The cycling internet has split itself into two camps, and both of them are shouting.
Camp one says carbohydrate is king. Load up before the ride, smash gels every twenty minutes, hit 120 grams an hour, and if you bonk it is because you did not eat enough. Camp two says go keto, become a fat-burning machine, run on your own body fat, and never bonk again. Between them, the average cyclist trying to fuel properly is left wondering if carbs are essential or poison.
Here is the thing nobody tells you: both camps are right, in very specific circumstances, and catastrophically wrong when they try to apply their answer to every situation. The evidence on fat adaptation, low-carb training, and periodised nutrition is far more nuanced than either side wants to admit. And that nuance is where the useful stuff lives.
Let me break this down.
What Fat Adaptation Actually Means
Your body runs on two primary fuel systems during exercise: fat and carbohydrate. At low intensities — easy Zone 2 riding, conversational pace, the kind of effort where you can chat about the weekend — fat provides the majority of the energy. As intensity climbs, carbohydrate takes over. By the time you are above threshold, carbohydrate is doing almost all the heavy lifting.
The crossover point is the exercise intensity at which carbohydrate becomes the dominant fuel. Below it, fat is the primary contributor. Above it, carbohydrate runs the show. This crossover exists in every human being, but its position varies. A well-trained endurance athlete with good fat oxidation capacity might not cross over until 65-70% of VO2max. A carb-dependent rider might hit it at 45-50%. George Brooks at UC Berkeley first described this crossover concept in the 1990s, and it remains one of the most useful frameworks for understanding how the body selects fuel during exercise.
The numbers involved are staggering when you lay them out. Even a lean cyclist with 10% body fat carries somewhere around 40,000-60,000 calories of stored fat energy. Glycogen stores — muscle and liver combined — hold roughly 2,000-2,500 calories. The appeal of tapping into that fat reservoir is obvious. If you could run on fat at higher intensities, you would have a virtually unlimited fuel tank instead of a limited one.
Fat adaptation is the process of shifting that crossover point higher. Through training and dietary manipulation, you increase the rate at which your mitochondria can oxidise fat. The result: at a given sub-threshold intensity, you burn more fat and less glycogen than you did before. In theory, this spares your limited glycogen stores for when you actually need them — for the climb, the breakaway, the final sprint.
In theory. Because the research has shown that this adaptation comes with a cost, and the cost matters enormously for competitive cycling.
Louise Burke and the SUPERNOVA Studies
If one body of research has defined this conversation, it is Louise Burke's work at the Australian Institute of Sport. Burke, a professor of sports nutrition, ran a series of studies with elite race walkers — the SUPERNOVA trials — that are still the most rigorous examination of how chronic dietary manipulation affects athletic performance.
In the 2017 study, Burke and her team took elite race walkers and assigned them to one of three dietary conditions over a three-and-a-half-week training camp: a high-carbohydrate diet, a ketogenic low-carbohydrate high-fat diet, or a periodised approach that matched carbohydrate intake to training demands. All groups did the same training. All were meticulously controlled.
The results were clean and uncomfortable for the keto camp. The ketogenic group dramatically increased their fat oxidation rates. Their bodies became measurably better at burning fat. But when it came to race-pace performance — a 10km race walk at competitive intensity — the keto group performed worse than both the high-carbohydrate and periodised groups. They had become brilliant fat burners and worse athletes.
Burke's 2020 follow-up confirmed and extended these findings. Even with more time to adapt, the ketogenic athletes showed the same pattern: excellent fat oxidation, impaired race performance. The fat adaptation was real. The performance benefit was not.
Here is where it gets really interesting. The problem was not that these athletes lacked fitness. It was that their bodies had downregulated the machinery for burning carbohydrate at high rates. They had trained themselves out of the ability to use the very fuel system that powers race-pace efforts. Fat adaptation had come at the direct expense of carbohydrate oxidation.
The Problem with Chronic Low-Carb for Cyclists
Burke's findings point to a fundamental physiological constraint that chronic low-carb advocates consistently underestimate.
When you restrict carbohydrate for weeks or months, your body adapts by becoming less efficient at using carbohydrate. Specifically, the enzyme pyruvate dehydrogenase — the gatekeeper that allows carbohydrate to enter the energy production cycle at high rates — gets downregulated. Your body is not being efficient. It is losing a capacity.
The practical consequence for cycling: you cannot burn what you have trained your body to ignore.
Imagine you have spent three months on a ketogenic diet. Your fat oxidation is through the roof. You line up for a sportive with a big climb in the final thirty kilometres. The pace goes up, the gradient bites, and suddenly you need to produce power at 85-90% of VO2max for fifteen minutes. This is carbohydrate territory. Your muscles need to burn glucose at high rates to sustain this effort. But your pyruvate dehydrogenase is downregulated. Your carbohydrate oxidation machinery has been idling for months. You have the glycogen — you ate a big breakfast, you took gels — but the metabolic pathway to use it at the rate you need is impaired.
You crack. Not because you are unfit. Not because you did not eat enough. Because your body has forgotten how to use the fuel it needs at that intensity.
This is not a theoretical concern. The research consistently shows that chronic low-carb and ketogenic diets impair performance at intensities above roughly 80% of VO2max. For a cyclist, that covers everything that matters in a race: surges, climbs, breakaway efforts, and the final sprint. The very moments that separate finishing in the group from finishing off the back.
Reduced sprint capacity. Reduced ability to respond to attacks. Reduced power on climbs. Reduced capacity to sustain race-pace efforts. These are not minor trade-offs. They are the entire competitive picture.
Train-Low Is Not Low-Carb
This is the critical distinction that gets lost in the noise, and it is the difference between a useful training strategy and a dietary dead end.
Train-low does not mean eating a low-carb diet. It means performing specific training sessions with reduced glycogen availability to amplify the adaptive signal from that session. It is a training stimulus, applied with precision, not a blanket dietary restriction.
The logic: when you exercise with low muscle glycogen, certain molecular signals in the muscle are amplified. AMPK activation increases. PGC-1 alpha expression goes up. Mitochondrial biogenesis gets a stronger nudge. The aerobic training adaptation from that session is greater than if you had done the same session fully fuelled.
But — and this is the part the keto crowd ignores — this only applies to specific types of sessions. Easy, aerobic, Zone 2 work. The kind of riding where fat is already the primary fuel and where the molecular signals for aerobic adaptation are the goal. Apply the same low-glycogen approach to a high-intensity interval session and you get a worse session, a worse stimulus, and a worse adaptation. You have not enhanced the training. You have sabotaged it.
The train-low framework works precisely because it is selective. You deplete for the sessions where depletion enhances the signal. You fuel for the sessions where carbohydrate availability determines the quality of the work. The two are not in conflict. They are complementary parts of a periodised approach.
The Evidence-Supported Train-Low Protocols
Not all train-low strategies are created equal. Some have solid evidence behind them. Others are gym-bro folklore wearing a science costume. Here are the protocols with genuine research support.
Sleep-Low
The sleep-low protocol is one of the most studied and most practical train-low methods. The structure: perform a glycogen-depleting session in the evening — a hard ride, a session with plenty of high-intensity work — then restrict carbohydrate intake overnight. Sleep with your glycogen stores low. The following morning, complete an easy endurance ride in that glycogen-depleted state.
Marquet and colleagues published a 2016 study showing that three weeks of sleep-low training improved cycling performance, body composition, and markers of fat oxidation compared to the same training volume and diet with normal carbohydrate timing. Same total carbohydrate intake. Same training. Different timing. Better results.
The beauty of the sleep-low approach is its simplicity. You are not starving yourself. You are not cutting carbs out of your diet. You are shifting when you eat them. The hard evening session gets fully fuelled. The easy morning ride uses the depleted state as a stimulus. Then you refuel normally after the morning ride and the rest of the day proceeds as usual.
Fasted Morning Rides
The simplest version of train-low: ride easy in the morning before eating. Water and black coffee only. Keep the intensity strictly aerobic — Zone 2, conversational pace, no surges. Duration: sixty to ninety minutes for most riders, building gradually.
The evidence here is less dramatic than sleep-low because the glycogen depletion is modest (you still have liver and muscle glycogen from the previous day's eating). But the combination of overnight fasting and easy riding does create a low-insulin, low-glycogen environment that amplifies some of the molecular signals for mitochondrial adaptation.
The key constraint: easy means easy. The moment you surge — up a hill, chasing a group, sprinting for a village sign — you have moved out of the zone where this works and into the zone where you are just doing a bad interval session without fuel.
Twice-a-Day Training
The twice-a-day model uses the first session to deplete glycogen and the second session to train in the depleted state. Morning: a harder ride that burns through glycogen. Restrict carbohydrate between sessions. Afternoon: an easy endurance ride in the depleted state.
This is the approach that many professional teams and sports institutes use, and it has good evidence behind it from researchers like John Hawley. The practical problem is that it requires two sessions in a day, which puts it out of reach for most amateur riders with jobs and families.
Restricting Carbohydrate During Easy Endurance Rides
Perhaps the most accessible train-low strategy: when you are doing an easy, long endurance ride, simply do not take on carbohydrate during the ride. Ride on water. Eat normally before and after. Let the ride itself create the mild depletion.
This is a low-risk, low-drama version of train-low that most riders can implement immediately. The condition, again, is that the ride stays easy. If you are riding four hours in Zone 2, skipping the energy drink and gels is a reasonable train-low stimulus. If you are riding four hours with a group and doing surges on every climb, you need to fuel.
Sam Impey and Periodised Nutrition
The most practical framework for applying all of this comes from Sam Impey's research at Loughborough University. Impey's work centred on a concept called "fuel for the work required" — the idea that carbohydrate intake should be matched to the demands of each specific training session, not set at a fixed daily level.
The science has finally caught up with what good coaches have been doing intuitively. Impey and his colleagues — working alongside James Morton, who developed much of the practical periodisation framework at Liverpool John Moores University — showed that periodising carbohydrate intake improved markers of fat oxidation and metabolic flexibility without the performance impairments seen in chronic low-carb approaches. Riders who ate more carbohydrate around hard sessions and less around easy sessions got the aerobic benefits of training with low glycogen and the performance benefits of training with high glycogen. They did not have to choose. The high-intensity work stayed high-quality because it was fuelled. The easy work amplified its aerobic signal because it was not over-fuelled.
What made Impey's contribution distinctive was the emphasis on thresholds. He identified that the muscle glycogen concentration at which the enhanced molecular signalling kicks in is somewhere below about 300 mmol per kilogram of dry weight. Above that threshold, the extra aerobic signal from training is minimal. Below it, the signal ramps up. This gives practitioners a target — you do not need to be fully depleted; you need to be below the threshold — and it explains why moderate train-low strategies can work without the misery of full glycogen depletion.
This is the middle path that the two warring camps refuse to acknowledge. You do not have to choose between "always high-carb" and "always low-carb." You can — and the evidence strongly suggests you should — vary your carbohydrate intake day to day and session to session based on what the training demands.
Impey's framework makes the daily decision straightforward. Hard intervals tomorrow? Eat carbohydrate today and fuel the session properly. Easy Zone 2 spin? Pull back the carbs a little. Race or event? Full fuelling, no question.
The total weekly carbohydrate intake might look similar to a standard sports nutrition recommendation. The difference is in the distribution. Instead of spreading carbohydrate evenly across every day, you concentrate it around the sessions that need it and pull it back from the sessions that benefit from its absence.
Asker Jeukendrup and the Performance Side
No discussion of carbohydrate and cycling is complete without Asker Jeukendrup's contribution. If Impey's work defines the periodised middle ground, Jeukendrup's research defines the ceiling: how much carbohydrate can a cyclist actually absorb and use during exercise, and what happens when you get the fuelling right?
Jeukendrup's lab established the protocols that underpin modern race fuelling. The key findings: trained cyclists can absorb and oxidise up to 90 grams of carbohydrate per hour when using a glucose-fructose mix (the dual-transporter model, where glucose uses the SGLT1 transporter and fructose uses GLUT5, allowing higher total absorption than either sugar alone). Higher carbohydrate intake during prolonged exercise consistently improves performance compared to lower intakes. His work also demonstrated that the gut itself is trainable — riders who practise high carbohydrate intake during training can increase their absorption capacity over a period of weeks, reducing the gastrointestinal distress that often limits fuelling in races.
This is directly relevant to the train-low conversation because it highlights a common mistake. Some riders adopt train-low strategies for all their rides, including the hard ones, and then wonder why they cannot tolerate 90 grams per hour on race day. Their gut has never practised absorbing carbohydrate at that rate. The hard, fuelled sessions are not just protecting performance in training — they are training the gut for race day.
This is the other half of the equation. Train-low strategies have their place in building aerobic capacity and metabolic flexibility. But when it is race day, when the efforts are hard, when the duration is long and the intensity is real — that is when you need every gram of carbohydrate your gut can process. The two approaches are not contradictory. They serve different purposes at different times.
The practical rule: train-low to build the aerobic engine. Race high to use it.
The Practical Framework: How to Periodise Carbs as a Cyclist
Enough theory. Here is how this works in a real training week.
The decision tree is simple. Before each session, ask one question: does this session require high-intensity effort, or is it easy aerobic work?
If the session is hard — intervals, tempo, race simulation, group ride with surges — fuel it. Eat carbohydrate in the meal before. Take 60-90 grams per hour during if it is longer than seventy-five minutes. Refuel with carbohydrate and protein within thirty minutes afterwards. Do not compromise the quality of the hard work by withholding fuel.
If the session is easy — Zone 2 endurance, recovery spin, conversational pace throughout — you have the option to reduce carbohydrate. Lighter breakfast or ride fasted. Water instead of energy drink. Food after the ride rather than during. This is not deprivation. It is matching the fuel to the demand.
If it is a race or event — full fuelling, every time, no exceptions. Carbohydrate-loading in the days before. Breakfast three to four hours before the start. 60-90 grams per hour during. This is not the time for train-low experiments.
A practical week might look like this:
Monday: rest day, moderate carbohydrate. Tuesday: hard interval session. High carbohydrate before, during, after. Wednesday: easy Zone 2 ride. Lighter breakfast, water on the ride, eat normally after. Thursday: tempo or sweet spot session. Full fuelling. Friday: rest day or very easy spin. Lower carbohydrate. Saturday: long endurance ride. If easy throughout, ride with reduced carbs for the first two hours, then fuel normally. If the ride includes hard efforts or group dynamics, fuel from the start. Sunday: race or hard group ride. Full fuelling.
Nothing radical. Nothing restrictive. Just matching the supply to the demand, session by session, across the week.
Who Should Avoid Train-Low Entirely
Train-low is not for everyone, and being honest about that is part of taking the evidence seriously.
Female cyclists at risk of relative energy deficiency in sport (RED-S). RED-S is a systemic condition caused by insufficient energy availability, and it carries serious consequences for bone health, hormonal function, immune function, and long-term performance. Any deliberate restriction of carbohydrate on top of existing low energy availability deepens the problem. If there is any question about whether your energy intake supports your training, the energy availability calculator is the place to start — not a train-low protocol.
Riders with a history of disordered eating. Train-low requires a healthy relationship with food and the ability to restrict and refuel without triggering restrictive patterns. For anyone with a history of disordered eating, the deliberate manipulation of carbohydrate around training sessions is a risk that outweighs any potential aerobic benefit.
Anyone in a heavy training block. If you are in a build phase with five or six quality sessions a week, your recovery demands are high and your glycogen needs are real. Adding deliberate depletion on top of an already demanding training load increases the risk of illness, overtraining, and accumulated fatigue. Save train-low experiments for lower-volume phases where the recovery cost is manageable.
Anyone already in a calorie deficit. If you are actively trying to lose weight and are eating below your maintenance calories, train-low is piling restriction on top of restriction. Get the energy balance right first. Pursue body composition goals and metabolic training goals separately, not simultaneously.
Masters riders who are under-recovering. Older athletes are already fighting harder to maintain muscle mass and recover from training. Restricting fuel around sessions without exceptional care can accelerate muscle loss and impair recovery. If you are over forty and considering train-low, keep it conservative — one or two fasted easy rides per week, always fully fuelling hard sessions, and monitoring your recovery markers closely.
What the Evidence Actually Tells Us
Strip away the ideology and the internet arguments, and the research points to a remarkably consistent picture.
Fat adaptation is real. You can train your body to burn more fat at a given intensity. The metabolic shift is measurable and meaningful for certain types of riding. But chronic low-carb and ketogenic approaches achieve this adaptation at the cost of the carbohydrate oxidation capacity you need for everything above moderate intensity. For competitive cyclists, that trade-off is a bad deal.
Train-low is a useful tool when applied with precision. Specific sessions performed with reduced glycogen can amplify the aerobic training signal. The sleep-low protocol, fasted morning rides, and reduced carbohydrate during easy endurance work all have evidence behind them. But they work because they are targeted — applied to the right sessions, at the right time, within a framework that still prioritises fuelling the hard work.
Periodised nutrition — fuel for the work required — is the cycling nutrition approach that squares the circle. It captures the aerobic benefits of occasional low-glycogen training without sacrificing the carbohydrate availability that powers race-pace efforts. It is flexible, sustainable, and supported by the work of researchers like Impey, Burke, and Jeukendrup.
The practical takeaway is not complicated. Fuel hard sessions. Reduce carbohydrate around easy rides when appropriate. Never restrict during races and events. And be honest about whether your individual circumstances — your training load, your energy balance, your health history — make train-low a suitable addition or an unnecessary risk.
The good news is that this is not a rigid protocol. It is a principle. Match the fuel to the work. The rest is details.
If you want to dig into how this fits your own training and nutrition, bring the question to the Roadman community on Skool. Real riders, real coaches, no ideology.