Fasting and Fitness: How to Train Without Crushing Your Hormones

Fasting: A Double-Edged Sword

Intermittent fasting is one of the most powerful tools for fat loss, insulin sensitivity, and longevity. But pairing fasting with the wrong training approach can backfire—leading to hormonal disruption, energy crashes, and stalled progress.

At Underpin Health & Fitness, we help clients use fasting strategically to unlock benefits without compromising strength or long-term health.

The Science of Fasted Training

When you train fasted:

• AMPK pathway activates → promotes fat oxidation and mitochondrial biogenesis

• Catecholamines rise → mobilize stored fat

• Cortisol spikes → provides energy but may suppress recovery if chronic

• Glycogen depletes → helpful for fat loss, harmful for strength cycles

Best Pairings: Fasting + Training

Fasted Cardio (Zone 2)

• Great for fat loss and mitochondrial health

• How to do it: 20–45 minutes at conversational pace, 2–3x per week

• Fuel: Black coffee, green tea, electrolytes (no sugar)

Fasted Strength Training

• OK if short (≤45 minutes) and not done daily

• Must be followed by a refeed with protein + carbs

• Best during maintenance, not heavy bulking or volume phases

Avoid Long Fasted HIIT or Heavy Lifting

• Spikes cortisol

• Can tank testosterone and thyroid function

• High risk of delayed recovery and overtraining symptoms

Hormonal Risks of Doing It Wrong

• Cortisol Dominance: Belly fat, poor sleep

• Thyroid Suppression: Low T3 → slow metabolism

• Sex Hormone Drops: Lower testosterone/estrogen → poor strength and libido

• Adrenal Fatigue Symptoms: Energy crashes, irritability, brain fog

How to Train With Fasting Safely

1. Align Training With Your Fast Window

• For 16/8 fasting, train near end of fast or just after eating

• For 24-hr+ fasts, do only low-intensity recovery (walking, yoga)

2. Strategic Breaking of Fast

Prioritize protein + carbs

• Protein: 20–40g

• Carbs: 30–50g

• Add electrolytes (magnesium, potassium)

3. Support Hormonal Balance

• Adaptogens: Ashwagandha, rhodiola

• Micronutrients: Zinc, B-complex, magnesium

• Lifestyle: 7–9 hrs of sleep, 1–2 full rest days per week

  
  

Advanced Layer: Peptides + Fasting

• MOTS-c: Boosts fat oxidation and insulin sensitivity

• SS-31 (Elamipretide): Protects mitochondria during energy stress

• BPC-157: Repairs gut lining and recovery under stress from fasting + training

  
  

This is the future of fasting—pairing traditional strategies with cellular optimization.

Who Should Be Careful With Fasted Training

• Women with irregular cycles (fasting can disrupt hormonal rhythm)

• Men with low testosterone symptoms

• Anyone under high stress or chronic sleep deprivation

Takeaway

Fasting can be a powerful tool—but without strategy, it risks hormonal imbalance and plateaued results. Use fasted cardio for fat loss and metabolic health, but fuel strength and performance with smart nutrition and recovery.

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At Underpin Health & Fitness, we help you find the right fasting + training balance that supports your goals and your hormones.

👉 Curious how fasting fits into your unique physiology? Book a consultation with Underpin Health & Fitness and we’ll tailor your approach for success.