Cholesterol

Cholesterol is a conditionally essential nutrient, which is needed for brain health, cell membranes, and making a variety of steroid hormones. It is also important for intra-cellular signaling between neurons.

Cholesterol is only one member of a large family of related compounds called sterols. All sterols share the same 17-carbon "backbone" consisting of 4 rings (named A, B, C, and D). this basic structure is shown below:

This basic backbone is modified by attaching various "side chains" or "decorations" to different numbered positions and/or by removing hydrogen atoms to create double bonds or aromatic rings, which result in a number of different steroid hormones such as Vitamin D, estrogen, testosterone, cortisol, aldosterone, and progesterone. The complete structure of cholesterol is shown below:

In this case, decorations are added at the 3, 10, 13, and 17 positions, and a double bond is created between the 5 and 6 positions.

Dietary sources of cholesterol are only from animals. Vegans are at increased risk of deficiency.

It is possible for cholesterol to be too high, leading to cardiovascular complications, or too low. The exact healthy range is controversial, and may be tied to a patient's APOE genetics.

In general, steroids are not soluble in water, and are carried in the blood by various carrier proteins, such as HDL (high density lipoprotein, sometimes called "good cholesterol"), LDL (low density lipoprotein, sometimes called "bad cholesterol"), and SHBG (sex-hormone binding globulin). Blood lipid tests that report HDL, LDL, etc are not really reporting cholesterol directly - but instead the amount of these carrier protein complexes. In fact, HDL and LDL both transport cholesterol and other lipids, including fatty acids. HDL is considered "good" because it scavenges un-needed lipids from the cells, and carries them back to the liver for energy production or expulsion from the body via bile. On the other hand, LDL carries dietary lipids from the liver out to the cells of the body. In truth, both HDL and LDL are necessary and "good" - but their functions must occur in the proper balance.

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