Cholesterol is classified nutritionally as a lipid (fat). Terminology categorizing cholesterol into "good" and "bad" types is misleading because inherently, cholesterol is a single compound required for optimal health. Investigating the biological particles called lipoproteins, which transport cholesterol in the bloodstream, provides a better understanding of how cholesterol might positively or negatively impact health.
Cholesterol
Cholesterol is a wax-like compound, which is hydrophobic, meaning that it will naturally separate from a watery environment much like oil in a pond. Because the bloodstream is a watery environment, moving cholesterol within circulation requires it to be packaged in a special transport molecule, which is hydrophilic (water-loving). This particle operates like a bus, shuttling cholesterol where it needs to go, ultimately dropping it off where it serves a functional purpose in tissues.
Lipoproteins
Lipoproteins describe the buses that transport hydrophobic molecules. There are several types of lipoproteins, all of which contain some protein, fat, cholesterol, and a few additional compounds. Lipoproteins are named based on their density. The two most common are called low-density lipoproteins and high-density lipoproteins.
Low-density Lipoproteins (LDL)
Low-density lipoproteins are very light in weight, containing 10 percent fat, 25 percent protein and 45 percent cholesterol. The duty of LDL is to deliver cholesterol to tissues for use, explaining why nearly half of its total content is cholesterol.
LDL's Reputation as "Bad"
A moderate quantity of LDL particles in circulation is expected, since body tissues cannot survive without cholesterol delivery. Higher particle quantities, however, increase the risk of cardiovascular disease because excessive LDL particles "spill" their cholesterol into tissues and arteries, resulting in fatty plaque formation. Because of this spillage, LDL is often termed "bad cholesterol." The description is inaccurate, because the cholesterol in LDL is not different from that in any other lipoprotein, nor is LDL actually "bad" for health unless the particle number exceeds the recommended quantity per deciliter blood.
High-density Lipoproteins (HDL)
High-density lipoproteins are heavier in weight than LDLs because they contain 5 percent fat, 45 percent protein and 20 percent cholesterol. The role of HDL is to perform a function called "reverse cholesterol transport," which describes escorting cholesterol from the tissues to the liver for recycling.
HDL's Reputation as "Good"
HDL's role in removing cholesterol from tissues and arteries has earned it the term "good cholesterol." Again, the description is misleading since the cholesterol in this lipoprotein is identical to that in LDL, and because HDL's "cardioprotective" effects rely much more on the total number of available HDL particles than on the function of any single particle.
Lipoproteins and Health
Taken together, there is only one compound called cholesterol, and a moderate quantity of it is mandatory for body health. The increased percentage of cholesterol in LDL does not make it inherently worse for health than HDL, which has a lower percentage cholesterol. Rather, the total number of either particle will determine whether cardiovascular disease risk is present. LDLs in excess of normal (>130mg/dl) and HDLs lower than normal (<60mg/dl) are equally problematic risk factors for cardiovascular diseases, whereas maintaining optimal levels of both lipoproteins indicate good health pertaining to cholesterol and lipid transport in circulation.
References
- "Chemistry For Today;" Spencer Seager; 2008
- "Human Physiology;" Stuart Ira Fox; 2006
- Mayo Clinic
