Biosynthesis and Breakdown of Fat – Core Metabolic Pathways of Adipose Tissue

Every cell requires an uninterrupted supply of adenosine triphosphate (ATP) to maintain viability and physiologic work. The primary energetic macronutrients delivered via the diet are carbohydrates, fats, and proteins.

When caloric intake surpasses total daily energy expenditure, metabolic surplus is stored for future survival demands. Carbohydrates are converted to glycogen within the liver (approx. 100 g) and skeletal musculature (approx. 400 g). Once glycogen reserves become fully saturated, excess carbohydrates undergo de novo lipogenesis (DNL), becoming synthesized into fatty acids and condensed into triacylglycerols within adipocytes.

Lipogenesis is the anabolic sequence encompassing fatty acid biosynthesis and their esterification to glycerol to yield triacylglycerols (TAGs) in adipose tissue and the liver. The master regulatory hormonal driver of this cascade is insulin, secreted by pancreatic beta cells following postprandial glycemic rises.

Insulin facilitates glucose entry into adipocytes via GLUT-4 transporters, activates endothelial lipoprotein lipase (LPL) to extract fatty acids from circulating chylomicrons and VLDL, and simultaneously exerts potent inhibitory pressure on intracellular lipolytic enzymes, favoring fat accretion.

Lipolysis denotes the enzymatic catabolism of stored triacylglycerols within adipocyte lipid droplets into free fatty acids (FFAs) and glycerol. This mobilization occurs during negative energy balance, physical exertion, and prolonged fasting.

The lipolytic cascade is executed sequentially by three primary lipases:

• Adipose Triglyceride Lipase (ATGL) – initiates lipid mobilization by hydrolyzing TAG to diacylglycerol (DAG);
• Hormone-Sensitive Lipase (HSL) – cleaves DAG into monoacylglycerol (MAG), highly responsive to PKA-mediated phosphorylation;
• Monoacylglycerol Lipase (MGL) – hydrolyzes MAG into the final free fatty acid and glycerol backbone.

Released fatty acids bind to serum albumin, transporting to working tissues (skeletal muscle, myocardium, liver) to undergo mitochondrial beta-oxidation for ATP generation.

The homeostatic balance between lipogenesis and lipolysis is modulated by the endocrine system and autonomic tone in response to nutritional status and energetic strain.

Primary regulatory axes involve:

• Pro-lipolytic hormones – epinephrine, norepinephrine (via beta-adrenergic receptors), glucagon, growth hormone (GH), and thyroid hormones (T3/T4), which amplify intracellular cyclic AMP (cAMP) and stimulate Protein Kinase A (PKA);
• Anti-lipolytic factors – insulin acts as the quintessential physiological suppressor of lipolysis by activating phosphodiesterase 3B (PDE3B), which degrades cAMP and halts HSL activation;
• Dietary applications – implementing a structured caloric deficit, modulating glycemic index, and engaging in aerobic and resistance exercise optimize insulin dynamics to sustain lipolytic dominance.