Cardio vs Muscle Gains: Are You Under-Eating Your Way to Fit Fatigue?

Poppy Hawe
11 minutes ago
4 min read

If you’re an active woman juggling strength training and cardio, you might feel stuck. You train hard but feel soft, or you’re caught in a cycle of high volume workouts and under-eating. You might blame cardio for killing your muscle gains. But here’s the truth: cardio isn’t the enemy. The real problem is often under-eating.

Let’s unpack why your body might be breaking down muscle, what really happens when you add cardio to your routine, and how to fuel your hybrid training for the best results.

Why Your Body Breaks Down Muscle

Muscle loss happens when your body doesn’t have enough energy to meet its demands. If you’re not eating enough, your body looks for fuel wherever it can find it — including your hard-earned muscle. This is survival mode.

Cardio often gets blamed because it burns calories and increases energy needs. But the real culprit is under-fuelling. When your energy intake is too low, your body breaks down muscle to keep going.

Think of your body like a phone battery. If you don’t recharge it properly, it starts draining essential power. Muscle is a valuable energy source, so your body taps into it when food isn’t enough.

Transitioning to Hybrid Training Changes Muscle Stimulus

Switching from pure strength training to hybrid training (strength plus cardio) means your weekly muscle stimulus changes. You might do fewer heavy lifts or less volume focused purely on muscle growth.

This doesn’t mean cardio is destroying your muscle. It means your muscle-building stimulus is lower. Cardio and strength training are two different things. Cardio supports heart health and endurance but doesn’t provide the same muscle-building signals as lifting heavy weights.

If you want to keep or build muscle, you need to maintain a strong lifting routine alongside your cardio.

Energy Balance Is Key: Controlled Deficit vs Chronic Restriction

How you manage your calories makes a huge difference.

Controlled deficit: You eat slightly less than you burn but keep protein high and time your carbs around workouts. This supports fat loss while preserving muscle.
Chronic restriction: You eat too little for too long, often with high training volume. This puts your body in survival mode, leading to muscle loss and fatigue.

One of my clients reversed dieted from 1400 calories to 1900-2100 calories per day. She dropped 8% body fat but maintained her scale weight. This shows that eating more, not less, can help you lose fat and keep muscle.

What You Need for Body Recomposition

Body recomposition means losing fat and gaining or maintaining muscle at the same time. It requires:

High protein intake: Aim for 1.6-2.2 grams per kg of body weight daily.
Timed carbohydrates: Eat carbs around your workouts to fuel performance and recovery.
Adequate calories: Enough energy to support training and recovery.
Progressive overload: Gradually increase weights or reps in your strength sessions.
Real recovery: Sleep, rest days, and stress management.

High volume training combined with calorie restriction won’t get you there. You need to fuel your body properly and lift smart.

The Interference Effect Isn’t Your Biggest Problem

The interference effect is when too much cardio reduces muscle gains. It’s real but only happens at very high cardio volumes, like running multiple hours daily.

For most hybrid athletes, this isn’t an issue. The key is to prioritize lifting if muscle is your goal. Schedule your hardest lower body lifts and hardest running sessions at least 24-48 hours apart. This gives your muscles time to recover and grow.

Post-Workout Nutrition: The Phone Battery Analogy

After training, your muscles need refuelling. Think of glycogen like your phone’s battery. After heavy use, it needs a good charge.

Aim to eat carbs and protein within 60 minutes post-workout:

25-30 grams of protein to support muscle repair.
About 1 gram of carbohydrate per kg of body weight to replenish glycogen.

This combination helps your body recover faster and build muscle.

Ask Yourself: Am I Eating Enough?

The fear of eating more is common, but it often keeps women stuck in a frustrating cycle. If you’re training hard but feeling soft or fatigued, ask:

Am I eating enough to support my training?
Is my protein intake high enough?
Am I timing my carbs around workouts?
Am I allowing enough recovery?

If the answer is no, it’s time to rethink your nutrition.

Your Next Step as a Hybrid Athlete

Cardio isn’t killing your gains. Under-eating is. Focus on eating enough, prioritizing protein, timing carbs, and lifting with purpose. Recovery matters just as much as training volume.

If you want to feel strong, lean, and energized, start by tracking your intake for a week. Adjust calories to support your activity. Lift heavy, run smart, and fuel well.

Your body can handle hybrid training — but only if you give it the fuel it needs.

Ready to take your hybrid training to the next level? Nutrifit is here to support you with tailored nutrition plans designed for active women like you. Let’s build strength, endurance, and confidence together.