Will Strathdee

Selection candidates face long-range load carriage, sleep deprivation, caloric deficits, and back-to-back high-intensity demands. Under these conditions, metabolic flexibility becomes an important factor in sustained performance and fatigue resistance.

What Is Metabolic Flexibility?

Metabolic flexibility refers to the ability to switch between fat and carbohydrate as primary fuel sources depending on demand, intensity, and availability.

In a flexible system:

• At rest or low intensity, fat oxidation dominates.
• At high intensity, carbohydrate use rises sharply.
• During transitions (e.g. loaded hill climbs, sprinting to cover), the body shifts substrates without lag or excessive fatigue.

Inflexible metabolism leads to:

• Early carbohydrate depletion
• Reduced fat oxidation under stress
• Glycogen crashes and poor recovery

Physiological Mechanisms

Mitochondrial Adaptations

Endurance training increases mitochondrial density and efficiency — enhancing fat oxidation and reducing reliance on limited glycogen stores¹. These adaptations are driven by PGC-1α activation and are crucial for long-duration output.

Enzyme Systems

Key enzymes like carnitine palmitoyltransferase-1 (CPT-1) and pyruvate dehydrogenase (PDH) regulate the transition between lipid and carbohydrate metabolism². Regular aerobic work with periodised fueling sharpens these systems.

Hormonal Control

Insulin, glucagon, and catecholamines all influence substrate preference. Low insulin environments (e.g., fasted training) encourage fat use, while high-intensity stress shifts the balance toward carbohydrate.

Periodised Fuelling: Train Low, Perform High

Periodised fuelling refers to manipulating carbohydrate availability around training sessions to maximise metabolic adaptations without compromising performance. Rather than constantly training high or low carb, candidates strategically “train low” (low glycogen) to stimulate fat metabolism and mitochondrial growth, while “training high” when intensity or recovery demand it.

How It Works:

• Low-carbohydrate availability upregulates AMPK and PGC-1α — key signals for mitochondrial biogenesis and fat oxidation¹.
• High-carbohydrate sessions protect performance during strength and speed work, avoiding muscle breakdown or CNS fatigue.

Cognitive & Operational Benefits:

• Simulates operational energy stress: low food availability, long durations.
• Enhances fuel efficiency and decision-making during underfed training.
• Reduces dependence on external carbohydrate intake — critical when rations are limited or unavailable.

Why It Matters for SF Candidates

1. Sustain Workload in Energy Deficit

On endurance-heavy selection phases, caloric intake rarely meets output. The ability to burn fat becomes critical. Fat provides >9 kcal/g and is plentiful whereas Glycogen isn’t.

2. Operate Under Multiple Intensities

Candidates need to walk for hours, then sprint, then carry. This requires rapidly shifting between aerobic and anaerobic systems. Metabolic flexibility enables this shift without excessive fatigue.

3. Faster Recovery Between Events

Improved oxidative capacity clears lactate and Hydrogen Ions more efficiently between bouts, improving repeat performance in tasks.

How to Train It

1. Zone 2 Endurance Work

• Mechanism: Enhances mitochondrial biogenesis, fat oxidation, and cardiac output.
• Method: 45–90 mins of low-intensity running or rucking (Z2 = 60–70% HRmax).
• Example: Fasted 75-minute ruck, 15–20kg load, HR <140 bpm. Focus on nasal breathing and posture.

2. Fasted State Conditioning

• Mechanism: Increases reliance on fat as fuel; strengthens low-glycogen tolerance.
• Guidelines: Use 1–2 sessions/week early in the day. Avoid high-intensity work while fasted to prevent muscle breakdown.

3. “Train Low, Compete High” Strategy

• Method: Perform selected training sessions with reduced carbohydrate availability, but fuel appropriately for key lifts or intervals.
• Effect: Improves metabolic signalling without compromising performance adaptations³.

4. Threshold Intervals

• Goal: Raise the point where fat metabolism declines and carbohydrate dominates.
• Example: 4–5×6 min at 85–90% HRmax with 90 sec rest. Run or loaded march. Maintain steady pacing.
  Over time, this improves substrate economy at higher speeds.

Red Flags: Where Candidates Go Wrong

Mistake	Outcome
Chronic low-carb diet	Compromised glycogen availability for high-intensity work
No low-intensity conditioning	Poor fat oxidation efficiency
Overemphasis on lifting	Great strength, but limited substrate range and fatigue resistance

Application for Arduous Military Courses

Candidates must manage:

• 10–12+ hour days on foot
• Poor sleep and recovery
• Often <3,000 kcal/day intake vs >5,000 kcal output

A metabolically flexible candidate can:

Move longer and more efficiently on less fuel
Preserve cognitive and physical function under fatigue
Avoid mid-exercise glycogen crashes