Cholesterol - Sources, absorption, function, and metabolism
Absorption, Transport, and Storage
Cholesterol in the intestinal lumen typically consists of one third dietary cholesterol and two-thirds biliary cholesterol. The average daily diet contains 300–500 mg of cholesterol obtained from animal products. The bile provides an additional 800–1200 mg of cholesterol throughout each day as gallbladder contractions provide a flow of bile acids, cholesterol, and phospholipids to facilitate lipid digestion and absorption. Dietary cholesterol is a mixture of free and esterified cholesterol whereas biliary cholesterol is nonesterified and is introduced into the small intestine as a cholesterol–bile salt–phospholipid water-soluble complex. The only other source of intraluminal cholesterol is mucosal cell cholesterol, derived from either sloughed mucosal cells or cholesterol secreted by the mucosal cells into the intestinal lumen. Measurements of exogenous and endogenous cholesterol absorption in humans indicate that there is probably very little direct secretion of newly synthesized cholesterol from mucosal cells into the intraluminal contents.
Cholesterol absorption occurs primarily in the duodenum and proximal jejunum of the small intestine and is dependent on the presence of bile salts. In the absence of bile secretion, or in the presence of bile acid-binding resins, there is virtually no intestinal absorption of cholesterol. On average, humans absorb 50–60% of the intestinal contents of cholesterol, but there is a large inter-individual variance in absorption, with values ranging from as low as 20% to as high as 80%. Intestinal transit time is related to cholesterol absorption with slower transit times resulting in higher fractional absorption rates. Dietary factors that affect the relative percent absorption of cholesterol include the total mass of dietary cholesterol, the concentration of plant sterols in the diet, and the type and amount of dietary fiber. Studies suggest that the ratio of polyunsaturated to saturated fat (P:S) in the diet has little effect on cholesterol absorption rates in humans, nor does the amount of dietary fat.
Two interesting, and as yet undefined, aspects of cholesterol absorption are that it decreases as the mass of cholesterol increases above an intake of 1500 mg per day, and that the fractional absorption below this level is relatively constant for an individual. For example, at a daily cholesterol intake of 800 mg a subject might absorb 60% or 480 mg a day, whereas at a daily intake of 400 mg the absorption remains at 60%, equalling 240 mg a day absorbed. The quandary is, if the system can accommodate absorption of 480 mg at the high cholesterol intake, then why is the amount absorbed 240 mg at the low intake? Clearly the upper value of cholesterol absorption is achievable, yet at the lower intake level the absorption rate stays at a fixed fractional value. The mechanisms controlling this aspect of cholesterol absorption have not been defined.
Experimental evidence indicates that biliary cholesterol and dietary cholesterol are absorbed equally; however, the pattern of exogenous and endogenous cholesterol absorption differs along the length of the intestinal lumen. Dietary cholesterol enters the small intestine solubilized in the oil phase of the stomach digest, whereas the binary cholesterol enters in the micelle phase of the bile. This differential distribution results in a greater absorption of biliary cholesterol in the upper portion of the small intestine with dietary cholesterol absorption increasing as the oil phase of the intestinal contents are hydrolyzed. As the oil phase is reduced, dietary cholesterol moves from the oil phase to the aqueous micelle phase and becomes available for absorption. In the case of cholesteryl esters in the diet, it is necessary that the esters are hydrolyzed by pancreatic cholesterol esterase (CEase) before the cholesterol is available for absorption. Pancreatic CEase requires the presence of bile salts for activity and may play a key role in the actual absorption process.
The process, and selectivity, of sterol absorption involves a complex interplay of regulated transporters, transporting sterols into and out of the enterocyte, and the assembly and secretion of chylomicrons into the lymph. The enterocyte takes up both cholesterol and phytosterols from the intestinal lumen by what appears to be a common sterol transporter or permease in the brush border membrane. Preliminary studies suggest that the Neiman–Pick C1 Like 1 (NPC1L1) protein is involved in this process. Once the sterols enter the enterocyte, the ATP-binding cassette (ABC) hemitransporters ABCG5 and ABCG8 function in the apical excretion of sterols back into the intestinal lumen. The selectivity of this process accounts for the higher absorption rates of cholesterol (50–60%) compared to the phytosterols, which are very poorly absorbed. Loss of ABCG5/G8 function results in excessive absorption of both cholesterol and phytosterols. Studies in mice have shown that ABCG5/G8 are expressed primarily in the liver and intestine, are coordinately up-regulated at the transcriptional level by dietary cholesterol intake, and require the liver X receptor a (LXRa), a nuclear receptor that regulates the expression of a number of key genes involved in lipid metabolism.
Evidence is accumulating that the fractional cholesterol absorption rates are regulated by one or more genetic determinants. The apolipoprotein (apo) E phenotype has a significant effect on fractional cholesterol absorption and appears to play a major role in determining the plasma lipoprotein response to changes in dietary cholesterol intake. Men with the apoE4 allele have a high cholesterol absorption rate whereas those with the apoE2 allele have a low cholesterol absorption efficiency. The absorption values for the more common apoE3/3 fall between the apoE2 and apoE4 patterns. Polymorphisms of the apolipoprotein A-IV and of the lowdensity lipoprotein (LDL) receptor gene have also been related to differences in fractional cholesterol absorption. These genetic variants affecting cholesterol absorption no doubt play a significant role in determining an individual’s fractional absorption of cholesterol as well as accounting for much of the heterogeneity of plasma lipid responses to changes in dietary cholesterol intakes (see the Section on Dietary Cholesterol and Plasma Cholesterol below).
Exogenous Cholesterol Transport
Cholesterol is absorbed in the unesterified state, whereas the cholesterol secreted into the lymph is 70–80% esterified. This esterification process generates a concentration gradient of free cholesterol within the mucosal cell, which could facilitate absorption rates. Cholesterol is esterified in intestinal mucosal cells by acyl-coenzyme A: cholesterol acyltransferase-2 (ACAT- 2) to form cholesteryl esters, which are secreted from the basolateral surface of the enterocyte as part of the chylomicrons. At this stage it is assumed that cholesterol molecules from exogenous and endogenous sources are indistinguishable, and have similar effects on endogenous cholesterol and lipoprotein metabolism. Chylomicrons are large particles (470 nm in diameter) composed mainly of triacylglycerols (95% by weight) and containing 3–7% cholesterol by weight, the esterified cholesterol localized in the hydrophobic core and the free cholesterol primarily in the hydrophilic outer layer. The data indicate that the amount of dietary cholesterol consumed has little effect on the cholesterol content of chylomicrons. The chylomicrons are released from the intestinal cells, enter the lymphatic system and are transported via the lymphatics (thoracic duct) to the bloodstream. Because chylomicrons are too large to pass through the capillaries, this is the only mechanism by which they can enter the bloodstream.
In the plasma compartment the chylomicrons pick up a number of apolipoproteins, which are required for intravascular metabolism of the particles. The initial metabolism of chylomicrons involves hydrolysis of the associated triacylglycerols by endothelial cell lipoprotein lipase (LPL) located in adipose, muscle, and heart tissues which results in production of chylomicron remnants. The chylomicron remnants, depleted of triacylglycerol and enriched with cholesteryl ester, are taken up by the liver via the LDL receptor-related protein (LRP). The ligand for hepatic uptake of the chylomicron remnant appears from various transgenic mouse studies to be the apo-E moiety of the particle. The clearance of chylomicrons from the bloodstream is rapid, with particles having a half-life of less than an hour. The liver cannot take up native chylomicrons but rather takes up the chylomicron remnant, which has lost approximately 90% of its triacylglycerol content and become relatively enriched in free and esterified cholesterol through the actions of the plasma cholesteryl ester transfer protein (CETP), which transfers cholesteryl ester from HDL to the apo-B-containing lipoproteins.
The chylomicron remnants taken up by the liver are subjected to lysosomal hydrolysis resulting in the release of the absorbed dietary and biliary cholesterol into the hepatocyte as free cholesterol. The influx of cholesterol contained in the chylomicron remnant has the ability to affect a number of regulatory sites of hepatic cholesterol metabolism, which function to maintain cholesterol homeostasis in the liver. The liver has four primary fates for the newly delivered cholesterol: catabolism to bile acids; secretion as biliary cholesterol; storage in lipid droplets as cholesteryl ester; or incorporation into very low-density lipoprotein (VLDL) for secretion from the liver.
Tissue Uptake and Storage
The body pool of cholesterol is approximately 145 g with one third of this mass localized in the central nervous system. The remainder of the metabolically active cholesterol pool exists in the plasma compartment (7.5–9 g) and as constituents of body tissues. In humans, tissue cholesterol levels are relatively low, averaging 2–3 mg per gm# wet weight. Little information exists regarding changes in hepatic and extrahepatic tissue cholesterol concentrations with changes in dietary cholesterol intake. Animal studies, which are usually carried out using very high levels of dietary cholesterol, have shown that hepatic cholesterol can increase from 2-fold up to 10-fold, depending on the species and other dietary constituents, when dietary cholesterol is increased.
Tissue Cholesterol Synthesis
Cholesterol biosynthesis occurs in every nucleated cell in the body. Although it is often thought that the majority of cholesterol synthesis occurs in the liver, studies have shown that the bulk tissues of the body account for the overwhelming majority of endogenous cholesterol production. Hepatic cholesterol synthesis in humans is thought to contribute 10–20% of the total daily synthesis rate. Because the majority of cholesterol synthesis in the body occurs in extrahepatic tissues, and the only quantitatively significant site for excretion and catabolism of cholesterol is the liver, some 600–800 mg of cholesterol each day must be transported from peripheral tissues through the plasma compartment to the liver to account for daily cholesterol catabolism and binary secretion. Approximately 9 mg cholesterol per kilogram body weight is synthesized by peripheral tissues every day and must be moved to the liver for catabolism via a process termed ‘reverse cholesterol transport’ (RCT).
RCT describes the metabolism, and important antiatherogenic function, of the HDL-mediated efflux of cholesterol from nonhepatic cells and its subsequent delivery to the liver and steroidogenic organs for use in the synthesis of lipoproteins, bile acids, vitamin D, and steroid hormones. A cellular ABC transporter (ABCA1) mediates the first step of RCT involving the transfer of cellular cholesterol and phospholipids to lipid-poor apolipoproteins. Lecithin:cholesterol acyltransferase (LCAT) mediated esterification of cholesterol generates spherical particles that continue to expand with ongoing cholesterol esterification and phospholipid transfer protein (PLTP)mediated particle fusion and surface remnant transfer. Larger HDL2 particles are converted into smaller HDL3 particles when CETP facilitates the transfer of cholesteryl esters from HDL onto apo-B-containing lipoproteins. The scavenger receptor B1 (SR-BI) promotes selective uptake of cholesteryl esters into liver and steroidogenic organs whereas hepatic lipase (HL) and LPL mediated hydrolysis of phospholipids and triglycerides. SR-BI mediates the selective uptake of cholesteryl esters from HDL and also LDL into hepatocytes and steroid hormone- producing cells without internalizing HDL proteins, which can recycle through the RCT sequence moving cholesterol from peripheral tissues to the liver.
Regulation of Synthesis
The rate-limiting enzyme in cholesterol biosynthesis is 3- hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, a microsomal enzyme which converts HMG-COA to mevalonic acid in the polyisoprenoid synthetic pathway. Peripheral tissue cholesterol synthesis is much less responsive to regulatory factors compared with the liver, which is controlled by a variety of dietary, hormonal, and physiological variables. Studies indicate that endogenous cholesterol synthesis is significantly increased in obesity and in patients with the metabolic syndrome. Obesity, insulin resistance, and diabetes have pronounced effects on both cholesterol absorption and synthesis. Findings in type I diabetes appear related to low expression of ABCG5/G8 genes resulting in high absorption and low synthesis of cholesterol. Cholesterol absorption efficiency is lower and cholesterol synthesis higher in obese subjects with type II diabetes compared to obese subjects without diabetes, suggesting that diabetes modulates cholesterol metabolism to a greater extent than obesity alone. In a similar manner, low cholesterol absorption and high synthesis appears to be part of the insulin resistance (metabolic) syndrome.
Research shows that in most individuals, dietary cholesterol alters endogenous cholesterol synthesis and that this feedback regulation can effectively compensate for increased cholesterol input from dietary sources. The precision of these regulatory responses depends on a number of genetic factors, and data suggest that multiple genetic loci are involved. For example, family studies have shown that in siblings of low cholesterol absorption families, cholesterol absorption percentages are significantly lower, and cholesterol and bile acid synthesis, cholesterol turnover, and fecal steroids significantly higher than in siblings of high absorption families.
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 a 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. 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.
Bile Acid Synthesis
The results from numerous sterol balance studies carried out in subjects fed diets low and high in cholesterol indicate that in humans dietary cholesterol has little effect on fecal bile acid excretion rates. This finding is in striking contrast to results from studies in some rodent models, which show that intake of pharmacological doses of dietary cholesterol can result in several-fold increases in bile acid synthesis and excretion. In contrast, some rodent species and nonhuman primates have little if any increase in bile acid excretion with increased intakes of cholesterol. Although there have been a few reports of enhanced bile acid excretion on a high-cholesterol diet in some patients, this does not appear to be a major regulatory response in humans.
Biliary Cholesterol Secretion
The majority of cholesterol entering the intestinal tract is biliary cholesterol. Biliary cholesterol secretion averages 1000 mg per day as part of the bile system and enters as free cholesterol already solubilized with bile acids and phospholipids. Both cholesterol absorption by enterocyte and biliary cholesterol secretion by hepatic cells are regulated by expression of the half-transporters ABCG5 and ABCG8. Studies in animals have shown that treatment with a LXR agonist decreases cholesterol absorption, increases biliary cholesterol secretion, and increases fecal neutral sterol excretion. Studies in transgenic mouse models demonstrate that increased expression of ABCG5 and ABCG8 increases biliary neutral sterol secretion and reduces intestinal cholesterol absorption, leading to increased neutral sterol excretion and cholesterol synthesis.
The only route of significant cholesterol excretion is through fecal excretion of neutral sterols. The combination of unabsorbed biliary and dietary cholesterol accounts for the total neutral sterol output, and under most conditions equals 750–850 mg a day. Dietary patterns or drugs that interfere with intestinal cholesterol absorption result in increased fecal neutral steroid excretion. In the colon, intestinal bacteria are able to metabolize cholesterol to a variety of neutral steroids as well as to nonsteroid end products. There have been some studies suggesting that the intestinal metabolism of cholesterol by bacteria, which can be altered by diet and drugs, can influence endogenous cholesterol metabolism as well as plasma cholesterol levels. What these relationships might be and the mechanisms involved have not been defined.
Daily production of steroid hormones is quantitatively a very small fraction of the daily turnover of dietary and newly synthesized cholesterol in the body. For men, the average daily excretion of steroid hormones is approximately 50 mg per day, whereas in women the value can be substantially higher depending on the menstrual phase.
Bile Acid Synthesis
The enterohepatic circulation of bile acids is essential for fat and cholesterol digestion and absorption. Each day the bile acid pool (approximately 3–5 g) cycles through the intestine 6–10 times. The absorption of bile acids by the ileum is very effective and 98–99% of bile acids secreted in the bile are returned to the liver via the portal vein. The small amount of bile acids lost each day as fecal acidic steroids are replaced through the conversion of hepatic cholesterol to the primary bile acids, cholic acid, and chenodeoxycholic acid. This catabolism of cholesterol can be as little as 250 mg per day up to 500 mg per day depending on the diet. The bile acids represent the only major catabolic product of cholesterol metabolism and in humans account for some 25–30% of the daily loss of cholesterol from the body.
Very Low-Density Lipoprotein Synthesis
The endogenous pathway for cholesterol transport focuses on the liver with the synthesis and secretion of VLDL particles. Cholesterol in these triacylglycerol-rich particles comes from multiple sources: endogenous synthesis, diet, and plasma lipoproteins. Catabolism of VLDL by LPL leads to formation of intermediate-density lipoproteins (IDL), which can either be taken up by the liver or undergo further metabolism to form LDL. Low-density lipoproteins contain apo-B100 and account for 60–80% of the plasma cholesterol in most individuals. During lipolysis of VLDL triacylglycerol, the lipoproteins containing apo-B becomes enriched with cholesteryl ester through the plasma CETP-catalyzed net transfer of cholesteryl ester from HDL. This process, termed reverse cholesterol transport, moves cholesterol from extrahepatic tissues to HDL to VLDL-IDL-LDL and eventual uptake by the liver. Some 70% of the LDL degraded each day is degraded by the hepatic LDL receptor pathway.
Dietary Cholesterol and Plasma Cholesterol
The effect of dietary cholesterol on plasma cholesterol levels has been an area of considerable debate. In 1972 the American Heart Association recommended that dietary cholesterol intake should average less than 300 mg per day as part of a ‘heart-healthy’, plasma cholesterol-lowering diet. Since that initial recommendation, a number of other public health dietary recommendations in the USA have endorsed the 300 mg daily limit.
Interestingly, few dietary recommendations from other countries contain a dietary cholesterol limitation. The evidence for a relationship between dietary cholesterol and plasma cholesterol indicates that the effect is relatively small, and that on average a change of 100 mg per day in dietary cholesterol intake results in a 0.057 mmol l1 (2.2 mg dl1) change in plasma cholesterol concentrations.
Studies have also shown that the majority of individuals are resistant to the plasma cholesterol-raising effects of dietary cholesterol (nonresponders) and have less than the predicted response. In contrast, a segment of the population (estimated to be between 10% and 20%) are sensitive to dietary cholesterol (responders) and exhibit a greater than expected plasma cholesterol response to a change in dietary cholesterol intake.
To date there are no defined physiological or clinical characteristics to differentiate responders from nonresponders, but studies suggest that the apo-E phenotype plays a role, as does the clinical condition of combined hyperlipidemia. Data also suggest that sensitivity to dietary cholesterol is associated with sensitivity to dietary fat, and that overall adiposity may also play a role.
Although on a population basis the plasma cholesterol response to dietary cholesterol is relatively small, and in most epidemiological analyses not related to hypercholesterolemia, some individuals are sensitive to dietary cholesterol changes and, if hypercholesterolemic, would experience plasma cholesterol reduction with dietary cholesterol restrictions. For the majority, however, dietary cholesterol restrictions have little effect on plasma cholesterol levels.
Dietary Cholesterol Intake Patterns
Dietary cholesterol intakes in the USA have been declining, from an average of 500 mg a day in men and 320 mg a day in women in 1972 to levels in 1990 of 360 mg a day in men and 240 mg a day in women. This decline arises in part from dietary recommendations to the American public to reduce total and saturated fat intake and to reduce dietary cholesterol daily intake to less than 300 mg, and in part from the increased availability of products with reduced fat and cholesterol content. Major efforts in the early 1970s by public health agencies and advertising emphasized reducing dietary cholesterol as a means to lower plasma cholesterol levels, leading to a high degree of consumer concern regarding cholesterol containing foods and demand for low-cholesterol products. Today, practically all foods sold in the USA are labelled for their cholesterol content and their percentage contribution to the daily value of 300 mg for cholesterol.
Major Dietary Sources
The major sources of cholesterol in the diet are eggs, meat, and dairy products. A large egg contains approximately 185 mg of cholesterol and contributes some 30–35% of the total dietary cholesterol intake in the USA. Meat, poultry, and fish contribute 45–50%, dairy products 12–15% and fats and oils 4–6%. In the USA, the range of dietary cholesterol intakes is 300–400 mg per day for men and 200–250 mg per day for women; thus for much of the population the national goal of a dietary cholesterol intake of less than 300 mg a day has already been met.
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