Difficulty: Intermediate | Prerequisites: Part 1 of these notes (energy systems, crossover concept, RER). Familiarity with basic endocrine function and enzyme regulation is helpful.
Once you understand that the body shifts between fat and carbohydrate as exercise intensity changes (the crossover concept from Part 1), the next question is: what controls that shift? This set of notes covers the hormones that direct fuel mobilisation, the enzymatic machinery behind glycogen breakdown and synthesis, how muscle cells pull glucose from the blood during exercise, and the step-by-step process of lipolysis. This is the mechanistic layer underneath the broader fuel-selection patterns. If your course tests "how" and "why" rather than just "what," this is where most of those answers live.
Catecholamines, glucagon, and insulin are the main hormonal switches controlling whether the body burns fat or carbohydrate. Glycogen is broken down by glycogen phosphorylase and rebuilt by glycogen synthase, with several kinases acting as regulators. Muscle glucose uptake during exercise depends on GLUT4 transporters driven by calcium signalling and other non-insulin pathways. Fat mobilisation proceeds through a three-lipase cascade (ATGL, HSL, MGL) unlocked by catecholamine-driven PKA signalling.
Epinephrine (Epi) / adrenaline
A catecholamine hormone released from the adrenal medulla during exercise. It stimulates glycogenolysis in muscle and liver, and activates lipolysis in adipose tissue via hormone-sensitive lipase. In simple terms, this is the "go" signal that frees up both glucose and fat for fuel.
Norepinephrine (NE) / noradrenaline
A catecholamine released from sympathetic nerve endings and the adrenal medulla. Works alongside epinephrine to stimulate lipolysis and glycogenolysis. Think of it as the co-pilot to epinephrine, with similar fuel-mobilising effects.
Glucagon
A pancreatic hormone that stimulates the liver to release glucose via gluconeogenesis and glycogenolysis. In simple terms, glucagon tells the liver to top up blood sugar when levels start to drop during prolonged exercise.
Insulin
A pancreatic hormone that promotes glucose uptake into muscle and adipose tissue and stimulates glycogen synthesis. During exercise, insulin levels decrease, which removes the brake on lipolysis and allows free fatty acids to flow. Think of insulin as the storage hormone: it puts fuel away, and exercise turns it down so fuel can come back out.
Hormone-sensitive lipase (HSL)
The rate-limiting enzyme in lipolysis, activated by PKA-mediated phosphorylation. It cleaves diacylglycerols into monoacylglycerols and free fatty acids. In simple terms, HSL is the key bottleneck in releasing stored fat for energy.
Adipocyte triglyceride lipase (ATGL)
The lipase that initiates triglyceride breakdown by removing the first fatty acid chain, producing a diacylglycerol. Think of ATGL as the enzyme that takes the first bite out of stored fat.
Monoglycerol lipase (MGL)
The final lipase in the cascade, cleaving the last fatty acid from monoacylglycerol to release glycerol and a free fatty acid.
Perilipin
A protein coating lipid droplets in adipocytes that acts as a gatekeeper. When phosphorylated by PKA, perilipin changes shape and allows lipases (particularly HSL) access to the triglyceride core. In simple terms, perilipin is the lock on the fat storage door, and PKA is the key.
Protein kinase A (PKA)
A signalling enzyme activated by cyclic AMP (cAMP) downstream of catecholamine receptor binding. PKA phosphorylates both HSL and perilipin, enabling lipolysis. It also activates glycogen phosphorylase kinase.
Glycogen phosphorylase
The enzyme that catalyses glycogenolysis, breaking glycogen into glucose-1-phosphate. Activated by phosphorylation via CaMK (calcium/calmodulin-dependent kinase) and PKA pathways during exercise.
Glycogen synthase
The enzyme responsible for building glycogen from glucose units. Stimulated by insulin and inhibited by GSK3. Active primarily during recovery and the fed state.
GSK3 (glycogen synthase kinase 3)
A kinase that phosphorylates and inhibits glycogen synthase. During exercise, GSK3 activity is suppressed so that glycogen synthesis can proceed when conditions are right (e.g., during recovery).
CaMK (calcium/calmodulin-dependent protein kinase)
A kinase activated by rising intracellular calcium during muscle contraction. It phosphorylates glycogen phosphorylase, promoting glycogen breakdown, and contributes to GLUT4 translocation.
GLUT4
The insulin-sensitive glucose transporter found in muscle and adipose tissue. During exercise, GLUT4 is recruited to the cell membrane by calcium signalling and hypoxia-related pathways, independent of insulin. In simple terms, GLUT4 is the door that lets glucose into muscle cells, and exercise can open that door without needing insulin.
Free fatty acids (FFAs)
Fatty acids released from triglyceride stores via lipolysis, transported through the blood (bound to albumin) to working muscles for oxidation. Sometimes called non-esterified fatty acids (NEFAs).
Glycogenolysis
The breakdown of glycogen into glucose-1-phosphate, catalysed by glycogen phosphorylase. Provides rapid glucose for glycolysis during exercise.
Gluconeogenesis
The synthesis of new glucose from non-carbohydrate precursors (lactate, glycerol, amino acids), primarily in the liver. Important for maintaining blood glucose during prolonged exercise.
The hormonal environment during exercise shifts fuel availability:
Catecholamines (epinephrine and norepinephrine) rise with exercise intensity. They activate lipolysis via HSL and stimulate glycogenolysis in both liver and muscle.
Glucagon increases during prolonged exercise as blood glucose begins to fall. It acts on the liver to promote gluconeogenesis and glycogenolysis, maintaining blood glucose supply.
Insulin decreases during exercise. This is critical because falling insulin removes the inhibition on lipolysis, allowing FFAs to be mobilised from adipose tissue. Lower insulin also reduces glucose uptake by non-exercising tissues, sparing glucose for working muscles.
HSL activation follows a specific signalling chain:
Catecholamines bind beta-adrenergic receptors on adipocytes.
This activates adenylyl cyclase, raising intracellular cAMP.
cAMP activates PKA.
PKA phosphorylates both HSL (activating it) and perilipin (removing the physical barrier over lipid droplets).
HSL then accesses and cleaves diacylglycerols.
The net hormonal picture during exercise: high catecholamines + high glucagon + low insulin = maximal fuel mobilisation from both glycogen and fat stores.
Glycogenolysis (breakdown):
Glycogen phosphorylase cleaves glucose units from glycogen as glucose-1-phosphate, which is converted to glucose-6-phosphate for entry into glycolysis.
In muscle, this glucose-6-phosphate is used locally for energy. In liver, it can be converted to free glucose and exported to maintain blood sugar.
Glycogen phosphorylase is activated by phosphorylation through two main pathways: CaMK (responding to calcium release during contraction) and PKA (responding to catecholamines).
Glycogen synthesis (rebuild):
Glycogen synthase catalyses the addition of glucose units to a growing glycogen chain.
Insulin stimulates glycogen synthase activity, making the post-exercise, fed state the prime window for glycogen resynthesis.
GSK3 normally inhibits glycogen synthase by phosphorylating it. During and after exercise, GSK3 activity is suppressed (e.g., by Akt/PKB signalling downstream of insulin), allowing glycogen synthase to become active.
Practical takeaway: exercise turns on glycogen breakdown (via CaMK and PKA); recovery plus food turns on glycogen rebuilding (via insulin and GSK3 suppression).
During exercise, working muscles dramatically increase their glucose uptake through several mechanisms, many of which are insulin-independent:
Increased glucose concentration gradient: as muscle consumes glucose intracellularly, the gradient from blood to muscle steepens, driving more glucose in.
Calcium (Ca2+) signalling: each muscle contraction releases calcium from the sarcoplasmic reticulum. This calcium activates CaMK, which triggers GLUT4 translocation to the cell surface.
Hypoxia: during intense exercise, local oxygen tension drops. Hypoxia-responsive pathways (involving AMPK) recruit additional GLUT4 transporters to the membrane.
Increased insulin sensitivity: exercise-induced myokines enhance muscle sensitivity to whatever insulin is present, so even low insulin levels promote meaningful glucose uptake.
The rate-limiting steps for muscle glucose uptake are GLUT4 density on the cell membrane and the signalling cascades (calcium, AMPK) that drive transporter translocation.
This is clinically significant: exercise can lower blood glucose in people with type 2 diabetes through these insulin-independent pathways, which is why physical activity is a cornerstone of diabetes management.
Triglycerides stored in adipocytes are broken down through a three-step lipase cascade:
Step 1 (ATGL): removes the first fatty acid from the triglyceride, producing a diacylglycerol. ATGL has the highest affinity for intact triglycerides and initiates the process.
Step 2 (HSL): removes the second fatty acid from the diacylglycerol, producing a monoacylglycerol. HSL is the rate-limiting step and is tightly regulated by PKA phosphorylation.
Step 3 (MGL): removes the final fatty acid from the monoacylglycerol, releasing glycerol and the last FFA.
The end products: three free fatty acids (transported via albumin to working muscles for beta-oxidation) and one glycerol molecule (sent to the liver for gluconeogenesis or glycolysis).
Perilipin's gatekeeper role is essential. In the resting state, perilipin coats lipid droplets and physically blocks lipase access. Catecholamine-driven PKA activation phosphorylates perilipin, causing it to change conformation and allow HSL to reach the stored triglycerides.
Fat oxidation is highest at low-to-moderate exercise intensities where FFA availability is high and the aerobic system has time to process them. At very high intensities, carbohydrate becomes the preferred substrate because glycolysis can generate ATP faster.
Lipolysis signalling cascade (simplified):
Catecholamines → beta-adrenergic receptor → adenylyl cyclase → ↑cAMP → PKA activation → phosphorylation of HSL + perilipin → lipase access to triglyceride → FFA + glycerol release
Glycogenolysis activation:
Muscle contraction → ↑Ca2+ → CaMK activation → glycogen phosphorylase activation → glycogen → glucose-1-phosphate
Catecholamines → PKA → glycogen phosphorylase kinase → glycogen phosphorylase activation (runs in parallel with the calcium pathway)
The insulin-independent glucose uptake pathway explains why a brisk walk after a meal helps control blood sugar in people with insulin resistance: the muscle contractions recruit GLUT4 without relying on a normal insulin response. Understanding the lipolysis cascade is relevant to pharmacology as well; beta-blockers (which block beta-adrenergic receptors) can blunt lipolysis and reduce exercise capacity, which is why athletes on these medications may notice changes in body composition and performance.
Students frequently assume insulin must be present for muscles to take up glucose during exercise. Muscle contraction activates GLUT4 translocation through calcium and AMPK pathways independently of insulin. Insulin helps, but it is not required.
Another common error is treating ATGL and HSL as interchangeable. They act on different substrates in sequence: ATGL on triglycerides, HSL on diacylglycerols. HSL is the rate-limiting step, not ATGL.
Students sometimes believe glucagon acts on muscle. Muscle cells lack significant glucagon receptors. Glucagon's effects on fuel mobilisation are predominantly hepatic (liver).
GSK3 confuses students because it inhibits glycogen synthase, which sounds counterintuitive. The logic: GSK3 is active at baseline, keeping glycogen synthesis suppressed. Insulin signalling (via Akt) suppresses GSK3, thereby releasing the brake on glycogen synthase. During exercise recovery, this is how the body switches back to "refuelling mode."
⚠️ The catecholamine → cAMP → PKA → HSL/perilipin signalling chain is a high-yield exam target. Be able to list each step and explain what happens if one component is blocked.
⚠️ Know the difference between the three lipases (ATGL, HSL, MGL) and which step each one catalyses.
⚠️ Be prepared to explain insulin-independent glucose uptake in muscle and name at least two mechanisms (calcium signalling and hypoxia/AMPK).
⚠️ Glycogen phosphorylase vs glycogen synthase: know which is active during exercise versus recovery, and which kinases regulate each.
⚠️ Understand why insulin decreases during exercise and what effect this has on both glucose and fat metabolism.
True or false: Glucagon acts directly on skeletal muscle to promote glycogenolysis.
Fill in the blank: The rate-limiting enzyme in lipolysis is ________.
True or false: Perilipin must be phosphorylated by PKA before HSL can access stored triglycerides.
Fill in the blank: The enzyme that breaks down glycogen into glucose-1-phosphate is ________.
True or false: During exercise, insulin levels increase to promote glucose uptake by working muscles.
Answers: 1. False (glucagon acts on the liver, not muscle). 2. Hormone-sensitive lipase (HSL). 3. True. 4. Glycogen phosphorylase. 5. False (insulin decreases during exercise; glucose uptake is driven by insulin-independent mechanisms).
Q: Trace the signalling pathway from catecholamine release to free fatty acid mobilisation from adipose tissue.
A: Catecholamines (epinephrine and norepinephrine) bind beta-adrenergic receptors on adipocytes, activating adenylyl cyclase. This raises intracellular cAMP, which activates protein kinase A (PKA). PKA phosphorylates both hormone-sensitive lipase (HSL), activating it, and perilipin, causing a conformational change that removes the physical barrier over lipid droplets. This allows HSL to access diacylglycerols and release free fatty acids, which are then transported via albumin to working muscles.
Q: Why can exercise lower blood glucose even when insulin levels are low?
A: Muscle contraction raises intracellular calcium, which activates CaMK and triggers GLUT4 translocation to the cell membrane independently of insulin. Additionally, local hypoxia activates AMPK, further recruiting GLUT4 transporters. Exercise also increases insulin sensitivity via myokines. These insulin-independent pathways allow substantial glucose uptake even with declining insulin.
Q: Compare the roles of glycogen phosphorylase and glycogen synthase. When is each most active, and what regulates them?
A: Glycogen phosphorylase breaks down glycogen into glucose-1-phosphate and is most active during exercise. It is activated by phosphorylation via CaMK (calcium from muscle contraction) and PKA (catecholamine signalling). Glycogen synthase builds glycogen from glucose units and is most active during recovery in the fed state. It is stimulated by insulin signalling, which suppresses GSK3 (a kinase that would otherwise inhibit glycogen synthase).
Q: A student says "ATGL is the rate-limiting enzyme in lipolysis because it acts first." Correct this statement.
A: While ATGL does initiate lipolysis by cleaving the first fatty acid from triglycerides, it is HSL that is the rate-limiting step. HSL is tightly regulated by PKA phosphorylation and the perilipin gatekeeper mechanism. The rate at which HSL is activated determines the overall pace of triglyceride breakdown, making it the bottleneck rather than ATGL.
The hormonal responses described here are the mechanistic drivers behind the crossover concept covered in Part 1: rising catecholamines and falling insulin at higher intensities shift fuel preference from fat to carbohydrate. The glucose uptake pathways connect to clinical exercise physiology and diabetes management. The glycogen metabolism section ties into dietary strategies for carbohydrate loading and recovery nutrition, which are typically covered in sports nutrition units. Lipid metabolism connects forward to Part 3's discussion of ketone body production during carbohydrate restriction.
epinephrine, adrenaline, norepinephrine, noradrenaline, catecholamines, glucagon, insulin, hormone-sensitive lipase, HSL, ATGL, adipocyte triglyceride lipase, monoglycerol lipase, MGL, perilipin, PKA, protein kinase A, cAMP, beta-adrenergic receptor, glycogen phosphorylase, glycogen synthase, GSK3, glycogen synthase kinase 3, CaMK, calcium calmodulin kinase, GLUT4, glucose transporter, glycogenolysis, gluconeogenesis, lipolysis, free fatty acids, FFA, NEFA, AMPK, fuel regulation, exercise endocrinology, EXS 101