Every time you eat, your body enters a different metabolic programme from the one it runs between meals. Understanding the fed and fasting states, glycogen storage and mobilisation, and the insulin-glucagon balance explains weight management, energy patterns, brain fog, blood sugar dysregulation, and the clinical rationale for time-restricted eating.
Every time you eat, your body enters a metabolic state governed by insulin. Every time you fast — overnight, between meals, during extended food restriction — it enters a different state governed by glucagon. These two states are not simply “fed” and “hungry.” They are fundamentally different metabolic programmes with different fuel sources, different enzyme activations, and different hormonal orchestration. Understanding them is the foundation of almost everything in clinical nutrition.
Most people — and, frankly, most standard dietary advice — treats the body as if it operates in a single metabolic mode with one dial: calories in vs calories out. The actual biochemistry describes a system with multiple modes, switchable by food timing and composition, with consequences for energy, body composition, inflammation, hormonal balance, and longevity that go far beyond the calorie equation.
Trigger: Blood glucose rises after a carbohydrate-containing meal. Pancreatic beta cells secrete insulin. Glucagon falls.
Liver: Glucose uptake via GLUT2 (insulin-independent). Glycogen synthesis activated (glucokinase, glycogen synthase). Glycolysis active. Excess glucose converted to acetyl-CoA and then fatty acids via de novo lipogenesis. VLDL assembled and exported to adipose tissue.
Muscle: GLUT4 translocates to membrane in response to insulin. Glucose enters for glycogen synthesis and energy. Amino acid uptake stimulated. Protein synthesis promoted (mTOR activated by insulin and amino acids).
Adipose tissue: GLUT4 translocates. Glucose and fatty acids (from VLDL) taken up. Triglyceride synthesis (lipogenesis). Lipolysis suppressed by insulin. Fat is being stored.
Brain: Glucose preferred fuel. GLUT1 and GLUT3 provide insulin-independent glucose uptake — the brain is always fuelled in the fed state.
Trigger: Blood glucose falls. Insulin falls. Glucagon rises from pancreatic alpha cells. Cortisol provides additional counter-regulatory support in extended fasting.
Liver: Glycogenolysis activated (glycogen phosphorylase) — liver glycogen broken down to maintain blood glucose. After 12–18 hours, liver glycogen depleted. Gluconeogenesis activated: glucose synthesised from amino acids (alanine, glutamine), glycerol (from fat breakdown), and lactate. Fatty acid oxidation (beta-oxidation) increases. Ketone body synthesis (acetoacetate, beta-hydroxybutyrate) for brain fuel in extended fasting.
Adipose tissue: Insulin withdrawal activates hormone-sensitive lipase (HSL). Triglycerides broken down to fatty acids and glycerol. Fatty acids released into circulation as albumin-bound free fatty acids. This is the fat-burning state.
Muscle: Switches to fatty acid oxidation for energy. Releases amino acids (alanine, glutamine) for hepatic gluconeogenesis — this is the muscle protein catabolism that occurs in extended fasting without adequate dietary protein.
Brain: Initially dependent on hepatic glucose (gluconeogenesis). After 3–4 days of fasting, adapts to use ketone bodies for 60–70% of energy needs, reducing gluconeogenic demand on muscle protein.
Glycogen is the storage form of glucose — a branched polymer of glucose units connected by alpha-1,4 and alpha-1,6 glycosidic bonds. The branching is critical: it allows rapid mobilisation from many points simultaneously when glucose is urgently needed.
The liver stores approximately 100g of glycogen (400 kcal). This is primarily blood glucose maintenance glycogen — its purpose is to supply the brain and red blood cells with glucose between meals and overnight. Liver glycogen is depleted within 12–18 hours of fasting.
Skeletal muscle stores approximately 400g of glycogen (1,600 kcal) in total, distributed across muscle fibres. Muscle glycogen is private to the muscle — it cannot be exported to maintain blood glucose because muscle lacks glucose-6-phosphatase (the enzyme needed to release glucose from cells). Muscle glycogen is used exclusively for muscle energy during exercise.
Waking fatigue and low morning energy often reflect overnight liver glycogen depletion — blood glucose has been maintained at the cost of gluconeogenesis and cortisol mobilisation. The cortisol awakening response (visible on DUTCH) is partly the body’s counter-regulatory response to overnight glucose demand.
After glycogen stores are full, excess glucose has nowhere to go but fat synthesis. Consistently overfilling glycogen — through high carbohydrate intake relative to expenditure — drives de novo lipogenesis and elevated triglycerides, visible on blood chemistry.
Liver glycogen depletion at 12–18 hours of fasting is what triggers the metabolic switch toward fatty acid oxidation and ketogenesis. Time-restricted eating windows that extend the overnight fast allow this switch to occur regularly, improving metabolic flexibility over time.
Whether that adds anything beyond eating less is graded in Fasting: Who It Helps, Who It Harms.
High-intensity exercise is glycogen-dependent. Adequate carbohydrate before and after high-intensity sessions supports muscle glycogen resynthesis. Chronic low carbohydrate intake in heavy training creates glycogen depletion, elevated cortisol, and eventually HPA axis suppression — visible on DUTCH.
The fed-to-fasting transition is orchestrated by the insulin:glucagon ratio rather than by either hormone alone. High insulin (after carbohydrate intake): activates glycogen synthesis, promotes fat storage, suppresses fat oxidation, promotes protein synthesis. Low insulin + high glucagon (during fasting): activates glycogenolysis and gluconeogenesis, promotes fat mobilisation, allows ketogenesis, activates autophagy (mTOR suppressed, AMPK activated).
The metabolic flexibility to move fluidly between these states — using glucose when available, switching to fat oxidation when not — is a measure of metabolic health. Some people, particularly those with insulin resistance, switch less efficiently; how common this is hasn’t been well measured. They remain glucose-dependent even in the fasting state, producing fatigue, hunger, and cognitive fog between meals that resolves only with the next glucose hit.
You are either in feast mode or famine mode, and every meal is a vote for which one your metabolism has practice at. Modern constant grazing practices feast mode all day. Metabolic flexibility requires regular practice of famine mode — not starvation, but the structured overnight fast that human physiology evolved with.
Blood chemistry: Fasting glucose and fasting insulin (both required — fasting glucose alone misses early insulin resistance). HOMA-IR (calculated from both). HbA1c (reflects average glucose over 3 months). TG:HDL ratio (metabolic flexibility and insulin sensitivity proxy). Metabolomix+ OAT: Krebs cycle intermediates (citrate, isocitrate, succinate, fumarate) reflect mitochondrial metabolic state. Elevated intermediates suggest metabolic blockade, often from nutrient cofactor insufficiency. Fatty acid oxidation markers (adipate, suberate, ethylmalonate) reflect whether beta-oxidation is functioning effectively — the key pathway of the fasting state. DUTCH: Cortisol pattern and metabolised cortisol — cortisol is the primary counter-regulatory hormone of the fasting state; dysregulated cortisol patterns disrupt fed-fasting transitions.
Ask the DH Concierge — trained on nearly forty years of working with people and the full biochemical foundation of the TDG approach.
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