Marathon Science Research Hub

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.

The Study That Kills the Dual Fuel Narrative

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.

What 75g of Ketones Did to 120 g/h of Carbs

Measure	Carbs Only	Carbs + Ketones	Difference
Ingested carb burned	1.50 g/min	1.35 g/min	-10% (P<0.01)
Oxidation efficiency	75%	67%	-8 percentage points
Blood glucose	4.9 mmol/L	4.4 mmol/L	-0.5 mmol/L
Time to exhaustion	349 sec	319 sec	No difference
GI symptoms	Low	Low	No difference

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.

Why Ketones Slow Down Your Carbs

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:

Gut slowdown. Ketones may delay stomach emptying and absorption. The carbs you swallowed sit in the gut longer instead of reaching the blood.
Liver traffic jam. Ketones suppress the liver's own glucose production. Less total glucose enters the bloodstream.
Glucose diversion. Blood markers suggest the liver reroutes glucose into a disposal pathway instead of sending it to muscles. The carbs get processed, just not for energy.

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.

How One Study Built a Billion-Dollar Narrative

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:

McCarthy et al. (2023): ketone monoester impaired 20-min time trial performance
Quinones et al. (2024): no time trial benefit despite elevated ketones
Stalmans et al. (2025): ketone ester impaired performance in low oxygen conditions
Martyn et al. (2026): blocked carb oxidation at race-level fueling

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 Decision: Should You Use Ketone Supplements?

Your Situation	Use Ketones?	Why	Do This Instead
Marathon/half with 60-120 g/h fueling	No	Interferes with the carbs you need	Invest in gut training + better carb blend
5K-10K racer	No	Too short for substrate issues	Caffeine 3-6 mg/kg, 30-60 min before
Ultra (>6h) at lower intensity	Maybe	Lower carb rate may reduce competition. Evidence still equivocal	Test in training first. Never race-day debut
Post-race recovery	Maybe	Early evidence on muscle growth / signaling, but no increase in muscle growth yet proven	Protein + carbs after exercise is cheaper and proven
Curious experimenter	Wait	No performance benefit at any dose tested in well-fueled athletes	Save $30-40. Buy more gels

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.)

Practical Takeaways

At 120 g/h carb intake, ketone monoesters cut oxidation efficiency from 75% to 67%. Your carbs become less effective.
The "dual fuel" narrative was built on one study using moderate carbs. At modern high-carb fueling rates, ketones compete instead of complement.
The interference is metabolic, not gastrointestinal. Ketones don't cause gut issues. They slow your gut's fuel delivery.
Save the money. $30-40 per serving for something that makes your other fuel work worse. Buy more gels or invest in gut training.

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

Boston Marathon Weekend

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.