occur at a daily carbohydrate intake of approximately 7-10 grammes per kilogramme body weight over 24 hours. For athletes engaging in intense daily training, this represents the target for maintaining optimal glycogen availability.

Practical carbohydrate recommendations by training intensity:

• Light training days (< 1 hour moderate intensity): 5-7 g/kg/day
• Moderate training days (1-2 hours mixed intensity): 7-10 g/kg/day
• Heavy training days (> 2 hours high intensity): 10-12 g/kg/day
• Pre-competition glycogen loading (36-48 hours before): 10-12 g/kg/day

Rethinking Breakfast Strategy

For athletes competing in afternoon or evening events, research suggests that even when lunch provides sufficient carbohydrates, a low-carbohydrate breakfast may reduce liver glycogen storage, negatively affecting endurance performance.

The key consideration isn’t necessarily the size of breakfast, but rather ensuring adequate total carbohydrate intake across the day. A well-trained athlete eating 500 grammes of carbohydrate daily can distribute this across multiple meals and snacks without requiring enormous single-meal intakes.

Strategic breakfast approaches for different scenarios:

Morning training sessions:

• Primary goal: Restore liver glycogen depleted overnight (20-30g carbohydrate minimum)
• Moderate carbohydrate intake (50-75g) 1-2 hours pre-exercise
• Focus on easily digestible sources to minimise gastrointestinal distress

Afternoon/evening training or competition:

• Ensure breakfast contains adequate carbohydrate (1-2 g/kg)
• Distribute remaining carbohydrate needs across lunch and pre-exercise snacks
• Meal timing strategies should align with individual preferences and digestive tolerance

The “Train-Low” Paradigm: Strategic Glycogen Manipulation

Glycogen as a Metabolic Signal

The glycogen granule is more than a simple fuel store; it’s a potent regulator of molecular cell signalling pathways that regulate the oxidative phenotype. This discovery has revolutionised our understanding of training adaptation.

Training with low muscle glycogen on occasions enhances intracellular signalling and consequent adaptations that upregulate the oxidative capacity of muscle cells and possibly improve endurance performance.

Evidence for Periodised Carbohydrate Availability

Several studies have demonstrated that strategically training with reduced glycogen stores can enhance specific training adaptations:

• Increased mitochondrial enzyme activity: Particularly citrate synthase and β-hydroxyacyl-CoA dehydrogenase
• Enhanced fat oxidation capacity: Allowing greater glycogen sparing during competition
• Improved metabolic flexibility: Better ability to utilise different fuel sources

One study showed that the leg that commenced 50% of training sessions with low muscle glycogen demonstrated a superior increase in maximal activity of both citrate synthase and β-hydroxyacyl-CoA dehydrogenase compared with the contralateral leg training with high glycogen.

Implementing Train-Low Strategies Safely

“Sleep-low” protocol:

1. Evening high-intensity or glycogen-depleting session
2. Restrict carbohydrate overnight
3. Morning training session with reduced glycogen availability
4. Resume normal carbohydrate intake post-training

Important caveats:

• Should not be used before important training sessions or competitions
• Many studies using low glycogen availability were of short duration and showed no changes or even decreases in performance in some cases
• Best reserved for steady-state sessions below lactate threshold

Sex Differences and Individual Variability

The Female Athlete Consideration

The study under discussion examined only male cyclists—a significant limitation. Research has demonstrated that adjusting the carbohydrate-to-fat ratio and the type of carbohydrate intake can mitigate the impact of gender and menstrual cycles on glycogen storage and substrate utilisation.

Female athletes may experience:

• Menstrual cycle effects: Hormonal fluctuations affecting glycogen metabolism across cycle phases
• Different substrate preferences: Greater reliance on fat oxidation at given exercise intensities
• Potentially different glycogen storage kinetics: Though research remains limited

Individual Response Variability

Unfortunately, many variables including training parameters (time, intensity, frequency, type, rest between bouts) and nutritional factors (type, amount, timing) varied considerably between studies, making valid inferences difficult.

Athletes should consider:

• Training status: Elite versus recreational athletes have vastly different glycogen storage capacities
• Dietary history: Recent carbohydrate intake patterns
• Training phase: Base building, intensive preparation, or competition periods
• Individual gastrointestinal tolerance: Some athletes cannot tolerate large pre-exercise carbohydrate intakes

Beyond Glycogen: Other Roles of Pre-Exercise Carbohydrate

Blood Glucose Maintenance

Even when glycogen stores are optimal, pre-exercise carbohydrate intake serves additional purposes:

When blood glucose levels decrease below 3.9 mmol/L during exercise, the brain’s motor control centres and the nervous system’s capacity to recruit muscle fibres are compromised. Pre-exercise carbohydrate helps maintain blood glucose, supporting:

• Central nervous system function
• Motor control and coordination
• Perceived exertion and mental focus
• Decision-making during competition

The Carbohydrate Mouth-Rinse Effect

Simply rinsing the mouth with carbohydrate solutions without swallowing has been shown to improve aspects of exercise performance such as endurance capacity and bench-press repetitions to failure. This phenomenon demonstrates that carbohydrate’s performance benefits extend beyond simple fuel provision, involving neural mechanisms that influence effort perception and motor output.

Immune Function Support

Differential effects on markers of immune function from supplemental carbohydrate in the pre-exercise period have been found, with carbohydrate offering more favourable impacts on salivary immunoglobulin A during high-repetition, high-volume resistance exercise. For athletes training intensively, maintaining immune function is critical for training consistency and athlete availability.

Glycogen Supercompensation: When Maximum Storage Matters

The Classic Carbohydrate Loading Protocol

Glycogen supercompensation results from a combination of ample rest, reduced training volume, and the consumption of a high-carbohydrate diet. For events lasting longer than 90 minutes, where glycogen depletion becomes performance-limiting, supercompensation strategies remain valuable.

Modified carbohydrate loading protocol:

• Days 4-7 before competition: Normal training with moderate carbohydrate (7-8 g/kg/day)
• Days 2-3 before competition: Reduced training volume with high carbohydrate (10-12 g/kg/day)
• Day 1 before competition: Rest or very light activity with continued high carbohydrate
• Competition day: Moderate carbohydrate breakfast (1-2 g/kg) 3-4 hours pre-event

Practical Challenges and Solutions

Athletes attempting supercompensation may encounter:

Gastrointestinal distress:

• Solution: Gradually increase carbohydrate intake over several days
• Choose easily digestible sources (white rice, pasta, bread, sports drinks)
• Moderate fibre intake in the final 24-48 hours

Weight gain from glycogen-bound water:

• Expected: 1-2 kilogrammes temporary weight increase
• This demonstrates that adjusting pre-exercise glycogen levels to competition requirements may provide an attractive weight management strategy in weight-bearing sports, particularly in athletes with high resting glycogen levels
• Consider whether carrying extra weight offsets performance benefits for your specific event

Future Directions and Unanswered Questions

Research Gaps

Several important questions remain:

1. Optimal measurement timing: Would extending observation beyond three hours reveal delayed glycogen storage?
2. Female athlete responses: Do women show similar glycogen ceiling effects?
3. Different athletic populations: How do findings translate to team sport athletes, strength athletes, or ultra-endurance specialists?
4. Long-term implications: Does chronic high glycogen availability affect metabolic health or longevity?

Emerging Technologies

Current methods for glycogen testing, such as Bergström muscle biopsy, magnetic resonance spectroscopy, and musculoskeletal high-frequency ultrasound, face challenges in enabling rapid, accurate, and real-time monitoring during training and competition.

Advances in continuous glucose monitoring and non-invasive glycogen assessment may soon provide athletes with real-time feedback on fuel availability, enabling more precise nutritional strategies.

Practical Takeaways: Evidence-Based Recommendations

For Well-Trained Endurance Athletes

1. Focus on consistent daily carbohydrate intake (7-10 g/kg/day during training) rather than obsessing over pre-exercise meals
2. Recognise that morning glycogen stores are likely adequate if you consumed sufficient carbohydrate the previous day
3. Prioritise liver glycogen restoration with modest carbohydrate intake (20-50g) before morning sessions
4. Reserve large pre-exercise carbohydrate meals for situations following glycogen depletion or before very long/intense efforts
5. Monitor your individual response through performance metrics and subjective feedback

For Recreational Athletes and Beginners

1. Don’t assume findings from elite athletes apply equally to your situation
2. Build glycogen storage capacity through consistent training before manipulating carbohydrate availability
3. Ensure adequate total daily carbohydrate before concerning yourself with precise timing
4. Use a sports nutrition professional to personalise recommendations based on your training load and goals

For Coaches and Sports Nutritionists

1. Assess each athlete’s training status and dietary patterns before making carbohydrate recommendations
2. Educate athletes about the difference between acute pre-exercise nutrition and overall carbohydrate periodisation
3. Implement train-low strategies judiciously and monitor for negative effects on performance or recovery
4. Consider the broader nutritional picture including protein, micronutrients, and overall energy availability

Conclusion

The revelation that high-carbohydrate breakfasts don’t increase glycogen in well-trained athletes shouldn’t be interpreted as “carbohydrates don’t matter.” Rather, it highlights a more nuanced understanding: when athletes are already glycogen-replete through consistent training and adequate daily carbohydrate intake, additional acute loading provides minimal benefit.

This finding reinforces several fundamental principles:

• Consistency trumps crisis management: Regular adequate carbohydrate intake maintains high glycogen availability without requiring extreme pre-exercise loading
• Training status matters: Adaptations from endurance training fundamentally alter glycogen metabolism
• Context determines strategy: The optimal approach varies based on recent training, dietary intake, competition demands, and individual characteristics

My Thoughts

The context matters in Sports Nutrition , as sports nutrition science evolves, we move away from one-size-fits-all recommendations toward personalised, context-dependent strategies. The glycogen ceiling phenomenon is one example of how our deepening understanding of exercise metabolism enables more sophisticated, evidence-based approaches to fuelling athletic performance. For athletes and practitioners seeking to optimise performance, the message is clear: build your nutritional foundation through consistent daily habits, understand your individual needs, and apply acute strategies strategically rather than reflexively.

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References and Further Reading

1. Original study: PMID 41115061 – High-carbohydrate breakfast and glycogen storage in trained cyclists
2. Fundamentals of glycogen metabolism for coaches and athletes
3. Regulation of muscle glycogen metabolism during exercise
4. Glycogen availability and skeletal muscle adaptations
5. Carbohydrate supplementation approaches for elite endurance athletes

For personalised sports nutrition guidance tailored to your specific needs, contact at Smart Nutrition International.