Metabolism and Excretion

The body's metabolic processes cannot break the sterol rings of cholesterol and therefore must either catabolize cholesterol to other products, which can be excreted in the urine or feces, or directly excrete cholesterol in the bile, with a fraction of the biliary cholesterol lost daily as fecal neutral sterols. In humans, the major route of excretion is as biliary cholesterol (two-thirds of the total lost each day), with catabolism to bile acids and bile acid excretion being the second most important route, accounting for approximately one-third of the daily turnover.

For all practical purposes, the body must excrete daily an amount of neutral and acidic sterols equivalent to the combined inputs of total dietary and newly synthesized cholesterol. Given an average fecal excretion of 1020 mg per day with 250 mg as acidic sterols, it can be calculated that the 770 mg per day excreted as neutral steroids comes from unabsorbed biliary

(650 mg) and unabsorbed dietary (120 mg) cholesterol (Table 1) It is easy to see that even small changes in the daily balance between a cholesterol input and output value of 1020 mg per day could, over years, result in significant tissue cholesterol accumulation.

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