24.3 Lipid Metabolism

Lipid metabolism is how your body breaks down fats for fuel and builds new fats when energy is abundant. These processes maintain energy balance, provide alternative fuel during fasting, and support functions like hormone production and cell membrane structure.

Lipid Metabolism

Energy Extraction from Fats

Triglycerides are the primary form of stored fat in the body. Each triglyceride consists of three fatty acid chains attached to a glycerol backbone. These fatty acids can be saturated (no double bonds between carbons) or unsaturated (one or more double bonds).

To use stored fat for energy, the body first has to break triglycerides apart through a process called lipolysis. This happens in adipose tissue and is stimulated by hormones like glucagon and epinephrine. Lipolysis splits triglycerides into free fatty acids and glycerol, which are then released into the bloodstream.

The free fatty acids travel to cells and enter the mitochondria, where they undergo beta-oxidation. Here's how that works:

1. A fatty acid chain is chopped into two-carbon units, each forming a molecule of acetyl-CoA.
2. Each round of chopping also produces one NADH and one $FADH_2$.
3. The acetyl-CoA molecules enter the citric acid cycle (Krebs cycle), generating more NADH and $FADH_2$.
4. NADH and $FADH_2$ feed into the electron transport chain to produce ATP.

This is why fats are such an energy-dense fuel source. A single long-chain fatty acid goes through many rounds of beta-oxidation, yielding far more ATP than a single glucose molecule.

Ketogenesis in Energy Production

When glucose is scarce (during fasting, starvation, or very low-carbohydrate diets), the liver ramps up ketogenesis. In this process, the excess acetyl-CoA generated from beta-oxidation is converted into ketone bodies instead of all entering the citric acid cycle. The three ketone bodies are:

• Acetoacetate
• Beta-hydroxybutyrate
• Acetone

Why does this matter? The brain normally runs on glucose and cannot directly use fatty acids for energy because fatty acids don't cross the blood-brain barrier. Ketone bodies, however, can cross the blood-brain barrier. During prolonged fasting, ketone bodies become the brain's primary alternative fuel, covering up to about 75% of its energy needs.

Ketone Body Utilization in Tissues

The liver produces ketone bodies but cannot use them itself. Instead, it releases them into the bloodstream, where they travel to tissues like the brain, heart, and skeletal muscle.

Once ketone bodies arrive at target tissues, they're converted back into acetyl-CoA through these steps:

1. Acetoacetate is converted to acetoacetyl-CoA by the enzyme succinyl-CoA:3-ketoacid CoA transferase (SCOT). The liver lacks this enzyme, which is why it can't use its own ketone bodies.
2. Acetoacetyl-CoA is split into two molecules of acetyl-CoA by the enzyme thiolase.
3. The acetyl-CoA enters the citric acid cycle, producing NADH and $FADH_2$ for ATP generation via the electron transport chain.

This system serves two purposes: it provides energy to tissues that can use ketone bodies, and it spares glucose for tissues like red blood cells that depend on it exclusively.

Lipogenesis Process and Regulation

When you consume more carbohydrates than your body needs immediately, the excess energy doesn't just disappear. Through lipogenesis, the liver and adipose tissue convert surplus carbohydrates into fatty acids and ultimately triglycerides for long-term storage.

Key steps of lipogenesis:

1. Excess glucose is broken down through glycolysis and the pyruvate dehydrogenase complex, producing acetyl-CoA.
2. Acetyl-CoA carboxylase (ACC) converts acetyl-CoA into malonyl-CoA. This is the rate-limiting (and most regulated) step.
3. Fatty acid synthase (FAS) builds palmitic acid, a 16-carbon saturated fatty acid, from malonyl-CoA and acetyl-CoA.
4. Palmitic acid can then be elongated or desaturated to form other fatty acids (e.g., stearic acid, oleic acid).
5. These fatty acids are esterified with glycerol to form triglycerides for storage.

Regulation of lipogenesis:

• Insulin stimulates lipogenesis by activating ACC and FAS. This makes sense: insulin rises after a carbohydrate-rich meal, signaling that energy is abundant.
• Glucagon and epinephrine inhibit lipogenesis by inactivating ACC. These hormones signal low energy or stress states, when the body should be burning fat, not making it.
• High-carbohydrate diets promote lipogenesis by providing a steady supply of acetyl-CoA precursors.

Lipid Transport and Metabolism

Lipids are hydrophobic, so they can't just dissolve in blood. The body packages them into lipoproteins, complex particles with a hydrophilic outer shell and a lipid-rich core. Major types include chylomicrons (carry dietary fats from the gut), VLDL, LDL, and HDL, each with different roles in lipid transport.

Cholesterol is a lipid with several critical functions. It's a structural component of cell membranes (where it modulates fluidity), and it serves as the precursor for all steroid hormones (like cortisol, estrogen, and testosterone) and for bile acids.

Bile acids are synthesized from cholesterol in the liver and secreted into the small intestine. They emulsify dietary fats, breaking large fat globules into smaller droplets so that lipases can digest them efficiently.

Phospholipids form the structural backbone of cell membranes. Each phospholipid has a hydrophilic head and two hydrophobic fatty acid tails, which is what allows them to spontaneously form the lipid bilayer. Their composition also influences membrane fluidity and function.