The Trade-Offs of Fueling With Carbohydrate

Why the strategy that protects performance can also create new limits.

If you’ve been anywhere near endurance sport discourse lately, you’ve seen it. Carbohydrate intake during exercise has turned into a proxy war. One side frames high fueling rates as non-negotiable: more carbohydrate, better performance. The other side argues athletes have become dependent on constant intake, that adaptation changes the equation, and that glycogen may not be the primary limiter.

The most recent flare-up traces back to work associated with Tim Noakes and colleagues, arguing that athletes may not need large doses to keep blood glucose stable, and that drops in glucose—rather than glycogen depletion—may be the more immediate performance threat in certain conditions. That claim hit the internet the way gasoline hits a campfire: instant certainty, instant mockery, and very little context.

Here’s the problem with the way this debate is usually framed. It assumes one side must be right and the other must be foolish or corrupt. That explanation is too easy and rarely true. What’s more likely is that both sides are right in the scenario they’re picturing, and wrong when they apply their scenario to everyone else.

The more adult question is not “Are carbs good or bad?” It’s: what bottleneck are we trying to relieve today? Because solving one constraint often exposes another.

The bottleneck model: why carbs work (and why they stop working)

Carbohydrate during endurance exercise is one of the most reliable acute performance tools we have. It’s not controversial that ingesting carbohydrate can improve performance, particularly as duration increases. Traditional guidelines commonly recommend roughly 30–60 g/h for many endurance sessions, with higher targets for longer events, and up to about 90 g/h when athletes use “multiple transportable carbohydrates” (e.g., glucose + fructose) and train the gut to tolerate it ¹.

That “90 g/h” ceiling isn’t a moral boundary. It’s largely a plumbing issue: intestinal absorption and transport. Jeukendrup’s work is the classic reference point here—using multiple transportable carbohydrates can increase exogenous carbohydrate oxidation beyond what a single source can achieve, and it can also improve tolerance when executed well ².

So why does high carbohydrate intake help? Two reasons, both bottlenecks.

First, at higher intensities, carbohydrate oxidation yields more energy per unit of oxygen than fat oxidation. When you’re near the limit of oxygen delivery and utilization, “efficiency” matters. In that world, carbohydrate availability is a way to reduce one constraint: the cost of producing power.

Second, carbohydrate during prolonged efforts can protect blood glucose and reduce the likelihood that performance deteriorates because the system becomes unstable. This is not just “fuel in the tank.” It’s keeping the control panel from dimming.

But here’s the part the internet tends to ignore:

once you push intake too high, the bottleneck often stops being energy and shifts towards the gut.

Your body is not designed to digest large amounts of sugar while working hard. During intense exercise, blood is redistributed away from the gastrointestinal tract toward working muscle and skin ^3–5. That splanchnic hypoperfusion can impair barrier function and increase permeability—what’s often discussed as “exercise-induced gastrointestinal syndrome.”

That’s the major trade-off.

Carbohydrate can protect performance by easing one limiter.

But the act of consuming and processing it under stress can create a new limiter.

If you’ve ever increased carbohydrate intake during training and felt stronger physiologically but worse operationally—sloshing stomach, nausea, cramps, urgent bathroom math—that’s not weakness. It’s your physiology.

And the higher the heat stress, dehydration, intensity, and mechanical jostling, the more that gut constraint can dominate the day ^6,7.

So when people argue about “optimal grams per hour,” they are often arguing about different sports.

A road cyclist doing a long stage with repeated surges and a fast finish is playing a game where carbohydrate availability can decide the result. A marathoner running near threshold for a long time is also heavily dependent on carbohydrate, particularly late in the race. But a long ultra-distance athlete operating below threshold for many hours may be limited less by carbohydrate delivery and more by pacing, fatigue resistance, and their ability to keep the gut functional.

Same nutrient. Different game being played.

A new study that changes how we think about “enough”

Now we can turn to the study anchoring this discussion: the crossover trial by Prins and colleagues in trained triathletes ⁸.

The design is unusually controlled. Athletes completed six weeks on either a high-carbohydrate diet of roughly 380 grams per day or a very-low-carbohydrate diet of about 40 grams per day. Energy intake, training load, and fat-free mass were held stable, allowing the comparison to reflect metabolic strategy rather than weight change.

After each phase, participants arrived fasted and performed a ride to exhaustion at seventy percent of VO₂max.

During the effort they received either placebo or carbohydrate at a rate of 10 grams per hour, a quantity far below modern race-fueling recommendations.

Two findings matter.

First, once athletes had adapted to their respective diets, time-to-exhaustion performance was similar between the high- and low-carbohydrate conditions.

Second, the small carbohydrate provision prevented hypoglycemia and improved performance by roughly twenty-two percent, and the magnitude of that benefit was comparable regardless of dietary background.

If pre-exercise glycogen availability were the dominant regulator of fatigue in this setting, the high-carbohydrate diet should have produced superior results. It did not.

The outcome changed not because the tank was fuller, but because the line stayed stable.

In this context, the performance-saving effect of carbohydrate did not require massive intake. It required continuity. A modest supply was enough to prevent the system from slipping into a state associated with deterioration.

There is another observation that deserves careful consideration. Continuous glucose monitoring revealed that regulation in the low-carbohydrate condition appeared unsettled early but normalized after approximately four weeks, coinciding with stabilization in circulating ketones.

This is important because studies that terminate during the transition may capture instability rather than the true adapted state. A difficult first two weeks does not necessarily predict long-term capacity.

None of this proves that carbohydrate is unnecessary. What it demonstrates is more precise and therefore more useful: when the primary threat to performance is blood-glucose instability rather than maximal substrate throughput, the minimum effective dose may be far smaller than commonly assumed.

And that brings us directly back to the central theme.

Every solution has a cost.

Uniting both worlds: how to fuel like an adult

The practical takeaway is not to pick a tribe. It’s to build a decision process.

Carbohydrate is powerful because it can move the point of failure. The mistake is assuming the failure disappears. It relocates.

A good fueling plan starts by identifying which limiter is most likely to matter in your situation:

If your sport or race demands repeated efforts above threshold, tactical surges, or a high-intensity finish after long duration, carbohydrate availability can be the constraint worth buying down—even if it costs you some digestive strain. In those moments, the tradeoff can be worth it, and higher intake targets have justification ².

If your session is steady aerobic work, long base mileage, or training where the goal is completion and adaptation—not winning a sprint—then chasing maximal intake may be solving a problem you don’t actually have. In that case, the more intelligent play is often to find the smallest intake that prevents the drift you fear: the slide in pace, coordination, mood, or cognition that shows up before the legs truly fail.

That is exactly why the Prins study is so interesting. It suggests that in some endurance contexts, the “performance protection” role of carbohydrate can be achieved with a very modest dose that is less likely to create new problems.

And the “cost” side of the trade-off deserves equal respect. High carbohydrate fueling increases exposure to practical constraints: gut tolerance, absorption ceilings, heat stress interactions, and behavioral dependence on constant feeding. Exercise itself can compromise GI integrity via reduced blood flow; piling large doses of carbohydrate on top of that stress can turn the gut into the limiter, regardless of what your legs are capable of.

So, the next move isn’t to adopt 10 g/h as a rule or 90 g/h as a badge of honor. It’s to learn where your line sits.

Experiment in training. Not on race day. Use stable sessions to test different intakes and see what changes: output, perceived exertion, and GI symptoms. Treat gut tolerance as trainable, but not infinitely elastic. Keep high intake where it clearly pays—hard long days, key race rehearsals, and competition. Keep lower or minimalist approaches where the goal is training quality and consistency, and where gut strain is more likely to cost you than help you.

That is the reconciliation.

High carbohydrate fueling is a legitimate performance tool.

Low carbohydrate adaptation can preserve endurance capacity in many settings.

And in the middle, there is a large, underappreciated space: minimal carbohydrate dosing that stabilizes the system without creating new limits.

Carbohydrate doesn’t need a religion. It needs a job description.

The art of fueling is deciding which limit you are most prepared to manage.