Cholesterol
Cholesterol is an essential component of all animal cells and, therefore, is ingested in foods of animal origin. The average daily intake in a typical American diet is estimated to be 600 mg to 800 mg, but undoubtedly higher as well as lower intakes are common. Some cholesterol is also secreted into the intestinal tract in bile and becomes mixed with the dietary cholesterol. It exists either free or esterified \vith fatty acid, but the latter are cleaved in the intestine by pancreatic cholesterol ester hydrolase before absorption. The amount of cholesterol present in the intestines is small compared to the triglycerides and their digestion products, and it is easily solubilized within the bile salt-lipid micelles from which it is absorbed. It is well known that both dietary fat and bile stimulate cholesterol absorption. In addi tion to facilitating the solubilization of cholesterol, bile salts are required to activate the pancreatic cholesteryl ester hydrolase. Onlv free cholesterol is believed to be absorbed into the mucosal cell.
Further mixing of dietary with endogenous cholesterol occurs in the mucosal cell, which actively synthesizes this compound. Some intracellular reesterification of free cholesterol also takes place, and both free and esterified cholesterol are incorporated into chylomicrons (60% 80% esterified) and the VLDL.
The extent to which dietary cholesterol is absorbed in humans seems to be variable, and the information available is somewhat contradictory, especially about the effect of the level of dietary cholesterol on the percentage and absolute amount of cholesterol that reaches the blood.
The results of several studies support the view that within the common range of cholesterol intake in American diets the amount of cholesterol absorbed is directly proportional to the dietary intake. The percentage of ingested cholesterol absorbed seemed to average 40% to 50% of the intake. In a number of studies, cholesterol absorption in patients with diagnosed hypercholesterolemia has been found to be normal.
It is generally agreed that with high cholesterol intakes there is a gradual decrease in the percentage absorbed,17 but the total absorption still increases with intake. The amount and type of triglycerides ingested simultaneously also influences cholesterol absorption.
