Since cholesterol is essential for all animal life, each cell is capable of synthesizing it by way of a complex 37-step process, beginning with the mevalonate pathway and ending with a 19-step conversion of lanosterol to cholesterol. Furthermore, it can be absorbed directly from animal-based foods.A human male weighing 68 kg (150 lb) normally synthesizes about 1 gram (1,000 mg) per day, and his body contains about 35 g, mostly contained within the cell membranes. Typical daily cholesterol dietary intake for a man in the United States is 307 mg.
Most ingested cholesterol is esterified, and esterified cholesterol is poorly absorbed. The body also compensates for any absorption of additional cholesterol by reducing cholesterol synthesis. For these reasons, cholesterol in food, seven to ten hours after ingestion, has little, if any effect on concentrations of cholesterol in the blood. However, during the first seven hours after ingestion of cholesterol, as absorbed fats are being distributed around the body within extracellular water by the various lipoproteins (which transport all fats in the water outside cells), the concentrations increase. It is also important to recognize, however, that the concentrations measured in the samples of blood plasma vary with the measurement methods used. Traditional, cheaper methods do not reflect (a) which lipoproteins are transporting the various fat molecules, nor (b) which cells are ingesting, burning or exporting the fat molecules being measured as totals from samples of blood plasma.
Cholesterol is recycled in the body. The liver excretes it in a non-esterified form (via bile) into the digestive tract. Typically, about 95% of the excreted cholesterol is reabsorbed by the small intestine back into the bloodstream
Plants make cholesterol in very small amounts. Plants manufacture phytosterols (substances chemically similar to cholesterol), which can compete with cholesterol for reabsorption in the intestinal tract, thus potentially reducing cholesterol reabsorption. When intestinal lining cells absorb phytosterols, in place of cholesterol, they usually excrete the phytosterol molecules back into the GI tract, an important protective mechanism. The intake of naturally occurring phytosterols, which encompass plant sterols and stanols, ranges between ~200–300 mg/day depending on eating habits. Specially designed vegetarian experimental diets have been produced yielding upwards of 700 mg/day.
