Effect of changing dietary fat saturation on low-density lipoprotein metabolism in man

A closer look at diversity and performance in family firms

Purpose

Ownership structure plays a significant role in determining board demographic diversity. However, it is still unclear how different ownership configurations impact the structures of firm's boards and how board diversity influences firm performance. This study aims to investigate the relationship between family ownership and board diversity. Therefore, in this study, the authors argue that family firms have a lower level of board demographic diversity (in terms of age, gender and nationality) than non-family firms and that board diversity moderates the relationship between ownership and firm performance.

Design/methodology/approach

To test the authors’ hypotheses, we draw data from a sample of 341 German family and non-family firms for a period of five years.

Findings

The results show that family firms are less diverse in terms of age, gender and nationality diversity than non-family firms.

Originality/value

This study contributes to the general understanding of family firms and in particular the role ownership plays in shaping board demographic diversity.

The homeoviscous adaptation to dietary lipids (HADL) model explains controversies over saturated fat, cholesterol, and cardiovascular disease risk

SFAs play the leading role in 1 of the greatest controversies in nutrition science. Relative to PUFAs, SFAs generally increase circulating concentrations of LDL cholesterol, a risk factor for atherosclerotic cardiovascular disease (ASCVD). However, the purpose of regulatory mechanisms that control the diet-induced lipoprotein cholesterol dynamics is rarely discussed in the context of human adaptive biology. We argue that better mechanistic explanations can help resolve lingering controversies, with the potential to redefine aspects of research, clinical practice, dietary advice, public health management, and food policy. In this paper we propose a novel model, the homeoviscous adaptation to dietary lipids (HADL) model, which explains changes in lipoprotein cholesterol as adaptive homeostatic adjustments that serve to maintain cell membrane fluidity and hence optimal cell function. Due to the highly variable intake of fatty acids in humans and other omnivore species, we propose that circulating lipoproteins serve as a buffer to enable the rapid redistribution of cholesterol molecules between specific cells and tissues that is necessary with changes in dietary fatty acid supply. Hence, circulating levels of LDL cholesterol may change for nonpathological reasons. Accordingly, an SFA-induced raise in LDL cholesterol in healthy individuals could represent a normal rather than a pathologic response. These regulatory mechanisms may become disrupted secondarily to pathogenic processes in association with insulin resistance and the presence of other ASCVD risk factors, as supported by evidence showing diverging lipoprotein responses in healthy individuals as opposed to those with metabolic disorders such as insulin resistance and obesity. Corresponding with the model, we suggest alternative contributing factors to the association between elevated LDL cholesterol concentrations and ASCVD, involving dietary factors beyond SFAs, such as an increased endotoxin load from diet-gut microbiome interactions and subsequent chronic low-grade inflammation that interferes with fine-tuned signaling pathways.

Role of Dietary Macronutrients and Fatty Acids in Obesity and Metabolic Risk in Older Adults

The aim of the study was to examine the role of dietary consumption of different types of fatty acids on metabolic risk factors and regional fat deposition in older men and women. We hypothesized that saturated fatty acid (SFA) intake, monounsaturated fatty acid (MUFA) and low intake of polyunsaturated fatty acids (PUFA) would be associated with markers of insulin resistance, hyperlipidemia, and hypertriglyceridemia. Sedentary, overweight and obese (body mass index: 29-48 kg/m²) adults (N=20) aged 45-78 years underwent two-hour oral glucose tolerance test, blood draw, DXA scan, and completed seven-day diet records. Subjects had low fitness levels (VO₂ max=23.5 ± 2.4 mL/kg/min) and high total body fat (43.5 ± 1.7%). The average macronutrient composition of the diet was high in fat as a percent of total kcal (35.5%). The ratio of MUFA to PUFA was associated with serum cholesterol (r=0.48, P=0.03) and tended to be associated with higher fasting glucose (r=0.42, P=0.06) and glucose at 120 min (r=0.43, P=0.06). PUFA intake as a percentage of fat intake was associated with lower serum cholesterol (r=-0.44, P=0.05). Therefore, dietary MUFA intake unbalanced by PUFA may confer increased risk for diabetes among obese, sedentary individuals. Future investigation of food sources, or context of dietary lipids, could lead to individualized dietary recommendations to promote healthy eating habits and potentially alter metabolic risk.

Substitution of dietary ω-6 polyunsaturated fatty acids for saturated fatty acids decreases LDL apolipoprotein B-100 production rate in men with dyslipidemia associated with insulin resistance: a randomized controlled trial

Background

The substitution of omega (ω)-6 (n-6) polyunsaturated fatty acids (PUFAs) for saturated fatty acids (SFAs) is advocated in cardiovascular disease prevention. The impact of this substitution on lipoprotein metabolism in subjects with dyslipidemia associated with insulin resistance (IR) remains unknown.

Objective

In men with dyslipidemia and IR, we evaluated the impact of substituting ω-6 PUFAs for SFAs on the in vivo kinetics of apolipoprotein (apo) B-containing lipoproteins and on the intestinal expression of key genes involved in lipoprotein metabolism.

Design

Dyslipidemic and IR men (n = 36) were recruited for this double-blind, randomized, crossover, controlled trial. Subjects consumed, in a random order, a fully controlled diet rich in SFAs (SFAs: 13.4% of energy; ω-6 PUFAs: 4.0%) and a fully controlled diet rich in ω-6 PUFAs (SFAs: 6.0%; ω-6 PUFAs: 11.3%) for periods of 4 wk, separated by a 4-wk washout period. At the end of each diet, the in vivo kinetics of apoB-containing lipoproteins were measured and the intestinal expression of key genes involved in lipoprotein metabolism was quantified in duodenal biopsies taken from each participant.

Results

The substitution of ω-6 PUFAs for SFAs had no impact on TRL apoB-48 fractional catabolic rate (Δ = -3.8%, P = 0.7) and production rate (Δ = +1.2%, P = 0.9), although it downregulated the intestinal expression of the microsomal triglyceride transfer protein (Δ = -18.4%, P = 0.006) and apoB (Δ = -16.6%, P = 0.005). The substitution of ω-6 PUFAs for SFAs decreased the LDL apoB-100 pool size (Δ = -7.8%; P = 0.005). This difference was attributed to a reduction in the LDL apoB-100 production rate after the substitution of ω-6 PUFAs for SFAs (Δ = -10.0%; P = 0.003).

Conclusions

This study demonstrates that the substitution of dietary ω-6 PUFAs for SFAs decreases the production and number of LDL particles in men with dyslipidemia and IR. This trial was registered at clinicaltrials.gov as NCT01934543.

Determining the mechanisms of dietary turnip rapeseed oil on cholesterol metabolism in men with metabolic syndrome

We have earlier reported the reduction of total cholesterol, low-density lipoprotein (LDL) cholesterol and oxidized LDL caused by short-term modification of diet with cold-pressed turnip rapeseed oil (CPTRO) instead of butter. The aim of this supplementary study was to determine whether the beneficial effects resulted from altered cholesterol metabolism during the intervention.Thirty-seven men with metabolic syndrome (MetS) completed an open, randomized and balanced crossover study. Subjects' usual diet was supplemented with either 37.5 g of butter or 35 mL of CPTRO for 6-8 weeks. Otherwise normal dietary habits and physical activity were maintained without major variations. Serum non-cholesterol sterols were assayed with gas-liquid chromatography and used as surrogate markers of whole-body cholesterol synthesis and absorption efficiency. Serum proprotein convertase subtilisin/kexin type 9 (PCSK9) concentration was analyzed with Quantikine ELISA Immunoassay. Serum cholesterol synthesis markers and serum cholestanol (absorption marker), all as ratios to cholesterol, did not differ between the periods. Serum campesterol and sitosterol ratios to cholesterol were significantly increased after the administration of CPTRO resulting from the increased intake of 217 mg/day of plant sterols in CPTRO. Serum PCSK9 concentration did not differ between CPTRO and butter periods.The reduction in serum cholesterol by 7.2% after consumption of rapeseed oil could not be explained by changes in cholesterol absorption, synthesis or PCSK9 metabolism in MetS.ClinicalTrials.gov NCT01119690.

Effect of dietary Fatty acids on human lipoprotein metabolism: a comprehensive update

Dyslipidemia is a major risk factor for cardiovascular disease (CVD). Dietary fatty-acid composition regulates lipids and lipoprotein metabolism and may confer CVD benefit. This review updates understanding of the effect of dietary fatty-acids on human lipoprotein metabolism. In elderly participants with hyperlipidemia, high n-3 polyunsaturated fatty-acids (PUFA) consumption diminished hepatic triglyceride-rich lipoprotein (TRL) secretion and enhanced TRL to low-density lipoprotein (LDL) conversion. n-3 PUFA also decreased TRL-apoB-48 concentration by decreasing TRL-apoB-48 secretion. High n-6 PUFA intake decreased very low-density lipoprotein (VLDL) cholesterol and triglyceride concentrations by up-regulating VLDL lipolysis and uptake. In a study of healthy subjects, the intake of saturated fatty-acids with increased palmitic acid at the sn-2 position was associated with decreased postprandial lipemia. Low medium-chain triglyceride may not appreciably alter TRL metabolism. Replacing carbohydrate with monounsaturated fatty-acids increased TRL catabolism. Trans-fatty-acid decreased LDL and enhanced high-density lipoprotein catabolism. Interactions between APOE genotype and n-3 PUFA in regulating lipid responses were also described. The major advances in understanding the effect of dietary fatty-acids on lipoprotein metabolism has centered on n-3 PUFA. This knowledge emphasizes the importance of regulating lipoprotein metabolism as a mode to improve plasma lipids and potentially CVD risk. Additional studies are required to better characterize the cardiometabolic effects of other dietary fatty-acids.

Saturated Fats Versus Polyunsaturated Fats Versus Carbohydrates for Cardiovascular Disease Prevention and Treatment

The effects of saturated fatty acids (SFAs) on cardiovascular disease (CVD) risk are modulated by the nutrients that replace them and their food matrices. Replacement of SFAs with polyunsaturated fatty acids has been associated with reduced CVD risk, although there is heterogeneity in both fatty acid categories. In contrast, replacement of SFAs with carbohydrates, particularly sugar, has been associated with no improvement or even a worsening of CVD risk, at least in part through effects on atherogenic dyslipidemia, a cluster of traits including small, dense low-density lipoprotein particles. The effects of dietary SFAs on insulin sensitivity, inflammation, vascular function, and thrombosis are less clear. There is growing evidence that SFAs in the context of dairy foods, particularly fermented dairy products, have neutral or inverse associations with CVD. Overall dietary patterns emphasizing vegetables, fish, nuts, and whole versus processed grains form the basis of heart-healthy eating and should supersede a focus on macronutrient composition.

Effects of Therapeutic Lifestyle Change diets high and low in dietary fish-derived FAs on lipoprotein metabolism in middle-aged and elderly subjects

The effects of Therapeutic Lifestyle Change (TLC) diets, low and high in dietary fish, on apolipoprotein metabolism were examined. Subjects were provided with a Western diet for 6 weeks, followed by 24 weeks of either of two TLC diets (10/group). Apolipoprotein kinetics were determined in the fed state using stable isotope methods and compartmental modeling at the end of each phase. Only the high-fish diet decreased median triglyceride-rich lipoprotein (TRL) apoB-100 concentration (-23%), production rate (PR, -9%), and direct catabolism (-53%), and increased TRL-to-LDL apoB-100 conversion (+39%) as compared with the baseline diet (all P < 0.05). This diet also decreased TRL apoB-48 concentration (-24%), fractional catabolic rate (FCR, -20%), and PR (-50%) as compared with the baseline diet (all P < 0.05). The high-fish and low-fish diets decreased LDL apoB-100 concentration (-9%, -23%), increased LDL apoB-100 FCR (+44%, +48%), and decreased HDL apoA-I concentration (-15%, -14%) and PR (-11%, -12%) as compared with the baseline diet (all P < 0.05). On the high-fish diet, changes in TRL apoB-100 PR were negatively correlated with changes in plasma eicosapentaenoic and docosahexaenoic acids. In conclusion, the high-fish diet decreased TRL apoB-100 and TRL apoB-48 concentrations chiefly by decreasing their PR. Both diets decreased LDL apoB-100 concentration by increasing LDL apoB-100 FCR and decreased HDL apoA-I concentration by decreasing HDL apoA-I PR.

Low-fat diets for acquired hypercholesterolaemia

BACKGROUND

Hypercholesterolaemia, characterised by raised blood cholesterol levels, is not a disease itself but a metabolic derangement that often contributes to many diseases, notably cardiovascular disease. In most cases, elevated cholesterol levels are associated with high-fat diet, especially saturated fat, coupled with an inactive lifestyle. Less commonly, raised cholesterol may be related to an inherited disorder, familial hypercholesterolaemia. This systematic review is only concerned with acquired hypercholesterolaemia.

OBJECTIVES

To assess the effects of low-fat diets for acquired hypercholesterolaemia and to investigate the incidence of adverse effects from low-fat dietary interventions. We planned to compare the relative effectiveness of low-fat diets with calorie-restricted diets for acquired hypercholesterolaemia. We also wanted to look into the relative effectiveness of low-fat diets and pharmacological interventions for acquired hypercholesterolaemia.

SEARCH STRATEGY

Studies were obtained from computerised searches of The Cochrane Library, MEDLINE, EMBASE and databases of ongoing trials. Date of last search was February 2010.

SELECTION CRITERIA

Otherwise healthy adults (equal to or greater than 18 years) with acquired (not familial) hypercholesterolaemia. We defined hypercholesterolaemia as either total cholesterol greater than 5.2 mmol/L, LDL-cholesterol greater than 3.0 mmol/L, HDL-cholesterol less than 1.0 mmol/L or a combination thereof, although investigators' definitions were also accepted. We wanted to include any low-fat dietary intervention, like low-fat and low-saturated fat diets, intended to lower serum total and LDL-cholesterol or to raise HDL-cholesterol. A low-fat diet was considered as a fat calorie intake less than 20% of the total calories. The minimum duration of the intervention had to be six months. We excluded studies in unhealthy people.

DATA COLLECTION AND ANALYSIS

Two authors were planned to independently assess risk of bias and extract data.

MAIN RESULTS

No study met our inclusion criteria.

AUTHORS' CONCLUSIONS

Well designed, adequately powered randomised controlled trials investigating patient-relevant outcomes of low-fat diets for otherwise healthy people with hypercholesterolaemia are required.

Saturated fat, carbohydrate, and cardiovascular disease

A focus of dietary recommendations for cardiovascular disease (CVD) prevention and treatment has been a reduction in saturated fat intake, primarily as a means of lowering LDL-cholesterol concentrations. However, the evidence that supports a reduction in saturated fat intake must be evaluated in the context of replacement by other macronutrients. Clinical trials that replaced saturated fat with polyunsaturated fat have generally shown a reduction in CVD events, although several studies showed no effects. An independent association of saturated fat intake with CVD risk has not been consistently shown in prospective epidemiologic studies, although some have provided evidence of an increased risk in young individuals and in women. Replacement of saturated fat by polyunsaturated or monounsaturated fat lowers both LDL and HDL cholesterol. However, replacement with a higher carbohydrate intake, particularly refined carbohydrate, can exacerbate the atherogenic dyslipidemia associated with insulin resistance and obesity that includes increased triglycerides, small LDL particles, and reduced HDL cholesterol. In summary, although substitution of dietary polyunsaturated fat for saturated fat has been shown to lower CVD risk, there are few epidemiologic or clinical trial data to support a benefit of replacing saturated fat with carbohydrate. Furthermore, particularly given the differential effects of dietary saturated fats and carbohydrates on concentrations of larger and smaller LDL particles, respectively, dietary efforts to improve the increasing burden of CVD risk associated with atherogenic dyslipidemia should primarily emphasize the limitation of refined carbohydrate intakes and a reduction in excess adiposity.

Differential influence of different dietary fatty acids on very low-density lipoprotein secretion when delivered to hepatocytes in chylomicron remnants

The influence of dietary fats carried in chylomicron remnants on the hepatic secretion of very low-density lipoprotein (VLDL) was investigated using chylomicron remnant-like particles (CRLPs) and cultured rat hepatocytes as the experimental model. Chylomicron remnant-like particles containing triacylglycerol (TG) from palm, olive, or corn (enriched in saturated, monounsaturated, or n-6 polyunsaturated fatty acids) oil, respectively, were incubated with cultured hepatocytes for 5 hours. The medium was then removed and replaced with medium without CRLPs; and the secretion of TG, cholesterol, and apolipoprotein B48 during the following 16 hours was determined. Secretion of TG into the d less than 1.050-g/mL fraction containing VLDL was unaffected by olive CRLPs, but was significantly increased in cells exposed to palm or corn CRLPs in comparison with both olive CRLPs and control incubations without CRLPs. Secretion of apolipoprotein B48, however, was not changed by any of the CRLP types. Apolipoprotein B messenger RNA levels were decreased by olive and corn CRLPs, and 3-hydroxy-3-methylglutaryl coenzyme A reductase messenger RNA abundance was increased by palm CRLPs; but expression of other genes involved in the regulation of VLDL secretion was unaffected. These findings demonstrate that CRLPs enriched in saturated fatty acids or n-6 polyunsaturated fatty acids increase the secretion of TG in VLDL, possibly because of the secretion of larger particles, whereas those enriched in monounsaturated fatty acids have no effect. Thus, different dietary fats have differential effects on VLDL secretion directly when delivered to the liver in chylomicron remnants.

Dietary monounsaturated fat activates metabolic pathways for triglyceride-rich lipoproteins that involve apolipoproteins E and C-III

BACKGROUND

Dietary monounsaturated fat (MUFA) and complex carbohydrates have different effects on triglyceride-rich lipoprotein (TRL) metabolism.

OBJECTIVE

We hypothesized that apolipoprotein (apo) E and apo C-III might be involved in these dietary effects because of their crucial role in TRL metabolism.

DESIGN

Twelve adults consumed, for 3 wk each, 2 isocaloric diets: first a carbohydrate-rich diet (48% complex carbohydrate, 8% MUFAs) and then a MUFA-rich diet (31% complex carbohydrate, 24% MUFAs) 12 mo later. The dietary composition of other macronutrients in the 2 diets was similar. Body weight was kept constant. Postprandial apo B kinetic studies using stable-isotope tracers were performed after each dietary intervention. Multiple VLDL, intermediate-density lipoprotein (IDL), and LDL fractions were prepared on the basis of apo E and apo C-III contents.

RESULTS

The MUFA diet increased by approximately 4-6-fold, the secretion of VLDLs and IDLs containing both apo E and apo C-III (E+CIII+) (P < 0.05). These are TRLs that mostly cleared from the circulation and are minor precursors of LDL. The MUFA diet also decreased by 60% (P < 0.05) the secretion of the TRLs without apo E or apo C-III (major precursors of LDL in plasma) and decreased their flux to LDLs. Total LDL flux did not change because the MUFA diet increased the flux to LDL from E-CIII+ TRLs, a process that requires the removal of apo C-III. In addition, the MUFA diet significantly increased the TRL fractional catabolic rate by 50% and doubled the percentage of TRLs that were cleared rather than being converted to LDLs.

CONCLUSION

MUFA intake activates synthetic and rapid catabolic pathways for TRL metabolism that involve apo E and apo C-III and suppresses the metabolism of more slowly metabolized VLDLs and IDLs, which do not contain these apolipoproteins.

Increase of serum cholesterol levels by heat-moisture-treated high-amylose cornstarch in rats fed a high-cholesterol diet

The effects of four cornstarches containing various contents of resistant starch on serum and liver cholesterol levels in rats fed a high-cholesterol diet were investigated. Male Sprague Dawley rats (aged 4 weeks) were divided into four groups (n = 7) and fed high-cholesterol diets containing 15% of cornstarch (CS), heat-moisture-treated CS (HCS), high-amylose CS (HA), or heat-moisture-treated HA (HHA) for 21 days. The results showed that the serum and hepatic level of total cholesterol, LDL-cholesterol, and triglyceride in rats of the HHA group and their arteriosclerosis index were significantly higher, suggesting that HHA increases the risk of arteriosclerosis under a high-cholesterol dietary condition. No significant between-group differences were noted in the levels of plasma mevalonic acid and hepatic HMG-CoA reductase mRNA, whereas fecal cholesterol excretion was significantly higher in the HHA group, indicating that the elevation of the serum and liver cholesterol levels was not due to the promotion of liver cholesterol synthesis and cholesterol absorption in the intestine.

Lipid peroxidation and oxidant stress regulate hepatic apolipoprotein B degradation and VLDL production

How omega-3 and omega-6 polyunsaturated fatty acids (PUFAs) lower plasma lipid levels is incompletely understood. We previously showed that marine omega-3 PUFAs (docosahexaenoic acid [DHA] and eicosapentaenoic acid) stimulate a novel pathway, post-ER presecretory proteolysis (PERPP), that degrades apolipoprotein B100 (ApoB100), thereby reducing lipoprotein secretion from liver cells. To identify signals stimulating PERPP, we examined known actions of omega-3 PUFA. In rat hepatoma or primary rodent hepatocytes incubated with omega-3 PUFA, cotreatment with the iron chelator desferrioxamine, an inhibitor of iron-dependent lipid peroxidation, or vitamin E, a lipid antioxidant, suppressed increases in thiobarbituric acid-reactive substances (TBARSs; a measure of lipid peroxidation products) and restored ApoB100 recovery and VLDL secretion. Moreover, omega-6 and nonmarine omega-3 PUFA, also prone to peroxidation, increased ApoB100 degradation via intracellular induction of TBARSs. Even without added fatty acids, degradation of ApoB100 in primary hepatocytes was blocked by desferrioxamine or antioxidant cotreatment. To extend these results in vivo, mice were infused with DHA, which increased hepatic TBARSs and reduced VLDL-ApoB100 secretion. These results establish a novel link between lipid peroxidation and oxidant stress with ApoB100 degradation via PERPP, and may be relevant to the hypolipidemic actions of dietary PUFAs, the basal regulation of ApoB100 secretion, and hyperlipidemias arising from ApoB100 overproduction.

Comparison of the effects of dietary n-3 and n-6 polyunsaturated fatty acids on very-low-density lipoprotein secretion when delivered to hepatocytes in chylomicron remnants

The effects of chylomicron remnants enriched in n-3 or n-6 polyunsaturated fatty acids (derived from fish or corn oil respectively) on the secretion of very-low-density lipoprotein (VLDL) lipid and apolipoprotein B (apoB) by rat hepatocytes in culture was investigated. Remnants were prepared in vivo from chylomicrons obtained from rats given an oral dose of fish or corn oil and incubated with cultured hepatocytes for up to 16 h. The medium was then removed and the secretion of cholesterol and triacylglycerol into the whole medium or the rho<1.050 g/ml fraction during the following 7-24 h was determined. After exposure of the cells to fish-oil as compared with corn-oil remnants, secretion of both cholesterol and triacylglycerol into the whole medium was decreased by 25-35%, and secretion into the rho<1.050 g/ml fraction was decreased by 20-25%. In addition, the levels of apoB48 found in the rho<1.050 g/ml fraction were significantly lower in cells treated with fish-oil rather than corn-oil remnants, although the levels of apoB100 remained unchanged. The expression of mRNA for apoB, as determined by reverse-transcriptase PCR, however, was not significantly changed after exposure of the cells to both types of remnants. These results demonstrate that the effects of dietary n-3 polyunsaturated fatty acids in depressing hepatic VLDL secretion occur directly when they are delivered to the liver from the intestine in chylomicron remnants, and that the secretion, but not the synthesis, of apoB is targeted.

Regulation of very low density lipoprotein apo B metabolism by dietary fat saturation and chain length in the guinea pig

Studies investigated the effects of dietary fatty acid composition and saturation on the regulation of very low density lipoprotein (VLDL) apo B flux, clearance, and conversion to low density lipoprotein (LDL) in guinea pigs fed semipurified diets containing 15% (w/w) corn oil (CO), lard (LA), or palm kernel oil (PK). Plasma cholesterol levels were highest with dietary PK (3.1 +/- 1.0 mmol/L) followed by LA (2.4 +/- 0.4 mmol/L) and CO (1.6 +/- 0.4 mmol/L) intake. VLDL particles were larger (P < 0.05) in the LA (78 +/- 7 nm) and PK (69 +/- 10 nm) groups compared to animals fed CO (49 +/- 5 nm). VLDL-apo B fractional catabolic rates (FCR) were highest in guinea pigs fed the LA diet (P < 0.05) and VLDL apo B flux, estimated from VLDL 125I-apo B turnover kinetics, were higher in LA compared to PK or CO fed guinea pigs. In the case of PK consumption, the kinetic estimates of VLDL apo B flux significantly underestimated rates compared to direct VLDL apo B secretion measurements and LDL turnover analyses. These data demonstrate that differences in the composition and amount of saturated fatty acids have differential effects on VLDL apo B flux, catabolism, and conversion to LDL which, together with changes in LDL receptor-mediated catabolism, determine plasma LDL cholesterol levels in guinea pigs. The data also indicate that kinetic analysis of VLDL metabolism in PK fed animals is inaccurate possibly due to the presence of a small, nonequilibrating pool of newly synthesized VLDL which is rapidly converted to LDL.

Dietary fat saturation modifies the metabolism of LDL subfractions in guinea pigs

The effects of dietary fat saturation on the metabolism of low-density lipoprotein (LDL) subfractions were measured in adult male guinea pigs fed semipurified diets containing 15% (wt/wt) corn oil (CO; 58% linoleic acid), lard (24% palmitic/14% stearic acid), or palm kernel oil (PK; 52% lauric/18% myristic acid). Animals fed the CO diet had lower plasma total cholesterol levels than guinea pigs fed the PK or lard diets (P < .01). Plasma LDL-1 (d = 1.019 to 1.05 g/mL) concentrations were 3.5- and 2.4-fold higher in animals fed the PK diet compared with the CO and lard groups, respectively, while LDL-2 (d = 1.05 to 1.09 g/mL) concentrations were not different among groups. For all dietary fat groups LDL-1 had a higher molecular weight and a larger diameter than LDL-2. LDL fractional catabolic rates (FCRs) varied, depending on both the diet and the LDL subfraction. Animals fed the polyunsaturated CO diet had a more rapid LDL FCR than animals from the other two groups (P < .01). Within the same diet group, LDL-2 exhibited a slower turnover rate than LDL-1 in animals fed the PK diet, while no differences in LDL subfraction FCR were found in the CO and lard groups. Animals fed the PK and lard diets did not exhibit significant modifications in the density distribution of LDL subfractions over a period of 33 hours. In contrast, animals fed the CO diet exhibited a shift of more buoyant to denser LDL particles, suggesting that differences in LDL intravascular processing are mediated by dietary fat saturation. In vitro LDL binding to hepatic membranes confirmed the in vivo data with an increased expression of apolipoprotein B/E receptors (Bmax) in animals fed the CO diet (P < .01). Hepatic apolipoprotein B/E receptors exhibited less affinity for LDL-2 in the PK group, a result consistent with the less rapid turnover of LDL-2 in PK-fed animals. The results suggest that dietary fatty acids varying in saturation and composition have distinctive atherogenic potentials. The lowest plasma LDL cholesterol concentrations mediated by CO intake could in part be explained by induced changes in the composition and processing of LDL subfractions, resulting in faster LDL turnover rates in addition to increased expression of hepatic apolipoprotein B/E receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Cholesterol absorption, elimination, and synthesis related to LDL kinetics during varying fat intake in men with different apoprotein E phenotypes

Cholesterol absorption, fecal elimination, and synthesis and low density lipoprotein (LDL) metabolism were measured in 29 middle-aged men while on their normal diet and a diet low in fat and cholesterol, and the obtained values were related to apoprotein (apo) E phenotypes. Basal cholesterol absorption efficiency was positively related to production rate (PR) for LDL apo B and negatively to cholesterol synthesis (measured by fecal steroids and dietary cholesterol), which in turn was negatively associated with the LDL level and positively with the fractional removal (FCR) of LDL apo B. The apo E subscript (e.g., E2/2 = 1, E2/3 = 2, etc.) was positively associated with cholesterol absorption and the LDL apo B and cholesterol levels and negatively with cholesterol synthesis and FCR for LDL apo B. Effective bile acid and cholesterol synthesis, fecal elimination of cholesterol, removal of LDL apo B, and low cholesterol absorption characterized men with the epsilon 2 allele. Reduction of dietary fat and cholesterol intakes lowered LDL cholesterol levels and cholesterol absorption but increased cholesterol synthesis proportionally to the apo E subscript; the FCR and PR for LDL apo B were significantly increased and decreased, respectively. The decrease in absorption was related to enhanced removal of LDL apo B and synthesis of cholesterol. During the modified diet, cholesterol metabolism was poorly related to LDL, apo E phenotypes, and LDL apo B kinetics. A positive correlation of cholesterol absorption with dietary fat intake in combined studies suggests that a dietary fat reduction-associated decrease in LDL cholesterol is at least partly caused by reduced cholesterol absorption.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of a high-carbohydrate, low-saturated-fat diet on apolipoprotein B and triglyceride metabolism in Pima Indians

The mechanisms by which high-carbohydrate, low-saturated-fat diets lower LDL cholesterol (LDLC) concentrations are unknown. In this study, kinetics of VLDL, intermediate density lipoprotein (IDL), and LDL apoprotein B and VLDL triglyceride were determined in seven nondiabetic (ND) and seven non-insulin-dependent diabetic (NIDDM) Pima Indian subjects on high-fat and high-carbohydrate (HICHO) diets. Metabolic changes were similar in ND and NIDDM. On the HICHO diet, LDLC decreased (131 +/- 8 vs. 110 +/- 7 mg/dl, P less than 0.0001) in all subjects. Mean fasting and 24-h triglyceride (TG) concentrations were unchanged, as were mean production rates and fractional clearance rates (FCR) of VLDL apoB and VLDL TG. The mean VLDL apoB pool size (303 +/- 20 vs. 371 +/- 38 mg, P = 0.01) increased owing to a decrease in the mean transport rate (10.7 +/- 1.1 vs. 8.4 +/- 0.9 mg/kg fat-free mass (ffm) per day, P less than 0.0001) and the mean rate constant (2.3 +/- 0.2 vs. 1.5 +/- 0.2, P less than 0.001) for the VLDL apoB to IDL apoB conversion pathway. The mean transport rate of VLDL apoB to LDL apoB via IDL (10.2 +/- 0.9 vs. 8.0 +/- 0.8 mg/kg ffm per day, P less than 0.001) decreased. Mean LDL apoB concentrations decreased (70 +/- 5 vs. 61 +/- 5 mg/dl, P less than 0.001) on the HICHO diet. Means for total LDL apoB transport rate, LDL apoB FCR, and LDLC/apoB ratios were unchanged. In summary, the HICHO diet decreased the activity of mechanisms that convert VLDL to LDL, which contributed to the decrease in LDLC in all subjects. There was also evidence in some subjects for increased activity of LDL apoB clearance mechanisms, and a decrease in the LDLC to apoB ratio.

Dietary polyunsaturated fats of the W-6 and W-3 series reduce postprandial lipoprotein levels. Chronic and acute effects of fat saturation on postprandial lipoprotein metabolism

The chronic and acute effects of different types of dietary fat on postprandial lipoprotein metabolism were studied in eight normolipidemic subjects. Each person was placed for 25 d on each of three isocaloric diets: a saturated fat (SFA), a w-6 polyunsaturated fat (w-6 PUFA) and a w-3 polyunsaturated fat (w-3 PUFA) diet. Two vitamin A-fat loading tests were done on each diet. The concentrations in total plasma and chylomicron (Sf greater than 1,000) and nonchylomicron (Sf less than 1,000) fractions of retinyl palmitate (RP) were measured for 12 h postprandially. Compared with the SFA diet, the w-6 PUFA diet reduced chylomicron and nonchylomicron RP levels 56 and 38%, respectively, and the w-3 PUFA diet reduced these levels 67 and 53%, respectively. On further analysis, the main determinant of postprandial lipoprotein levels was the type of fat that was chronically fed, which appeared to mediate its effect by changing the concentration of the endogenous competitor for the system that catabolizes triglyeride-rich lipoproteins. However, there was a significant effect of the acute dietary fat load, which appeared to be due to a differential susceptibility to lipolysis of chylomicrons produced by SFA as opposed to PUFA fat loads. The levels of postprandial lipoproteins are determined by the interaction of these chronic and acute effects.

Interaction of dietary cholesterol and triglycerides in the regulation of hepatic low density lipoprotein transport in the hamster

These studies report the effects of dietary cholesterol and triglyceride on rates of receptor-dependent and receptor-independent LDL transport in the liver of the hamster. In animals fed diets enriched with 0.1, 0.25, or 1% cholesterol for 1 mo, receptor-dependent LDL transport in the liver was suppressed by 43, 63, and 77%, respectively, and there were reciprocal changes in plasma LDL-cholesterol concentrations. In addition, dietary triglycerides modified the effect of dietary cholesterol on hepatic LDL transport and plasma LDL concentrations so that at each level of cholesterol intake, polyunsaturated triglycerides diminished and saturated triglycerides accentuated the effect of dietary cholesterol. When animals were raised from weaning on diets containing small amounts of cholesterol, the decline in receptor-dependent LDL transport was nearly abolished by the addition of polyunsaturated or monounsaturated triglycerides, but was markedly augmented by the addition of saturated lipids. When animals raised on diets containing cholesterol and saturated triglycerides were returned to the low cholesterol, low triglyceride control diet, hepatic receptor-dependent LDL transport and plasma LDL-cholesterol concentrations returned essentially to normal within 2 wk. Neither receptor-independent LDL transport nor the receptor-dependent uptake of asialofetuin was significantly altered by dietary cholesterol or triglyceride suggesting that the effect of these lipids on hepatic LDL receptor activity was specific and not due to a generalized alteration in the physiochemical properties of hepatic membranes. These studies demonstrate the important role of saturated triglycerides in augmenting the effect of cholesterol in suppressing hepatic LDL receptor activity and elevating LDL-cholesterol levels.

Heterogeneity of cholesterol homeostasis in man. Response to changes in dietary fat quality and cholesterol quantity

Studies were carried out to examine the effects of dietary fat and cholesterol on cholesterol homeostasis in man. 75 12-wk studies were carried out during intake of 35% of calories as either saturated or polyunsaturated fat, first low and then high in dietary cholesterol. Dietary fat and cholesterol intakes, plasma lipid and lipoprotein levels, cholesterol absorption and sterol synthesis in isolated blood mononuclear leukocytes were measured during each diet period. In 69% of the studies the subjects compensated for the increased cholesterol intake by decreasing cholesterol fractional absorption and/or endogenous cholesterol synthesis. When an increase in plasma cholesterol levels was observed there was a failure to suppress endogenous cholesterol synthesis. Plasma cholesterol levels were more sensitive to dietary fat quality than to cholesterol quantity. The results demonstrate that the responses to dietary cholesterol and fat are highly individualized and that most individuals have effective feedback control mechanisms.

Effect of dietary fat saturation on plasma lipoproteins and high density lipoprotein metabolism of the rhesus monkey

Rhesus monkeys were fed corn or coconut oil-based diets for 3-6 mo to determine effects on the composition of all lipoprotein classes and on the metabolism of high density lipoproteins (HDL). Major findings included the following. Coconut oil feeding increased concentrations of all classes of plasma lipoproteins without altering lipoprotein size, suggesting an increase in particle number. The percentage of saturated fatty acids in the cholesteryl esters (CE) of low density lipoproteins (LDL) and HDL reached 40% with coconut oil feeding. This value probably constitutes a minimum estimate of the CE which were of intracellular rather than intraplasmic origin. The CE in LDL and HDL were nearly identical suggesting virtually complete equilibration by the core lipid transfer reaction. The CE in very low density lipoproteins, in contrast, were significantly more saturated than those in LDL and HDL irrespective of diet. Lower HDL levels on the corn oil diet were associated with higher fractional catabolic rates for both apolipoprotein A-I (0.42 vs. 0.31 d-1) and apolipoprotein A-II (0.45 vs. 0.30 d-1).

Regulation of the production and catabolism of plasma low density lipoproteins in hypertriglyceridemic subjects. Effect of weight loss

In subjects with hypertriglyceridemia, plasma concentrations of low density lipoprotein (LDL) cholesterol are often normal or reduced. Perturbations that alter plasma very low density lipoprotein (VLDL) concentrations are associated with opposite changes in plasma LDL levels. To determine the mechanisms regulating plasma LDL levels, we used 131I-VLDL and 125I-LDL to measure the fractional catabolic rates (FCR), production rates (PR), and rates of interconversion of apoprotein B (apo B) in VLDL, intermediate density lipoprotein, and LDL in six hypertriglyceridemic subjects pre- and post-weight reduction. [2-3H]glycerol was used to quantitate VLDL triglyceride PR. All data are presented as mean +/- SD. Percent ideal body weight fell from 132 +/- 17.9 to 119 +/- 15.9% in the group, P less than 0.05. After weight loss, plasma VLDL triglyceride (486.0 +/- 364.1 vs. 191.3 +/- 65.4 mg/dl, P less than 0.05) and VLDL apo B (32.2 +/- 12.0 vs. 14.8 +/- 6.8 mg/dl, P less than 0.05) concentrations were reduced. VLDL triglyceride PR also fell after weight reduction (56.6 +/- 39.0 vs. 28.6 +/- 23.1 mg/kg per h, P less than 0.05), as did VLDL apo B PR (47.9 +/- 41.4 vs. 19.0 +/- 14.1 mg/kg per d, P less than 0.05). Pre-weight loss, plasma LDL cholesterol and apo B levels were low-normal or reduced (64.0 +/- 12.6 and 58.4 +/- 11.9 mg/dl, respectively) despite normal or elevated LDL apo B PR (17.4 +/- 7.2 mg/kg per d). The reduced cholesterol and apo B levels were associated with increased FCRs (0.68 +/- 0.29 d-1) and reduced cholesterol/protein ratios (1.01 +/- 0.18) in LDL. The plasma levels of LDL cholesterol and apo B rose after weight reduction (84.8 +/- 24.9, P less than 0.05; and 69.5 +/- 14.3 mg/dl, P less than 0.05, respectively, vs. base line). These increased concentrations resulted from a combination of events. First, the FCR for LDL apo B fell in five of six subjects with a significant reduction for the group as a whole (0.48 +/- 0.11 d-1, P less than 0.05 vs. base line). Second, the cholesterol/protein ratio increased in all six subjects with a significantly greater mean after weight loss (1.25 +/- 0.27, P less than 0.05 vs. base line). In contrast, the LDL apo B PR fell or was essentially unchanged in the six subjects after weight loss (mean, 14.4 +/- 2.8 mg/kg per d; NS vs. pre-weight loss). The changes in LDL catabolism and composition were associated with changes in the source of LDL apo B. Pre-weight loss, 73.3% of LDL was derived from VLDL, while 26.7% was directly secreted into plasma. Post-weight reduction, VLDL-derived LDL fell to 46.8% of total, while direct secretion accounted for 53.2% of LDL production. These changes were significant; P < 0.95. Thus, all subjects had direct secretion of LDL apo B and the magnitude of this source of VLDL triglyceride secretion. These results indicate that the regulation of plasma LDL levels in hypertriglyceridemic subjects is quite complex and that the rise in LDL levels after weight loss results from reduction in the fractional catabolism of this lipoprotein. The fall in the FCR is associated with changes in the source of LDL and in its composition.

Receptor-mediated catabolism of low density lipoprotein in man. Quantitation using glucosylated low density lipoprotein

Low density lipoprotein (LDL) catabolism occurs by LDL receptor-dependent and LDL receptor-independent pathways. We have shown previously that nonenzymatic glucosylation of LDL in the presence of cyanoborohydride irreversibly blocks the lysine residues of LDL. Glucosylated LDL (GLC-LDL) was not degraded by the LDL receptor of fibroblasts, and its degradation by macrophages was similar to that of native LDL. This suggested that GLC-LDL should be a good tracer of LDL receptor-independent catabolism, and if combined with a tracer of total LDL catabolism, should enable one to calculate the extent of LDL receptor-dependent catabolism. To determine the contribution of each pathway in man, we prepared (125)I-GLC-LDL and (131)I-control LDL and simultaneously determined the fractional catabolic rate (FCR) of each tracer in four subjects. In preliminary experiments, we showed that the conditions for glucosylation did not affect LDL turnover. In the four subjects, the FCR for total LDL catabolism ranged from 0.345 to 0.724 d(-1) with a mean of 0.57+/-0.16 d(-1). The FCR of GLC-LDL varied from 0.071 to 0.141 d(-1) with a mean of 0.11+/-0.03 d(-1). The latter is similar to the FCR reported for native LDL in subjects with homozygous familial hypercholesterolemia, supporting the interpretation that GLC-LDL traces only the receptor-independent pathway. Despite the wide range of total LDL catabolism in these subjects. LDL receptor-independent catabolism accounted for only 19.5-20.6% of total catabolism. In turn, LDL receptor-dependent catabolism accounted for 80% of total clearance in each person. Furthermore, while the decay curve of LDL showed the usual biphasic pattern, the decay curve of GLC-LDL was monoexponential in each subject even when followed for as long as 48 d. This suggests that LDL receptor activity is responsible for the biphasic nature of LDL decay. These studies emphasize the central role of LDL receptor activity in normal LDL metabolism in man.