Estimated read time: 3.51 minutes (about as long as it takes you to try to rip open that gel mid long run 🤔)

Hey Performance Nerds! Jonah here. 🤓

Most runners grab the same gel for every run and never switch. But the right gel changes with how many carbs an hour you take.

Here's what you'll learn today:

(Augie eats the same breakfast before a 10-minute walk and a 2-hour hike. Cute in a dog, costly in a runner.)

I keep asking myself a question I don't have a clean answer for.

Is endurance worth it?

That sounds strange to say, because I love this right now more than I know how to explain.

I love running right now. I love double days. I love finishing one run, eating, answering emails, then spending the afternoon thinking about the next one.

I love the science of it. I read a paper and immediately wonder what it would feel like in my own body.

Carbs. Caffeine. Heat. Sleep. Strength. Pacing.

All these small variables make training feel less like a plan and more like a living experiment.

I love talking to researchers who have spent their careers studying the things most people treat as background noise.

That part lights me up. But the other part is real too.

Endurance asks for a lot. More than the workouts.

It asks for nights you skip because the next morning's run matters more.

Fewer random drinks. Fewer late dinners. Fewer yeses to friends you love seeing.

It asks you to organize normal life around a body you're trying to protect.

I am 30, and sometimes I wonder if I am missing some normal version of being 30.

Then I go run again, and the question gets harder.

Because I don't feel trapped by it. Most days, I feel lucky. Curious. Obsessed in a way that feels healthy and maybe not entirely sane.

I don't have a tidy answer.

Part of me thinks this is what devotion costs. Part of me worries I use training to avoid other parts of life. Most days, I think both are probably true.

That tension is a lot of why Marathon Science exists. Marathon Science is me trying to understand this pursuit in public.

It is for people who give more to running than they can neatly justify.

So if you've felt that tradeoff, you're not weird. Maybe you've wondered if the long run was worth missing the night out.

Or if protecting tomorrow's session made you a better athlete and a more distant friend.

I get it in a practical way. I set the early alarm and hope the trade still feels worth it when it rings.

If this has been on your mind, hit reply and tell me.

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.

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.

Glucose is a simple sugar (rice, potatoes, pasta), and maltodextrin is a chain of glucose.

Both share one lane into your blood, a transporter called SGLT1.

That lane tops out near 60 grams of carbs an hour for most runners. Push past that on glucose alone, and the extra can often just sit in your gut.

Fructose is another type of carb, the kind you get from fruit or juice. It rides a separate lane, GLUT5.

Open that second lane and more total carb reaches your working muscles as usable energy.

Jentjens and Jeukendrup (2004) showed two lanes move more fuel than one.

So the real question isn't your race. It's your grams per hour.

For most marathoners, aim for 60 to 90 grams an hour. That's the target.

Some elite, high-output runners who have gut-trained for it push toward 120. The evidence doesn't support going past that.

Half marathon and Hyrox sit lower, around 30 to 60. Below 60, a single-source gel keeps up fine.

For shorter, harder efforts of roughly 1 to 2 hours, a single-source gel may even be the smarter pick. One half-marathon study found that adding fructose slightly lowered carb-burning rate versus glucose alone (Lee et al., 2014). The effect is small and the evidence is early, so treat it as a maybe, not a rule.

Know your target rate before you head out. Your race distance and intensity set it.

Then pick your gel by the grams of carb per hour you plan to take, and trust it.

Below 60 grams an hour, taste and convenience matter most. A single-source glucose or maltodextrin gel keeps up.

For shorter, high-intensity races, there's early evidence a glucose-only gel may keep your carb-burning slightly higher. Loose evidence, but worth a test.

At 60 to 90, read the label for "glucose plus fructose" or "maltodextrin plus fructose." Both 2:1 and 1:0.8 work. I use 1:0.8 based on the newer literature.

Above 90, favor a roughly 1:0.8 ratio, and only if you've trained your gut for it. At that intake, the extra fructose helps you oxidize more and leaves less sitting in your gut.

Gut training may cut stomach issues and raise what you tolerate.

Think of it as teaching your gut to handle more fuel, not magically absorb it.

Practice your race gel at your race rate. (Race day is a terrible time to learn your gut disagrees.)

"Do I need a fancy dual-carb gel for an easy long run?" → Usually not. Under 60 grams an hour, a single-source gel keeps up. Save the dual-source for race pace.

"Is more fructose always better?" → Not exactly. A 2:1 blend works fine. I still lean 1:0.8 above 60 grams an hour, and especially above 90. It may leave less sitting in your gut.

"Will a dual-source gel fix my GI issues?" → It may help you take in more with less discomfort. But the bigger race-day driver is blood pulled from your gut during hard efforts, not your gel.

Bottom line: The question isn't which gel is best. It's how many grams an hour you're trying to take in.

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.

Most of you treated blackcurrant like a recovery supplement.

That is the trap. The more interesting case is not recovery. It is how the body uses fuel during steady endurance work.

The correct answer?
A. It may increase fat use during steady exercise, which could help preserve carbohydrate availability in ultra-style efforts

The current evidence is most interesting around fat oxidation, which means using fat as fuel during exercise.

New Zealand blackcurrant extract may help runners use more fat during prolonged moderate exercise. That could matter in ultra-style efforts, where carbohydrate availability can become one of the limiters.

That does not make it a magic race-day booster. The useful claim is narrower: it may shift fuel use in a way that could help preserve carbohydrate.

Here's how the votes shook out:

Option D winning makes sense.

Most supplements get sold through recovery promises. Less soreness, faster bounce-back, better next-day legs. That mental model is familiar.

But for this question, recovery was the weaker read. The better answer was about the fuel mix during exercise, not what happens between workouts.

What about performance?

That part is still less settled. Some studies show performance signals, some do not, and the mechanism is still being worked out.

So the smart use case is not "take this and get faster." It is "this may be relevant for long steady efforts where fuel availability matters."

Bottom line?
Blackcurrant is most interesting as a possible fuel-use lever for long steady work, not as a proven shortcut for speed, heat, or recovery.

I checked my Power Duration Curve this week before planning my next marathon block. My Critical Power is 317 watts right now.

That number is not a trophy. It is the threshold Stryd uses to build my power zones.

Stryd defines Critical Power as an estimate of the hardest effort I can hold for about 40 minutes.

Above it, fatigue starts stacking fast. Below it, I can stack more work because fatigue does not pile up as quickly.

That matters for the marathon. Marathon pace has to live below CP. Raise CP, and the same marathon power costs a smaller slice of my threshold. That is the quiet math behind getting faster.

Why it matters:

Before your next block, check your CP and plan the week by zones, not vibes.

👉 Try it on your next run