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OCR for page 140
DRIs: THE ESSENTIAL GUIDE TO NUTRIENT REQUIREMENTS 140 CHOLESTEROL C holesterol plays an important role in steroid hormone and bile acid biosynthesis. It also serves as an integral component of cell membranes. Most people absorb between 40 and 60 percent of ingested cholesterol. Such variability, which is probably due in part to genes, may contribute to the individual differences that occur in plasma cholesterol response to dietary cholesterol. All tissues are capable of synthesizing enough cholesterol to meet their metabolic and structural needs. Consequently, there is no evidence for a bio- logical requirement for dietary cholesterol. Neither an Estimated Average Re- quirement (EAR), and thus a Recommended Dietary Allowance (RDA), nor an Adequate Intake (AI) was set for cholesterol. Much evidence indicates a positive linear trend between cholesterol intake and low density lipoprotein (LDL) cholesterol concentration, and therefore an increased risk of coronary heart disease (CHD). A Tolerable Upper Intake Level (UL) was not set for cholesterol because any incremental increase in cholesterol intake increases CHD risk. It is recommended that people maintain their di- etary cholesterol intake as low as possible, while consuming a diet that is nutri- tionally adequate in all required nutrients. High amounts of cholesterol are found in liver and egg yolk. The main adverse effect of dietary cholesterol is increased LDL cholesterol concentration, which could result in an increased risk for CHD. CHOLESTEROL AND THE BODY Function Cholesterol is a sterol that is present in all animal tissues. Tissue cholesterol occurs primarily as free (unesterified) cholesterol, but is also bound covalently (via chemical bonds) to fatty acids as cholesterol esters and to certain proteins. Cholesterol is an integral component of cell membranes and serves as a precur- sor for hormones such as estrogen, testosterone, and aldosterone, as well as bile acids. Absorption, Metabolism, Storage, and Excretion Cholesterol in the body comes from two sources: endogenous and dietary. All cells can synthesize sufficient amounts of cholesterol for their metabolic and

OCR for page 140
PART II: CHOLESTEROL 141 structural needs. Dietary cholesterol comes from foods of animal origin, such as eggs, meat, poultry, fish, and dairy products. Dietary and endogenous cholesterol are absorbed in the proximal jejunum, primarily by passive diffusion. Cholesterol balance studies show a wide varia- tion in the efficiency of intestinal cholesterol absorption (from 20 to 80 per- cent), with most people absorbing between 40 and 60 percent of ingested cho- lesterol. Such variability, which is probably due in part to genetic factors, may contribute to the differences seen among individuals in plasma cholesterol re- sponse to dietary cholesterol. In addition, cholesterol absorption may be re- duced by decreased intestinal transit time. In the body, cholesterol can be stored in the liver; secreted into the plasma in lipoproteins, primarily very low density lipoproteins (VLDL); oxidized and secreted as bile acids; or directly secreted into the bile. Free and esterified cholesterols circulate principally in LDL in the blood. The body tightly regu- lates cholesterol homeostasis by balancing intestinal absorption and endogenous synthesis with hepatic excretion and bile acids derived from hepatic cholesterol oxidation. Increased hepatic cholesterol delivery from the diet and other sources results in a complex mixture of metabolic effects that are generally directed at maintaining tissue and plasma cholesterol homeostasis. Observational studies have shown that increased dietary cholesterol intake leads to a net increase in plasma LDL cholesterol concentrations. DETERMINING DRIS Determining Requirements All tissues are capable of synthesizing enough cholesterol to meet their meta- bolic and structural needs. Consequently, there is no evidence for a biological requirement for dietary cholesterol. Neither an Estimated Average Requirement (EAR), and thus a Recommended Dietary Allowance (RDA), nor an Adequate Intake (AI) was set for cholesterol. However, it is recommended that people maintain their dietary cholesterol intake as low as possible, while consuming a diet nutritionally adequate in all required nutrients. The UL The Tolerable Upper Intake Level (UL) is the highest level of daily nutrient intake that is likely to pose no risk of adverse effects for almost all people. Much evidence indicates a positive linear trend between cholesterol intake and LDL cholesterol concentration, and therefore an increased risk of CHD. A UL was not set for cholesterol because any incremental increase in cho- lesterol intake increases CHD risk. Because cholesterol is unavoidable in ordi-

OCR for page 140
DRIs: THE ESSENTIAL GUIDE TO NUTRIENT REQUIREMENTS 142 nary non-vegan diets, eliminating cholesterol in the diet would require signifi- cant dietary changes. These changes require careful planning to ensure adequate intakes of proteins and certain micronutrients. Still, it is possible to eat a low- cholesterol, yet nutritionally adequate, diet. DIETARY SOURCES Foods Foods of animal origin contain cholesterol. High amounts are found in liver and egg yolk. Moderate amounts are found in meats, some types of seafood, including shrimp, lobster, certain fish (such as salmon and sardines), and full- fat dairy products. ADVERSE EFFECTS OF CONSUMPTION The main adverse effect of dietary cholesterol is increased LDL cholesterol con- centration, which could result in an increased risk for CHD. Serum HDL con- centration also increases, although to a lesser extent, but the impact of such a diet-induced change in CHD risk is uncertain. Studies have shown that serum cholesterol concentrations increase with increased dietary cholesterol and that the relationship of blood cholesterol to the risk of CHD progressively increases. On average, an increase of 100 mg/day of dietary cholesterol is predicted to result in a 0.05–0.1 mmol/L increase in total serum cholesterol, of which ap- proximately 80 percent is in the LDL fraction. There is also increasing evidence that genetic factors underlie a substantial portion of the variation among individuals in response to dietary cholesterol. Although mixed, there is evidence that increases in serum cholesterol concen- tration due to dietary cholesterol are blunted by diets low in saturated fat, high in polyunsaturated fat, or both. No consistent significant associations have been established between di- etary cholesterol intake and cancer, including lung, breast, colon, and prostate cancers.

OCR for page 140
PART II: CHOLESTEROL 143 KEY POINTS FOR CHOLESTEROL Cholesterol plays an important role in steroid hormone and bile 3 acid biosynthesis and serves as an integral component of cell membranes. Because all tissues are capable of synthesizing enough 3 cholesterol to meet their metabolic and structural needs, there is no evidence for a biological requirement for dietary cholesterol. Neither an EAR, RDA, nor AI was set for cholesterol. 3 Much evidence indicates a positive linear trend between 3 cholesterol intake and LDL cholesterol concentration, and therefore increased risk of CHD. It is recommended that people maintain their dietary 3 cholesterol intake as low as possible, while consuming a diet nutritionally adequate in all required nutrients. A UL was not set for cholesterol because any incremental 3 increase in cholesterol intake increases CHD risk. High amounts of cholesterol are found in liver and egg yolk. 3 Meats, some types of seafood, including shrimp, lobster, and certain fish, as well as full-fat dairy products contain moderate amounts of cholesterol. The main adverse effect of dietary cholesterol is increased LDL 3 cholesterol concentration, which could result in an increased risk of CHD.