Of all the nutrition mistakes marathon runners make, restricting carbohydrates is among the most common and the most costly. It is also one of the most understandable — diet culture has spent decades telling everyone that carbohydrates are the enemy, that cutting them leads to leanness, and that the most disciplined athletes avoid them. None of this is true for someone training for 26.2 miles, and understanding why requires getting clear on what carbohydrates actually do in the body during sustained endurance exercise.
This is not a post about whether carbohydrates are healthy in the abstract. It is about what happens physiologically when a marathon runner does not eat enough of them — and what happens when they do.
What Carbohydrates Actually Do
Carbohydrates are the body’s preferred and primary fuel source for exercise above easy aerobic intensity. When you eat carbohydrates, they are broken down into glucose and either used immediately for energy or stored as glycogen — primarily in the muscles and liver — for later use.
Muscle glycogen is the fuel your body draws on during moderate to hard running efforts. The liver glycogen maintains blood glucose levels between meals and provides fuel to the brain, which is also entirely dependent on glucose and cannot use fat as a direct fuel source. When both stores are adequately stocked, you have the fuel to run hard, sustain pace, and maintain focus through the back half of a long run.
Fat is also used for fuel during running, but primarily at lower intensities. As pace increases and effort climbs, the contribution of fat to energy production drops and the contribution of carbohydrates rises steeply. By the time you are running at your marathon goal pace or above, carbohydrates are doing the overwhelming majority of the work.
This is not a theory. It is exercise physiology, and it is among the most consistently established findings in sports nutrition science.
What Happens When You Run Low
Most runners who have trained for a marathon have experienced it at some point — the wall, the bonk, the point somewhere in the mid-to-late miles where everything falls apart. Legs stop responding. Pace drops involuntarily. The brain goes foggy. What is happening is not a fitness problem. It is a fuel problem. Glycogen stores have been depleted to the point where the body can no longer sustain the work, and the brain — running low on glucose — is signaling distress.
Hitting the wall at mile eighteen is the dramatic version of glycogen depletion. But the more common version is subtler and more insidious: chronic mild carbohydrate restriction across a training cycle that slowly degrades performance, recovery, and adaptation without producing a single obvious crisis moment.
A runner who has been cutting carbs — eating low-carb meals, skipping pre-run fuel, avoiding rice and pasta out of habit — is arriving at most training sessions with partially depleted glycogen stores. The sessions feel harder than they should. The intervals do not produce the quality they are supposed to. Recovery takes longer. The long run at the end of the week feels like a slog rather than a training stimulus. And because none of this has a single obvious cause, it tends to be attributed to overtraining, poor sleep, life stress, or simply a bad running week.
The actual cause is frequently the fuel deficit that has been accumulating for weeks.
Carbohydrate Needs During Marathon Training Are Not Modest
One of the consistent surprises for runners who start working with a sports dietitian is how much carbohydrate adequate fueling for marathon training actually requires. The numbers are significantly higher than most people assume, and they scale with training load in ways that daily habits rarely account for.
General sports nutrition guidelines recommend five to seven grams of carbohydrate per kilogram of body weight per day for moderate training loads, and six to ten grams per kilogram for high-intensity or high-volume training. During peak marathon training weeks — when long runs are approaching twenty miles and total weekly mileage is high — some runners’ needs climb to eight to twelve grams per kilogram per day.
For a 130-pound runner, six grams per kilogram — the midpoint of the moderate training range — works out to roughly 355 grams of carbohydrate daily. That is approximately five cups of cooked pasta, or eight cups of cooked rice, or about ten to twelve medium pieces of fruit — on top of everything else they are eating. At peak training weeks, that same runner’s needs climb toward 475 to 710 grams per day. Most runners who are nominally “eating enough” are eating considerably less carbohydrate than this, particularly if they have absorbed any low-carb messaging.
These numbers are not a prescription for every meal of every day. They are a framework for understanding the magnitude of the fuel requirement and why running a meaningful deficit is easy to do accidentally.
Carbs and Training Adaptation
Beyond fueling individual sessions, carbohydrate availability affects the quality of training adaptation — the actual physiological changes that make you fitter.
Hard training sessions — tempo runs, intervals, long runs at progressive pace — require adequate carbohydrate availability to perform at the intended quality. When glycogen is low going into a hard session, the body cannot sustain the pace that makes the workout a productive stimulus. You are physically incapable of running at the intensity that produces adaptation. The session becomes a junk mile at slow pace rather than the quality work it was designed to be, and the training benefit is largely lost.
After a hard training session, carbohydrate intake in the recovery window — particularly within the first thirty to sixty minutes post-run — replenishes glycogen stores and initiates the repair processes that make training adaptation possible. Skipping post-run carbohydrates to avoid eating “too much” delays recovery and leaves the body less prepared for the next session.
Over a sixteen or twenty-week training cycle, the compounded effect of session after session at degraded quality due to low carbohydrate availability is significant. Fitness that should have been built is not built. Race day performance reflects not just the training that was completed, but the quality of it.
What About Fat Adaptation and Training Low?
There is a real concept in exercise physiology called metabolic flexibility — the body’s ability to use fat more efficiently as a fuel source — and there is a training approach called “train low” that deliberately uses low glycogen sessions to stress fat oxidation pathways. This approach has some evidence behind it and is used strategically by some elite endurance athletes.
But there are important caveats that tend to get lost when this idea filters down to recreational runners.
Training low is a specific, periodized protocol in which low-glycogen sessions are strategically placed within a larger training plan that includes plenty of adequately fueled sessions. It is not the same as simply eating low-carb across the board and training in a chronically depleted state. Research on train-low protocols consistently shows that key quality sessions — the ones that drive adaptation and race fitness — need to be performed well-fueled to produce the intended training effect. The performance of those sessions is impaired when carbohydrate availability is low.
For the vast majority of recreational marathon runners, the risk-to-benefit math on chronic low-carb training does not work out. The sessions are harder, adaptation is compromised, injury risk goes up, and race performance suffers. Fat adaptation is not a meaningful limitation for marathon performance in runners who are simply eating enough carbohydrates to support their training.
Carb Loading: What It Actually Means
Carb loading — the practice of increasing carbohydrate intake in the days before a race — has been a staple of marathon preparation for decades, and the underlying physiology is sound. Maximizing glycogen stores before a long race delays the onset of depletion and reduces the likelihood of hitting the wall.
The modern evidence-based approach to carb loading involves increasing carbohydrate intake to around ten to twelve grams per kilogram over the final one to three days before a marathon, while tapering training volume. This tops off both muscle and liver glycogen stores so that race day begins with the fuel tanks as full as possible.
A few important practical notes. First, carb loading does not work if your baseline dietary carbohydrate intake is already low — there is nothing to load onto. Runners who restrict carbohydrates throughout training cannot compensate with two days of pasta before the race and expect the same result as runners who have been fueling adequately all along. Second, carb loading often causes a temporary weight gain of two to four pounds due to water retention associated with glycogen storage. This is expected, normal, and does not reflect fat gain. Third, the night before a race is not the time for a huge unfamiliar meal — carb loading should happen over two to three days, not in one sitting at the pre-race dinner.
Practical Carbohydrate Sources for Marathon Runners
High-carbohydrate eating does not have to mean eating pasta at every meal, and it does not require choosing foods you do not enjoy. The goal is consistent carbohydrate availability across the day, with additional emphasis around training sessions.
Grains and starches — rice, pasta, bread, oats, quinoa, potatoes, sweet potatoes — are the backbone of carbohydrate intake for most runners and deserve to be at the center of the plate rather than treated as an optional addition.
Fruit provides carbohydrates alongside vitamins and micronutrients, digests quickly, and is particularly useful as pre-run fuel when the goal is easy-to-digest energy without a heavy stomach.
Legumes — lentils, beans, chickpeas — provide carbohydrates alongside protein and fiber, making them useful at meals that do not immediately precede a run.
Sports nutrition products — gels, chews, sports drinks, energy bars — are useful for fueling during long runs and races when solid food is not practical, but they are supplements to an overall carbohydrate-sufficient diet, not the primary strategy.
The timing of carbohydrate intake matters as much as the total. Including a meaningful carbohydrate source at every meal and snack — not just at dinner — distributes fuel availability across the day and prevents the late-day glycogen deficit that often drives evening overeating and the loss of control around food that many runners experience.
The Relationship Between Carbs and the Runner’s Relationship With Food
One more thing worth naming directly. Many runners who restrict carbohydrates are doing so in the context of a complicated relationship with food and body composition — the belief that leaner is faster, that eating carbohydrates will undermine weight management, or that the discipline of avoiding them is part of what makes a good athlete.
This belief system produces real harm. Chronic carbohydrate restriction in a training context drives the hunger and loss of control that many runners experience as evidence they need to restrict more. It contributes to the cycles that exhausts athletes mentally and emotionally. And for female athletes especially, it is a meaningful driver of the underfueling that produces hormonal disruption, bone stress injuries, and the performance decline that is the opposite of what the restriction was supposed to produce.
Eating enough carbohydrates to support marathon training is not a lack of discipline. It is the discipline of fueling the work correctly. The runners who perform best over long athletic careers are almost universally the ones who have figured this out.
If you are training for a marathon and want to build a nutrition plan that actually supports the work you are putting in, a free connect call is the place to start.
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Frequently Asked Questions
How many carbs should I eat while training for a marathon? General sports nutrition guidelines recommend five to seven grams of carbohydrate per kilogram of body weight per day for moderate training loads, and six to ten grams per kilogram for high-intensity or high-volume training. During peak marathon training weeks — when long runs are approaching twenty miles and total weekly mileage is high — some runners’ needs climb to eight to twelve grams per kilogram per day.
Should I go low-carb to lose weight before my marathon? Pursuing significant weight loss through carbohydrate restriction during a marathon training cycle is a strategy that tends to produce worse performance, slower recovery, higher injury risk, and a more difficult training experience — while not reliably producing the intended result. Carbohydrate restriction during heavy training degrades training quality and adaptation, meaning the fitness you are trying to build is undermined by the fuel deficit. If body composition is a goal alongside performance, working with a registered dietitian to find an approach that does not compromise the training is strongly recommended.
What should I eat before a long run? Three to four hours before a long run, a meal of three hundred to five hundred calories centered on carbohydrates, and a snack 1-2 hours prior, within 60 minutes carbs only — toast with peanut butter and banana, oatmeal with fruit, a bagel — provides the glycogen top-off that supports performance over the full duration of the run. Within thirty to sixty minutes before the run, if needed, something lighter and quick to digest — a banana, a gel, a small handful of crackers. The morning workout nutrition post covers pre-run timing in more detail.
When should I start carb loading before a marathon? Most evidence supports beginning carb loading two to three days before the race, increasing carbohydrate intake to approximately ten to twelve grams per kilogram while reducing training volume during the taper. A single large pasta dinner the night before is not carb loading — it is one meal. The process needs to happen over several days to meaningfully top off glycogen stores. If this seems overwhelming and it’s your first time, starting with a ‘baby’ carb load can help. Eight grams per kilogram of carbohydrates per day would be a good place to start.
Can I eat too many carbs during marathon training? In practice, most marathon runners are not eating too many carbohydrates — they are eating too few. Excess carbohydrate intake beyond what can be stored as glycogen is converted to fat, but this requires consistently eating far more than training demands, which is uncommon in athletes with high weekly mileage. For runners who are genuinely eating well above their needs, a registered dietitian can help calibrate intake to match training load precisely. But for the vast majority of recreational marathon runners, the more relevant question is whether they are eating enough.