Fueling for the challenge

The Science Behind High-Watt Endurance: Why Carb Loading and Glycogen Storage Matter

Ever tried to hold a high wattage for hours and felt like your legs were turning into bricks halfway through? Whether you're powering through endless climbs on a bike or grinding out a long-distance run, the ability to sustain high output over time is one of the most challenging aspects of endurance sports. And here’s the kicker: your performance hinges less on willpower and more on biochemistry — specifically, your fuel strategy.

This post unpacks the science behind high-watt endurance exercise and why carb loading and glycogen storage are not just helpful, but absolutely essential. Whether you're chasing a personal best or simply want to avoid the dreaded bonk, understanding how your body uses fuel can be a total game changer.

Understanding High-Watt Exercise
Let’s start with watts. In endurance sports like cycling, running with a power meter, or rowing, wattage is a measure of how much power you’re generating. Think of it like the horsepower of your body. The higher the watts, the more work you’re doing. And the more watts you want to sustain, the more energy you need to burn.

But maintaining high wattage isn’t just about leg strength or lung capacity — it’s a massive metabolic demand. The body needs a constant supply of energy to keep those watts up, especially during extended efforts. That’s where your fuel systems come into play, and where the real science begins.

The Body’s Fuel Systems Explained
Your body has three primary energy systems: the phosphagen system (ATP-PC), glycolysis, and oxidative phosphorylation. The phosphagen system is your quick-burst energy — the stuff you tap into for sprints or short, explosive efforts. Glycolysis comes into play next, using glucose (a carbohydrate) to produce energy over a longer period but still relatively quickly. Finally, oxidative phosphorylation is the long-game system that uses oxygen to burn carbs and fats for extended energy.

When you're riding at high watts or running at threshold pace, your body heavily relies on glycolysis — in other words, carbohydrates. Fat metabolism is much slower and less efficient at higher intensities. That’s why your diet leading into long workouts matters so much. If your glycogen stores are low, your ability to produce and maintain power takes a nosedive.

Carb Loading: Science or Myth?

Carb loading gets thrown around a lot in endurance circles, often accompanied by images of athletes inhaling bowls of pasta the night before a big race. But is it legit? Yes — with a caveat. Carb loading is rooted in real science, but it's often misunderstood and poorly executed.

At its core, carb loading is the strategic increase of carbohydrate intake in the days leading up to an endurance event to maximize glycogen stores in muscles and the liver. Glycogen is your body’s most efficient and accessible energy source during high-intensity exercise. The more glycogen you have stored, the longer you can perform before fatigue sets in.

Here's the science: one gram of stored glycogen holds about 3–4 grams of water, which means effective carb loading also slightly increases water retention — something that can actually help in hot conditions. More importantly, well-executed carb loading has been shown to improve endurance performance by up to 2–3%, which can mean the difference between holding pace and hitting the wall.

But here’s what carb loading is not:

• It’s not effective if you haven’t tapered training slightly to allow glycogen to accumulate.

• It’s not magic if your baseline carb intake is already low.

Effective carb loading usually starts 2–3 days before the event, combined with a reduction in training intensity. Carb intake during this phase often increases to around 8–12 grams per kilogram of body weight per day. That’s a lot of carbs — and for good reason. You’re stocking the shelves for a major metabolic demand.

The takeaway? Carb loading isn’t a myth. It’s a well-supported endurance strategy — but only when done right.

Carb Timing and Training Zones

Carbs aren’t just about the night before. Timing your carbohydrate intake based on your training intensity and duration is one of the smartest things you can do to optimize performance and recovery.

Here’s the basic principle: the higher the intensity, the more your body relies on carbohydrates. Low-intensity, zone 1–2 efforts (easy rides, long slow runs) primarily burn fat. But once you move into tempo (zone 3), threshold (zone 4), or VO2 max (zone 5) work, your body shifts heavily toward carbs as its preferred fuel.

Why does this matter? Because if you don’t fuel correctly for the zone you're training in, you’re either leaving performance on the table or risking fatigue and under-recovery.

Before a workout:
For anything longer than 90 minutes or above moderate intensity, eating carbs 1–3 hours beforehand can significantly improve output. A small meal with 1–2 grams of carbs per kilogram of body weight is a good place to start.

During a workout:
Once you’re out on the road or trail, your carb needs depend on duration and intensity:

• <60 minutes: water is fine.

• 60–90 minutes at high intensity: 30–45g of carbs/hour.

• 90–180 minutes: aim for 60–90g/hour.

• 3 hours: train yourself to tolerate up to 90–120g/hour using mixed sources (glucose + fructose).

After a workout:
This is the “glycogen window.” Your muscles are most receptive to replenishment in the first 30–60 minutes post-exercise. Aim for a carb-rich snack or meal (1–1.2g/kg), ideally paired with some protein (about 20–25g) to assist muscle repair.

Matching your carb intake to your training zones helps avoid the trap of underfueling — something especially common among endurance athletes trying to manage body weight. Remember, you're not just eating for today’s workout. You’re fueling tomorrow’s capacity.

Bringing It All Together

At the end of the day, sustaining high-watt performance in endurance sports isn’t just about how hard you train — it’s about how smart you fuel. Your body runs on glycogen when the intensity climbs, and if that tank runs dry, your power drops fast. Carb loading, timed nutrition, and knowing your training zones aren’t hacks — they’re foundational to going longer, stronger, and feeling good while doing it.

This isn’t about chasing numbers for the sake of it. It’s about showing up to the start line with a body that’s ready, a brain that’s sharp, and a fuel tank that won’t quit halfway. If you’re already pushing yourself in training, give your nutrition the same respect — because performance is built on preparation, and carbs are the key to unlocking it.