Estimated read time: 5.37 minutes (About as long as it takes to calculate the per-gram cost of your ketone drink and then cry. 🤔)

Hey Performance Nerds! Jonah here. 🤓

Ketone monoester drinks cost $30-40 per serving. The pitch: stack them with your carb gels for "dual fuel" performance.

A new study tested this at the top of today's fueling ceiling. The ketones didn't add fuel. They slowed down the fuel you already paid for.

Here's what you'll learn today:

(Augie's never tried a ketone drink. He fuels exclusively on whatever falls off the kitchen counter.)

I'm giving away the new COROS Pace 4 GPS watch. AMOLED screen, $230 value, yours for free.

Refer the most new subscribers by Monday (Boston Marathon day) and it's on your wrist.

Winner announced next Thursday. First of many giveaways.

P.S. I'll be in Boston this weekend. The Running Channel is hosting a live Q&A on marathon nutrition myths. Saturday April 18, 6:30 PM. 73 Newbury St.

We'll hang out after with drinks and good conversation. Would be great to see some of you there.

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.

In 8 trained cyclists on 120 g/h carbs, adding 75g of ketone monoester cut ingested-carb burn by 10%.

Oxidation efficiency dropped from 75% to 67%.

Of every gram of carb you swallow, only 67 cents worth gets burned when ketones are present. Without ketones, 75 cents.

Full price for fuel your body can't fully use.

The study: Martyn et al. (2026), a clean crossover trial from Liverpool John Moores and Bath, published in the Journal of Applied Physiology. 8 trained cyclists (VO2max ~66), glycogen-loaded, cycling for 3 hours just below threshold on 120 g/h maltodextrin-fructose.

Time to exhaustion after 3 hours? No benefit from adding ketones.

The interference was worst in hours 1-2, when blood ketone levels peaked at ~2.5 mmol/L. By hour 3, the gap narrowed as ketone levels fell.

The ketones didn't cause gut problems. They caused metabolic problems.

Your gut absorbs carbs through two sets of transporters. Think of them as pipes.

One handles glucose. The other handles fructose. A maltodextrin-fructose blend at 120 g/h maxes out both pipes.

Adding ketones appears to slow the flow through those pipes. Three things happen:

One more finding: a marker (3-methylhistidine) suggests the body might be increasing muscle protein breakdown. The theory is that it compensates for glucose it couldn't access normally.

This mechanism is still proposed, not confirmed.

Study limits: 8 cyclists (not runners), single-blind, one ketone dose. Large effect sizes, small sample.

Cox et al. (2016) is the paper that launched a supplement industry, published in Cell Metabolism. 12 elite cyclists took ketone monoester during a 120-minute time trial and produced roughly 2% better endurance performance with glycogen sparing.

The detail everyone missed: Cox used moderate carb intake, not 120 g/h.

In theory, ketones could give you a second fuel source. In practice, the research hasn't shown a performance benefit at any carb intake.

Cox is the outlier. At 60-120 g/h, where most competitive runners actually fuel, ketones don't add to your carbs.

They slow them down.

Since Cox, the evidence has been consistently negative:

The one exception: Poffe et al. (2021). Ketone monoester plus sodium bicarbonate boosted 15-minute time-trial power inside a simulated race.

Bicarbonate may rescue the pH-related issues, but it doesn't fix the carb oxidation suppression Martyn found.

The governing body has already moved. The UCI now does not recommend the inclusion of ketone monoesters in professional riders' nutritional plans.

Transparency note: Cox et al. included inventors of the ketone monoester formulation among its authors, a significant conflict of interest. Morton and Gonzalez, who led the Martyn study, have no industry ties to ketone companies.

The money argument: one ketone serving ($30-40) buys 15-20 gel packets. The gels deliver more usable energy, are proven to work, and won't interfere with themselves. (Your wallet called. It agrees.)

Bottom line: Ketone monoesters at race-relevant carb intakes don't add fuel. They slow down the fuel you already have.

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 runners associate muscle damage with hard effort. But the biggest damage often shows up when the effort feels controlled… and that’s where this one trips people up.

The correct answer?
D. Sustained downhill running (≥3–5% grade) at steady pace.

Downhill running loads your muscles eccentrically, meaning they’re lengthening under tension to brake each step. That creates far more structural damage than uphill or flat running, even when your heart rate feels easy.

Here’s how the votes shook out:
⬜️ A. Steady uphill running at moderate effort (Zone 2–3) ⛰️ – 22
⬜️ B. Flat interval workouts at 5-10K pace with full recovery 🏃‍♂️💨 – 23
⬜️ C. Flat terrain intervals at marathon pace 🛣️ – 2
🟩 D. Sustained downhill running (≥3–5% grade) at steady pace ⬇️🔥 – 282 ✅

A lot of you got this right, which tells me you’ve felt it.

That “my legs are wrecked but my lungs were fine” feeling the day after a downhill run? That’s eccentric damage in action.

What about uphill?

It feels harder, but it’s mostly concentric work. Your muscles are shortening, not braking, so damage is lower even when effort is higher.

Bottom line?
If your legs feel unusually trashed after an “easy” run, check the elevation profile, not the pace.

I ran 90 minutes last Sunday at 78% of my Critical Power. Felt easy, and I barely touched my gels.

Wednesday's tempo sat at 95% CP for 40 minutes. Same legs, very different fueling demand.

Carb oxidation isn't a switch that flips at CP. It's a dial that climbs as intensity climbs relative to your CP.

Well below CP, fat still contributes meaningfully and carb demand stays lower. Closer to CP, and especially above it, carb oxidation tends to dominate.

That's even more true when the session runs long. Heat, glycogen status, and training state nudge the picture too.

That's why I lean on Stryd's Critical Power as my reference point. It tells me where a session sits on that intensity continuum, so I can match fueling to metabolic demand.

Why it matters:

Know where each session sits relative to your CP. Then fuel the demand, not just the distance.

👉 Try it on your next run