Fueling science has come a long way in recent years. We now know that carbohydrates are essential for endurance performance, and that a 1:0.8 ratio of glucose to fructose allows for optimal absorption.
This science has shaped the current fueling guidelines: consume up to 90 grams of carbohydrates per hour during intense endurance efforts.
But do these guidelines apply equally to all athletes?
Could some athletes naturally oxidize more carbohydrates than others? Do others burn less? And if we could personalize these guidelines based on each athlete’s actual carbohydrate oxidation rate, could we improve performance?
A new study led by Dr. Tim Podlogar and Professor Gareth Wallis explores this possibility. Let’s take a look at the study and what it means for the future of fueling.
Prefer to listen? In the most recent Nduranz podcast, we sit down with Dr. Tim and Professor Wallis to talk about the details of this study and what it means for the future of fueling. Listen here.
Fueling Guidelines: Where We Are Now
In the past, athletes were told to fuel with up to 60 grams of carbohydrates per hour. But newer research on glucose and fructose absorption in the gut has shifted the current recommendation to up to 90 grams per hour.
But Professor Wallis says that these fueling guidelines are still fairly generic. As a result, many athletes ignore the guidelines and there’s a trend of athletes fueling with very high amounts—often fueling with 120 grams or more per hour.
The Tradeoffs of Underfueling vs. Overfueling
Professor Wallis says that, for many athletes, increasing carbohydrate intake can be a good thing. It can help them move from underfueling to a point where they’re actually oxidizing more fuel, which leads to better performance.
But he also warns that high carbohydrate intake might have the negative effect of increasing glycogen usage.
“We're trying to preserve glycogen stores. Overdosing with carbohydrates can cause a paradoxical increase in the use of existing glycogen stores. And that's been seen in two or three studies. We don’t know whether there's a performance consequence, but it's a risk.”
Importantly, Dr. Tim notes tolerance does not equal absorption. Some of the carbohydrates might still be sitting in the stomach or gut. Some might get absorbed but not oxidized, especially if the energy demand isn’t high enough. It could be stored in non-exercise tissues. And we don’t yet know whether any of that has consequences—good or bad.
Professor Wallis says it’s a question of trade-offs. What’s the relationship between how much fuel you can tolerate, absorb, and actually use?
“If you’re not getting GI distress, it’s tempting to say, ‘Well, it’s fine.’ But is it?”
The Solution: Personalized Fueling
Personalized fueling means adjusting your carbohydrate intake based on your own physiology—not just general guidelines. Instead of targeting 60 or 90 grams per hour because that’s what’s commonly recommended, you’d fuel based on what your body actually uses.
This is what Dr. Tim and Professor Wallis set out to explore in their latest study.
How the Study Was Done
Dr. Tim, Professor Wallis and their colleagues came up with a straightforward protocol to figure out how much ingested carbohydrates an athlete actually uses during exercise. You can read the entire study here.
The Test
Athletes ran or cycled at a steady pace—about 90–95% of their first lactate threshold (LT1)—for 2 to 2.5 hours. This low-to-moderate intensity was key: it kept things stable so the data would be clean and reliable.
The Fuel: A Traced Glucose Drink
During the session, athletes drank a glucose-based sports drink containing a tiny amount of a carbon isotope tracer. This tracer let researchers track how much of the ingested carbohydrate was actually used for fuel.
Athletes were intentionally given more carbohydrates than needed to ensure their bodies had enough available fuel to reach their maximum limit.
Breath Test = Real-Time Fuel Data
Throughout the session, the athletes’ expired breath was collected. That’s how the researchers could measure:
- Total carbohydrate burned
- How much came from the drink vs. stored glycogen
The Goal: Find the Peak
The test revealed each athlete’s peak exogenous carbohydrate oxidation rate—basically, the maximum amount of ingested carbohydrates their body could use at that intensity.
And once you know your peak rate, you can reverse-engineer the exact amount of carbohydrate you need to consume to perform at your best.
This is the foundation of personalized fueling.
Key Findings: One Size Doesn’t Fit All
The study revealed one clear truth: you can’t predict how many carbohydrates an athlete uses just by looking at them.
“The numbers we're seeing in the lab are all over the place. There is no real big relationship between body size, body mass, fitness level, the duration of the training volume, or the amount of carbohydrates they have in the diet on the ability to oxidize ingested glucose.” - Dr. Tim Podlogar
This completely shatters the myth that an elite athlete who is used to high-carbohydrate fueling would oxidize fuel better than an amateur—and shows why testing personal rates is key.
Because of this wide variability, standard fueling recommendations—like 90g/h—might be too much for some, and not enough for others. Without testing, athletes may end up:
- Underfueling, missing out on potential performance gains
- Overfueling, increasing the risk of GI issues or wasting precious glycogen
That’s why personalized testing is key to optimizing fueling.
Real-Life Applications
Once athletes know their personalized oxidation rate, they could create a fueling strategy that makes sense for their physiology. For some, the number might mean increasing fueling amounts to reach peak oxidation rates. Other athletes may need to fuel less.
For example:
- In triathlon, it might help avoid overfueling on the bike—so you don’t suffer GI issues on the run from having too many undigested carbohydrates in the gut.
- In stage races, it could help athletes stay in energy balance across multiple days or weeks—without unnecessary fuel overload that might actually reduce glycogen availability over time.
- In cycling, where intensity can vary drastically, athletes could match fueling to inclines and descents.
“I think the most important thing is for athletes and coaches to have peace of mind that what they're doing actually makes sense.” – Dr. Tim Podlogar
Carbohydrate Personalization Is Actionable
Dr. Tim notes that we already have a lot of personalized data—blood glucose, sweat rates, core body temperature… But a lot of this data isn’t actually useful.
For example, we can test an athlete’s core body temperature during races. But you can’t stop in the middle of a race to ask for more ice—so the data isn’t actionable. He says it often ends up being “testing for the sake of testing.”
This isn’t the case with carbohydrate personalization. The data is actionable because you can personalize the dosage.
Once you know your optimal carb dose, you can:
- Train the gut to reach that number comfortably
- Fuel smarter on key training days and race days
- Adapt based on terrain, effort, or even weather conditions
Looking Ahead: The Future of Fueling
Both Tim and Gareth believe this type of testing could become widespread. As technology improves and costs come down, we might see:
- At-home testing kits
- More real-time data integration
- Field-based testing—not just lab-based testing protocols
It’s not available to most athletes yet—but we’re getting close.
“It’s a bit like glucose monitors. Twenty years ago, you had to go to a doctor. Now anyone can do it.” – Professor Wallis
Want to hear more details? Listen to the full podcast here.