Fueling the 3 Energy Systems: How to Eat for Performance at Every Intensity
If you're chasing performance—whether you're a weekend warrior, competitive athlete, or somewhere in between—understanding how to fuel your body for different types of activity is a game-changer. As a nutrition coach, one of the first things I help clients understand is how their energy systems work and how nutrition ties directly into performance, recovery, and sustainability.
Let’s break down how to fuel your three energy systems like a pro—and when to lean into carbs, fats, or a combo of both.
Understanding Your Body's Energy Systems
Your body has three primary energy systems:
Phosphagen System (ATP-PCr) – Short bursts of high-intensity effort (think max-effort lifts or sprints under 10 seconds).
Glycolytic System (Anaerobic) – Moderate to high intensity, lasting up to 2 minutes (400m sprint, HIIT circuits).
Oxidative System (Aerobic) – Longer duration, lower intensity (distance running, cycling, hiking).
Each system taps into different fuel sources and understanding how to eat to support each one can help optimize your performance.
Carbs: The High-Octane Fuel for Intensity
When intensity ramps up, your body wants carbs. Period.
Carbohydrates are your body’s go-to source of energy when training at high intensities—especially during anaerobic efforts and fast-paced aerobic work. They're stored in muscles and the liver as glycogen, which can be quickly broken down to fuel those mid-range efforts like tempo runs, weightlifting sessions, or team sports.
Whether you're sprinting, hitting a CrossFit WOD, or playing a fast-paced sport, carbs are crucial. Under-fueling with carbohydrates often leads to early fatigue, decreased power output, and even increased risk of injury.
How to fuel this system:
Include carbs in your pre-workout meal 1.5 - 2 hours before training (e.g., oatmeal, rice, potatoes, fruit).
Post-workout, replenish with a mix of carbs and protein to restore glycogen and aid recovery.
Don’t fear carbs—they’re your friends when used properly.
Fat: The Slow Burn for Endurance
For lower intensity, longer-duration exercise, your body shifts its preference toward fat as a fuel source. Think hiking, Zone 2 cardio, or longer rides and runs where you're not going all-out.
The oxidative system favors fat oxidation when there's plenty of oxygen available and the demand for quick energy isn't as high. This is also why endurance athletes often train at lower intensities to "teach" the body to burn fat more efficiently.
How to fuel this system:
Prioritize healthy fats in meals if your training is longer and steady (avocados, nuts, olive oil, fatty fish).
Avoid super high-fat meals right before training—they can slow digestion and make you feel sluggish.
Stay consistent with overall caloric intake so your metabolism remains optimized.
The Combo: Carbs + Fat for That Sweet Spot
Most real-world training and athletic scenarios live in the grey area—somewhere between all-out intensity and chill-paced effort. This is where mixing fuels becomes key.
Mid-duration workouts (45–90 minutes), tempo runs, team sports, or strength training circuits often use both the glycolytic and oxidative systems. This means your body is pulling from both carbs and fats to meet energy demands.
How to fuel this system:
Start with a balanced pre-workout meal: protein, carbs, and some fat.
During training over 60 minutes, consider small carb-rich snacks (sports drinks, gels, bananas).
Post-workout recovery still leans on carbs + protein, but don’t shy away from some fat as well.
It's Not Just About Duration—Intensity Matters
Here’s the kicker: fueling energy systems isn’t only about how long you’re working out—it’s also about how hard you're going.
A 30-minute easy jog will burn more fat than a 30-minute interval run. Conversely, a two-hour walk uses more fat than a two-hour soccer match where you're constantly sprinting, jogging, and walking.
That’s why a good online nutrition coach will assess not just how long you train, but what that training looks like minute-to-minute.
TL;DR:
Short & intense = carbs.
Long & easy = fats.
In between = both.
Smart Fueling Is Sports Nutrition 101
If you want to improve performance, nutrition coaching can be the secret weapon you didn’t know you needed.
Fueling properly means:
More consistent energy during workouts.
Faster recovery so you can train again tomorrow.
Better results from the same amount of effort.
Less bonking, cramping, and burnout.
Working with an online nutritionist or online nutrition coach allows you to build custom fueling strategies based on your personal goals, body type, schedule, and sport. Cookie-cutter plans won’t cut it. You need nutrition that matches your life and your workouts.
Ready to Fuel Like a Pro?
Whether you're chasing a PR, training for your first race, or just want to feel better during workouts, fueling your energy systems correctly is key.
If you're tired of guessing, Googling, or hoping your nutrition is “good enough,” let’s change that.
I’ll help you cut through the noise and dial in what actually works. Reach out today for personalized nutrition coaching—because your goals deserve more than guesswork.
👉 Click here to schedule your free strategy call and let’s build a plan that fuels your goals, your training, and your life.