Study Notes – Fat Burning & Exercise Physiology

1. Basics of Fat (Adipose Tissue)

• Fat = adipose tissue (used interchangeably).

• Locations:

  • Subcutaneous (hypodermis, under the skin).

  • Around organs.

  • Inside liver.

  • In skeletal muscle.

• Visible fat concern: hypodermis (superficial layer).

• Rectus abdominis (“six-pack”) is present in everyone → visibility depends on thickness of subcutaneous fat.

2. Energy Sources for Exercise

• Skeletal muscle uses:

  • Carbohydrates (glucose/glycogen).

  • Fats (fatty acids).

  • Proteins (rare, usually only when carbs/fats limited).

• Ratio of fats:carbs depends on exercise intensity.

3. Fat Burning Zone Concept

• At rest → high % fat used (≈70–80%), but total calories burned is very low.

• With increasing intensity:

  • % fat decreases, % carbs increases.

  • Total calories burned increases, so absolute fat burned can be higher at moderate intensities.

• Very high intensity → mostly carbs, almost no fat.

4. Example Data (Male, 40s, marathon trainee)

• Slightly above rest:

  • ~78% fat, ~22% carbs.

  • Very low total calorie burn (~1.4 kcal/min).

• Moderate intensity (~127 bpm):

  • 40% fat, 60% carbs.

  • Higher total burn (~11.5 kcal/min).

  • Absolute fat burned (~4.7 kcal/min) > rest.

• High intensity:

  • % fat continues dropping.

  • Absolute fat burned peaks at moderate intensity zone (often called Zone 2).

5. Zone 2 Training (Fat Burning Zone)

• Definition: moderate intensity, heart pumping but sustainable.

• Talk test: can hold a conversation.

• Blood lactate ~2.0 mmol/L.

• Adaptations:

  • ↑ capillary density in muscles.

  • ↑ mitochondria in muscle cells.

  • Improved fat utilization efficiency.

6. Key Takeaways on Fat Loss

• Total caloric expenditure matters more than fat:carb ratio.

• Zone 2 is useful, but not superior for fat loss compared to higher intensity, if calories burned are equal.

• Study evidence:

  • Group doing Zone 2 (longer sessions) vs. group doing high-intensity (shorter, harder sessions).

  • Both lost fat at same rate when total calories burned matched.

7. Pros & Cons

• High Intensity:

  • Time-efficient.

  • Burns more calories quickly.

  • Risks: overtraining, fatigue.

• Zone 2 / Low Intensity:

  • Sustainable, improves endurance.

  • Builds mitochondrial efficiency, heart health.

  • Less calorie-dense per unit of time.

8. Endurance Relevance

• Carbohydrate storage limited (~500 g glycogen ≈ 2000 kcal).

• Fat stores vast (~100,000 kcal).

• Zone 2 training teaches body to spare carbs by using more fat, beneficial for endurance athletes (marathon, triathlon).

9. Post-Exercise Fat Burn

• EPOC (Excess Post-Exercise Oxygen Consumption):

  • After intense workouts, body continues to burn fat aerobically while restoring ATP and oxygen balance.

10. Spot Reduction Myth

• Cannot target fat loss in specific areas with exercise.

• Genetics and sex influence fat storage/reduction patterns.

• Building muscle underneath fat (e.g., abs) may increase visibility, but doesn’t change fat distribution.

11. Practical Advice

• Fat loss = consistency + calorie expenditure.

• Best exercise = one you enjoy and can sustain regularly.

• Blend of:

  • Zone 2 (endurance & fat utilization).

  • High intensity (time efficiency, calorie burn).

  • Strength training (muscle mass, metabolic health).