Fueling Performance: Why Carbohydrates Matter for Exercise

May 1

When it comes to optimizing athletic performance — whether you’re lifting heavy, sprinting, or logging miles on the trail — carbohydrates play a leading role. As the body’s preferred energy source during high-intensity and endurance exercise, understanding how carbs are stored, used, and replenished can be the difference between crushing your training or crashing midway through.

Carbs = Performance Fuel

Carbohydrates (CHO) are primarily stored in three forms:

Blood Glucose = ~5 grams
Liver Glycogen = 75-100 grams — helps regulate blood sugar levels
Muscle Glycogen = 300-400 grams — the primary fuel for working muscles

During moderate to high-intensity exercise, muscle glycogen is your “MVP.” At intensities above 65-70% of the VO2max, your body shifts toward using muscle glycogen as the predominant fuel. Once it is depleted, it pulls from blood glucose and eventually liver glycogen — unless you’ve topped off with a carbohydrate-rich snack or drink.

But here’s the catch: once your glucose stores run out, you simply can’t maintain the same energy output. That’s why you might feel light-headed, confused, clammy or “hit the wall” during long bouts of training — especially if you're fasted or under-fueled.

The Case for Daily Carbs

To keep your performance sharp and recovery on track, daily carbohydrate intake matters. Depending on your training volume, your needs can vary widely.

Low-intensity or skill-based training = 3-5 grams CHO / kg body weight
Moderate training (~60 minutes/day) = 5-7 grams CHO/ kg body weight
High-intensity or 1-3 hr/day endurance training = 6-10 grams CHO/ kg body weight
Extreme volume (4-5 hrs/day) = 8-12 grams CHO/ kg body weight

Rather than calculating carbs as a percentage of total calories, which can be vague, using gram-based target based on body weight can be a more actionable approach.

What About Carb Loading?

If you’re raining for a race, long ruck, or endurance event lasting over 90 minutes, you may have heard of carbohydrate super compensation - aka carb loading.

Carb loading increases muscle glycogen stores, allowing you to train longer before fatigue sets in. Here’s how to do it effectively:

Classic Method:

3-day low-carb “depletion” phase
Followed by 3-day high-carb “loading” phase

This approach can cause fatigue and performance dips during the depletion phase, so it’s not ideal for everyone.

Modern Method:

Taper training 1-3 days
Consume 10g/kg of CHO per day
No exercise 24-36 hours before the event

This updated approach improves endurance performance by 2-3% and delays fatigue by up to 20%.

Takeaways for Athletes

Muscle glycogen is your main energy tank during high-intensity efforts.
Carb needs depend on training volume — use g/kg as a guide.
Carb loading can help for events over 90 minutes, but must be done strategically.
Fasted training or low-carb diets reduce your capacity for intense or long-duration efforts.

Whether you’re a runner, lifter, or hybrid athlete, carbs aren’t just fuel — they’re foundational.