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The 5 Zone Energy System: Finding Your Threshold

Aug 6, 2025 | Articles, Hormones

How Anaerobic Threshold, Lactic Acid, and Energy Systems Can Transform Your Training and Hormones

Introduction: Why Your Cardio Strategy Might Be Holding You Back

You’ve probably heard terms like anaerobic threshold or Zone 2 training tossed around at your gym or in fitness podcasts. Maybe you’ve felt the “burn” during sprints or struggled to build endurance  but never really understood what your body was doing or why your results plateaued.

Whether your goal is fat loss, endurance, hormone balance, or longevity, the key to unlocking your next level might lie in understanding your body’s energy systems and how they intersect with training thresholds.

This post breaks it all down in plain language backed by science and gives you an edge. At 1st Optimal, we don’t just guess. We use lab testing, performance data, and functional medicine to personalize your training, recovery, and hormonal strategy.

Table of Contents

  1. Anaerobic Threshold and Lactic Acid: The Basics
  2. The Three Energy Systems You Must Understand
  3. Thresholds, Zones, and How to Target the Right One
  4. Cardio and Hormones: Cortisol, Testosterone, and Fat Loss
  5. Aerobic Training (Zone 2): The Mitochondria Miracle
  6. Interval Training and Threshold Workouts: When and How
  7. Peptides and Cardio Recovery: What the Science Says
  8. Case Study: Two Clients, Two Threshold Approaches
  9. Advanced Applications: Cardio Periodization and Lab-Guided Performance
  10. Interpreting Functional Lab Data for Training
  11. The Menopause and Midlife Factor
  12. Building Your Personalized Threshold Plan with 1st Optimal
  13. FAQs: Your Top Training Questions Answered
  14. Final Thoughts: Building a Smarter, Hormone-Friendly Cardio Plan
  15. Call to Action
  16. References

Anaerobic Threshold and Lactic Acid: The Basics

The anaerobic threshold (AT) is the intensity at which your body transitions from aerobic to anaerobic energy systems. At this point, oxygen is no longer able to meet energy demands, and your body begins producing more energy from glucose through glycolysis, a faster but less efficient process.

The result? A sharp rise in lactate levels, increased breathing rate, and that classic burning sensation in your muscles.

Contrary to old-school thinking, lactic acid isn’t bad. In fact, lactate is a fuel, not a toxin.

  • Lactic acid is rarely present in the blood.
  • Lactate is the form your body actually uses and recycles.
  • The term “lactate threshold” (LT) is now more accurate than “anaerobic threshold.”

The Three Energy Systems You Must Understand

Your body uses different systems to fuel different types of movement. These overlap but can be trained individually:

 

1. Phosphagen System (ATP-PC)

  • Duration: 0–10 seconds
  • Fuel: ATP and creatine phosphate
  • Used for: sprinting, Olympic lifts, explosive work

 

2. Glycolytic (Anaerobic) System

  • Duration: 30 seconds to 2 minutes
  • Fuel: glucose
  • Produces: lactate
  • Used for: 400m sprint, circuit training

 

3. Oxidative (Aerobic) System

  • Duration: 2 minutes to several hours
  • Fuel: fat and carbs with oxygen
  • Used for: steady-state cardio, Zone 2 walks, long runs

Thresholds, Zones, and How to Target the Right One

 

What is the aerobic threshold (AeT)?

  • The lowest intensity where lactate starts to accumulate.
  • Occurs around 60–70% VO₂ max.
  • Where fat-burning is most efficient.

 

What is the lactate threshold (LT)?

  • Occurs at 75–85% VO₂ max.
  • Lactate accumulation outpaces clearance.
  • Key point for performance conditioning.

 

Heart Rate Training Zones:

  • Zone 1: <60% HRmax – Recovery
  • Zone 2: 60–70% HRmax – Aerobic fat burn
  • Zone 3: 70–80% HRmax – Tempo (can fatigue recovery)
  • Zone 4: 80–90% HRmax – Threshold zone
  • Zone 5: >90% HRmax – VO₂ max, anaerobic bursts

Cardio and Hormones: Cortisol, Testosterone, and Fat Loss

Your hormonal response to cardio depends entirely on the type, intensity, and your recovery habits.

 

Zone 2 training supports:

  • Lower resting cortisol
  • Increased insulin sensitivity
  • Enhanced mitochondrial function
  • Fat oxidation

 

High-intensity interval training (HIIT) causes:

  • Short-term cortisol spikes
  • Growth hormone and testosterone release
  • Increased GLP-1 and metabolic flexibility

 

But overdoing HIIT without recovery? That leads to:

  • Burnout
  • Hormonal crashes
  • Fat retention

Aerobic Training (Zone 2): The Mitochondria Miracle

Zone 2 is the base-building zone. It’s where:

  • Mitochondria multiply
  • Fat becomes a primary fuel
  • Blood sugar improves
  • Endurance and recovery peak

 

How to do it:

  • 45–60 minutes steady pace
  • Heart rate 60–70% of max
  • You should be able to hold a conversation

 

Ideal for:

  • Weight loss
  • Hormonal balance
  • Cardiovascular longevity

Interval Training and Threshold Workouts: When and How

 

Why interval training works:

  • Improves VO₂ max
  • Raises lactate threshold
  • Burns calories efficiently
  • Supports anabolic hormones

 

Protocols:

  • 4x4s: 4 mins high, 3 mins rest × 4
  • Sprint Intervals: 30s all-out, 4 min rest × 5
  • Lactate Repeats: 5 min just above threshold × 3–5

Use these no more than 2x/week, especially if recovering from burnout or in perimenopause.

Peptides and Cardio Recovery: What the Science Says

We frequently use peptide therapies to support energy system training and recovery.

 

Peptides to know:

  • BPC-157: Soft tissue healing
  • Tesamorelin: Growth hormone boosting, fat loss
  • GHK-Cu: Mitochondrial regeneration
  • CJC-1295/Ipamorelin: Fat loss + recovery

 

Combined with threshold-focused training, peptides improve:

  • Fat burning
  • Inflammation control
  • Cardiovascular performance
  • Hormone signaling

Case Study: Two Clients, Two Threshold Approaches

 

Mark, 50 – Stressed entrepreneur

  • Issue: Low testosterone, belly fat, high insulin
  • Plan: 3x Zone 2, 1x HIIT, Tesamorelin
  • Result: -12 lbs fat, +3 lbs muscle, energy up

 

Lauren, 43 – Former CrossFitter

  • Issue: Sleep disruption, hormonal imbalance
  • Plan: Replaced HIIT with Zone 2, added GHK-Cu
  • Result: HRV increased, weight normalized, better sleep

Advanced Applications: Cardio Periodization and Lab-Guided Performance

Don’t just guess. Periodize your cardio with intent:

Cycle Example:

  • Weeks 1–4: Base Phase – Zone 2
  • Weeks 5–8: Threshold + intervals
  • Week 9: Recovery
  • Week 10: Reassess with labs or VO₂ max

 

Add:

  • Peptides for mitochondria or fat loss
  • Wearable data (HRV, sleep, readiness)

Interpreting Functional Lab Data for Training

Functional medicine lets us use biomarkers to make cardio more efficient.

 

Key Labs:

  • Cortisol – DUTCH or saliva
  • Free + Total Testosterone
  • HRV – via Oura, WHOOP
  • Glucose + Insulin
  • VO₂ Max + Lactate Testing

We tailor workouts and recovery based on lab-verified readiness, not guesswork.

The Menopause and Midlife Factor

 

What changes?

  • Estrogen loss → fewer mitochondria
  • Fat metabolism slows
  • VO₂ max drops
  • Sleep gets worse

 

Our approach:

  • Hormone replacement or peptide support
  • Gentle Zone 2 cardio
  • Resistance training
  • Interval training only when HRV is high

 

Hormones and cardio must work together, not in conflict.

Building Your Personalized Threshold Plan with 1st Optimal

We combine:

  • Lab Testing – metabolic, hormones, inflammation
  • VO₂ and Lactate Testing – real-time threshold zones
  • Peptide Support – for recovery and hormone balance
  • Wearable Data – HRV, readiness, sleep
  • Coaching – to keep you accountable and adjust weekly

FAQs: Your Top Training Questions Answered

Q: What’s the difference between anaerobic and aerobic training?

A: Anaerobic = short bursts without oxygen. Aerobic = long, steady movement with oxygen.

Q: Is lactic acid bad?

A: No. Lactate is recycled as fuel.

Q: Best cardio for fat loss?

A: Zone 2 burns fat. Intervals increase metabolism post-workout.

Q: Can I do cardio every day?

A: Yes, if you alternate intensities and listen to your body.

Q: What’s the best way to test my threshold?

A: VO₂ max, lactate testing, or a 20-minute time trial.

Final Thoughts: Building a Smarter, Hormone-Friendly Cardio Plan

Cardio without context can burn you out.

But when personalized with labs, thresholds, and hormones, cardio becomes:

  • More efficient
  • Less stressful
  • Better for weight loss and longevity

Don’t guess. Test, train, and optimize with the team at 1st Optimal.

🧪 Want to test your anaerobic threshold or hormone profile?

📅 Book a consult with our performance team

🧬 Order your custom metabolic lab kit

🎯 Take our free quiz

References

  1. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4241848/
  2. https://pubmed.ncbi.nlm.nih.gov/29985345/
  3. https://journals.physiology.org/doi/full/10.1152/japplphysiol.00041.2006
  4. https://pubmed.ncbi.nlm.nih.gov/28422553/
  5. https://www.ncbi.nlm.nih.gov/books/NBK544280/
  6. https://www.jospt.org/doi/abs/10.2519/jospt.1994.19.6.371
  7. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5927566/
  8. https://pubmed.ncbi.nlm.nih.gov/33540251/
  9. https://pubmed.ncbi.nlm.nih.gov/32840286/
  10. https://journals.sagepub.com/doi/10.1177/1941738120951417
  11. https://pubmed.ncbi.nlm.nih.gov/34100993/
  12. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6163023/
  13. https://pubmed.ncbi.nlm.nih.gov/29758272/
  14. https://pubmed.ncbi.nlm.nih.gov/31971936/
  15. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7936502/
  16. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4415003/
  17. https://pubmed.ncbi.nlm.nih.gov/36175990/
  18. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3772611/
  19. https://pubmed.ncbi.nlm.nih.gov/26787157/
  20. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8879322/
  21. https://pubmed.ncbi.nlm.nih.gov/11346390/
  22. https://pubmed.ncbi.nlm.nih.gov/34324743/
  23. https://pubmed.ncbi.nlm.nih.gov/24532846/
  24. https://pubmed.ncbi.nlm.nih.gov/27624883/
  25. https://pubmed.ncbi.nlm.nih.gov/33012335/
  26. https://pubmed.ncbi.nlm.nih.gov/34951136/
  27. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6835901/
  28. https://pubmed.ncbi.nlm.nih.gov/31230279/
  29. https://pubmed.ncbi.nlm.nih.gov/35295445/
  30. https://pubmed.ncbi.nlm.nih.gov/36974683/