Mostly insoluble in water, it travels in the blood stream in the form of lipoproteins. Initially, it is carried from the intestinal mucosa to the liver in chylomicrons. In the liver it is converted into low-density lipoprotein (LDL) to carries cholesterol to the body cells, while high-density lipoprotein (HDL) carries it back to the liver for excretion.
It is interesting to note that the cholesterol in LDL cholesterol and the cholesterol in HDL cholesterol are identical. The only difference between the two is the carrier molecule (i.e. the lipoprotein).
A large part of this mechanism was clarified by Dr Michael S. Brown and Dr Joseph L. Goldstein, who received the Nobel Prize in Physiology or Medicine for their work in 1985.
A study by Ockrene et al showed that there are seasonal variations in cholesterol levels in humans. Although no explanation could be found, it explains why more patients may be diagnosed with elevated cholesterol levels in winter.
Desirable level corresponding to lower risk for heart disease
200-239
5.2-6.2
Borderline high risk
>240
>6.2
High risk
However, as today's testing methods determine LDL ("bad") and HDL ("good") cholesterol separetely, this simplistic view has become somewhat outdated. The desirable LDL level is considered to be 75-130 mg/dl (1.9-3.3 mmol/L), and a ratio of total cholesterol to HDL—arguably the most useful measure—of less than 5:1 is thought to be healthy.
Ockene IS, Chiriboga DE, Stanek EJ 3rd, Harmatz MG, Nicolosi R, Saperia G, Well AD, Freedson P, Merriam PA, Reed G, Ma Y, Matthews CE, Hebert JR. Seasonal variation in serum cholesterol levels: treatment implications and possible mechanisms. Arch Intern Med 2004;164:863-70. PMID 15111372.
Cholesterol is a 
is primarily synthesized from