Summary
Highlights
This video is the second part of a three-part series on metabolism, focusing on lipid metabolism. Fats are a powerful energy source, providing more than twice the energy of carbohydrates. Before fats can be used, they must be broken down.
Fats are broken down in the small intestine into fatty acids and glycerol. Bile salts emulsify fats, and lipases digest them. Once absorbed, these fatty acids are either stored or converted into energy.
Fats are hydrophobic and need lipoproteins to travel through the bloodstream. There are four main types of lipoproteins: chylomicrons (transport dietary fats), VLDLs (very low-density lipoproteins, transport endogenous lipids), LDLs (low-density lipoproteins, deliver cholesterol to cells, often called 'bad cholesterol'), and HDLs (high-density lipoproteins, collect excess cholesterol and return it to the liver, known as 'good cholesterol').
Fats are stored in adipose tissue as triglycerides, serving as a long-term energy reserve. When energy is needed, triglycerides are broken down into fatty acids and glycerol through lipolysis, catalyzed by lipases. Glycerol can be converted to glyceraldehyde-3-phosphate, which can then be used to create glucose (gluconeogenesis) or enter the catabolic pathway to produce pyruvic acid.
Fatty acids are catabolized in the mitochondria through beta-oxidation, breaking them down two carbons at a time to produce acetyl coenzyme A. Acetyl coenzyme A then enters the Krebs cycle, generating NADH and FADH2, which power the electron transport chain for ATP production. Fatty acids yield significantly more ATP than glucose due to their long hydrocarbon chains.
When glucose is scarce (e.g., fasting, low-carb diets), the liver converts excess acetyl coenzyme A from fatty acid breakdown into ketone bodies, which serve as an alternative fuel for brain, muscles, and heart. Excessive ketone production, as in uncontrolled diabetes, can lead to ketoacidosis, a dangerous drop in blood pH.
When energy intake exceeds demand, the body converts excess glucose or amino acids into fatty acids through lipogenesis, primarily in the liver. Acetyl coenzyme A from glucose metabolism synthesizes new fatty acids, stored as triglycerides in adipose tissue, ensuring energy availability for later.
Lipid metabolism allows the body to switch between carbohydrates and fat for energy, sustaining long periods without food, powering endurance activities, and storing energy efficiently. Understanding this process helps in making informed choices about diet, exercise, and metabolic health.