Endurance Fueling Guide: Why Carbs Power Performance

Key Takeaways

• • Hard endurance efforts rely mostly on carbs, supplying more than 80% of your energy.

• • Your body has limited carb stores, so it burns more fat during easier sessions to save carbs for tough efforts.

• • Even with lots of endurance training, carbs power your high-speed work.

• • High-fat and ketogenic diets increase fat burning but don’t improve high-intensity performance and can reduce your ability to use carbs.

• • Ketone drinks raise blood ketones but deliver mixed performance results and can cause gut issues.

• • Carbs before and during competition help you maintain the high outputs needed to perform your best.

When the workout gets tough—think long climbs, fast tempo work, breakaways, or late-race pushes—your body leans almost entirely on carbohydrate. During these high-intensity efforts, more than 80% of your energy comes from carbs, mostly from stored muscle glycogen with support from blood glucose.

The catch? Your carb tank is small. You carry enough stored carbohydrate to fuel about 90 minutes of hard endurance exercise. Because that tank empties quickly, your body uses more fat during easier, lower intensity workouts to conserve carbs for when you push hard.

Training helps your body fine-tune this balance. Over time, your muscles:

• • Become more efficient at burning fat during easier efforts.

• • Use less glycogen and produce less lactate at the same pace you used to run or ride.

• • Are able to store more glycogen and use carbs more efficiently when you push the pace.

So yes, training improves your ability to use fat—but it also strengthens your ability to burn carbs when you need speed - and when intensity rises, the athletes who are trained and fueled for speed are the ones who win.

How Endurance Training Changes Fat and Carbohydrate Use

People often say that endurance training “teaches your body to burn fat.” That’s only partly true. Your body does learn to burn more fat during low and moderate intensities, but when you’re working hard, carbs remain the main fuel source.

One study had athletes train for nine weeks and measured how their bodies used fuel before and after training. After the training block:

• • They burned more fat at the same easy-to-moderate pace.

• • But at a challenging pace, their fuel use looked the same as before—carbs still provided most of the fuel.

• • Total energy use went up because the athletes were fitter and could push harder.

Additional research confirms it: even after plenty of endurance training, carbs remain the dominant fuel during demanding sessions. Your body may become more efficient overall, but high performance still depends on carbs.

Why Carbohydrates Dominate Fuel Use at Race Pace

To understand what truly fuels performance, look at what happens during race-like efforts. Studies of trained runners and cyclists show that during long, hard efforts—like 60-, 90-, or 120-minute time trials—carbs supply roughly 85–95% of the total energy. Even when athletes were fasted, or when fat availability was manipulated, the body still turned to carbs first when intensity stayed high.

In one set of cycling and running studies:

• • Carb use averaged around 4–5 grams per minute during hard efforts.

• • Fat use barely reached 0.2–0.4 grams per minute.

That gap is massive. It shows that fat simply can’t supply energy fast enough when the pace gets serious. Carb is the fuel that lets athletes surge, hold fast paces, and finish strong.

This is the central message of the entire body of research: if you want to perform well at high intensity, you need carbohydrates available—before, during, and after key efforts.

Why High-Fat and Ketogenic Diets Fail at High-Intensity Performance

Because the body stores so much fat, high-fat diets and ketogenic diets seem appealing—more fuel, right? And yes, they do increase fat burning. But increasing fat burning doesn’t equal better performance.

In early research, just three days on a high-fat, low-carb diet made athletes:

• • Burn more fat at easy-to-moderate intensities

• • Feel that the same workload was harder

• • Rely on a fuel that produces large amounts of energy, but takes longer to convert into usable power

A more extreme test—28 days of a ketogenic diet in trained cyclists—made things even clearer:

• • Four out of five athletes saw their aerobic capacity drop.

• • Their bodies relied heavily on fat, but carb use plummeted.

• • In a steady endurance test, average performance looked unchanged, but individual results were all over the map—some got worse, one improved dramatically and skewed the data.

The problem is simple: these tests weren’t done at true race intensity. When elite athletes were actually tested at race pace, the picture sharpened:

A study on world-class race walkers found that after three weeks on a low-carb, high-fat diet:

• • Fat burning increased to the highest levels ever recorded in a study.

• • But oxygen cost increased—meaning athletes had to work harder just to maintain the same pace.

• • Only the athletes who followed high-carb or strategically timed carbs (periodized carbs) improved their real race performance.

• • The high-fat group did not improve, despite burning tons of fat.

Other studies tested a short high-fat diet followed by a day of carb loading (so athletes started with full glycogen stores). Even then, the downside remained:

• • The body reduced its ability to use carbohydrate efficiently.

• • A key enzyme that helps turn carb into usable energy dropped by about 50%.

• • Carbs stayed “blocked,” even when they were available.

In cycling tests, this blunted carb metabolism showed up as:

• • Lower power during high-intensity sprints

• • Reduced ability to hit race-critical surges

One 100 km cycling trial found that overall time didn’t suffer after a high-fat diet, but sprint power was significantly worse. And sprint power is what wins races.

The takeaway: High-fat and ketogenic diets boost fat burning, but they don’t help you race faster—and they can make it harder to hit the high-intensity efforts that decide performance.

What Athletes Should Know About Ketone Drinks

Ketone supplements—ketone esters and diesters—have become popular because they raise blood ketone levels without extreme dieting. But higher ketone levels don’t automatically lead to better performance.

Here’s what the research shows:

• • In a fasted 30-minute rowing test, one ketone drink improved performance by 1–2%.

• • In a cycling test with 60 minutes of steady work plus a 30-minute time trial, a ketone + carbohydrate drink raised blood ketones to levels expected to help performance and improved results by about 2%.

• • But the results varied widely from athlete to athlete.

Other research in professional cyclists tells a very different story. After taking a ketone diester alongside a typical pre-race breakfast and caffeine:

• • Cyclists had higher ketone levels

• • Lactate levels after the effort were lower

• • But performance dropped

• • Riders produced less average power

• • And many experienced gut discomfort

Other studies also reported nausea, vomiting, and diarrhea with ketone drinks.

So while ketones change fuel use, they don’t reliably improve performance—especially in real-world race conditions where carbs are available and athletes need sustained power.

How to Use Carbs Strategically in Training and Racing

Sports nutrition today focuses on “carbohydrate availability”—making sure you have the right amount of carbs for the right type of training or racing.

That means:

• • You don’t need high carbs for every session.

• • Some workouts may intentionally use low carbs to create specific training adaptations.

• • Carbs can be delayed after some sessions to influence recovery and adaptation.

• • Intake should match training goals, intensity, and event demands.

But when the goal is performance—when the session is hard, or when you’re racing—there’s no substitute for carbs.

The research consistently shows:

• • Muscles rely heavily on carbs during high-intensity exercise, even in highly trained athletes.

• • High-fat approaches reduce the body’s ability to burn carbs when you need them most.

• • Ketone drinks don’t provide dependable performance benefits and can come with side effects.

• • Carbs before and during competition support the high energy output needed to perform well.

Training lets you experiment with fuel. Racing demands carbs.