Maximizing Workout Efficiency: Understanding Energy Systems in Exercise

As a clinical nutritionist and personal trainer, my goal is to help you understand not just what to do in the gym, but how your body works during every rep, run, and rest. Whether you're aiming to lose fat, gain lean muscle, or simply feel more energised, knowing how your body uses energy is key to training smarter—not harder.

In this post, we’ll explore the science behind how your body fuels movement, what systems are activated during different types of exercise, and how to optimise your workouts for both performance and results.

Glycogen: Your Body’s First Responder

At the start of exercise—especially higher-intensity workouts like lifting or sprinting—your muscles primarily rely on glycogen. This is a stored form of carbohydrate found in your muscles and liver. It's fast, efficient, and provides the quick energy you need when intensity is high.

Your liver also plays a backup role by maintaining blood glucose levels, ensuring your brain and muscles stay fuelled during ongoing activity.

Fat: The Endurance Fuel

As you settle into longer or lower-intensity exercise, your body gradually shifts to burning fat for energy. This usually happens after 20–30 minutes of steady-state activity like walking or gentle cycling.

Fat provides more energy per gram than carbohydrates, but it’s slower to break down—making it more useful for sustained efforts rather than explosive ones.

Do We Switch Between Fuels?

Yes! Your body constantly shifts between glycogen and fat depending on:

• Exercise intensity and duration

• Your fitness level

• Recent nutrition

Weights Before Cardio? Yes—Here’s Why

Sequence	Muscle Performance	Fat Burning	Hormonal Response
Weights → Cardio	Stronger lifts	Increased	Higher testosterone & GH
Cardio → Weights	Weaker lifts	Less	Elevated cortisol earlier

Weights-first lets you hit resistance training with full glycogen stores, while cardio afterwards can tap into more fat reserves once those stores are lower.

Protein: A Backup Fuel You Want to Avoid

When energy demands are high and glycogen is low, your body may start breaking down muscle protein to make glucose—a process called gluconeogenesis. This usually happens during:

• Long-duration endurance sessions

• Inadequate nutrition (especially low-carb diets)

• Chronic under-eating or overtraining

This isn’t ideal for health or physique goals, as it can lead to muscle loss, a slower metabolism, and poor recovery.

 How to Prevent It:

• Eat enough carbohydrates to support your workouts

• Prioritise adequate protein intake daily

• Time your meals/snacks around training for better performance and recovery

Every movement your body makes—whether it’s lifting weights, going for a walk, or stretching on a yoga mat—is powered by a smart, adaptive system working behind the scenes. By understanding how your body uses glycogen, fat, and protein during exercise, you can tailor your workouts and nutrition to support your goals more effectively.

Whether you're aiming to burn fat, build muscle, or simply feel more energised, the key is to train with awareness, eat with balance, and listen to your body.

Because when you move with purpose and fuel with intention, wellness becomes a way of life—not just a goal.