The mental image of carbo loading most runners carry around is a single oversized plate of pasta the night before a race, eaten until the runner feels uncomfortably full, washed down with a glass of wine that won’t help anything. This is not carbo loading. It is dinner.
Actual carbo loading is a 36-48 hour protocol with a specific gram-per-kilo target, distributed across multiple meals, paired with controlled fluid intake. Done well, it adds roughly 1.5-2 kg of body weight — most of it glycogen and the water bound to it — and meaningfully extends the duration you can hold race intensity before fueling becomes a question of survival rather than maintenance. Done badly, or done as a single Saturday-night ritual, it does nothing except make Sunday morning’s run harder.
This post breaks down what the protocol actually is, why it works, and how to do it without spending the last two days before a race feeling sick.
What the protocol actually is
The modern carbo-loading literature, anchored in work from Ron Maughan and Asker Jeukendrup’s groups through the 2000s and 2010s, converges on a fairly tight prescription. The target is 10-12 grams of carbohydrate per kilogram of body weight per day, sustained for 36-48 hours before the race.
For a 70 kg runner, that’s 700-840 grams of carbohydrate per day. For 48 hours.
That number is much higher than most runners realise. A typical mixed-diet day for an active person might land around 250-350 grams of carbohydrate — already higher than the general population, but well under half the carbo-load target. To hit 700+ grams in a single day, the runner has to make a deliberate, structural shift in what they eat. Pasta dinners alone won’t do it. The carbs have to be present in every meal, in every snack, and in fluids.
The window matters. Older protocols asked runners to deplete glycogen with a hard long run six or seven days out, then load aggressively over the final three days. This works but is unpleasant and adds injury risk. The modern consensus skips the depletion phase entirely for races up to and including the marathon — the taper does most of the depletion-reduction work on its own. The high-carb window starts on the Friday morning of a Sunday race and runs through Saturday night.
Why it works
Glycogen is the body’s accessible carbohydrate store, held in muscle (about 400-500 grams in a trained endurance athlete) and liver (about 80-110 grams). Race-pace running burns through this store at roughly 100 grams per hour at marathon pace for an average runner, faster at faster paces. Without supplementation, the tank empties somewhere between 90 and 120 minutes of running.
Two things happen when you carb-load aggressively for 36-48 hours:
Muscle glycogen supercompensates. The skeletal muscle responds to repeated high-carb meals by storing glycogen at concentrations above its normal baseline. A well-trained marathoner who normally carries 400 g of muscle glycogen can push that to 550-600 g with a proper load. That extra 150-200 grams of glycogen is the difference between running out of fuel at mile 18 and having reserves to spare at mile 24.
Liver glycogen fills. Liver glycogen is what stabilises your blood sugar overnight and during the early miles of the race. A runner who skips dinner, runs in the morning, and doesn’t load is starting the race with a half-empty liver. The first symptom is light-headedness in the first 5 km; the second is over-reliance on muscle glycogen, accelerating its depletion.
The water that comes with the glycogen — roughly 3 grams of water per gram of stored glycogen — is also useful. It bumps total body water, improves the buffer against dehydration in the early miles, and is part of why a properly loaded runner feels heavier and slightly bloated on race morning. That feeling is correct. It means the protocol worked.
What 700-800 grams of carbohydrate actually looks like
This is where most runners undershoot. Hitting the target requires conscious effort because energy-dense, low-volume carb sources have to make up most of the load — there isn’t physically enough room in the gut to load via vegetables and salads.
A working day plan for a 70 kg runner targeting ~750 grams:
- Breakfast (~180 g): large bowl of oatmeal with honey and banana, glass of orange juice, two slices of toast with jam.
- Mid-morning snack (~80 g): a sports drink mix and a granola bar.
- Lunch (~200 g): big bowl of pasta or rice with a low-fat sauce, bread roll, fruit juice.
- Afternoon snack (~120 g): bagel with honey, banana, sports drink.
- Dinner (~170 g): more pasta or rice, bread, fruit-based dessert, dilute sports drink with the meal.
Notice what is not on that plan. There is very little fat. Very little protein. Very little fibre. This is deliberate. Fat slows gastric emptying and crowds out room for carbohydrate. Protein in large amounts does the same and isn’t doing meaningful glycogen work. Fibre adds bulk without calories, makes you feel full prematurely, and risks gut distress on race morning.
Race-week eating is the one block of the cycle where a “healthy plate” model is actively counterproductive. You want refined carbs, low fibre, low fat, low protein, and you want them in volume.
Where drink mixes and starches earn their place
This is where most runners find the math impossible without help. Eating 700+ grams of carbs in solid food for two days running is genuinely hard. The gut runs out of capacity, the jaw runs out of motivation, and the appetite collapses before the target is hit.
Liquid carbohydrate is the way through. A 500 ml serving of a high-carb sports drink can deliver 80-100 grams of carbohydrate with no chewing, in five minutes, and largely without filling the stomach the way a meal does. Add two or three drink-mix servings across the day and the carbohydrate budget gets a lot more achievable.
A few options that actually deliver the gram counts:
Maurten Drink Mix 320 → — 80 g of carbs per 500 ml serving in a 16% hydrogel solution. The 0.8:1 maltodextrin-to-fructose ratio engages both transporters, which matters more during the race itself but is also useful in the load. Probably the cleanest single-serving way to add 80 g of carbs without filling the stomach.
Tailwind Endurance Fuel → — 27 g of carbs per scoop, mixed to taste. Easier on the wallet for high-volume use across a full loading window, and the electrolytes are useful as you bump fluid intake. Two scoops in a bottle gives you 54 g of carbs with sodium thrown in.
UCAN SuperStarch → — 26 g per packet, slow-release. Useful as a late-evening dose before bed because it doesn’t spike blood sugar the way a maltodextrin drink does — the sleep-disruption profile is gentler. Not ideal as the primary load source because of the per-serving gram count, but a good last-meal-of-the-day tool.
Skratch Labs Sport Hydration → — 22 g of carbs per scoop, real-fruit flavour, optimised for hydration. The lowest carb density of the four, but the most palatable across many servings — relevant when you are drinking sports drink for the fifth time in a day and the gel-sweet flavours have stopped going down.
The right answer is usually a stack of two: one high-carb option as the primary load tool (Maurten 320 or two-scoop Tailwind), and a lower-density, easier-drinking option (Skratch or single-scoop Tailwind) for the rest of the day.
Hydration during the load
The water that binds to stored glycogen has to come from somewhere. If you don’t drink more, the body will pull it from existing tissue, leaving you slightly dehydrated entering race morning.
The general rule during a load is: drink to clear urine, then drink slightly more. Sports drink at meals replaces fluid and adds carbs at the same time, which makes the load and the hydration single tasks. Plain water between meals is fine; do not chug it.
Sodium intake matters during the load and is often overlooked. The extra fluid you’re taking in needs sodium to be retained rather than excreted, and the standard sports drink will not give you enough on its own at the volumes a real load involves. A salt-shake on dinner, a sodium-loaded electrolyte drink in the afternoon, salted snacks (pretzels, salted crackers) at the lower-volume eating moments — all of these help.
If your morning urine is dark and you’ve felt thirsty all day, you’re under-hydrated. If you’re peeing every 30 minutes and it looks like water, you’ve over-hydrated and diluted your sodium — back off the volume slightly.
Race morning is not part of the load
A common mistake: treating race morning as the “final” carbo-loading meal. It isn’t. The protocol ends the night before. Race morning is a separate calculation — a 100-300 gram carbohydrate intake, 3-4 hours before the gun, with the exact amount depending on body weight, race start time, and stomach tolerance. We’ll cover race-morning eating in detail in the next post in this cluster.
What matters for the load is that by Saturday night, you’ve hit the target. By Sunday morning, the work is done — the glycogen is stored, the liver is full, the body water is up, and the only remaining job is to eat a sensible pre-race breakfast that tops up liver glycogen without overloading the gut.
If you have done the load properly, you should weigh 1.5-2 kg more on Sunday morning than you did on Friday morning. That’s the signal it worked. If you weigh the same, you did not eat enough.
Where the calculator fits
Running Momma’s fueling calculator handles race-day intake — the gels, drinks, and sodium during the run itself — but doesn’t yet generate carbo-loading plans. That work is on the runner. The two things that matter most for getting it right are honest measurement (weigh yourself Friday morning and Sunday morning) and the willingness to eat past the point where eating stops being fun. For two days. The races where you regret it are the ones where you didn’t.
And once you’ve got the loading right, the race-day arithmetic — how many gels you actually need — becomes a much more forgiving problem. A runner with full glycogen who under-fuels by 100 grams across a marathon will probably finish. A runner who didn’t load and also under-fuels won’t.
What is your honest gram-per-day intake the Saturday before your last marathon? If you can’t answer that, you didn’t load. You ate pasta.
This guidance describes general carbohydrate-loading practice for healthy adult endurance athletes. If you have diabetes, insulin resistance, a gastrointestinal condition, or are pregnant, talk to a clinician before making large short-term changes to your carbohydrate intake.