Cholesterol and triglyceride fatty acid synthesis in apolipoprotein E2-associated hyperlipidemia

L-rhamnose and lactulose decrease serum triacylglycerols and their rates of synthesis, but do not affect serum cholesterol concentrations in men

Colonic short-chain fatty acids (SCFA) may affect hepatic lipid metabolism. Lactulose increases colonic acetate production, whereas L-rhamnose increases propionate. To test the effects of oral L-rhamnose and lactulose for 28 d on fasting concentrations and hepatic synthesis of lipids in humans, 18 men were administered 25 g/d of L-rhamnose, lactulose, or d-glucose for 4 wk in a partially randomized crossover design, with blood collected from fasting subjects on the first and last day of each period. Cholesterol and triacylglycerol (TG) synthesis rates were determined using deuterated water uptake rate over the last 24 h of each period. Postprandial blood lipids, and glucose and insulin were assessed in 11 subjects on d 28. Fasting serum cholesterol was unchanged; however, when expressed as a percentage change, TG were decreased, relative to baseline (P < 0.04), by L-rhamnose (-10%) and lactulose (-10%), compared with D-glucose, which increased serum TG (+11%). Net TG-fatty acid (TGFA) synthesis on d 28 was lower with L-rhamnose (2.42 +/- 0.38 g/d) and lactulose (2.62 +/- 0.35 g/d) than with D-glucose (2.96 +/- 0.31 g/d, P < 0.01). We conclude that these results do not support a primary role for propionate in the cholesterol-lowering effect of soluble fiber. However, both lactulose and L-rhamnose lowered serum TG (expressed as a percentage change) and TGFA synthesis, compared with d-glucose, which increased them. Although these data are consistent with inhibition of TGFA synthesis by SCFA, other aspects of the metabolism of these sugars cannot be ruled out as putative agents of their TG-lowering effects.

Soy protein reduces triglyceride levels and triglyceride fatty acid fractional synthesis rate in hypercholesterolemic subjects

To examine the effects of protein source and isoflavones on triglyceride (TG) fatty acid (TGFA) and cholesterol biosynthesis, subjects (>50 years, LDL cholesterol >130 mg/dl) underwent a four-phase randomized cross-over feeding trial. Diets contained either isolated soy protein or common sources of animal protein (25 g/1000 kcal), without or with isoflavones (49 mg/1000 kcal) and were each fed for 6 weeks. Blood samples from 20 hyperlipidemic subjects (6M, 14F, 62 +/- 9 years, BMI 26 +/- 3 kg/m(2), LDL cholesterol >160 mg/dl after feeding animal protein without isoflavones) were selected to measure TGFA fractional synthetic rate (TGFA-FSR) and free cholesterol fractional synthetic rate (FC-FSR) over 24h as deuterium oxide uptake into TGFA and free cholesterol. Soy protein reduced TG by 12.4% (P < 0.0001), total cholesterol by 4.4% (P < 0.001), and LDL cholesterol by 5.7% (P = 0.003) compared to animal protein. The TGFA-FSR was reduced by 13.3% (P = 0.018) and FC-FSR was increased by 7.6% (P = 0.017) after the soy protein relative to the animal protein. Isoflavones had no significant effect on TG and TGFA-FSR. Isoflavones reduced total cholesterol levels by 3.1% (P = 0.009) but had no significant effect on LDL, HDL cholesterol levels, or FC-FSR. These data demonstrate that dietary protein type modulates circulating TG and cholesterol levels in hypercholesterolemic individuals by distinct mechanisms.

Dietary sitostanol reciprocally influences cholesterol absorption and biosynthesis in hamsters and rabbits

The aim of this study was to examine the efficacy of variable dietary sitostanol (SI) concentrations on cholesterol absorption, synthesis and excretion rates in two animal models. Hamsters and rabbits were fed semi-purified diets supplemented with cholesterol and 1% (w/w) phytosterols containing either 0.007, 0.17, 0.8 or 1% (w/w) SI. The control (0% (w/w) SI) groups consumed the same diets but no phytosterols were added. The dual-isotope plasma ratio of [13C]- and [18O]cholesterol and deuterium incorporation methods were applied to measure simultaneously cholesterol absorption and fractional synthesis, respectively. Plasma total cholesterol levels were lower in rabbits and hamsters fed 0.8 and 1% (w/w) SI, respectively, as compared to their controls. Percent cholesterol absorption was lower (P = 0.03) in hamsters fed 1% (w/w) SI (42.5 +/- 13.3%) than control (65.1 +/- 13.4%). Moreover, cholesterol excretion in the feces was 77 and 57% higher (P = 0.017) in the 1% (w/w) SI- relative to control- and 0.17% (w/w) SI-fed groups, respectively. In rabbits, cholesterol excretion was 64% higher (P = 0.018) in 0.8% (w/w) SI- compared with control-fed groups. Fractional synthesis rate was higher (P = 0.033) in hamsters fed 1% (w/w) SI (0.116 +/- 0.054 pool day(-1)) as compared to control (0.053 +/- 0.034 pool day(-1)). However, cholesterol synthesis rates did not vary among groups fed variable concentrations of SI. In rabbits, percent cholesterol absorption and its fractional synthesis rate varied but did not reach significance. Fractional synthesis rate in hamsters was correlated (r = -0.32, P = 0.03) with percent cholesterol absorption. In conclusion, dietary SI exhibited a dose-dependent action in inhibiting cholesterol absorption while increasing cholesterol excretion and upregulating cholesterogenesis in hamsters resulting in lower circulating lipid levels.

Use of deuterium oxide to measure de novo fatty acid synthesis in normal subjects consuming different dietary fatty acid composition1

The effect of dietary linoleic (C18:2n-6) and palmitic acids (C16:0) on rate of hepatic de novo fatty acid synthesis was assessed in normal subjects. The diet was formulated to provide combinations of high and low levels of C18:2n-6 and C16:0. After 21 days of diet treatment, plasma triacylglycerol level and incorporation of deuterium into the plasma very low density lipoprotein triacylglycerol (VLDL-TG) pool over 24 hours was measured. Plasma triacylglycerol levels were within the normal range. Increasing dietary intake of linoleic acid decreased plasma triacylglycerol level when subjects consumed a low level of dietary palmitic acid. The relative and net amount of de novo synthesized fatty acid in the plasma VLDL-TG pool was not influenced by the diet treatments. A relationship between plasma triacylglycerol level and rate of hepatic de novo fatty acid synthesis was observed.

Lack of interaction of apolipoprotein E phenotype with the lipoprotein response to lovastatin or gemfibrozil in patients with primary hypercholesterolemia

The magnitude of serum lipid changes in response to hypolipidemic drugs varies considerably between individuals. These differences may be due to interactions between genetic and environmental factors that effect drug bioavailability or the capacity of the lipid-regulating enzyme and receptor targets to be affected. The apolipoprotein E (apoE) gene locus has been examined in this regard, but reports are conflicting on the effect of its variability on the response to hypolipidemic drugs. We investigated the effect of apoE polymorphism on the serum lipid response to the hepatic hydroxymethyl glutaryl coenzyme A (HMG CoA) reductase inhibitor lovastatin and the fibric acid derivative gemfibrozil. Lipoprotein changes were assessed after 12 weeks of therapy in 106 patients with primary hypercholesterolemia and combined hyperlipidemia treated with lovastastin and in 63 given gemfibrozil therapy. No significant effect of the apoE phenotypes E3/2, E3/3, or E4/3 on the heterogeneity of lipid responses to either drug was found.

The contribution of newly synthesized cholesterol to biliary cholesterol in healthy humans

Hypersecretion of biliary cholesterol appears to be the key defect in the pathogenesis of cholesterol gallstones, and this may be due to an enhanced synthesis of cholesterol. To measure fractional syntheses of biliary and plasma cholesterol, five male and 3 female healthy humans with an intact enterohepatic circulation were infused intravenously with [1-13C]acetate for 15 h. Samples of duodenal bile and blood were taken hourly and an enteral formula diet was given. Free cholesterol mass distribution was analyzed by gas chromatography mass spectrometry. The Mass Isotopomer Distribution Analysis (MIDA) technique allowed to calculate fractional synthesis. After 6 hours of infusion, the [13C]label of the cytosolic acetate pool reached a plateau of approximately 12%. Individual fractional cholesterol synthesis is plasma and bile correlated significantly (6-15 h) and amounted to 4.2% and 5.3% after 15 h, respectively. It may be concluded from this study, that newly synthesized cholesterol is secreted into bile to a higher extent than into plasma.

Postprandial vitamin A and squalene clearances and cholesterol synthesis off and on lovastatin treatment in type III hyperlipoproteinemia

Postprandial fat clearance and absorption, fecal elimination and synthesis of cholesterol, bile acid synthesis, and cholesterol precursors and plant sterols in serum were studied in five patients with type III dyslipoproteinemia off and on lovastatin. The basal values were related to those in nontreated normolipidemic control subjects with apolipoprotein E3/3 phenotype (apo E3 controls, n = 16). On regular home diets, cholesterol precursor concentrations and cholesterol precursor/cholesterol ratios were high in the type III group. However, cholesterol absorption efficiency, bile acid and cholesterol synthesis measured with sterol balance technique and the precursor sterol/plant sterol ratios in serum were similar to the control values, suggesting that cholesterol absorption and metabolism was normal in these subjects. Lovastatin normalized the increased lipoprotein concentrations and reduced biliary cholesterol secretion, absolute absorption of cholesterol, precursor sterol/cholesterol and precursor sterol/plant sterol ratios in serum, fecal neutral and total sterol outputs and cholesterol synthesis. Lovastatin had no effect on cholesterol absorption efficiency or bile acid synthesis. Despite normalization of the triglyceride-rich lipoprotein levels by lovastatin, the postprandial vitamin A and squalene peak concentrations and the areas under the curves remained above the control ranges. The findings show that in type III hyperlipidemia, the precursor sterol/cholesterol ratios do not predict cholesterol synthesis. The latter, bile acid synthesis, precursor sterol/plant sterol ratios in serum, and cholesterol absorption are normal under basal conditions. The normalization of increased lipids by lovastatin is mainly due to reduced synthesis and absolute absorption of cholesterol, while the retarded postprandial fat clearance was not normalized by the drug.

Meal frequency influences circulating hormone levels but not lipogenesis rates in humans

To determine whether human lipogenesis is influenced by the frequency of meal consumption, 12 subjects were divided into two groups and fed isocaloric nutritionally adequate liquid diets over 3 days, either as three larger diurnal (n = 6) or as six small, evenly spaced (n = 6) meals per day. On day 2 (08:00 h) of each diet period, 0.7 g deuterium (D) oxide/kg body water was administered and blood was collected every 4 hours over 48 hours for measurement of plasma insulin and glucose-dependent insulinotropic polypeptide (GIP) levels. At each time point, the incorporation of D into plasma triglyceride fatty acid (TG-FA) was also determined by isotope ratio mass spectrometry after TG-FA extraction and combustion/reduction. Insulin and GIP levels were elevated over daytime periods in subjects fed three versus six meals per day. Contribution of de novo synthesis to total TG-FA production was not significantly different for days 2 and 3 in subjects consuming three (6.56% +/- 1.32% and 6.64% +/- 2.08%, respectively) and six (7.67% +/- 2.29% and 7.88% +/- 1.46%, respectively) meals per day. Net TG-FA synthesis rates over days 2 and 3 were 1.47 +/- 0.33 and 1.55 +/- 0.53 g/d, respectively, for subjects fed three meals per day, and 1.64 +/- 0.47 and 1.69 +/- 0.30 g/d for subjects fed six meals per day. These findings suggest that consuming fewer but larger daily meals is not accompanied by increases in TG-FA synthesis, despite the observation of hormonal peaks.

Response of cholesterol synthesis to cholesterol feeding in men with different apolipoprotein E genotypes

To investigate the influence of dietary cholesterol level and apolipoprotein (apo) E polymorphism on cholesterol synthesis, seven apo E2/- and six apo E4/- normolipidemic subjects consumed self-selected diets containing low cholesterol ([LC] 250 mg/d) and high cholesterol ([HC] 800 mg/d) levels for approximately 20 days. On day 20, subjects were given 0.7 g deuterium oxide (D2O)/kg body water followed by maintenance doses. Cholesterol synthesis was measured as the uptake rate of D into plasma free cholesterol over 24 hours. Serum total cholesterol levels were higher (P < .05) in the apo E4/- versus apo E2/- group over both dietary periods. No influence of dietary cholesterol content on serum levels was observed, nor was there an effect of apo E genotype or dietary cholesterol level on cholesterogenesis. However, a genotype-independent association was observed between both cholesterogenesis (P < .001) and the increase in cholesterogenesis (P = .05) with the change in serum total cholesterol level subsequent to high-cholesterol feeding. These findings suggest that (1) apo E genotype is not associated with cholesterol synthesis rate in subjects on self-selected diets, and (2) hyporesponders to a dietary cholesterol challenge display higher synthetic rates than hyperresponders. The observation of lower cholesterol synthesis in individuals with the largest increases in serum cholesterol level after a dietary cholesterol challenge suggests a passive rather than dominant role of cholesterol synthesis in regulating serum levels.

Human cholesterol synthesis measurement using deuterated water. Theoretical and procedural considerations

Human cholesterogenesis is measurable as the rate of incorporation of deuterium derived from deuterium oxide (D2O) within the body water pool into plasma or erythrocyte cholesterol pools. Oral D2O equilibrates across body water, thus enabling extracellular sampling of pools (such as urine) to serve as accurate indicators of intracellular deuterium enrichments at the point of synthesis. Required doses of D2O fall below the threshold associated with negative side effects. Deuterium/carbon incorporation ratios into cholesterol during biosynthesis have been established that are applicable in humans. Models using unconstrained and constrained curve fitting permit improved flexibility in interpretation of deuterium-uptake kinetics. However, sample-size restrictions presently limit the ability of the technique to examine the kinetics within individual lipoprotein species. Correction of enrichment data for proton exchange during combustion and reduction phases of sample preparation is an additional important procedural concern. In summary, the deuterated-water procedure is a useful tool in studies of human cholesterol synthesis that offers the advantages of short measurement interval, relative noninvasiveness, and provision of a direct index of synthesis in comparison with other available techniques.

Measurement of endogenous synthesis of plasma cholesterol in rats and humans using MIDA

We used the mass isotopomer distribution analysis (MIDA) technique to measure endogenous synthesis of plasma cholesterol in vivo in rats and normal human subjects. Sodium [1-13C]- or [2-13C]acetate was infused, and plasma free cholesterol was analyzed by gas chromatography-mass spectrometry. Frequencies of mass isotopomers M0-M4 (mass-to-charge ratio 368-372) were quantified. The enrichment of the true precursor for cholesterol synthesis (acetyl-coenzyme A in contributing tissues) was determined using the MIDA method. This technique remains mathematically valid even if more than one tissue contributes to circulating free cholesterol. The fractional contribution (f) from endogenous synthesis to free cholesterol in normal women (n = 5) was 2.48 +/- 0.39% after 7 h in the postabsorptive state and 1.27 +/- 0.41% after 8 h of refeeding. In ad libitum-fed rats (n = 12), f was 2.89 +/- 0.44% after 12 h, whereas administration of recombinant tumor necrosis factor increased this value fourfold. Next, the rate constant (k) for removal of labeled free cholesterol from plasma was calculated. Higher masses (M2-M4) were followed to avoid the problem of persistent label incorporation. During the 60 h after cessation of [13C]acetate infusions, k was 0.02490 +/- 0.00298/h in humans. Using these values of k and f, absolute cholesterogenesis was 568 +/- 55 mg/day in normal women (follicular menstrual phase), similar to prior estimates based on whole body sterol balances. Women also exhibited a diurnal variation for endogenous cholesterol synthesis (34.6 +/- 5.4 mg/h nighttime vs. 15.9 +/- 5.2 mg/h daytime) consistent with current knowledge about rhythms in cholesterogenesis. Checks on the model were internally consistent (e.g., comparisons among different isotopomers for calculating precursor enrichment). We conclude that fractional and absolute endogenous cholesterol synthesis can be measured using stable isotopes in vivo by the MIDA technique.