Estimated read time: 5.34 minutes (about as long as it takes to eat that pre-race protein shake that's doing nothing for your glycogen stores 🤔)

Hey Performance Nerds! Jonah here. 🤓

Most runners carb load before every race. The science says most of them don't need to.

Your glycogen stores last roughly 75-90 minutes of hard running. Below that threshold, your normal diet handles it.

Here's what you'll learn today:

(Augie has never carb-loaded in his life. He does, however, attempt to steal every pre-race bagel within reach.)

Today's newsletter breaks down when to carb load. But carb loading is one piece of race fueling.

How many gels do you actually need? When should you take them? Caffeinated or not? What do you eat race morning? How do you train your gut so it doesn't fall apart at mile 18?

We've been building a fueling calculator that answers all of it. Plug in your race, your body weight, your gut history. It gives you a personalized protocol. Every threshold pulled from published research. 485 tests. No guessing.

Before we ship it, I want to build the right thing first. One question:

P.S. If you’re deep into software, coding, and running, and the idea of turning cutting-edge research into tools runners actually use gets you fired up, I want to talk.

I’m looking for a CTO to help bring these projects to life as Marathon Science grows rapidly.

If that sounds like you, just hit reply.

The gel I race with (and why):

BETA FUEL: delivers 80g of carbs per serving through a 1:0.8 maltodextrin-to-fructose ratio.

That ratio is why I've never had a GI issue with it in 3 years of racing. Fast absorption, no gut bombs. I don't earn a commission on this. I negotiated 15% off for you because I believe in the science behind it.

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💡 See this week’s full Stryd training tip at the end of this newsletter.

Your body stores roughly 500g of glycogen across muscles and liver. That's enough fuel for about 75-90 minutes of moderate-to-hard running (Thomas et al. 2016, Burke et al. 2011).

If your race is shorter than that, your tank is already full from normal eating.

When glycogen runs low, your body shifts to burning fat. Fat burns too slowly to sustain your target race pace.

That's the wall. Carb loading delays it by topping off your fuel stores before the gun goes off.

Modifiers that change your target:

GI comfort beats perfect numbers. When in doubt, go lower.

Find your weight. Find your target. Done.

A 75 kg runner targeting 10 g/kg needs 750g of carbs.

That's not a pasta dinner. That's an entire day of focused eating.

The 7-day depletion/loading protocol is outdated.

Bussau et al. (2002) found that a single day of 10 g/kg with minimal exercise matched the old multi-day approach in trained cyclists. No starving yourself. No depletion runs.

Areta & Hopkins (2018) reviewed 181 muscle biopsy studies. Two factors predicted glycogen storage more than anything else.

Your fitness level and how many carbs you eat beforehand.

Fitter runners have greater storage capacity. And research consistently shows 2-3 days of high carb intake paired with a taper fills the tank.

Most runners obsess over which gel to take at mile 18. Tim Podlogar, a sports nutrition researcher who advises Tour de France teams, says they're solving the wrong problem.

"The more impactful variable is the 24 hours before the gun goes off."

That easy run or set of strides the morning before your race isn't just a shakeout. Brief exercise appears to prime your muscles to absorb and store more fuel.

Think of it as opening the door before you start shoveling carbs through it.

Podlogar uses this with Tour de France teams. A short activation session in the morning, then aggressive carb intake the rest of the day.

If you've never carb loaded before, consider spreading intake across 36 hours instead. Your gut will thank you.

My marathon protocol: activation strides the morning before, then high-carb simple foods the rest of the day. For half marathons, just a carb-focused dinner and normal breakfast.

I don't load for anything shorter. (Your Strava friends bringing pasta to a 10K don't need to either.)

Loading-friendly foods:

Skip these during loading:

Simple sugars pack more fuel per bite than complex carbs. Lower fiber means less fullness and more room for carbs.

"Will carb loading make me gain weight?"

Temporarily, yes. Water follows glycogen into your muscles. Expect 1-2 kg on the scale. It's fuel, not fat. It disappears during the race.

"Is the pasta dinner enough for my marathon?"

Probably not. A single pasta dinner provides roughly 200g of carbs. A 70 kg runner targeting 10 g/kg needs 700g. You need an entire day of carb-focused eating.

"What about the depletion phase? My coach told me to run long 3 days before and eat low-carb."

Outdated protocol. Bussau et al. (2002) showed you can get the same loading from 24 hours of high-carb eating without depleting first.

"I always feel bloated race morning."

Bottom line: The question isn't whether to carb load. It's whether your race is long enough to need it. Most races aren't.

Forward this to your running buddy who brings a pasta pot to every 5K.

-Jonah

Welcome to the prove you’re a nerd section. Each week, I ask a question about a common running science myth.

Answer correctly, and you’ll be entered into a weekly raffle to win a package of Jonah’s favorite supplements.

You trained your gut for 90g carbs per hour… then race day hits 75°F and suddenly it feels impossible. Most of you saw right through why.

The correct answer?
C. Blood is pulled away from your gut to cool you, so less of what you take in actually gets absorbed 🌡️🩸 ✅

When core temperature rises, your body prioritizes cooling over digestion. Blood gets redirected to the skin, which means your gut gets less support to absorb carbs efficiently. Same intake, less delivery.

Here’s how the votes shook out:
⬜️ A. Carbs sit longer in your stomach in the heat, leading to sloshing and poor absorption 🤢🥤 – 5
⬜️ B. Your body switches to burning more fat in the heat, so ingested carbs aren’t used as much 🔥🥑 – 3
🟩 C. Blood is pulled away from your gut to cool you, so less of what you take in actually gets absorbed 🌡️🩸 – 231 ✅
🟨 D. Heavy sweating makes it harder for your body to break down carbs during exercise 💧⚠️ – 15

A few of you went with the “sloshing stomach” explanation.

That can happen. But it’s usually the downstream effect, not the root cause. If absorption slows because blood flow drops, carbs back up and then you feel it.

What about sweating?

Fluid loss matters, but it doesn’t directly limit carb breakdown. The bottleneck is delivery and absorption, not your ability to process carbs once they’re in.

Bottom line?
If fueling falls apart in the heat, it’s not your gut training, it’s blood flow. Adjust intake, slow slightly, and respect the heat.

I ran 20 miles last week at marathon pace. The day before, I didn't eat enough carbs. Life got busy, meals got skipped, and I started that run on half-full glycogen stores.

I felt it by mile 14. But Stryd saw it earlier. My Ground Contact Time climbed steadily through the back half of the run. Each stride spent a little longer on the ground.

That's my muscles losing elastic energy return and grinding through each foot strike instead of springing off.

GCT rises when fatigue builds. Low glycogen likely speeds that up. When your muscles run low on fuel, they lose the ability to contract quickly, so they compensate by spending more time on the ground.

What you see in your metrics can trace back to what you ate yesterday.

Why it matters:

Before your next long run, nail your carbs the day before. Then check your GCT drift afterward. (Your legs will thank your pasta bowl.)

👉 Try it on your next run