You know the feeling. Seventy kilometres into a ride, the legs go from compliant to concrete. The group rolls away on a climb you normally clear without thinking. Your brain gets foggy, your mood drops, and the last thirty kilometres become an exercise in damage limitation.
That is glycogen depletion. And it is fixable.
Not with a new training plan. Not with a lighter bike. With food — specifically, with an understanding of how carbohydrate is stored in your body, how fast different types of riding burn through it, and how to reload it before the next session. Glycogen management is the foundation that every other fuelling decision sits on top of. It is a core concept in cycling nutrition, and once you understand it, everything else about how you fuel on and off the bike makes more sense.
What glycogen actually is — and why it matters more than you think
Glycogen is the stored form of carbohydrate in the body. When you eat carbohydrate — rice, pasta, bread, fruit, a gel — your body breaks it down into glucose, uses what it needs immediately, and packs the rest into long chains of glucose molecules called glycogen. Those chains sit in two places: your muscles and your liver.
The split matters.
Muscle glycogen is local fuel. It sits inside the muscle fibres themselves and can only be used by the muscle it lives in. Your quads cannot borrow glycogen from your arms. Your body stores approximately 400-500 grams of glycogen across all skeletal muscle, though trained endurance athletes can push that closer to 600 grams with chronic loading. At roughly 4 calories per gram, that is 1,600-2,000 calories of on-board carbohydrate fuel sitting in the muscles.
Liver glycogen is the central bank. It stores 80-110 grams — roughly 320-440 calories — and its primary job is maintaining blood glucose levels. When blood sugar drops, the liver breaks down its glycogen and releases glucose into the bloodstream. This is what keeps your brain functioning, your central nervous system firing, and your coordination intact. When liver glycogen runs out, you do not just lose power. You lose the ability to think clearly, hold a line, and make decisions. That is the cognitive component of the bonk that riders describe as "the lights going out."
Add those together and you have approximately 2,000-2,500 calories of stored carbohydrate. That sounds like a lot until you consider what cycling actually costs.
A 75 kg rider putting out 250 watts — a solid threshold effort — burns roughly 900 calories per hour. At that intensity, carbohydrate is the dominant fuel source, contributing 70-85 per cent of the total energy. Run the arithmetic and your glycogen stores last about 90-120 minutes of hard riding. Less if you started with partially depleted stores. Less again if you did not eat breakfast.
This is not a theoretical problem. This is the reason you blew up on that sportive. This is why the third interval set feels impossible on a day you skipped lunch. Every fuelling decision you make — before, during, and after the ride — is about managing this finite reserve.
How fast you burn through it: intensity changes everything
The rate at which your body drains glycogen is not fixed. It is almost entirely dictated by intensity. And the relationship is not linear — it is exponential. Ride a bit harder and you burn disproportionately more glycogen.
Zone 2 (endurance pace, 55-75% of FTP). At this intensity, fat is the primary fuel source. Your body burns roughly 30-40 grams of carbohydrate per hour, with fat contributing the majority of the energy. This is why you can ride zone 2 for four or five hours without bonking, even with modest fuelling. Glycogen is being used, but slowly. A well-fuelled rider starting with full stores has 8-12 hours of zone 2 fuel on board before depletion becomes a factor — assuming they eat something along the way.
Tempo and sweet spot (76-90% of FTP). The crossover point. Carbohydrate contribution climbs to 50-65 per cent of total energy. You are burning 60-90 grams of carbohydrate per hour. At this intensity, your 2,000+ calories of stored glycogen lasts roughly 3-4 hours. This is the danger zone for sportive riders — the effort feels manageable, so they do not eat enough, and by hour three the wheels come off.
Threshold (91-105% of FTP). Carbohydrate dominates at 70-85 per cent of total energy production. You are burning 100-150 grams of carbohydrate per hour. Glycogen stores last approximately 90-120 minutes. This is why time trials and hard group rides have a natural ceiling — if you have not eaten enough before and during, you will hit the wall between 60 and 90 minutes.
VO2max efforts (106-120% of FTP). Nearly 100 per cent carbohydrate fuelled. Burning rates hit 150-200+ grams per hour during the effort itself, though these intervals are short enough that total glycogen cost per session is manageable. The issue is cumulative: three sets of 4x4-minute VO2max intervals in a week, on top of normal training, can leave glycogen stores chronically depressed if you do not refuel aggressively between sessions.
The practical takeaway is that intensity, not duration, is what empties the tank. A four-hour zone 2 ride is less glycogen-expensive than a 90-minute threshold session. But the four-hour ride feels harder to recover from because it takes longer. Both need refuelling. The threshold session needs it more urgently.
The bonk, explained
The bonk is not a single event. It is two parallel failures happening at the same time.
First, muscle glycogen drops below a critical threshold — roughly 25-30 per cent of capacity, as measured in biopsy studies by Ørtenblad and colleagues. At that point, the muscle's ability to contract at high force declines sharply. Not gradually. Sharply. The calcium release mechanism in the muscle fibre depends on local glycogen availability, and when it falls below the threshold, the fibre cannot produce force even if you are screaming at your legs to go harder.
Second, liver glycogen runs out and blood glucose falls. This is the central fatigue component. Your brain runs almost exclusively on glucose, and when blood glucose drops below approximately 3.5 mmol/L, the brain begins to downregulate motor unit recruitment. It is a protective mechanism — the brain is reducing power output to preserve glucose for its own survival. You experience this as confusion, emotional volatility, dizziness, and the overwhelming desire to stop.
Both of these happen faster than most riders expect. And both are preventable with the right fuelling strategy.
Carb loading protocols that actually work
If you have been riding for more than a few years, you have probably heard of carb loading. You may also have heard that it requires a brutal six-day protocol involving three days of deliberate glycogen depletion followed by three days of massive carbohydrate intake. That was the Bergstrom-Hultman protocol from the 1960s, and while it works, it is miserable. Three days of depletion riding while restricting carbohydrate makes you irritable, fatigued, and prone to illness. It is also unnecessary.
The modern evidence, particularly the work by Bussau and colleagues published in the International Journal of Sport Nutrition and Exercise Metabolism, showed that you can achieve the same glycogen loading effect with a 48-hour protocol. No depletion phase. Just two days of high carbohydrate intake combined with reduced training volume.
The numbers: 10-12 grams of carbohydrate per kilogram of body weight per day, for two days before the event. For a 75 kg rider, that is 750-900 grams of carbohydrate per day. It sounds absurd. It is a lot of food. But the research is clear — it works, and it produces glycogen stores comparable to the old six-day protocol.
What 800 grams of carbohydrate in a day actually looks like
This is where most riders fail the protocol. They know the number. They cannot imagine eating that much. So they aim for it, undershoot by 300 grams, and wonder why they do not feel loaded on race morning.
A practical day at roughly 800 grams of carbohydrate for a 75 kg rider:
Breakfast: Large bowl of porridge (80g oats) with honey, banana, and a glass of orange juice. ~120g carbohydrate.
Mid-morning snack: Two thick slices of white toast with jam, plus a sports drink. ~80g carbohydrate.
Lunch: Large plate of white pasta with tomato sauce and bread on the side. ~150g carbohydrate.
Afternoon snack: Two rice cakes with honey, a banana, and a bottle of sports drink. ~100g carbohydrate.
Dinner: Large bowl of white rice with chicken and vegetables, plus a bread roll. ~150g carbohydrate.
Evening snack: Bowl of cereal with milk, toast with jam. ~100g carbohydrate.
Across the day: Two bottles of sports drink (500ml each at standard concentration). ~100g carbohydrate.
Total: approximately 800 grams. Notice the theme: white carbohydrate, low fibre, low fat. This is not a healthy eating day. This is a performance loading day. The low fibre keeps gut volume down. The low fat maximises carbohydrate density per meal. You will feel full. That is normal.
Two critical notes. First, reduce training volume sharply during these two days. A 20-30 minute spin with a few openers is fine. Anything longer burns through the glycogen you are trying to store. Second, this protocol is for target events — a key sportive, a race, a Gran Fondo you have been building toward. It is not for every Sunday ride.
Train-low strategies: powerful but dangerous for masters riders
The opposite end of the spectrum from carb loading is deliberate glycogen depletion during training. Train-low strategies involve completing specific sessions with reduced glycogen availability to force metabolic adaptations that improve fat oxidation and mitochondrial function.
The science is real. Sam Impey and colleagues at Loughborough University published a series of papers showing that training with low glycogen availability upregulates AMPK signalling, increases PGC-1alpha expression (a key driver of mitochondrial biogenesis), and improves the muscle's capacity to oxidise fat at moderate intensities. Riders who train low develop a broader metabolic flexibility — they can use fat more efficiently at higher percentages of their VO2max, preserving glycogen for the moments that matter.
The most practical version is the sleep-low/train-low protocol. The structure:
- Complete a hard evening session (intervals, tempo, or sweet spot) that depletes glycogen.
- Eat a protein-rich, low-carbohydrate dinner — something like grilled chicken with vegetables, no rice or pasta.
- Sleep. Your body does not replenish muscle glycogen overnight without carbohydrate.
- Wake up and ride in the morning, in a glycogen-depleted state. Keep this ride in zone 1-2 only. Sixty to ninety minutes.
- Eat a carbohydrate-rich breakfast immediately after the morning ride.
This works because the overnight depletion creates the metabolic signalling environment without requiring you to suffer through a fully fasted hard session. The morning ride is easy by design — you are not trying to produce power, you are trying to force fat oxidation under glycogen stress.
The risk equation for over-40 riders
Train-low protocols carry costs. Cortisol rises. Immune function drops. Protein breakdown increases. Perceived exertion for the same workload goes up. These costs are manageable in a 25-year-old professional with a recovery infrastructure. They are less manageable in a 45-year-old with a full-time job, broken sleep, and the accumulated stress load that comes with being a human adult.
The guidance from the research and from coaches who work with masters athletes — people like Phil Sheridan at Hexis and the nutritionists working with the Irish Cycling Federation — is consistent:
- Use train-low sparingly. Once per week, maximum, during a base or general preparation phase.
- Never on consecutive days. The immune suppression from a single train-low session takes 24-48 hours to resolve.
- Never during high-intensity blocks. VO2max and threshold work require glycogen. Trying to do interval work in a depleted state does not make you tougher. It makes the intervals worse and the adaptation smaller.
- Never if you are feeling run-down, fighting illness, or in a period of high life stress. The cortisol from train-low stacks on top of existing cortisol. If the pile is already high, do not add to it.
- Always refuel aggressively after the depleted session. The metabolic benefit comes from the training stimulus, not from prolonged deprivation. Eat carbohydrate within 30 minutes of finishing.
For most masters cyclists, one sleep-low session per week during a base phase — and zero during build and race phases — is the right dose. The adaptation is real. The risk of overdoing it is also real.
Glycogen supercompensation: loading beyond your baseline
Supercompensation is the body's overreaction to depletion. When muscle glycogen drops to low levels and is then replenished with high carbohydrate intake, the muscles do not just refill to their previous capacity. They overshoot, storing 20-40 per cent more glycogen than baseline.
This is the physiological mechanism behind carb loading, and it was first demonstrated by Bergstrom and Hultman in their landmark biopsy studies. The muscle, having been depleted, upregulates glycogen synthase — the enzyme responsible for packing glucose into glycogen chains — and increases glycogen storage for a brief window.
The window matters. Supercompensation peaks at approximately 24-36 hours after the reloading phase begins and lasts roughly 48-72 hours. After that, glycogen levels gradually return to baseline as the enzyme activity normalises. This is why the timing of carb loading relative to your event is critical. Load too early, and the supercompensation effect has faded by race day. Load too late, and you have not had enough time to fill the stores.
The practical protocol for race week:
Four days out: Normal training, normal eating.
Three days out: Final hard session — a threshold or VO2max workout that creates meaningful glycogen depletion. This is the trigger for the supercompensation response.
Two days out: Begin the 48-hour loading phase. High carbohydrate intake (10-12 g/kg/day), reduced training, low fibre, low fat. The goal is maximum glycogen storage.
One day out: Continue loading. Short spin with openers if desired. Evening meal should be carbohydrate-heavy but familiar — nothing new.
Race morning: Top up with a carbohydrate-rich breakfast 2-3 hours before the start. This is not loading — the loading is done. This is topping up liver glycogen, which depletes overnight during sleep.
The result, as measured in muscle biopsy studies, is glycogen stores of 600-800 mmol/kg dry weight, compared to a normal resting level of 400-500 mmol/kg. For a 75 kg rider, that is roughly 600-700 grams of stored glycogen instead of 400-500 grams. An extra 200 grams of on-board fuel. At threshold intensity, that is an additional 20-30 minutes before depletion. In a long race, that is the difference between finishing with the group and crawling home alone.
Daily carb targets by training load
This is where glycogen management becomes a daily practice rather than a race-week protocol. The single biggest fuelling error in amateur cycling is eating the same amount of carbohydrate every day regardless of what the training demands.
Asker Jeukendrup's work, refined over two decades, provides the framework that most sports nutritionists now use. The numbers are expressed as grams of carbohydrate per kilogram of body weight per day, and they scale with training load:
| Day type | Carb target (g/kg/day) | 75 kg rider | Example foods | |---|---|---|---| | Rest day | 3-5 | 225-375g | Normal meals, no extra carb loading | | Easy ride day (60-90 min Z2) | 5-7 | 375-525g | Add a portion of rice or pasta to two meals | | Moderate day (90-120 min mixed) | 6-8 | 450-600g | Larger portions at all meals, snacks between | | Hard session day (intervals, race-pace) | 7-10 | 525-750g | Carb-dense meals, recovery drink, snacks | | Pre-race / race day | 10-12 | 750-900g | Full loading protocol as described above |
The range in each band exists because individuals differ in body composition, metabolic efficiency, and training history. A rider with higher muscle mass stores more glycogen and may need the upper end. A lighter rider or one with better fat oxidation may sit lower.
The key principle is periodisation. Fuel for the work. A rest day does not need 10 g/kg. A hard interval day cannot survive on 3 g/kg. Match the intake to the demand, and the glycogen management takes care of itself.
This is a topic that comes up constantly in the Not Done Yet community, because most riders significantly underestimate carbohydrate needs on hard training days and overeat on rest days. Getting the periodisation right — eating more on hard days, less on easy days — is one of the most impactful changes a masters cyclist can make.
Recovery fuelling: the first two hours
After a hard ride, glycogen synthase activity is elevated for approximately two hours. During this window, the rate at which muscles convert ingested carbohydrate into stored glycogen is roughly 50 per cent faster than at baseline. This is the recovery window, and it is real — not marketing from supplement companies.
The target: 1-1.2 grams of carbohydrate per kilogram of body weight within 30 minutes of finishing, repeated at 60 and 120 minutes if full recovery is needed before the next session. For a 75 kg rider, that is 75-90 grams immediately post-ride, plus another 75-90 grams over the next two hours.
A practical recovery meal: a large bowl of rice with protein, a recovery drink, and a piece of fruit covers roughly 100 grams. Follow it with a normal carbohydrate-rich dinner two hours later and you are on track.
This matters most when you are training on consecutive days. If you ride hard on Tuesday evening and again on Wednesday morning, the recovery window between sessions is your lifeline. Skip it and Wednesday's session starts with half-empty tanks. Nail it and you start refuelled.
If you have 24 hours or more between sessions, the urgency drops. Total carbohydrate intake across the day matters more than the timing of the first meal. But for the back-to-back training that masters programmes often require — two quality sessions in 48 hours — the recovery window is non-negotiable.
Putting it together: a week of fuelling for a masters cyclist
Theory is useful. Practice is what gets you to the finish. This is what glycogen-aware fuelling looks like across a typical training week for a masters rider doing 8-12 hours.
Monday — rest day
No structured training. Walk the dog, stretch, live your life.
Carb target: 3-4 g/kg (225-300g for a 75 kg rider). Normal meals. Protein and vegetables are the priority. Carbohydrate comes from what naturally appears — potatoes with dinner, fruit with breakfast, bread with lunch. No need to add sports drinks or extra rice. Glycogen stores are still replenishing from the weekend, and rest-day eating supports that without excess.
Tuesday — evening intervals (75 minutes, including warm-up)
VO2max or threshold intervals. High glycogen demand.
Carb target: 8-10 g/kg (600-750g). Front-load the carbohydrate toward the session. Porridge for breakfast. Pasta or rice at lunch. A carb-rich snack 90 minutes before the session — a couple of rice cakes with honey, a banana, a sports drink. During the session, sip a bottle of drink mix (40-60g carbohydrate). After the session, recovery meal within 30 minutes: rice, protein, vegetables.
Wednesday — easy spin (60-75 minutes, zone 2)
Recovery from Tuesday's session. Low intensity.
Carb target: 5-6 g/kg (375-450g). Normal meals with slightly larger carbohydrate portions than a rest day. No need for on-bike fuelling if the ride is under 75 minutes. Breakfast before, normal lunch, normal dinner. The priority is completing the glycogen refuelling that started after Tuesday's intervals.
Thursday — tempo ride or sweet spot (90 minutes)
Moderate-high glycogen demand.
Carb target: 7-8 g/kg (525-600g). Similar to Tuesday but slightly lower because the session is less glycolytically demanding. Eat well across the day. Fuel the ride with a bottle of drink mix. Recovery meal afterward.
Friday — rest day or very easy spin (30-45 minutes)
Pre-weekend recovery.
Carb target: 4-5 g/kg (300-375g). Similar to Monday. Normal meals. If you are riding on Saturday and Sunday, start increasing carbohydrate slightly at Friday dinner — an extra portion of rice or pasta. Not full loading, just priming.
Saturday — long ride (3-4 hours, mostly zone 2 with some tempo)
The big endurance ride. Moderate glycogen demand sustained over a long period.
Carb target: 7-9 g/kg (525-675g). Carbohydrate-rich breakfast 2-3 hours before. During the ride: 40-60 grams per hour from drink mix, gels, bars, or real food. This is a long zone 2 ride, not a race — you do not need 90+ grams per hour, but you do need consistent intake to avoid depleting stores that you need for Sunday. Recovery meal within 30 minutes of finishing. Carb-rich dinner.
Sunday — group ride or race-pace effort (2-3 hours)
The hardest ride of the week. Highest glycogen demand.
Carb target: 8-10 g/kg (600-750g). Treat this like Tuesday: front-loaded carbohydrate, fuelling during the ride at 60-90 grams per hour, recovery meal within 30 minutes. If this is a race or hard group ride, consider the 10 g/kg end of the range. The refuelling after this session is critical for starting Monday's rest day on the right foot.
The weekly pattern
Notice the shape: low-moderate-high-moderate-low-high-high. Carbohydrate follows the training load. Rest days are low. Hard days are high. The total weekly carbohydrate intake for this rider might be 3,200-3,800 grams across the week — an average of 460-540 grams per day. But no single day looks like that average. Some days are 225 grams. Some days are 750. The average is meaningless. The daily match between fuel and work is everything.
This is glycogen management in practice. Not a single number. Not a fixed meal plan. A responsive system that puts carbohydrate where it is needed and pulls it back where it is not.
The common mistakes — and how to fix them
Eating the same carbohydrate every day. The most prevalent error. A rider eating 400 grams of carbohydrate per day, every day, is overfuelling rest days and underfuelling hard days simultaneously. Match the intake to the work.
Skipping the recovery window. Finishing a hard ride and waiting two hours before eating costs you glycogen synthesis rate at exactly the moment it is highest. Keep a recovery drink or meal ready.
Trying to carb load the night before. One big pasta dinner the night before a race does not constitute carb loading. Loading takes 48 hours and requires 10-12 g/kg per day. A single large meal might deliver 200 grams. You need 750-900 across the day, repeated for two days.
Chronically training depleted. Some riders wear low fuelling as a badge of toughness. The result is poor session quality, elevated cortisol, suppressed immune function, and a body that never fully recovers between sessions. Fuel the work. Save the depletion for specific, controlled train-low sessions.
Ignoring liver glycogen. You can wake up with full muscle glycogen and empty liver glycogen — the liver depletes overnight to maintain blood glucose during sleep. Skipping breakfast before a hard morning session means your brain starts running on fumes even if your legs feel fine. Eat before hard sessions. Always.
Glycogen is not complicated. It is a tank. You fill it, you drain it, you fill it again. The art is in matching the fill-drain-refill cycle to your training, your racing, and your recovery. Get that right and the bonks stop, the third-hour fade disappears, and Thursday's session does not suffer because you failed to eat on Tuesday night.
That is what fuelling is. Not a diet. Not a protocol. A daily decision about where the carbohydrate goes.