Lipolysis and Fatty Acid Mobilization

Core Idea

Lipolysis is the breakdown of triglycerides into glycerol and free fatty acids, catalyzed by hormone-sensitive lipase (HSL) in adipose tissue. Epinephrine and glucagon activate HSL via PKA-mediated phosphorylation; insulin inhibits it. Released fatty acids bind albumin and are transported to liver and muscle for oxidation.

Explainer

Your body stores energy primarily as triglycerides — three fatty acid chains esterified to a glycerol backbone — packed into lipid droplets inside adipocytes. When energy demand rises (during fasting, exercise, or stress), those stored fats must be broken down and shipped to tissues that can oxidize them. This breakdown process is lipolysis, and understanding it means following a hormonal signal from the bloodstream all the way to the release of free fatty acids.

The signaling cascade works through the hormone-signaling mechanisms you already know. During fasting, the pancreas releases glucagon; during exercise or stress, the adrenal medulla releases epinephrine. Both hormones bind G-protein-coupled receptors on adipocytes, activating adenylyl cyclase, which raises intracellular cAMP. Rising cAMP activates protein kinase A (PKA), which phosphorylates two key targets: hormone-sensitive lipase (HSL) and perilipin, the protein coating the lipid droplet surface. Phosphorylated perilipin changes conformation, exposing the triglyceride core, while phosphorylated HSL translocates from the cytosol to the droplet surface. The process is actually sequential: adipose triglyceride lipase (ATGL) removes the first fatty acid (producing diacylglycerol), HSL removes the second (producing monoacylglycerol), and monoacylglycerol lipase (MGL) removes the third. The net result is one glycerol and three free fatty acids per triglyceride molecule.

Insulin acts as the brake on this system. In the fed state, insulin activates phosphodiesterase 3B, which degrades cAMP, shutting down PKA and keeping HSL dephosphorylated and inactive. This is why lipolysis is suppressed after meals and activated during fasting — the insulin-to-glucagon ratio is the master switch. In insulin resistance, this brake weakens: adipocytes release fatty acids even when blood glucose is high, flooding the liver with lipid and contributing to fatty liver disease and dyslipidemia.

Once released, free fatty acids face a transport problem — they are hydrophobic and would be toxic to membranes at high concentrations. The solution is serum albumin, a large plasma protein with multiple fatty acid binding sites. Albumin ferries fatty acids through the blood to the liver (for ketogenesis or re-esterification) and to skeletal and cardiac muscle (for beta-oxidation). Glycerol, being water-soluble, travels freely to the liver, where glycerol kinase phosphorylates it to glycerol-3-phosphate, feeding it into glycolysis or gluconeogenesis. This division of labor — fatty acids for oxidation, glycerol for glucose production — makes lipolysis a critical node connecting fat metabolism to carbohydrate metabolism during fasting.