How Many Carbs Do Endurance Athletes Really Need?

Have endurance athletes gone from too little to too much?

For years, endurance athletes were told to train hard, stay light and treat hunger as a minor inconvenience. The result was predictable: many people tried to run, ride and race while chronically underfuelled.

Now the pendulum has moved. High-carbohydrate fueling is everywhere, and numbers such as 90, 100 or even 120 grams per hour can start sounding less like tools and more like entry requirements.

That creates an obvious question—especially when the companies explaining the benefits also sell the fuel: who genuinely needs the upper end, and who is simply buying more gels?

Your body never flips from fat to carbohydrate

One of the most persistent exercise myths is that the body burns fat first and later switches to carbohydrate. Biology is less tidy. Fat and carbohydrate contribute together; their proportions shift with intensity, duration, training status and fuel availability.

At an easier pace, oxidative metabolism can supply more of the required energy and fat can make a larger contribution. As the effort rises, carbohydrate becomes increasingly useful because it can support faster ATP production. This is why an athlete can carry a huge amount of stored energy as body fat and still slow dramatically when glycogen and blood-glucose availability fall.

Having energy stored is not the same as being able to release it quickly enough for the work.

What bonking actually feels like

Aaron describes bonking as more than ordinary tiredness. Power drops, coordination and decision-making deteriorate, and light-headedness or shaking can appear as blood glucose falls. The brain is part of the performance system too.

That distinction matters because athletes often interpret a fueling failure as poor fitness, weak motivation or a bad day. Training cannot express itself properly when the body lacks accessible fuel for the requested intensity.

There is no single correct number

Carbohydrate guidance needs context. A short easy session, a three-hour endurance ride and a high-intensity race do not create the same demand. Neither do an elite cyclist’s training week and a recreational runner’s weekend long run.

Aaron’s practical hierarchy is duration first, then intensity. For shorter work, an athlete may not need fuel during the session at all. As duration and intensity increase, carbohydrate becomes more valuable, and upper-end targets may make sense for athletes completing long, hard events who have trained their gastrointestinal system to tolerate them.

The important phrase is may make sense. A target that helps an elite athlete protect race pace is not automatically necessary for every gym session or easy jog.

Your gut is trainable—but race conditions still matter

At higher intakes, the type of carbohydrate matters because glucose and fructose use partly different intestinal transport routes. Combining them can increase total carbohydrate delivery beyond what glucose alone typically supports.

But eating a certain amount of carbs is not the same as absorbing and using it comfortably. Aaron recommends gradually practising the intended race strategy during relevant training sessions. The gut can adapt to repeated carbohydrate exposure, just as the muscles adapt to training.

Even a well-rehearsed plan can fail in heat or dehydration. Reduced blood flow to the gut, altered gastric emptying and the accumulated stress of racing can change what is tolerable. A race day fueling plan therefore needs room for conditions and might have to be adjusted on the fly.

Underfueling is bigger than one bad race

Race-day performance is only part of the story. If energy intake repeatedly fails to cover both training and normal physiology, the body begins making trade-offs. Recovery, bone health, immune function, reproductive function, cognition and mood can all suffer.

Low carbohydrate availability can contribute, particularly during high-volume endurance training, but it is not identical to low energy availability. Carbohydrate is one part of total energy intake, and the health picture cannot be reduced to one macronutrient.

Women sometimes receive a more visible warning through menstrual disruption, but men can also develop endocrine and health consequences. Persistent fatigue, irritability, poor recovery and declining performance should not be celebrated as badges of honor.

Why fasted training became attractive

The logic behind fasted training sounds plausible: train with less carbohydrate, increase the signal for fat oxidation and teach the body to spare glycogen. Acute metabolic adaptations are real, but translating them into better performance is less straightforward.

Aaron’s concern is practical. If restriction lowers the quality of important sessions, increases stress or makes it harder to meet total energy needs, the theoretical advantage may not survive contact with the athlete’s actual week. For most people, simply fueling the work is a more reliable route than trying to make every session a metabolic challenge.

Science became a product in Aaron’s kitchen

The founder story began with a wonderfully practical complaint: high-carbohydrate fueling was becoming popular, but existing products made it expensive and awkward. Consuming four to six small gels per hour was not particularly elegant.

Aaron began reverse-engineering products and testing formulations while completing his PhD. He wanted 50 grams of carbohydrate, a thin texture, a useful glucose-to-fructose ratio, no gelling agents and no preservatives. One early maltodextrin formulation famously changed from liquid into something closer to rock candy during shipping.

That failure taught him something the ingredient label did not: two ingredients with the same broad name can behave very differently inside a real product. The laboratory supplied the principles. Repeated testing, manufacturing constraints, cost and athletes supplied the rest of the education.

Takeaway

The most useful message here is not “eat more carbs.” It is “match fuel to the work.”

Endurance sport needed to correct a culture that praised restriction and normalized depleted athletes. But correction can become overcorrection when one upper-end number is detached from duration, intensity, body size, training volume, gut tolerance and the athlete’s actual goal.

A good fueling experiment does not begin with obedience to a magic target. Pick a repeatable session. Increase carbohydrate modestly. Keep the effort and hydration reasonably similar. Then watch more than pace: perceived effort, hunger, mood, gut comfort and recovery all carry information.

The goal is not maximum intake. It is enough accessible energy to do the work, recover from it and remain healthy enough to keep going.