Cholesterol: Animal-Specific Sterol Essential for Life

Cholesterol is a sterol lipid essential for all animal life. The statement that it is found in "all living organisms" is inaccurate — cholesterol is animal-specific. Plants synthesize phytosterols (β-sitosterol, campesterol); fungi use ergosterol; bacteria use hopanoids.

Biological Roles in Animals

• Membrane fluidity & integrity: Modulates phospholipid packing, buffering against temperature extremes.
• Precursor for: Steroid hormones (cortisol, aldosterone, sex steroids), vitamin D₃ (via 7-dehydrocholesterol + UV), bile acids.
• Lipid rafts: Platforms for signaling proteins.
• Myelin sheath component.

Sources

• Endogenous synthesis: Liver, intestines, adrenals, skin (~1 g/day). Rate-limited by HMG-CoA reductase.
• Dietary: Animal foods only (meat, dairy, eggs, shellfish). Plants contain zero cholesterol.

Clinical Relevance

• Elevated LDL-C → atherosclerosis.
• Statins inhibit HMG-CoA reductase → lower synthesis.
• Phytosterols (plant sterols) compete with cholesterol absorption → lower LDL-C.

Cholesterol is vital for animals; plants, fungi, and bacteria use different sterols.