Why Carb Loading Still Matters

Carb loading is one of the few performance interventions in endurance sport with decades of evidence behind it — and one of the most consistently butchered in practice. Athletes overdo it. They underdo it. They start too early. They start too late. They eat the wrong foods. They add protein. They add fiber. They show up to the start line bloated, heavy, slightly nauseated, and convinced that "carb loading just doesn't work for them."

Carb loading absolutely works. The execution is just usually wrong.

The purpose of carb loading is simple: maximize muscle and liver glycogen so you arrive at the start line with a full tank. For any race longer than approximately two hours, your stored glycogen becomes a performance-limiting variable. The fuller the tank, the longer you can hold race pace before fatigue sets in.

For races under two hours — a 5K, 10K, half marathon at most paces, sprint triathlon — formal carb loading is unnecessary. Eat carb-forward the day before, sleep well, and execute race morning properly. That's enough.

For races over two hours — marathon, 70.3, IRONMAN, ultra runs, gravel races, gran Fondo — carb loading is one of the highest-leverage things you can do in the final 48 hours. Done right, it adds meaningful glycogen and noticeable performance. Done wrong, it adds weight, gut distress, and a worse start than if you hadn't bothered.

The Science: Glycogen & Supercompensation

Glycogen is the storage form of carbohydrate in your body. It lives in two places: your muscles (roughly 400–500 g in a trained endurance athlete) and your liver (roughly 80–100 g). Muscle glycogen fuels working muscle. Liver glycogen maintains blood glucose, which fuels the brain and supports working muscle when blood sugar runs low.

At normal carbohydrate intake levels — say, 5–6 g/kg/day — your glycogen stores sit at somewhere between 60% and 80% of capacity. That's the baseline a well-fueled training athlete walks around with.

Carb loading is the deliberate process of pushing those stores from 60–80% capacity up toward 100% — and, in trained athletes, slightly past 100%. That extra storage is called glycogen supercompensation, and it's a real, repeatable physiological response to a high-carbohydrate diet combined with reduced training volume in the final 36–48 hours before a race.

The mechanism is straightforward. When you eat carbohydrate, your body releases insulin. Insulin activates glycogen synthase, the enzyme responsible for packing glucose into glycogen storage. When you reduce training volume, you stop drawing down glycogen as quickly as you store it. The result: stored glycogen accumulates beyond its normal resting level.

How much extra? In well-trained athletes, muscle glycogen can rise from a typical 400 g to as high as 600–700 g across 36–48 hours of proper loading. That's an extra 800–1,200 kcal of immediately available race-day fuel sitting in your muscles. For a marathon or longer race, that is not a small number.

The science is clear, well-replicated, and old. What's changed in the last two decades isn't whether loading works — it's how we execute it. (Bergström & Hultman 1966; Sherman 1981; Burke 2017; Jeukendrup 2014)

What the Old Methods Got Wrong

If you've been racing long enough, or if you've read older endurance books, you've encountered some version of the "depletion-then-load" protocol: a week before the race, hammer a long workout to deplete glycogen, then eat low-carb for three days, then switch to high-carb for the final three days to "supercompensate" the empty tank.

This protocol — the classic Scandinavian or Bergström model — was the gold standard in the 1960s and 70s. It is now outdated.

Here's why:

1. The depletion phase is unnecessary. Trained athletes can fully load glycogen without first depleting it. Modern research (Sherman et al., Burke et al.) has shown that simply eating high-carb for 36–48 hours, combined with a taper in training volume, achieves the same glycogen levels as the depletion-then-load protocol — without the downside.
2. The depletion phase is risky. Spending three days in glycogen depletion just before a major race is a recipe for poor sleep, irritability, immune suppression, mood crashes, and increased injury risk. None of that is what you want in race week.
3. Loading for a full week causes problems. Athletes who try to "load" for five or seven days end up gaining weight, feeling sluggish, and arriving at the start line bloated. Excess carbohydrate beyond what your muscles can store gets converted to fat or held as water. Neither of those is performance-enhancing.

The other common mistake — the opposite of overdoing it — is starting too late. Athletes who carb-load only on race morning, or who try to cram everything into the final 12 hours, never get the supercompensation effect. Glycogen storage takes time. You can't fill the tank in one breakfast.

The modern protocol fixes both errors. No depletion phase. No week-long load. Just a focused 36–48 hour window of intentional high-carb eating, paired with a normal race-week taper.

The Modern 36–48 Hour Protocol

The modern carb load is a two-day intervention. T-2 (two days out) and T-1 (one day out). That's it. No more, no less, for most athletes.

The protocol looks like this:

• T-3 and earlier: Normal eating. Carb-forward but not loading. Continue regular meal patterns.
• T-2 (two days before race): Begin the load. Carbohydrate intake jumps to your target g/kg/day. Reduce fat, protein, and fiber. Reduce training volume to a short, easy session.
• T-1 (day before race): Sustain the load at the same g/kg/day. Continue low-fiber, low-fat, moderate-protein eating. Training is minimal — short shakeout or full rest.
• Race morning: Top-off meal at T-180 minutes (3 hours before start), small T-60 meal, T-10 universal gel. Race morning is its own protocol — covered in the Race Morning Playbook.

The two-day structure works for one specific reason: muscle glycogen storage saturates fairly quickly in trained athletes. By the end of the second day of high-carb eating with reduced training, you've reached supercompensation. A third day adds water weight and GI burden but not much additional glycogen.

T-2 and T-1 are mirror images

Don't try to load harder on T-1 than T-2. The athletes who try to "make up for it" on T-1 always end up with GI distress at the start line. The two days should look very similar in carbohydrate volume — same g/kg target, same food choices, same eating cadence.

The structural difference is training: T-2 may include a short opener or shakeout; T-1 is shorter still or full rest. That's the only meaningful change between the two days.

How Much Carbohydrate, Really?

This is where most athletes get into trouble. The numbers in the research literature — 10–12 g/kg/day — are real, but they are also intimidating, and for many athletes they are not the right starting point.

The ANC framework uses tiered loading targets, calibrated to the athlete's race distance, training history, and GI tolerance. Here's the general structure:

For most athletes reading this playbook, the right answer is somewhere between Gentle and Standard — not Aggressive or Advanced. The Aggressive and Advanced tiers exist for athletes who have demonstrated, through deliberate gut training, that they can tolerate that volume of carbohydrate without distress.

A practical example. A 70 kg (154 lb) athlete loading at the Standard tier (10 g/kg/day) is eating 700 g of carbohydrate per day. That's approximately 2,800 kcal from carbs alone — and that's just T-2 and T-1, separately. It is a lot of food. Most athletes underestimate the volume required to actually hit these numbers.

The honest reality: many athletes who think they "carb load" are actually loading at 5–6 g/kg/day — which is roughly normal eating. They're not loading at all. They're just eating dinner. That's why their performance gains feel modest.

What to Eat (and What to Avoid)

The volume of carbohydrate required to actually hit a loading target is larger than most athletes appreciate. To get there comfortably, you have to choose foods that are carb-dense, low in fiber, low in fat, and easy to digest. Bulky, fibrous, fat-laden foods will fill your stomach long before they fill your glycogen.

Foods that work well

• White rice, white pasta, white bread, bagels. The classic loading staples. Carb-dense, low fiber, easy to digest.
• Potatoes (no skin), white sweet potato (no skin). Carb-dense, mild on the gut.
• Bananas (ripe), applesauce, canned pears, peeled fruit. Low-fiber fruit options.
• Honey, jam, syrup, sugar-sweetened drinks. Concentrated carbohydrate sources to layer onto meals.
• Sports drinks, juice, soda (yes, really). Liquid carbs are an underrated way to bridge between meals.
• Pretzels, white crackers, plain bagel chips, fig bars (low-fiber varieties). Easy snacks between meals.
• Pancakes, waffles, white toast with jam. Breakfast staples that scale well.

Foods to keep small or avoid during the load

• High-fiber foods: whole grains, brown rice, whole wheat pasta, beans, lentils, raw vegetables, salads, bran, fibrous fruits with skin. Fiber is healthy in normal training — it is the wrong call during the load window. Save it for after the race.
• High-fat foods: avocado, nut butters in volume, fatty meats, fried foods, heavy cream sauces. Fat slows gastric emptying, takes up space, and crowds out carbs.
• Excessive protein: oversized steak dinners, big chicken portions, large protein shakes. A moderate amount of lean protein is fine and supports recovery, but protein should not be the centerpiece of loading meals.
• Anything new. The two days before a race are not the time to try the new pasta place, the new bread brand, or the recovery shake your friend just bought. Stick to foods you eat regularly.
• Excess dairy if you don't tolerate it well. Even athletes who normally handle dairy fine can run into trouble when the gut is being asked to process a high carb volume on top of lactose.

Carb loading is not a window for nutritional virtue. It is a window for performance. Eat the white bread.