Estimated read time: 4.5 minutes (about as long as it takes to talk yourself into a fourth bagel the night before a marathon🤔)

Hey Performance Nerds! Jonah here. 🤓

This one is the most-requested guide I've ever written. Back in the inbox on purpose for race week.

Boston just ran. London goes Sunday. Most runners taper their training and then wing the most important meal week of the year.

Carb loading is one of the few nutrition moves that actually moves marathon times. Most runners still get the dose, the timing, or the food choices wrong.

Here's what you'll learn today:

(Augie has zero loading protocol. He just eats my dropped bagel crumbs and somehow still beats me up the stairs.)

I've been building something behind the scenes for months.

You know the newsletter. Every Thursday, I break down the research and try to make it useful for your training.

But there's a layer underneath that I've never been able to share in a weekly format. The tools, the researcher conversations, the full analysis that gets cut down to fit a 3-minute read.

I've been working on a way to give you all of it.

I'm not ready to share the details yet. But people on this list will get first access and a rate that won't be available later.

More soon.

Jonah

P.S. Augie just walked across my keyboard while I was writing this. He has zero interest in the project. But he does appreciate the extra hours I've been spending at my desk. More lap time.

(He’s not a lap dog).

Huge congrats to Michael Potter, who won the Coros watch from last week's referral drawing. I'll email you directly to sort out delivery.

To everyone who shared the newsletter last week, thank you. This list grows because you pass it along, and that means more than you know.

If you ran Boston on Monday, congratulations. Whatever the day threw at you, you showed up and did the work.

And to everyone toeing the line in London this Sunday, you're in Augie’s head all week. Go get it.

Carb loading was the #1 reply-thread question this week, so that's where we're going today.

The gel I race with (and why):

BETA FUEL: delivers 40g 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.

Marathon Science subscribers get 15% off one-time purchases with code JONAH26. Live now through May 31.

The running power meter trusted by serious athletes — is now an official performance sponsor of Marathon Science, delivering real-time pacing precision so you can train smarter and race faster.

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

For a marathon, target 8-12 grams of carbs per kg of bodyweight per day during loading.

That's roughly 3.6-5.5 grams per pound. New to loading or sensitive gut? Start at the bottom.

Your muscles store glycogen as their preferred race fuel. When you top off those tanks, you delay the moment your body switches to slower-burning fat.

That switch is the wall.

We don't need the old 5-day depletion ritual either. Trained runners can fill their glycogen stores in 24-48 hours with high-carb intake and a taper.

For me, at roughly 77 kg, that's 800 grams of carbs per day for two days.

That is 13 bagels.

I am personally responsible for at least one bagel store owner's kid's college tuition.

Find your weight. Find your number. Eat.

First-timers and anyone with GI history should anchor to the Low column (or closer to 6-8g of carbs per kg per day).

Experienced loaders with a trained gut can push toward the High column.

Most runners try to hit 800 grams through pasta alone. By dinner you feel like a stuffed turkey and you still aren't close.

Liquid carbs solve this. They digest faster and take up less stomach space, so you can stack fuel all day without feeling miserable.

Useful options:

Translation: more fuel in, less bloat. (Your stomach will thank you. Your splits might too.)

Do not eat your vegetables.

Okay, you should eat them most of the year. But carb loading and broccoli are a bad couple. (Sorry mom. Look away for a second.)

Fiber slows gastric emptying and adds bulk that fills you up before your glycogen tank does. Beans, raw veggies, whole grains, and bran cereals all do this.

Fat does the same thing. It sits in the stomach for hours, displacing the simple carbs you actually need.

Your fat stores are already enormous. Even lean runners carry tens of thousands of calories of fat, while glycogen stores hold roughly 2,000.

Top off the small tank. Don't add fuel to the one that's already overflowing.

Cut these for 48 hours:

Lean on white rice, white bread, bagels, pasta, juice, gummy candy, sports drinks. The blander, the better.

(And no, I will not be telling my mom that broccoli got benched.)

I have a lot of regrets.

The "big daddy" pizza the night before Boston last year is near the top of the list. Mile 12 still gives me nightmares.

Race week is the wrong time to debut a new food or restaurant. Your gut needs predictability.

Two reasons:

Stick with foods you've eaten in training. White rice, plain pasta, bagels, pancakes, oatmeal, bananas, rice cakes. Boring is the goal.

Never used liquid carb mixes? Test them on long-run weekends in the month before your race. Race week is for execution, not experiments.

The pizza was really good though. (Worth it? My splits voted no.)

No bagel is safe within 48 hours of one of my marathons.

Bottom line: Carb loading isn't a giant pasta dinner. It's a focused 24-48 hour push of simple, low-fiber, low-fat carbs your gut already trusts.

Forward this to the friend who's still planning a "carbo-load steakhouse dinner" the night before Sunday's race.

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.

Going low-carb might feel like you’re “teaching your body to burn fat,” but your brain isn’t nearly as flexible. Most of you nailed this one, which tells me the energy systems are clicking.

The correct answer?
A. To provide a backup fuel for the brain when liver glycogen runs low (since fat can’t directly fuel it) 🧠⚡ ✅

Your brain can’t run on fat directly. When liver glycogen drops, your body produces ketones as an alternative fuel to keep your brain functioning without dipping blood glucose too low. It’s a survival mechanism, not a performance hack.

Here’s how the votes shook out:
🟩 A. To provide a backup fuel for the brain when liver glycogen runs low (since fat can’t directly fuel it) 🧠⚡ – 143 ✅
⬜️ B. To increase fat oxidation efficiency in muscles and spare glycogen for later efforts 🔥🏃‍♂️ – 19
⬜️ C. To reduce overall energy expenditure and slow metabolism during starvation 🐢📉 – 16
⬜️ D. To buffer blood acidity caused by increased fat metabolism and prevent fatigue 🧪🛡️ – 14

A few of you went with fat oxidation.

Makes sense. Ketones get lumped into the whole “fat-burning” conversation. But they’re not really about making your muscles more efficient, they’re about keeping your brain online when carbs are scarce.

There’s one nuance worth knowing.

Yes, muscles can use ketones. But in endurance performance, they don’t meaningfully replace the need for carbs at marathon pace. You still need carbs to run fast.

Bottom line?
Ketones aren’t a performance upgrade, they’re your body’s backup plan to keep your brain fueled when carbs run low.

I watched Boston on Monday. Heartbreak Hill took its usual toll.

Most of the late-race blowups I saw traced back to one mistake. Runners paced by goal time. Their bodies paced by effort. When those two stopped agreeing, the wheels came off.

Pace lies on hills, wind, and heat. Heart rate drifts 10-15 bpm across a marathon, even at honest effort. Neither one tells you what your body is actually spending.

Stryd Power does. It reads the cost of each step in real time. No guessing.

Critical Power is the anchor. Marathon effort lives just below CP, inside the metabolic steady-state zone. Amateurs target 85-93%. Elites push 90-97%.

Why it matters:

Racing London this week, or just survived Boston? Build your plan around Critical Power, not a fresh-legs goal pace.

👉 Try it on your next run