Marathon Carb Loading: The Modern 36-Hour Protocol That Replaces Depletion

Educational journalism, not medical advice. Every claim here is checked against its cited sources by editor Tim Bunce — a health writer, not a physician. It isn’t specific to your situation: for health decisions, talk to your own clinician. How we work →

Why glycogen matters at marathon distance

Muscle glycogen stores in a normally-fed adult are roughly 1,500-2,000 kcal worth — sufficient for 90-120 minutes of running at marathon pace. Marathon pace times exceed this in most runners. Without carb loading, runners typically “hit the wall” (rapid pace deterioration as glycogen depletes) around 30-35km. With proper carb loading, super-compensated muscle glycogen extends the glycogen window to roughly 35-40km, and intra-race carbohydrate consumption fuels the remainder. The performance impact is large — well-loaded runners typically maintain pace where unloaded runners decelerate significantly Burke 2011.

The modern protocol

• 36-48 hours before race: begin carb loading. No depletion phase needed.
• 8-12g carbohydrate per kg body weight daily. For a 70kg athlete: 560-840g/day. For an 80kg athlete: 640-960g/day. These are large numbers requiring planning.
• Distribute across 4-6 meals. 100-150g of carbohydrate per meal, plus snacks. Eating one giant pasta dinner doesn’t deliver enough.
• Cut fat and fibre. Both slow gastric emptying and produce GI volume that interferes with race-day comfort. The pre-race diet should be lower than normal in vegetables, beans, whole grains; higher in white rice, pasta, bread, low-fibre cereals, juice, sports drinks.
• Finish loading 12-16 hours before the start. The night-before dinner can be normal-sized. Race morning should be a small, easy meal (200-400 kcal of simple carbohydrate, 2-4 hours before the start).
• Hydrate with electrolytes throughout the loading period. Glycogen storage requires water (3g water per gram of glycogen). Expect 1-2kg of water-weight gain from a successful load.

During-race carbohydrate

• 60-90g of carbohydrate per hour, beginning around 30-45 minutes into the race.
• Use multi-transportable mixes (glucose:fructose ratio of roughly 2:1). Glucose alone is absorbed at a maximum of 60g/hr; adding fructose uses a separate transporter, raising the absorption ceiling to 90+g/hr Jentjens 2004.
• Practice the strategy in training. Gut tolerance to high carbohydrate intake is trainable; race-day GI distress is usually from untrained gut handling.
• Sources: sports gels, energy drinks, banana, dates, hard candy. Solid food works for some runners but produces more GI issues at race pace.

“Modern carb-loading protocols using 8-12g/kg of carbohydrate over 36-48 hours achieve muscle glycogen super-compensation equivalent to the older depletion-loading protocols, with significantly better gastrointestinal tolerance and reduced training disruption.”

— Burke et al., J Sports Sci, 2011 view source

Common errors

• Carbohydrate dose too low. “Just eat more pasta” falls short of 8-12g/kg in most cases. Track the actual intake during the loading period.
• Too much fibre. Whole-grain pasta, brown rice, beans, vegetables — nutritious normally, problematic the day before a marathon. Switch to white versions.
• Race-morning carb-loading. Too late. The intestinal absorption and glycogen synthesis timeline doesn’t allow meaningful loading on race day.
• Untrained gut for intra-race carbohydrate. 60-90g/hr is well-tolerated only with prior training. Practice fuelling at race intensity during long training runs.
• Carb-loading for short events. 5k, 10k, half marathon don’t need carb-loading — normal mixed diet provides sufficient glycogen for events under 90 minutes.

Who actually needs this

• Marathon and longer (42km+): yes, carb-loading produces meaningful benefits.
• Half marathon (21km): probably not. Most well-trained athletes complete a half marathon within their existing glycogen window. A normal mixed diet pre-race is sufficient.
• Long-course triathlon (Olympic+): yes, similar protocol.
• Ultra-distance events: yes, with adapted nutrition during the event.
• Cycling stage races / multi-day events: daily carbohydrate intake during the event matters more than pre-event loading.

Practical takeaways

• Modern carb loading: 8-12g/kg/day of carbohydrate for 36-48 hours before the race. No depletion phase needed.
• Distribute carbohydrate across 4-6 meals; one large dinner isn’t enough.
• Cut fat and fibre during the loading period to reduce GI volume and race-day discomfort.
• During the race: 60-90g carbohydrate/hour using multi-transportable mixes (glucose:fructose 2:1).
• Practice intra-race fuelling in training — the gut handles high intake only with prior adaptation.

Going the other direction — deliberately training with low glycogen to nudge adaptation — has its own evidence and trade-offs: see glycogen-depletion workouts (training low).

Where the protocol came from — and why the brutal "depletion" phase died

The 36-hour plan in this article is the great-grandchild of a much harsher regimen. In the late 1960s, Swedish physiologists Jan Bergström and Eric Hultman used a needle-biopsy technique to sample muscle before and after exercise and made a landmark discovery: if you first exhausted a muscle's glycogen with hard exercise and then ate a high-carbohydrate diet for several days, that muscle rebuilt its glycogen to roughly double its normal level. Crucially, the rebound happened only in muscles that had been emptied first, not in rested limbs. This rebound — called glycogen supercompensation (storing more than baseline) — became the scientific basis for carb-loading, and the team's companion work tied higher muscle glycogen directly to longer time-to-exhaustion Solem 2025.

A 2025 systematic review and meta-analysis of 30 studies (319 participants) quantified just how large that rebound is. After glycogen-depleting exercise followed by a high-carbohydrate diet, muscle glycogen climbed by roughly 270 mmol per kg dry weight after cycling and about 157 mmol per kg after running, settling near an apparent ceiling of about 700 mmol per kg dry weight — evidence that the muscle has a finite "tank" that loading fills but cannot overflow indefinitely Solem 2025. The same review confirmed that deeper depletion before loading produced a bigger supercompensation, which is exactly why the original protocols prescribed an exhausting depletion bout days before the race Solem 2025.

So why has the modern protocol dropped that depletion phase entirely? Because the practical cost outweighed the small extra gain. The classic depletion ride or run left runners exhausted, irritable, and prone to illness or injury in the final taper week — the worst possible moment. Contemporary reviews of carbohydrate strategies for elite endurance athletes now recommend simply loading with 10–12 g of carbohydrate per kg of body weight per day, started 36–48 hours out, with no depletion phase at all; well-trained, tapered athletes fill their tanks adequately without first emptying them Cao 2025. The depletion step survives today mainly as a laboratory tool for studying glycogen, not as race preparation Solem 2025.

Why women may need to load differently

Most carb-loading guidance was written from studies on men, and the numbers do not transfer cleanly. The pivotal evidence comes from a 2001 trial by Tarnopolsky and colleagues: when female endurance athletes raised the percentage of carbohydrate in their diet without eating more total food, their muscle glycogen did not increase the way men's did. Only when the women increased their total energy (calorie) intake alongside the higher carbohydrate proportion did their muscle glycogen rise meaningfully Tarnopolsky 2001. The practical lesson: for a woman, "eat a higher-carb diet" is not enough — she has to eat more food overall during loading days, not just swap fat for carbohydrate within the same calorie budget.

A review of sex differences in carbohydrate metabolism reached the same conclusion: women can supercompensate their glycogen comparably to men, but appear less responsive to simply raising the carbohydrate fraction of the diet, and tend to oxidise proportionally less carbohydrate (and more fat) at a given exercise intensity Tarnopolsky 2006. Targeting the same per-kilogram dose used in this article (8–12 g/kg) is reasonable for women, provided total energy intake genuinely rises to support it Tarnopolsky 2006. Menstrual-cycle hormones add a smaller layer: estrogen tends to nudge metabolism toward fat use, so glycogen storage and use can vary modestly across the cycle, though this is a fine-tuning detail rather than a reason to abandon loading Tarnopolsky 2006.

There is a safety edge here worth naming. Many female endurance runners already under-eat relative to their training, a state linked to Relative Energy Deficiency in Sport (RED-S) and its knock-on effects on bone, hormones, and immune health Lodge 2023. A woman who treats carb-loading as merely "more rice, less fat" inside an already-restricted intake may fail to load and stay chronically under-fuelled. If you menstruate, are recovering from disordered eating, are pregnant, or have a condition such as diabetes, treat the loading week as a reason to talk with a sports dietitian or your clinician rather than improvising Lodge 2023.

The scale won't budge in your favour — here's why that's normal

Nearly every runner who loads correctly steps on the scale the morning of the race and panics: they are 1–2 kg heavier than usual. This is expected and not fat gain. Glycogen is stored in muscle and liver bound to water, and the body retains roughly 3 grams of water for every gram of glycogen laid down — a ratio first described in human muscle work by Olsson and Saltin and still the standard rule of thumb Shiose 2022. Fill a depleted glycogen tank and you necessarily carry several hundred grams of extra glycogen plus its associated water, which a narrative review found adds about 1.0–1.5 kg of body mass, almost entirely as water rather than fat Shiose 2022.

That bound water is a feature, not a bug. As you burn glycogen during the race, the water it held is released and becomes available to your circulation and sweat — a small built-in hydration reserve. It is worth being honest about the evidence, though: the exact water-to-glycogen ratio is not settled. The same review notes human measurements ranging widely (from about 3:1 up to far higher values) depending on how much fluid was consumed during recovery, and some studies could not cleanly separate glycogen-bound water from water simply retained after drinking Shiose 2022. The takeaway for race morning is simple: a couple of kilograms of "loading weight" signals that the protocol worked. Do not try to diet it off in the final days, and do not interpret it as having eaten too much.

"Training the gut": why 90 g/hour is a skill, not a given

The during-race section above recommends 60–90 g of carbohydrate per hour from a glucose-fructose mix. What it doesn't say is that the high end of that range is a trained capacity, not something most runners can do on race day cold. The ceiling exists because glucose is absorbed across the gut wall by a transporter called SGLT1, which saturates at roughly 60 g/hour; pour in more single-source carbohydrate and the excess simply sits in the gut, drawing in water and causing the bloating, cramping, and diarrhoea runners dread Jeukendrup 2017. Adding fructose helps because fructose rides a different transporter, GLUT5, opening a second absorption lane; this is the mechanism behind the multiple-transportable-carbohydrate mixes that allow oxidation to climb toward 90 g/hour, and it is why the 90 g/hour figure is reserved for events lasting two hours or more Jeukendrup 2017.

The gut, importantly, is adaptable. Repeatedly practising high carbohydrate intakes during long training runs appears to increase the number of those intestinal transporters and improve gastric emptying and comfort, so the same dose that wrecks an untrained stomach becomes tolerable Jeukendrup 2017. This is the literal meaning of "train the gut," and it is why coaches insist on rehearsing race fuelling in the weeks beforehand. Practical guidance from reviews of elite endurance fuelling reinforces the same principle from both ends of the day: keep pre-race meals familiar, low in fat and fibre, and easy to digest, and build intra-race tolerance gradually rather than debuting a 90 g/hour plan on race morning Cao 2025. A useful self-test in the final block of training is whether you can comfortably take in 60–90 g/hour on a long run; if your gut rebels, scale back the race-day target rather than pushing through.

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