Dietary regulation of cholesterol metabolism

Decreased plasma cholesterol concentrations after PUFA-rich diets are not due to reduced cholesterol absorption/synthesis

Plasma cholesterol concentrations increase with consumption of high saturated fatty acid (SFA) and decrease with high polyunsaturated fatty acid (PUFA) diets, leading to shifts in lipid levels consistent with reduction in heart disease risk. Direct measurements of cholesterol absorption, one of the key regulators of plasma cholesterol levels, have not been performed in humans after consumption of high PUFA diets. Thus, cholesterol absorption and fractional synthesis rates (FSRs) were measured in 16 healthy adults (8 males and 9 females) using a randomized cross-over study with a diet containing high (PUFA/SFA) P/S ratio (2:1) and a low P/S ratio (0.5:1). Cholesterol absorption and fractional cholesterol synthetic rates were measured using stable isotopes after 20 days of dietary intervention. Diet did not affect cholesterol absorption or synthesis. There was a significant decrease in plasma cholesterol concentrations (P < 0.02), specifically LDL-cholesterol (P < 0.02), without a change in HDL-cholesterol or triacylglycerol concentrations. Intraluminal cholesterol solubilization and plasma sterol (cholesterol biosynthetic intermediates and plant sterols) levels were not affected by diet. Thus, consumption of diets with a high P/S ratio reduces plasma total and LDL-cholesterol concentrations independent of shifts in cholesterol absorption or synthesis.

Evolution of cholesterol management therapies: exploiting potential for further improvement

The development of therapies for the control of atherosclerosis is an important example of the drug discovery process. Findings from population studies initiated 4 decades ago have been gradually translated to today's therapies, the most effective of which are the statins. Driven by an increased understanding of the atherosclerotic process and shortfalls of current treatments, the development of therapies for cholesterol control continues, with new agents in the pipeline promising therapeutic benefits over existing treatments. The advent of new statins such as rosuvastatin and pitavastatin, with improved efficacy for reducing cholesterol and the ability to positively affect other disease risk factors, may exploit the potential for improvement of current treatments.

Dietary saturated fatty acids increase cholesterol synthesis and fecal steroid excretion in healthy men and women

In a strictly controlled 6-week trial with 47 healthy volunteers we have determined the effect of replacement of polyunsaturated by saturated fatty acids on the fecal steroid excretion and on the rate of whole body cholesterol synthesis, as measured both by the sterol balance method and by the concentration of the cholesterol precursor lathosterol in serum. Subjects were fed mixed natural diets, of which the total fat content was kept constant at 45% energy. Consumption of polyunsaturated fatty acids, mainly linoleic acid, was 21% energy for the first 3-week period (P:S ratio 1.9), and 5% of energy (P:S ratio 0.2) for the next 3-week period, or vice versa. Cholesterol intake as determined by analysis of duplicate diets was 41 mg MJ-1 (about 500 mg day-1) during both periods. Feces were collected for 5 days at the end of both periods. The steroid composition of the feces was not affected by the change of diets. The fecal excretion of neutral steroids was significantly higher on the low P:S high-saturated-fat (2.25 +/- 0.68 mmol day-1) than on the high P:S high-linoleic-acid diet (2.00 +/- 0.69 mmol day-1; P < 0.01). The excretion of bile acids was similar (0.77 +/- 0.40 and 0.79 +/- 0.41 mmol day-1, respectively). The cholesterol balance and the rate of cholesterol synthesis were higher during the low P:S (1.86 +/- 0.83 mmol day-1) than during the high P:S period (1.55 +/- 0.85 mmol day-1; P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Relation between the frequency of colorectal adenoma and the serum cholesterol level

Several investigators have reported an association between low serum cholesterol levels and an increased frequency of colorectal cancer. Because low cholesterol levels may be a result of an established cancer, we have investigated the relation between serum cholesterol levels and the frequency of colorectal adenomas, which are thought to be precursors of colon cancer. We prospectively studied 1083 consecutive patients who underwent colonoscopy (241 of whom were excluded because of malignant disease, chronic inflammatory bowel disease, familial polyposis, or partial colectomy). In the remaining 842 patients, analysis of covariance was performed to evaluate the contribution of serum cholesterol to the risk of colorectal adenoma. Serum cholesterol levels were significantly and positively associated with the frequency of colorectal adenoma in subjects of both sexes. After adjustment for age and body-mass index, this positive association remained significant between the top quintile and the lowest quintile for serum cholesterol, with regard to the total study group (odds ratio, 2.0; 95 percent confidence limits, 1.1 and 3.6) and men only (odds ratio, 2.2; 95 percent confidence limits, 1.0 and 4.8). We conclude that there is not an inverse correlation between serum cholesterol levels and the risk of colorectal adenomas; on the contrary, there appears to be a small positive association.

Effects of dietary cholesterol and fatty acids on plasma lipoproteins

The effects of dietary cholesterol and fatty acids on low density and high density lipoproteins (LDL and HDL) were studied in 20 young men. After 2-3 wk of evaluations on ad lib. diets, basal diets, which consisted of 15% protein, 45% carbohydrates, 40% fat, and 300 mg/day of cholesterol, were given for 4-5 wk (Basal). The ratio of dietary polyunsaturated to saturated fatty acids (P/S) for different groups of subjects were 0.25, 0.4, 0.8, or 2.5. 750 and 1,500 mg/d of cholesterol were added to the basal diets as 3 and 6 eggs, respectively. Total cholesterol and LDL cholesterol were lower in all subjects on the basal diets than on the ad lib. diets. Addition of 750 mg cholesterol to the diet with P/S = 0.25-0.4 raised LDL cholesterol by 16 +/- 14 mg/dl to 115% of basal diet values (n = 11, P less than 0.01); 1,500 mg increased LDL cholesterol by 25 +/- 19 mg/dl to 125% (n = 9, P less than 0.01). On the diet with P/S = 0.8, 750 mg produced insignificant increases in LDL cholesterol, but 1,500 mg produced increases of 17 +/- 22 mg/dl to 115% of basal (n = 6, P less than 0.02). On the P/S = 2.5 diet, neither 750 nor 1,500 mg produced significant changes. Thus, both the cholesterol contents and P/S ratios of diets were important in determining LDL levels. The lipid and apoprotein compositions, flotation rates, molecular weights, and binding by cellular receptors of LDL were virtually unchanged by the addition of cholesterol to the diets high in saturated fat. These diets, therefore, caused an increase in the number of LDL particles of virtually unchanged physical and biological properties. On the diet with low P/S ratio, HDL2 rose, whereas this effect was absent on diets with high P/S ratios. The response of LDL to dietary manipulations is consonant with epidemiologic data relating diets high in cholesterol and saturated fat to atherogenesis. The response of HDL2, however, is opposite to that of its putative role as a negative risk factor. Further work is needed to clarify this interesting paradox.

Influence of dietary fat saturation on lipid absorption in the rat

Flux (mumole/h) of triglycerides, phospholipids and cholesterol into the thoracic duct lymph was measured in rats receiving a constant intraduodenal infusion of a cholesterol-free oil rich in either polyunsaturated (P/S = 4.8) or saturated (P/S = 0.2) fatty acids. Rats had ad libitum access to a fat-free semi-synthetic diet throughout the experiment. Oils were infused at a rate equivalent to a 10% (w/w) fat-containing diet for at least 18 h prior to collection of lymph; both oils were compared in each animal. Although absorption of the infused oils approximated 100%, triglyceride flux was significantly lower during infusion of the saturated compared to the polyunsaturated oil. Phospholipid and total cholesterol fluxes were not significantly affected by the type of oil, but the percent of lymph total cholesterol which was esterified was slightly but significantly lower during infusion of the unsaturated oil. Using the molar phospholipid/triglyceride ratio as a index of lymph lipoprotein size, it was found that absorption of the oil rich in polyunsaturated fatty acids resulted in an increase in the mean size of lymph lipoproteins. The potential significance of an influence of dietary fat saturation on lymph lipoprotein size and cholesterol esterification for the ultimate metabolic fate of absorptive lipoprotein constituents is discussed.

Effects of long-term ingestion of soya phospholipids on serum lipids in humans

The average reduction of 22 percent in serum cholesterol and 26 percent in triglycerides in the participants in this study was achieved by a diet low in fat and cholesterol supplemented with lecithin. The results are important because all of the participants were able to follow the diet protocol while continuing their normal activities and did so willingly and eagerly over the entire 26 month period. Although the statistical analyses of the data in this small group do not enable us to conclude that lecithin added to the low fat diet actually produced the reduced levels of cholesterol and triglycerides, the trend is encouraging. Because the lecithin and low fat diet was tolerated well with no significant side effects for 2 years, we suggest that a long-term prospective randomized study with a larger group of patients, including appropriate linoleic acid controls, be carried out to confirm or deny these preliminary findings.

Influence of dietary oil, cholesterol, and soysterols on the fecal neutral and acidic steroid excretion in laying hens

The influence of dietary lipid factors (saturated and unsaturated oil, cholesterol, and plant sterols) upon the fecal neutral and acidic sterol excretion in the laying hen were investigated during a nine-week experimental period. Single Comb White Leghorn laying hens at thirty weeks of age were fed two basal diets containing 8.0% hydrogenated coconut oil or safflower oil, with or without supplementing 1.0% cholesterol, 2.0% soysterols, or combination of cholesterol and soysterols with each oil treatment. Feeding safflower oil increased the fecal excretion of bile acids and, to a lesser extent, catabolic neutral sterols as compared to hydrogenated coconut oil. The fecal excretion of sterol metabolites was further enhanced when soysterols and cholesterol were fed simultaneously. When soysterols were fed alone, endogenous cholesterol excretion in feces appeared to increase, but soysterol feeding with cholesterol did not retard the apparent absorption rate of cholesterol.

Swine in atherosclerosis research--Development of an experimental animal model and study of the effect of dietary fats on cholesterol metabolism

The effect of saturated (beef tallow) and polyunsaturated (corn oil) fat diets on plasma cholesterol, bile flow and composition, and excretion of fecal steroids, was studied in 5 female and 6 male miniature swine. The animals were fitted with a modified Baldwin T-tube in the common bile duct for bile sampling and a catheter in the duodenum for reinfusion. Plasma cholesterol levels were increased by beef tallow and decreased by corn oil. Both fats caused an increase in the secretion of bile, biliary lipids and bile acid pool size. The increases were significant when the source of fat was corn oil. The fecal neutral steroid excretion measured in the male swine was increased by both fats. However, only the polyunsaturated fat diet caused a significant increase in the fecal acidic steroid excretion.

Effects of saturated and unsaturated fats given with and without dietary cholesterol on hepatic cholesterol synthesis and hepatic lipid metabolism

Hepatic cholesterol synthesis was studied in rats after consuming diets of varying neutral lipid and cholesterol content. Cholesterol synthesis was evaluated by measuring 3-hydroxy-3-methylglutaryl-CoA reductase and by determining the rate of 3H-labeled sterol production from [3H]mevalonate. Results were correlated with sterol balance data and hepatic lipid content. Hepatic cholesterol synthesis was relatively great when cholesterol was excluded from the diet. The source of neutral dietary lipids, saturated vs. unsaturated, produced no change in hepatic sterol synthesis. Values for fecal sterol outputs and hepatic cholesterol levels were also similar in rats consuming either saturated or unsaturated fats. When 1% cholesterol was added to the diet, hepatic cholesterol synthesis was suppressed but the degree of suppression was greater in rats consuming unsaturated vs. saturated fats. This was associated with greater accumulation of cholesterol in livers from rats consuming unsaturates and a reduction in fecal neutral sterol output in this group as opposed to results from rats on saturated fats. Cholesterol consumption also altered the fatty acid composition of hepatic phospholipids producing decreases in the percentages of essential polyunsaturated fatty acids. It is concluded that dietary cholesterol alters cholesterol and fatty acid metabolism in the liver and that this effect is enhanced by dietary unsaturated fats.

Mode of action of chlorophenoxyisobutyric acid on cholesterol metabolism in man

In short-term trials chlorophenoxyisobutyric acid (CPIB) (Atromid-S) reduced the plasma cholesterol and triglyceride levels in eight subjects with type II and IV hyperlipidemias to an equal extent. In these subjects, who were maintained on constant solid food diets, CPIB administration resulted in increased excretion of fecal neutral and acidic sterols in the type II subjects only. There was an immediate increase in specific activity of plasma cholesterol in seven of the eight subjects, and a reduced rate of fall of specific activity in many of the subjects. It is suggested that CPIB inhibits the synthesis of cholesterol in vivo, and that the subsequent fall in plasma cholesterol is responsible for the release of cholesterol with higher specific activity from tissues into the plasma pool.

The effects of unsaturated dietary fats on absorption, excretion, synthesis, and distribution of cholesterol in man

Cholesterol balance studies were carried out in 11 patients with various types of hyperlipoproteinemia to determine the mechanism by which unsaturated fats lower plasma cholesterol. Unsaturated fats produced no increase in fecal endogenous neutral steroids in 10 of 11 patients and no decrease in absorption of exogenous cholesterol in 5 patients who received cholesterol in the diet. In 8 of 11 patients no changes occurred in excretion of bile acids during the period on unsaturated fat when plasma cholesterol was declining. However, in 3 of 11 patients small but significant increases in bile acid excretion were found during this transitional period; in 2 others increases also occurred after plasma cholesterol had become constant at lower levels on unsaturated fat.Since the majority of patients showed no change in cholesterol or bile acid excretions during the transitional period, we propose that when excretion changes did occur they were probably not the cause of the plasma cholesterol change. Furthermore, turnover data and specific activity curves suggested that cholesterol synthesis was not influenced by exchange of dietary fats. Thus, excluding changes in excretion and synthesis, we conclude that it is most likely that unsaturated fats cause plasma cholesterol to be redistributed into tissue pools. We have also examined the possibility that cholesterol which is redistributed into tissues could be secondarily excreted as neutral steroids or bile acids. In at least 5 of 11 patients excretion patterns were consistent with this explanation. However, we cannot rule out that excretion changes may have been due to alterations in transit time, to changes in bacterial flora, or to transitory changes in absorption or synthesis of cholesterol or bile acids. Our conclusion that unsaturated fats cause a redistribution of cholesterol between plasma and tissue pools points to the necessity in future to explore where cholesterol is stored, to what extent stored cholesterol can be mobilized, and to define the factors governing these fluxes.

Effects of dietary fats on plasma lipids and lipoproteins: an hypothesis for the lipid-lowering effect of unsaturated fatty acids

Several aspects of the effects of dietary fat on plasma lipids and lipoproteins were investigated in 12 subjects during the long-term feeding of formulas containing 40% of their calories as either saturated or unsaturated fats. The changes in fatty acid composition of plasma lipids, shown previously to occur after prolonged feedings of a dietary fat, required 10-14 days to be complete and were synchronous with the effect of the fat on plasma lipid concentrations. The change in lipid concentration occurred in low but not in high density lipoproteins. The effects on lipid levels of the low density lipoproteins were found to occur with little or no effect on the concentration of the protein moiety of these lipoproteins; as a result, cholesterol- and phospholipid to protein ratios in low density lipoproteins fell during unsaturated fat feeding. The effects of dietary fat on plasma phospholipids were studied in detail: the relative amounts of phosphatidylcholine, phosphatidylethanolamine, sphingomyelin, and lysophosphatidylcholine were unaffected by the type of dietary fat. However, the molecular species of phosphatidylcholine were markedly affected. More than 90% of the fatty acids at the alpha-position were saturated during both saturated and unsaturated feedings. In contrast, during unsaturated feedings, linoleate at the beta-position outnumbered oleate by approximately 4:1, whereas during saturated feedings these two types of fatty acids were present in nearly equal amounts.This paper also presents the following hypothesis for the lipid-lowering effect of unsaturated dietary fat: since unsaturated fatty acids occupy a greater area than saturated acids, they alter the spatial configuration of the lipids into which they are incorporated; as a result, fewer lipid molecules can be accommodated by the apoprotein of the low-density lipoproteins (LDL), and thus the lipid content of the lipoprotein is lowered. The experimental findings of this study, while not proving this hypothesis, are consistent with it.

Disorders of the serum lipoproteins. II. Hyperlipoproteinaemic states

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Effect of cholestyramine on the fecal excretion of intravenously administered cholesterol-4-14C and its degradation products in a hypercholesterolemic patient

The effect of cholestyramine on the fecal excretion of bile acids and neutral sterols was measured in a hypercholesterolemic patient on a low fat, high polyunsaturated fatty acid-containing diet after the intravenous injection of cholesterol-4-(14)C. A significant (16%) lowering of serum cholesterol concentration was accompanied by a 3.2-fold increase in fecal bile acid excretion but no change in neutral sterol output. The increased bile acid loss was adequate to account for the observed fall in serum cholesterol level. The implications of these findings were discussed.

Effect of dietary fat on the fecal excretion of cholesterol and its degradation products in man

Fecal bile acid and neutral sterol excretion rates were determined in five healthy young men when serum cholesterol changes were induced by isocaloric substitution of an unsaturated (safflower oil) for a saturated fat (butter). The isotope balance method was used after the intravenous injection of cholesterol-4-(14)C. A feces extraction method is presented which permits essentially complete separation of fecal neutral sterols and bile acids. There was a significant increase in the total excretion of the fecal end products of cholesterol metabolism from 966 +/- 42 mg/day on saturated fat to 1147 +/- 45 mg/day on unsaturated fat, and the increase was equally distributed between the neutral sterol and bile acid fractions. With the substitution of dietary fats, regardless of the sequence of their feeding, there was a 28% reduction in serum cholesterol concentration during ingestion of the unsaturated fat. There were reciprocal changes in serum cholesterol levels and fecal steroid excretion with the substitution of one type of fat for the other. The changes in plasma cholesterol content were more than adequately balanced by the reciprocal changes in fecal cholesterol end product excretion. The findings in this study agree with several previous reports in supporting the hypothesis that the hypocholesteremic action of dietary unsaturated fatty acids is associated with an increase in the fecal loss of bile acids and neutral sterols.