Cardio for Body Composition: When, Why and How to Use It

Let’s clear something up: cardio isn’t the enemy of gains, and it’s not the secret to fat loss either.

At Strength Lab, we coach clients across a range of goals—from bodybuilders in comp prep to busy professionals chasing fat loss—and cardio always sparks the same questions:

• “Should I do fasted cardio?”
• “What’s better, HIIT or steady-state?”
• “Do I need cardio if I’m already lifting?”
• “Is the ‘fat-burning zone’ real?”

This blog breaks down when cardio actually helps your body composition, the smartest ways to use it, and how to avoid the common traps that stall your progress.

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First Things First: What Is Body Composition?

Body composition refers to the ratio of fat mass to lean mass (muscle, bone, organs). It’s not about weight loss, it’s about improving the quality of your weight.

A lower body fat percentage with more muscle mass = better body comp.

To improve this, your training and nutrition need to:

• Stimulate muscle growth or retention
• Create a consistent calorie deficit (for fat loss) or surplus (for muscle gain)
• Support recovery and metabolic health

Cardio can help with one of those levers but only when used strategically.

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What Cardio Can (and Can’t) Do for Body Composition

What Cardio Can Do:

• Support a calorie deficit by increasing total daily energy expenditure
• Improve cardiovascular health and recovery between sets
• Enhance fat oxidation when paired with the right nutrition
• Aid stress relief and sleep when done at the right intensity

What Cardio Can’t Do:

• Spot reduce fat
• Replace strength training
• “Out-burn” a poor diet
• Guarantee muscle retention

If your main goal is fat loss or lean muscle, strength training is your foundation. Cardio is a support act, not the headliner.

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Different Types of Cardio and How They Fit

1. LISS (Low-Intensity Steady-State)

Think walking, cycling or incline treadmill at a steady pace.

When to use it:

• Post-strength training
• On rest days for active recovery
• First thing in the morning if preferred (fasted or not, see below)

Benefits:

• Low impact on joints and recovery
• Easy to sustain long-term
• Helps burn calories without added stress

2. HIIT (High-Intensity Interval Training)

Short bursts of high effort (e.g. sprints, bike intervals) followed by rest.

When to use it:

• 1-2x per week max
• When short on time but need high output
• For trained individuals who can recover well

Benefits:

• Time-efficient
• Burns calories post-session (EPOC effect)
• Mimics the metabolic demands of lifting

Warning: Too much HIIT can backfire if you’re already lifting heavy, under-recovered or in a deep calorie deficit.

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Fasted Cardio: Myth or Method?

The idea behind fasted cardio is that exercising on an empty stomach forces your body to burn more fat for fuel. Sounds good in theory. But in practice?

What the research shows:

• Total fat loss is the same whether cardio is fasted or fed—what matters is your overall calorie balance
• Fasted cardio may slightly increase fat oxidation during the session, but not enough to meaningfully change body comp
• If fasted cardio leads to lower performance or overeating later in the day, it can hurt your results

Verdict:
Do it if it suits your schedule or preference, not because it’s magic. Consistency beats timing.

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Cardio Timing: When to Do It

Post-Workout

• Great for adding calorie burn without stealing energy from strength training
• Ideal for moderate LISS (20-30 minutes walk or bike)

On Rest Days

• Helps keep activity high without hitting the weights
• Can improve recovery if done at low intensity (zone 2)

Separate Session

• If doing high-intensity cardio, keep it separate from lifting (either different time of day or alternate days)

Tip: Don’t overdo cardio volume at the expense of recovery or lifting quality.

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How Much Cardio Do You Really Need?

Here’s what we recommend at Strength Lab for different goals:

Fat Loss (with strength training)

• 2-4 LISS sessions per week (20-40 mins each)
• 0-2 HIIT sessions per week depending on recovery
• Focus on daily step count (8,000-12,000 as a baseline)

Muscle Gain / Recomp

• 1-2 LISS sessions per week
• Keep HIIT minimal unless sport-specific
• Prioritise food, sleep and muscle recovery

Competition Prep

• Cardio typically increases as calories drop
• LISS is preferred to preserve muscle and manage stress
• HIIT used sparingly based on energy availability 

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Debunking the Fat-Burning Zone

Yes, there is a “fat-burning zone” where a higher percentage of energy comes from fat (around 60-70% of max heart rate).

But here’s the truth:

• You burn more total fat over time by creating a consistent calorie deficit, not by chasing a heart rate zone
• HIIT burns more calories overall, but uses more carbs as fuel
• LISS burns more fat during the session, but total calorie burn is lower

So what matters? Your overall energy balance, training consistency and muscle retention. The best cardio is the one you can do consistently, that supports—not replaces—your resistance training.

 

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Final Word: Cardio Is a Tool, Not a Fix

Used well, cardio supports your body composition goals. Used poorly, it can burn you out or get in the way of progress.

The key? Match the type, amount and timing of cardio to your goals, lifestyle and recovery capacity.

If you’re unsure where to start or if cardio is helping or hurting your results, chat with one of our coaches. We’ll build a plan that works for your body, not against it.