Calculator 04 · Event day
Carb loading and race fueling calculator
Four numbers decide how the back half of a long event goes: what you load beforehand, what you eat on the morning, what you take in per hour while you are moving, and what you replace afterwards. Here are all four, with the reasoning attached.
What carb loading actually does
Muscle stores carbohydrate as glycogen, and a normally fed muscle holds enough for somewhere around ninety minutes to two hours of hard continuous work. Loading exploits the fact that muscle will store more than its usual amount if you feed it heavily while training volume is low — supercompensation. The current protocol is 10 to 12 grams of carbohydrate per kilogram of bodyweight per day for 36 to 48 hours before the event.[1]
The older protocols involving a depletion phase — three days of near-zero carbohydrate and hard training, then three days of gorging — are obsolete. They were unpleasant, they compromised the taper, and the modern version gets you to the same glycogen concentration without any of it. If someone is still recommending a depletion week, they are working from 1960s literature.
Two things to expect. First, the food volume is genuinely large: for a 70-kilogram athlete that is roughly 700 to 840 grams of carbohydrate a day, which is easier to reach through low-fibre, energy-dense choices and drinks than through brown rice and vegetables. Second, you will gain weight. Each gram of stored glycogen brings around three grams of water with it, so two to four pounds on the scale is normal and is the loading working, not a problem to correct.
Said the way I would say it out loud
Your muscles keep a fuel tank. Normally it is mostly full. Carb loading tops it past normal by eating a lot of carbs for a day or two while you are barely training. You will feel a bit heavy and puffy, and the scale will go up a few pounds — that is water coming along with the fuel, and it is exactly what is supposed to happen. It comes off during the event.
Who should not bother
Loading pays off for events long enough to genuinely deplete glycogen — roughly ninety minutes and beyond of sustained work. For a 5K, a CrossFit competition, a powerlifting meet or a two-hour team practice, you arrive a couple of pounds heavier with no fuel benefit to collect. Eat normally, keep carbohydrate reasonable in the day before, and spend the effort on sleep instead.
The pre-event meal
The guideline is 1 to 4 grams of carbohydrate per kilogram in the 1 to 4 hours before the start.[1] The convenient way to remember it is that the grams per kilogram roughly tracks the hours you have: four hours out you can handle four grams per kilogram, an hour out you want closer to one.
This meal is topping up liver glycogen more than muscle glycogen — the liver depletes overnight and is what keeps blood glucose steady in the early part of an event. Keep it low in fat and fibre, both of which slow gastric emptying and are the usual culprits behind race-morning gut trouble. Familiar food only. Race morning is not the time to discover how you respond to something new.
Fueling during, and where 60 and 90 grams come from
The rates in the calculator are not arbitrary; they come from the biology of intestinal transport. Glucose crosses the intestinal wall via the SGLT1 transporter, which saturates at an oxidation rate of about 60 grams an hour. Eat more glucose than that and it does not get absorbed faster — it sits in your gut, draws water in, and causes the distress endurance athletes know well.
Fructose uses a different transporter, GLUT5. Combining glucose and fructose therefore uses two roads instead of one, and total oxidation rates rise to around 90 grams an hour. This is why the guidance shifts at roughly the two-and-a-half-hour mark from "carbohydrate" to "multiple transportable carbohydrates" — the ratio commonly used is about two parts glucose to one part fructose.[2] Nearly every modern sports nutrition product is formulated this way, whether or not the label explains why.
Said the way I would say it out loud
Your gut has a limit on how fast it can take carbs in, and that limit is about 60 grams an hour if all the carbs are the same type. Mixing two types opens a second door and gets you to about 90. That is the entire reason sports drinks and gels contain both glucose and fructose instead of just sugar. It is not a marketing thing, it is a plumbing thing.
Train your gut, do not surprise it
Tolerance for carbohydrate during exercise is trainable. The intestinal transporters upregulate in response to regular exposure, and athletes who practise high intake rates in training tolerate rates on race day that would have wrecked them a few months earlier. Conversely, deciding on the start line to try 90 grams an hour for the first time is a well-established way to have a bad day.
Build up over four to six weeks. Start at a rate you handle comfortably, add roughly ten grams an hour every week or two, and do it in the sessions that most resemble your event in intensity and duration. Use the exact products you will use on the day, at the same concentration. Then take a small amount early and often — every fifteen minutes rather than a large hit every hour — because a steady trickle is easier on the gut and keeps blood glucose steadier.
Afterwards
If you have another session or event within about eight hours, aggressive refuelling matters: 1.0 to 1.2 grams of carbohydrate per kilogram per hour for the first four hours.[1] Moderate to high glycaemic index sources work best here, and adding protein helps if you cannot manage the full carbohydrate rate.
If your next hard effort is a day or more away, all of that urgency disappears. Glycogen resynthesis over twenty-four hours depends almost entirely on total carbohydrate eaten, not on how quickly you started. Eat a proper meal when you are hungry and stop worrying about the window.
Common questions
Does the type of carbohydrate matter for loading?
Less than the total. What matters is reaching 10 to 12 grams per kilogram, and that is hard with high-fibre foods because you run out of stomach before you run out of target. White rice, pasta, bread, potatoes without skins, juice, sports drinks, jam, honey and low-fibre cereals all do the job. This is the one time in the calendar when refined carbohydrate is the technically correct answer.
Why did I gain four pounds during loading?
Water bound to the glycogen you just stored, roughly three grams of water per gram of glycogen. It is the mechanism working, and it goes away as you burn through the stores during the event. Do not try to correct it.
What about "fat adaptation" or racing low-carb?
Fat adaptation reliably increases fat oxidation rates, which sounds promising, but it also impairs the muscle's ability to use carbohydrate at high intensities and has repeatedly failed to improve performance in events where intensity matters. For long, slow, low-intensity efforts the calculus is more debatable. For anything at or above a hard aerobic pace, carbohydrate availability remains the thing that decides how the last hour goes.
Can I go above 90 grams an hour?
Some ultra-endurance and professional cyclists now train up to 100 to 120 grams an hour with heavily fructose-weighted products and years of gut training behind them. It is real, and it is not where a recreational athlete should start. Get comfortable and reliable at 60 to 90 before you think about it.
Do I need sports products, or will real food work?
Real food works well up to moderate intensities and moderate rates — bananas, dates, rice cakes, boiled potatoes with salt, jam sandwiches. Gels and drinks win when intensity is high, when you need precise rates near the ceiling, or when chewing is impractical. Most people end up mixing the two, with real food early and liquid or gel later when the gut is less cooperative.
Sources for this page
- Burke LM, Hawley JA, Wong SHS, Jeukendrup AE. Carbohydrates for training and competition. Journal of Sports Sciences, 2011;29(sup1):S17–S27.
- Jeukendrup AE. A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports Medicine, 2014;44(Suppl 1):S25–S33. Full text
- Thomas DT, Erdman KA, Burke LM. Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic Performance. JAND, 2016;116(3):501–528. PMID 26891166
- Jeukendrup AE. Training the gut for athletes. Sports Medicine, 2017;47(Suppl 1):101–110.