TEF and Metabolic Flexibility: Burning the right fuel at the right time with minimal waste

  1. Thermic Effect of Food (TEF)

Definition: The energy required to digest, absorb, and metabolize food.

🔥 Thermic Effect by Macronutrient:

Macronutrient TEF % of Calories Consumed

Protein 20–30%

Carbohydrates 5–10%

Fats 0–3%

➡️ Protein has the highest thermic effect—this is part of why it's key in body recomposition and fat loss.

🔄 2. Metabolic Flexibility

Definition: Your body’s ability to switch between fuel sources: fat, glucose, ketones, and protein-derived substrates (GNG).

A metabolically flexible athlete can:

Burn glucose efficiently during sprints/lifting

Burn fat and ketones during long, steady-state efforts

Use gluconeogenesis (GNG) under carbohydrate scarcity (like on keto or carnivore)

🔋 3. Fuel Systems in Elite, Conditioned Athletes

🔹 Glycolysis (Carbs → Glucose → ATP)

Anaerobic, fast

Used for short, high-intensity bursts (e.g., sprints, heavy lifts)

Creates lactate and potential fatigue

🔹 Fat Oxidation (Lipolysis + Beta-Oxidation)

Aerobic, slow

Preferred during low to moderate intensity, long duration

Becomes more efficient on low-carb or keto diets

🔹 Ketosis

Liver converts fat into ketones (BHB, acetoacetate)

Brain and muscles adapt to use ketones

Best suited for ultra-endurance, fasting states, or when carbs are chronically low

Can spare muscle glycogen during low/moderate output

🔹 Gluconeogenesis (GNG)

Creation of glucose from non-carb sources (e.g., amino acids, lactate, glycerol)

Supports brain and red blood cell glucose needs in low-carb states

Highly active in keto-adapted athletes

🧠 4. Metabolic State in Highly Conditioned Athletes

Elite athletes often achieve:

Efficient fat oxidation, even when not in ketosis

Quick access to glycogen stores when needed

Better mitochondrial density and oxygen efficiency

Lower lactate accumulation at high intensities

Elevated GNG capacity without losing muscle due to hormonal signaling (like increased growth hormone and norepinephrine)

🎯 Carnivore / Keto Athlete:

When deeply adapted:

High TEF from protein can increase resting metabolic rate

Low insulin environment favors fat mobilization and ketone production

Metabolic flexibility improves if training stimulus remains high

GNG from protein and glycerol ensures brain + muscle glucose without carbs

➡️ Combine this with fasted training, strategic refeeding (cyclical keto), and hormone regulation, and you're training the body to become an engine of perpetual transformation—burning the right fuel at the right time with minimal waste.