Although it is much reviled, it seems hard to believe that one’s liver would pump toxins into one’s bloodstream for no reason.
Does LDL accomplish anything positive? If you had LDL levels of zero, would you be supremely healthy?
LDLs help distribute fatty acids around the body. Without it, the fats would agglutinate, being hydrophobic and all. It also helps transport cholesterol to the cells that need it, and cholesterol is necessary for stable cell membranes. Additionally, somehow, LDL helps prevent cancer, though no one knows why quite yet.
Like most nutritional things, you need the right amount of it (within a fairly large range). Not having enough is bad, and having too much is bad. If you don’t have any, you will soon be dead (but it is not a likely way to die).
And the cholesterol transported by LDL is the base steroid ring structure from which many hormones as well as Vitamin D are synthesized. Without zero LDL you would suffer from various deficiencies and be ill.
LDL carries cholesterol TO the cells of the body.
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ALL cells in the body need cholesterol to maintain their cell membranes. Without an intact membrane, a cell is kaput.
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Specialized cells in the endocrine glands (adrenal, ovaries, testes) need cholesterol in order to synthesize the hormones they secrete. Without adrenal hormones, you’d die. Without ovaries/testes, your bones would rot and you’d lose your so-called secondary sex characteristics.
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The liver actually has the greatest need for cholesterol. It uses cholesterol to manufacture bile. Without bile, you’d have a lot of trouble absorbing fats, especially fat-soluble vitamins. In addition, bile has other functions such as helping eliminate toxic substances from the body.
Why not simply have cholesterol float in the bloodstream to where it’s needed? Why do we need LDL particles (remember, LDL is a large particle which contains a lot more than cholesterol. Look here for a good schematic pic). -
LDL has chemicals on its surface which act like keys to unlock doors on cells; call them LDL-doors. Any cell that needs more cholesterol simply makes more LDL-doors. Cells that have too much cholesterol, can lower the number of their LDL-doors. If the number of LDL-doors on cells was fixed, cells would have trouble getting the the right amount of cholesterol. In the absence LDL particles to carry cholesterol (and those chemical keys), there would be no way to regulate how much cholesterol a cell gets.
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Cholesterol is oily. It won’t dissolve in the blood (which is mostly water). So, by burying cholesterol deep inside LDL particles and surrounding it by chemicals (proteins and phospholipids) that are water soluble, you can transport cholesterol throughout the body, despite the fact that cholesterol is insoluble in the blood.
Finally, just for interest and not relevant to your question, drugs like statins work by inhibiting the manufacture of cholesterol. So, if a liver cell, say, can’t make cholesterol because of the presence of a statin in the body, it’s only choice is to make more LDL-doors (okay LDL-receptors) and thereby pull in more LDL from the blood. That leads to a reduction in blood LDL levels. (Note that people with congenital absence of LDL-receptors, or born with non-functional LDL receptors, do not respond to statins).
ETA: LDL also delivers cholesterol to areas of injury (where new cells, and thus new membranes, will be needed). This is obviously of importance to wound healing etc. Unfortunately, this may also be part of the cause for cholesterol build-up in arteries (where it is delivered to sites of artery injury).
Are these functions unique to LDL, or can HDL perform the same functions?
All the lipoproteins (chylomicrons, HDL, LDL, IDL, VLDL) have in common the ability to transport insoluble fats in the water medium of the blood (with chylomicrons, VLDL, and IDL carrying triglyceride as well as some cholesterol). Their structures are also similar to one and other insofar as they have fats in their centers and various proteins and phospholipids on the outside (good picture in the Wiki link).
I should mention that in addition to helping solubilize the lipoprotein particles in the blood, the proteins (called apoproteins or apolipoproteins) in and on the lipoprotein particles act as ligands (“keys”) for various receptors (“doors”) and activators of important enzymes involved in triglyceride and cholesterol metabolism (e.g. apoC[sub]2[/sub], apoA-I).
By and large, HDL can also deliver cholesterol to the tissues of the body (such as liver and sites of steroid hormone production). However, classically, HDL has been viewed as primarily taking cholesterol FROM tissues in the body (such as from sites of “cholesterol build-up” in arteries) to the liver for excretion in the bile. This is called reverse cholesterol transport.
More recently, it has become clear that sometimes HDL gives cholesterol back to the other lipoproteins (in exchange for triglyceride) rather than taking the cholesterol to the liver for excretion. In some sense this process short circuits reverse cholesterol transport - the more this happens in a person’s body, the higher their risk of atherosclerosis. People who greatly lack the ability to do such short circuiting, more often live to 100 years or more than those who don’t. Drugs to block the process have been developed and held great promise. However, early clinical trials with those drugs have not lowered the risk of heart attacks and other manifestations of atherosclerosis.
I still hadn’t had my morning coffee when I posted earlier and forgot to answer this.
For the reasons indicated above, we all need some LDL. The problem seems to be that most of us have way too much. For example, an average North American adult’s LDL is at least 100 mg/dl (2.5 mmol/l) and probably closer to 125. Yet in early infancy, the average LDL level is, perhaps, one third of that. Obviously, there is no epidemic of cell membrane dysfunction, hormone or bile lack in (most) infants. They are designed to thrive with an LDL of less than 40 mg/dl (less than 1.0 mmol/l). And they do usually thrive.
By around six months of age, most infants have at least doubled their LDL levels, presumably due to dietary influences.
In fact, many adult mammals living in the wild, maintain LDL levels that are similar to those of humans in early infancy. And, again, most wild mammals aren’t suffering from any of the predicted effects of LDL cholesterol deficiency.
So, you need some LDL cholesterol, for sure. Trouble is you didn’t nearly as much as most of have.
According to this article, people with high LDL cholesterol build muscle faster, which suggests the purpose of LDL in repairing tissue:
Also, high LDL cholesterol levels in humans may be directly related to the inability to produce vitamin C, unlike the vast majority of other species:
The fact that we can now get enough vitamin C from the diet doesn’t change how LDL works, just as we still have appendices.
Although a Google search will give results like this which suggests that atherosclerosis is common in wild animals (remember also that humans are much more likely to die of old age and degenerative diseases).