You’re standing at mile 20 of your goal marathon, legs feeling surprisingly fresh, when suddenly it hits—that familiar fog, the heavy legs, the complete loss of power.
You’ve hit the wall. Hard.
Meanwhile, the runner next to you, who you’ve been pacing for the last hour, surges ahead like they just started the race.
That’s not surprising if you’ve been running for any number of years.
But here’s what might surprise you: the carb loading advice you followed might be decades out of date, and actually working against you.
The research on carbohydrate loading has evolved dramatically over the past two decades. What worked for marathoners in the 1960s isn’t necessarily what modern science tells us will optimize your performance today.
The Classic Protocol: Where Carb Loading Started (and Why We’ve Moved On)
Let’s look at where this whole carb loading thing came from.
Back in the 1960s, Swedish researchers discovered you could double your muscle glycogen stores with a strategic diet manipulation [1].
Imagine storing twice as much fuel in your muscles before a race.
The catch? The original protocol was brutal.
Days 1-3: Beat yourself up with hard training while eating almost zero carbs.
Days 4-7: Load up on carbs (60-70% of calories) while tapering.
British marathoner Ron Hill used this approach to win gold at the 1969 European Championships [2], and suddenly everyone wanted in on the secret.
But here’s what they didn’t advertise: the side effects were awful.
The high-fat, low-carb phase left runners with serious digestive issues.
That hard workout a week before race day? It compromised recovery when you needed it most.
And the carb-free days felt like torture (despite how tough we are, most runners found this protocol absolutely miserable).
The good news? We’ve figured out you don’t need the suffering.
Research in the following decades [3] showed you could achieve nearly the same glycogen levels without the depletion phase.
Even better: well-trained athletes could max out their stores in just 1-2 days instead of a full week.
What’s Actually Happening in Your Muscles
Here’s what’s actually happening when you carb load.
Research by Dr. Jørgen Jensen at the University of Oslo [4] revealed that a single bout of glycogen-depleting exercise triggers sustained activation of glycogen synthase and AMPK, along with elevated expression of proteins like GLUT1, GLUT4, and hexokinase II, the cellular machinery that determines glucose uptake capacity.
Think of it like this: when you deplete your glycogen stores through exercise, your muscle cells basically panic and upgrade their fuel storage capacity.
They increase the number of “doors” (glucose transporters) that let carbohydrates into the cell and ramp up the enzymes that convert those carbs into storable glycogen.
The numbers are impressive.
In trained and well-fed athletes, muscle glycogen concentration is roughly 150 mmol/kg wet weight following rest, but can reach supercompensated levels of 200 mmol/kg wet weight after a few days on high-carbohydrate diets, that’s a 30-40% increase in fuel storage.
Each gram of glycogen is stored with 3 grams of water, which explains the 1-2kg weight gain you might notice.
The Modern Approach: Simpler and Science-Based
The issue is that we now know you can achieve the same glycogen supercompensation without all the suffering.
Modern research shows [5] there’s no need to deplete glycogen stores before loading, and carb loading for just 24-36 hours is adequate, especially for trained athletes.
For most recreational marathoners, the sweet spot is 2-3 days pre-race.
For highly trained athletes, 1-2 days may suffice.
Data from multiple studies [6] shows the target is 8-12 grams of carbohydrate per kilogram of body weight daily during the loading phase.
If you’re a 70kg (154lb) runner, that means 560-840g of carbs per day.
To put that in perspective, it’s roughly 14 bagels worth of carbohydrate (not a recommendation, just so you understand the magnitude!).
Instead of three massive meals that leave you feeling stuffed and miserable, spread your carb intake throughout the day with constant snacking.
Research published in the Journal of Applied Physiology [7] shows you should focus on fast carbohydrates, refined sugars, white rice, and starches that digest easily.
White bread, pasta, potatoes, and rice should be your go-to foods.
Save the brown rice and whole grain bread for after the race.
Here’s what’s different from the classic protocol: NO depletion phase, NO extreme low-carb days, a shorter loading window (2-3 days vs. 7 days), and much easier on your digestion and recovery.
Does Race Distance Actually Matter?
Here’s where many runners go wrong: they apply marathon carb loading strategies to every race distance.
Let’s be clear about this.
For 5Ks and 10Ks: Skip the carb loading entirely.
Your normal glycogen stores can fuel 90 minutes of running, which means most runners never tap out during these shorter races.
Just eat normally, stay hydrated, and have a decent race-morning breakfast.
For half marathons: It depends on your finish time.
If you’re crossing the line in under 90 minutes, you probably don’t need to carb load.
Running 1:45 or slower? Consider a modified 2-3 day protocol with increased carbs in the final 24 hours.
For marathons: This is where carb loading becomes non-negotiable.
The research shows a 2-3% performance boost [8], and one study found properly carb-loaded marathoners ran 14% faster than those who didn’t.
That’s potentially 20+ minutes for many recreational runners.
The bottom line? Match your fueling strategy to your actual race demands, not what you see elite marathoners doing.
The Practical Game Plan for Busy Runners
Let’s make this practical.
You’ve got a full-time job, family obligations, and a race in 3 days.
Here’s exactly what to do.
Days 3-2 before race: Keep your easy running short and light.
Increase the carb proportion at each meal while decreasing fat and protein.
Research shows [9] you should make your biggest meal lunch the day before, not dinner, which allows proper digestion and glycogen storage rather than leaving you with a full stomach on race morning.
For race morning timing: a study demonstrated you should use 1g carb per kilogram of body weight for each hour before the race.
If you’re eating 3 hours out, that’s 3g/kg.
Two hours out? 2g/kg.
Top off with a gel 10 minutes before the start.
What about liquid calories? Field-based research [10] suggests sports drinks and fruit juices can reduce bloating while providing carbs, they’re easier on your digestive system than solid food alone.
The Train-Low Debate: Does It Actually Work?
But what if I told you some coaches are now recommending you do the opposite during training, strategically restricting carbs to improve performance?
The “train low, compete high” strategy involves restricting carbohydrates during specific training sessions to enhance fat metabolism and metabolic adaptations, while maintaining high carbohydrate availability for competitions.
Popular protocols include twice-a-day training with low glycogen for the second session, or fasted morning runs.
Unfortunately, the research here is more complicated than social media influencers would have you believe.
While a foundational 1983 study [11] found carbohydrate restriction resulted in adaptation that conserved carbohydrate stores, subsequent research has produced mixed results, some studies show benefits while others indicate these benefits don’t translate into long-term improved endurance performance, especially at the elite level.
The problems are real: a systematic review [12] found that training intensity is often compromised with depleted glycogen, and there are potential consequences including decreased performance capacity and reduced carbohydrate oxidation capability.
For most recreational runners with limited training time, the research suggests train-low strategies have a narrow application, if any.
Your Tuesday night track workout is hard enough without adding unnecessary metabolic stress.
Common Mistakes That Sabotage Your Loading
The biggest mistake? Research consistently demonstrates [13] that one big pasta dinner the night before won’t adequately replenish your muscle glycogen stores, you need 2-3 days of elevated intake.
One of the most common reasons carb loading doesn’t work is that athletes simply don’t consume enough carbohydrates.
Yes, it’s genuinely difficult to eat 560-840g of carbs per day.
But that’s what the research shows is necessary for full supercompensation.
Other pitfalls: trying new foods during loading week (stick with what you know), ignoring hydration (you need water to store that glycogen), and continuing high-volume training (which prevents the supercompensation effect).
The Bottom Line
The evolution of carb loading research gives us a clear message: you don’t need extreme depletion phases, week-long protocols, or complicated train-low strategies.
For most recreational runners, the modern approach is elegantly simple, 2-3 days of elevated carbohydrate intake combined with reduced training will maximize your glycogen stores.
Match your strategy to your race distance.
Skip carb loading for 5Ks and 10Ks.
Use a modified approach for half marathons.
Go all-in for marathons.
And always practice your protocol during training before using it on race day.
When you nail this piece of preparation, you’ll experience what it feels like to surge through mile 20 instead of hitting the wall.
That’s the difference proper fueling makes.
