Carb loading is a strategy where you dramatically increase your carbohydrate intake in the days before a long endurance event, pushing your muscles to store more fuel than they normally hold. The goal is to start your race with a full tank. Done right, it involves eating 10 to 12 grams of carbohydrate per kilogram of body weight each day for 36 to 48 hours before your event, while simultaneously cutting back on training volume.
Why It Works
Your muscles store carbohydrate in a form called glycogen, which serves as the primary fuel source during sustained exercise. Normally, your body regulates glycogen storage tightly, keeping levels within a set range. But a bout of hard exercise temporarily overrides that regulation. After you deplete glycogen through training, your muscles activate enzymes that drive glucose uptake and glycogen production at an elevated rate. When you then flood the system with carbohydrates, your muscles pack in more glycogen than they’d hold under normal conditions. This process is called supercompensation.
The practical takeaway: you need the combination of prior exercise (your training) and high carbohydrate intake (the load) to push glycogen above baseline. Simply eating a lot of pasta without the training stimulus won’t produce the same effect.
Who Benefits From Carb Loading
Carb loading provides the biggest advantage for events lasting longer than 90 minutes: marathons, half-ironman and full ironman triathlons, long-distance cycling races, ultramarathons, and competitive cross-country skiing. These are the events where glycogen depletion becomes a limiting factor, often experienced as “hitting the wall” or “bonking.”
For events under 90 minutes, a full loading protocol isn’t necessary. Eating a carbohydrate-rich diet in the 24 hours beforehand (6 to 12 grams per kilogram of body weight) is enough to top off normal glycogen stores, which will carry you through shorter efforts.
The Loading Timeline
The modern carb loading protocol is simpler than the old-school method, which involved a grueling depletion phase followed by several days of overeating. Current guidelines recommend starting 36 to 48 hours before your event. During this window, aim for 10 to 12 grams of carbohydrate per kilogram of body weight per day.
For a 70-kilogram (154-pound) runner, that works out to 700 to 840 grams of carbohydrate daily. For a 60-kilogram (132-pound) athlete, it’s 600 to 720 grams per day. These are large numbers, and hitting them requires deliberate food choices throughout the day, not just bigger meals.
At the same time, reduce your training volume significantly. If you normally run 10 miles, drop to a short, easy shakeout run or rest entirely. This taper serves a dual purpose: it prevents you from burning through the glycogen you’re trying to store, and it allows your muscles to fully replenish their energy reserves. Think of the final two days before a race as storage time, not training time.
What to Eat
The biggest mistake people make when carb loading is reaching for their usual “healthy” whole-grain, high-fiber foods. During the loading window, you want the opposite: low-fiber, easily digestible carbohydrate sources that won’t leave you bloated or cause stomach issues on race day. Fiber, fat, and excess protein all slow digestion and increase the risk of gastrointestinal distress.
Good choices include:
- Starches: white rice, white pasta, white bread, pancakes, baked or mashed potatoes (without skin), pita bread, cornflakes or Rice Krispies
- Sweets and snacks: jelly candy, golden syrup, vanilla yogurt, ice cream, Rice Krispie squares
- Drinks and fruit: pulp-free orange or apple juice, fruit without skin, sports drinks
- Quick meals: sushi rolls (skip the seaweed wrap if it bothers your stomach), white focaccia sandwiches with simple fillings
This isn’t the time to worry about eating “clean.” Your job is to get a high volume of carbohydrates into your muscles with minimal digestive stress. Spread your intake across meals and snacks rather than trying to cram it all into three sittings. A bowl of white rice with a little chicken, a stack of pancakes with syrup, juice between meals, and some jelly candy as snacks can help you hit the numbers without feeling miserable.
Reducing Fat and Fiber
To make room for all those carbohydrates, you’ll need to scale back on fat and fiber. Fat is calorie-dense but contributes nothing to glycogen storage, and it slows gastric emptying. A burger with fries might be high in carbs, but the fat content will leave you feeling heavy and can contribute to nausea.
Fiber creates similar problems. High-fiber foods like beans, whole grains, raw vegetables, and bran cereals increase stomach fullness, gas, and bloating. Research has consistently linked fiber and fat intake to greater gastrointestinal distress during exercise. Switching to refined, low-residue carbohydrate sources for just two days won’t affect your long-term nutrition, and it makes a real difference in how you feel at the starting line.
Expect Some Weight Gain
A successful carb load will add temporary weight, and that’s a sign it’s working. Every gram of glycogen your muscles store pulls along roughly 3 grams of water. For an athlete storing several hundred extra grams of glycogen, this can mean 2 to 4 pounds of additional body weight by race morning. Don’t let this shake your confidence. That water is bound inside your muscle cells, not sitting in your gut or under your skin. It will be used alongside the glycogen as fuel during the event, and you’ll be lighter by the finish than when you started.
Avoiding Stomach Problems
Gastrointestinal distress is the most common complaint during carb loading, and it’s almost always caused by food choices rather than the strategy itself. A few guidelines help:
- Avoid hypertonic sports drinks. Carbohydrate beverages above 6 to 8 percent concentration are linked to stomach upset and cramping. Stick to standard-strength sports drinks or dilute concentrated ones.
- Favor liquids and semi-solids over dense solids. Solid carbohydrate sources increase feelings of stomach fullness compared to gels, drinks, and softer foods. If you’re struggling to eat enough, juice, smoothies, and syrup-topped pancakes are easier to get down.
- Spread intake throughout the day. Six smaller meals and snacks will cause less discomfort than three enormous ones.
- Practice before a training day first. Never try a new carb loading protocol before your goal race. Test it before a long training run so you know which foods work for your body.
Considerations for Women
Most carb loading research has been conducted on male athletes, and there’s evidence that hormonal fluctuations across the menstrual cycle influence glycogen storage. Estrogen affects how the body uses and stores carbohydrate, and glycogen storage capacity appears to differ between the follicular phase (the first half of the cycle, before ovulation) and the luteal phase (the second half, after ovulation). The science isn’t settled enough to offer precise phase-specific protocols, but women who find carb loading less effective may want to track whether their menstrual cycle timing aligns with their event and experiment during training blocks to find what works.
A Sample Two-Day Plan
Here’s what a loading day might look like for a 70-kilogram athlete aiming for roughly 750 grams of carbohydrate:
- Breakfast: Large stack of pancakes with maple syrup, a glass of pulp-free orange juice, vanilla yogurt
- Mid-morning snack: White toast with jam, a second glass of juice
- Lunch: Large bowl of white pasta with a simple tomato sauce, white bread on the side
- Afternoon snack: Rice Krispie squares, sports drink
- Dinner: Big plate of white rice with a small portion of chicken, cooked vegetables without skin
- Evening snack: Bowl of cornflakes with milk, jelly candy
Repeat a similar pattern the following day, adjusting portions based on how you feel. The night before the race, eat your final large meal early enough to allow full digestion, typically finishing dinner at least 10 to 12 hours before your start time. A smaller, carb-rich breakfast on race morning (toast, a banana, a sports drink) tops off liver glycogen without overloading your stomach.