What myelin is made of

Myelin is the protective sheath surrounding many nerve fibers in the brain and spinal cord.

It is not 100% cholesterol. Myelin is composed of about 70–85% lipids and 15–30% proteins by dry weight. Of those lipids, cholesterol is a major component, but there are also glycolipids and phospholipids. Cholesterol typically makes up around 20–30% of the total lipid content.

Function of cholesterol in myelin

Cholesterol provides structural stability and helps maintain the integrity of the myelin sheath.

Myelin formation (myelination) is critical for efficient nerve signal conduction.

Effect of cholesterol-lowering drugs

Statins and other cholesterol-lowering drugs reduce cholesterol levels in the bloodstream, but the brain synthesizes most of its own cholesterol locally.

Blood cholesterol and brain cholesterol are largely separate because the blood-brain barrier prevents most circulating cholesterol from entering the brain.

In theory, certain cholesterol-lowering drugs that cross the blood-brain barrier could influence brain cholesterol metabolism, but most statins do not significantly lower cholesterol within the central nervous system in healthy adults.

The claim that myelin is "100% cholesterol" is factually incorrect, and the direct implication that taking cholesterol-lowering drugs will automatically damage myelin is an oversimplification. The brain largely regulates its own cholesterol supply independently of the bloodstream, so most standard statin treatments are unlikely to cause myelin loss in healthy individuals. However, more research is warranted into the effects of cholesterol-lowering drugs that do cross the blood-brain barrier.