Cholesterol

The general opinion about cholesterol is that our body needs it, but makes the required amount itself, that there are basically two kinds of cholesterol: "good cholesterol" and "bad cholesterol", and that too high a level of "bad cholesterol" is a risk factor, if not the cause of vascular diseases. (1)
Is cholesterol really the problem?

What is cholesterol?
Cholesterol is often classified as a fat (-like substance), because it is not water (or blood) soluble. It is present in the cell walls of every cell in the body. The highest concentration of cholesterol is found in the brain (10-20%) and other parts of the nervous system, as well as in the liver.

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What is cholesterol exactly?

Cholesterol is a sterol. In chemistry this means it has a steroid part (4 carbon rings) and a hydroxyl group (alcohol group).

What does it do?
Cholesterol has several functions. For example it plays an important role in cell membranes, it helps with digestion of fats and fat-soluble vitamins (conversion to bile acids), and is the precursor for the synthesis of vitamin D and the steroid hormones. These hormones include cortisol, aldosterone, and the sex hormones progesterone, the various estrogens, and testosterone.

Do we need cholesterol in our diet?
Because cholesterol is so important, the human body can produce most of the cholesterol it needs, through a long chain of chemical reactions. Likewise it can also produce vitamin B2, B3 and B5. The more dietary cholesterol we consume, the less our body needs to make.

Just as a vitamin C deficient diet can cause scurvy, a diet short in cholesterol can have several adverse effects. Low cholesterol levels inhibit brain-functioning (2), whereas a diet rich in cholesterol improves cognitive functions. (3) In elderly people lower cholesterol levels are associated with increased infection rates and heart failure risks. (4) As a warning sign, decreased cholesterol levels evoke depression and aggression. As such, a low cholesterol level is associated with violent (5-7) and suicidal behaviour. (8-10) This can be explained by the indirect stimulatory effect of cholesterol on serotonin activity. (11)
Cholesterol is therefore an essential nutrient.

The Good, the Bad and the Ugly
As cholesterol is an essential nutrient, how can it be good or bad, since there is only one molecule that carries the name cholesterol, and there is no dispute about what that molecule is, or what it looks like?

Fats and cholesterol are not soluble in water or blood, so in order to transport fats and cholesterol via the blood, the body needs to use transporters. These transporters are so-called lipoproteins, which, as their name suggests, are part fat and part protein. Four types exist, one of which is associated with good cholesterol: HDL, and another with bad cholesterol: LDL.

Labelling HDL and LDL "good" and "bad" is not logical, because the label refers to the substance transported, and not to the transporter itself. Nevertheless, high levels of LDL are associated with heart disease.
Labelling cholesterol itself "good" or "bad" is also not logical, because cholesterol is always the same molecule.
Since neither LDL itself, nor cholesterol can be held responsible, what is the guilty substance?

Cholesterol is not a very stable substance (due to the double bond in its molecule), and already oxidizes at a temperature of 100 degrees Celsius. (12)
Heating to this temperature and above (normal cooking temperatures) results in the oxidation of part of the dietary cholesterol into harmful unnatural oxysterols. When consuming 30% oxidized cholesterol, about 6% of these oxysterols actually get absorbed (13), causing an increase in the levels of cholesterol and oxysterols combined.

Similar yet different oxysterols are produced by the body itself, during the process of transformation of cholesterol into bile acids and vice versa. These natural oxysterols inhibit cholesterol production and stimulate transformation of cholesterol into bile acids. (15)

To conclude we can observe that "good" and "bad" cholesterol do exist, though not connected to HDL and LDL. "Good" cholesterol is raw cholesterol, and "bad" cholesterol is oxidized cholesterol i.e. unnatural oxysterols.

Unnatural oxysterols in food
Foods with the highest amounts of unnatural oxysterols are foods containing cholesterol that have been heated and/or frozen a number of times; products containing eggs (26), but especially dried food products such as dried egg, dried cheese, milk powder, and dried meat. (27)(28)(18)

Cholesterol in food
Plants do contain cholesterol, but only in small amounts.
Healthy dietary cholesterol is mostly present in raw egg yolks. Raw fish is also a good source. Care must be taken not to heat it in any way, to prevent the creation of unnatural oxysterols.

Conclusion
Cholesterol is not bad, it is essential for our health. Unnatural oxysterols created through the heating of (food containing) cholesterol are the most likely cause of health-related problems attributed to (LDL) cholesterol.
Dietary cholesterol is necessary for optimal health, and unless one has a rare regulatory disorder, consuming lots of raw cholesterol should never be problematic, as excess cholesterol is converted into bile acids.

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