Carbohydrates 101: The Science Behind Fueling for Performance
March 20th, 2025
Why Carbs Matter for Athletes
Carbohydrates are one of the most misunderstood nutrients in sports nutrition. Athletes are bombarded with conflicting advice—low-carb, high-carb, fasted training, keto, carb loading—so it’s no surprise that fueling strategies get confusing.
Here’s the reality: carbohydrates are the body’s primary energy source for high-intensity activity. When consumed in the right amounts, at the right times, and in the right forms, they optimize endurance, prevent fatigue, and accelerate recovery.
Mistiming, under-fueling, or consuming the wrong types of carbs can lead to early glycogen depletion, impaired muscle function, and even metabolic disruptions. Understanding how, when, and why to fuel with carbohydrates is fundamental to unlocking your best performance.
Why Carbohydrates Are the Preferred Fuel Source
At their core, carbohydrates are molecules made of carbon, hydrogen, and oxygen that provide 4 kcal per gram. While fat provides more energy per gram (9 kcal), carbs remain the dominant fuel source during endurance and high-intensity efforts. Here’s why:
1. Direct Glucose Availability & Glycogen Storage
Carbohydrates break down into glucose, which fuels muscle contraction and maintains blood sugar homeostasis. Excess glucose is stored as glycogen in muscles and the liver—serving as a rapid-access energy reserve.
- Muscle glycogen directly powers working muscles.
- Liver glycogen regulates blood glucose, preventing energy crashes.
- When glycogen stores are depleted, performance suffers, leading to fatigue, loss of coordination, and decreased power output (a.k.a. “bonking”).
2. Faster ATP Production with Less Oxygen Requirement
Energy production occurs through three primary pathways:
| System | Fuel Source | ATP Production Rate | When Used? |
|---|---|---|---|
| Phosphagen (ATP-PCr) | Creatine phosphate | Immediate (very high rate) | <10 sec, sprinting & max effort |
| Anaerobic Glycolysis | Glucose (from carbs) | Fast (moderate ATP yield) | 30 sec – 2 min, high-intensity bursts |
| Aerobic Metabolism | Glucose + Fat | Slower, but sustainable | >2 min, endurance efforts |
Carbohydrates require less oxygen to produce ATP compared to fats. During aerobic exercise, when oxygen availability is limited, carbs remain the most efficient fuel source. This is why the body prioritizes carbohydrates at higher intensities—they deliver energy faster with lower metabolic cost.
3. Carbohydrates Spare Protein for Muscle Repair
Without sufficient carbohydrate intake, the body begins catabolizing protein (via gluconeogenesis) to maintain blood glucose levels. This compromises muscle repair, adaptation, and recovery—negatively impacting long-term performance.
4. Fat Oxidation is Limited During High-Intensity Work
While fat is an excellent low-intensity fuel source, it cannot sustain energy demands at higher intensities.
- Fat metabolism is slower because it requires more oxygen per ATP molecule.
- During intense exercise (>70% VO₂ max), fat utilization drops while carb reliance increases.
- If carbohydrates are unavailable, performance declines due to reduced ATP availability.
Types of Carbohydrates & Their Role in Performance
Carbohydrates are categorized into simple (fast-digesting) and complex (slow-digesting) forms, each playing a distinct role in fueling strategy.
Simple Carbohydrates (Monosaccharides & Disaccharides)
These digest rapidly, providing quick energy for immediate use.
Ideal for: Pre-race top-offs, during workouts, and rapid post-exercise glycogen replenishment.
Sources: Sports drinks, gels, Skratch Labs Sport Drink Mix, fruit, honey.
Complex Carbohydrates (Polysaccharides)
These take longer to digest and provide a sustained energy release over time.
- Starches: Found in grains, potatoes, and legumes.
- Fiber: Supports digestion and moderates blood sugar spikes.
- Glycogen: The stored form of glucose in muscles and the liver.
Ideal for: Pre-race meals, glycogen loading, and long-duration endurance efforts.
Sources: Whole grains, oats, rice, lentils, quinoa, Skratch Labs Energy Bar Sport Fuel.
Carbohydrate Timing: When & How Much to Consume
Proper carb timing maximizes glycogen stores, stabilizes blood sugar, and optimizes energy availability throughout training and competition.
Pre-Training / Pre-Race
- 1-4g/kg body weight of carbohydrates 1-4 hours before exercise.
- Simple carbs closer to start time, complex carbs further out.
- Limit fiber to avoid GI distress.
Example: 1 cup cooked oatmeal + honey + Skratch Labs Sport Drink Mix (20g carbs).
During Exercise
- <60 minutes: Small amounts of fast-digesting carbs may help, but not essential.
- 1-2 hours: 30-60g/hr of carbohydrates.
- 2.5+ hours: 60-90g/hr, potentially up to 120g/hr for trained athletes.
- Mix glucose + fructose sources for enhanced absorption.
Example: Skratch Labs Super High-Carb Mix (100g) or Sport Chews + Drink Mix.
Post-Training / Recovery
- 1-1.2g/kg body weight of carbohydrates within 30 minutes post-exercise.
- Combining protein (~20-30g) + carbs enhances glycogen resynthesis.
Example: Skratch Labs Recovery Drink Mix (35g carbs, 8g protein) + banana.
How to Increase Carb Absorption & Gut Tolerance
For athletes pushing 90-120g/hr of carbs, gut training is essential to prevent GI distress and improve fuel absorption.
Stepwise Carb Loading for Gut Training:
1️⃣ Start at 30-40g/hr for shorter sessions.
2️⃣ Gradually increase by 10g every few weeks.
3️⃣ Use multiple carb sources (glucose + fructose) for improved digestion.
4️⃣ Practice race-day fueling strategies in training.
Key Takeaways: Science-Backed Fueling Strategies
✔ Carbs are the primary fuel source for endurance and high-intensity exercise.
✔ Carbohydrates produce ATP faster and with less oxygen compared to fats.
✔ Fueling properly preserves muscle mass and supports optimal recovery.
✔ Pre-race, intra-training, and post-exercise fueling are all crucial.
✔ Gut training allows athletes to tolerate higher carb intake for better endurance.
Final Thought:
Carbohydrates are not the enemy—they are a performance necessity. Training and racing without them is like trying to drive a race car with an empty tank.
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