Carbohydrate: friend or foe? Summary of research needs

This symposium evaluated the current state of science relative to the role of carbohydrates in human health and identified priority research topics to address gaps in our knowledge about carbohydrates and health. Future revisions of dietary guidelines will benefit from an expanded research agenda leading to a better understanding of the benefits and risks of consuming diets high in carbohydrates.

Beans, as a source of dietary fiber, increase cholecystokinin and apolipoprotein b48 response to test meals in men

Dry beans lower plasma cholesterol, an effect that has been associated with both the fiber and the protein content of beans. The objective of this study was to determine the acute hormone and lipid responses to a test meal that contained dry beans as a source of dietary fiber. A crossover design was employed in which men consumed the test meal and a control meal in random order. Both meals contained egg, bread, jelly, orange juice, milk and margarine. The high fiber meal contained white beans, whereas the low fiber (control) meal contained rice and dry milk. The men maintained their normal dietary pattern and fasted overnight before the study days. After a fasting blood sample was drawn, the men consumed the test meal and blood samples were collected over the next 6 h. Blood samples were analyzed for cholecystokinin (CCK), insulin and glucose. Plasma was separated into lipoprotein fractions and the triglyceride, cholesterol, apolipoprotein (apo) B100 and B48 content of triglyceride-rich lipoproteins determined. Insulin and glucose responses did not differ significantly between test meals; however, the CCK response was twice as high after the bean-containing meal than after the low fiber meal (P = 0.03). The increase in apo B48 concentration was significantly higher after the bean meal than after the low fiber meal (P < 0.05). Adding beans to a meal to increase fiber content prolongs the postprandial presence of intestinally derived lipoproteins and augments the CCK response to the meal.

Predictors of plasma triglyceride elevation in patients participating in a coronary atherosclerosis treatment program

PURPOSE

Low-fat, high-carbohydrate diets have been used successfully to prevent and treat coronary heart disease, although these diets have been shown to cause elevations in fasting plasma triglyceride concentrations. The present study investigated metabolic factors (glucose, insulin, body weight) associated with changes in plasma triglyceride concentrations in patients participating in a comprehensive, multidisciplinary program, which included the use of a very low-fat diet designed to regress atherosclerotic cardiovascular disease.

METHODS

Thirty-six patients were entered into the study and placed on a 10% fat diet. Body mass index and fasting plasma insulin, glucose, lipids, and apolipoproteins were assessed at entrance into and after 3 months of participation in the program. Statistical analysis (discriminant function analysis) was used to identify factors that predicted elevations in plasma triglyceride that occurred during therapy.

RESULTS

For the entire group, significant reductions in body weight (-2.4%), fasting glucose (-6%), total cholesterol (-8%), and low-density lipoprotein cholesterol (-11%) were observed, while insulin and triglycerides showed no significant changes. Twenty-one of the patients experienced an increase in fasting triglyceride concentration of 10% or greater.

CONCLUSIONS

Three variables (baseline body mass index and fasting triglyceride and insulin concentrations) accurately classified 90% of those who would experience a > or = 10% elevation in triglycerides (P = 0.0002) and 67% of those who experienced no change. The present analysis provides a practical algorithm for clinicians to predict which patients will experience significant elevations in plasma triglyceride concentration when undergoing risk factor reduction that includes the consumption of a very low-fat, high-carbohydrate diet.

Fiber, inulin and oligofructose: similarities and differences

The biological, chemical and physical properties of dietary fibers are associated with physiologic actions in the small and large intestine that have important metabolic implications for health. These properties of fiber include dispersibility in water, bulk, viscosity, adsorption and binding of compounds and fermentability. Dietary fructans share some of the properties of dietary fiber and thus are likely to have similar metabolic effects. Within the small intestine, properties such as dispersibility in water, bulking and viscosity are associated with slowing the digestion and absorption of carbohydrate and lipid and promoting nutrient absorption along a greater length of the small intestine. Both of these actions are related to cholesterol reduction and blunting of alimentary gylcemia. Although fructans are dispersible in water and will provide some bulk because they are nondigestible in the small intestine, they do not appear to be associated with significant increases in viscosity. Thus one would predict that any immediate effects on alimentary glycemia or on cholesterol reduction are likely to be modest compared with more viscous polysaccharides. Fermentability and bulking capacity of nondigestible carbohydrates define an essential role of fiber in maintaining gastrointestinal health. Within the large intestine, carbohydrates that are not digested in the small intestine are available for fermentation by the microflora present. Carbohydrates that are dispersible in the aqueous phase are more readily digested by microbes. A large body of evidence indicates that dietary fructans are digested in the large intestine, resulting in an increase in microbial mass and production of short-chain fatty acids.

(n-3) fatty acid supplementation in moderately hypertriglyceridemic adults changes postprandial lipid and apolipoprotein B responses to a standardized test meal

The effects of (n-3) fatty acids on the postprandial state were investigated by monitoring the alimentary responses to identical test meals fed to adults [n = 11; fasting triacylglycerol (TG) 2.55 +/- 0.24 mmol/L; mean +/- SEM] after a self-selected diet baseline period (BLP) and then after a 6-wk (n-3) fatty acid period (FOP) [ approximately 5.2 g (n-3) fatty acids] and a 6-wk control oil period (COP) administered in random order. Samples were drawn immediately prior to the test meal (time 0) and then hourly from 2 to 6 h postmeal. Postprandial plasma triacylglycerol (TG) and TG-rich lipoprotein (TRL) TG apo B48, and B100 absolute concentrations were significantly lower after FOP than after COP or BLP, while plasma cholesterol was unchanged. Normalizing the results as increments over time 0 eliminated the diet effect on all but plasma TG. Time remained a significant effect for plasma TG, TRL TG, and TRL TC. Finally, only absolute TRL B48 and absolute and incremental plasma TG concentrations displayed significant time-diet interactions. These results suggest that postprandial TRL apo B reductions are likely caused by (n-3) fatty acid suppression of both hepatic and intestinal apoB secretion/synthesis. Altered TRL metabolism, i.e. changes in postprandial TG, cholesterol, apo B48, and increase in LDL particle size, may represent an additional mechanism for the reduced heart disease risk associated with fish [(n-3) fatty acid] consumption.

Cholecystokinin and serotonin receptors in the regulation of fat-induced satiety in rats

The present study investigated the relationship between endogenous CCK and serotonin (5-HT) in fat-induced satiety. Male Wistar rats with duodenal cannulas were adapted to eating 6 h/day along with receiving an infusion of saline or one of two isocaloric solutions (10 ml, 1 kcal/ml, 0.45 ml/min) varying in fat and carbohydrate content (20 or 80% energy from fat). Rats were infused 10 min after food presentation. The satiation/satiety response was determined from measures of meal size (MS), intermeal interval (IMI), and total food intake (TFI). Infusion with either fat solution reduced MS compared with saline; however, the 80% fat infusate reduced TFI and lengthened the IMI compared with saline and the 20% fat infusate. CCK and 5-HT involvement in fat-induced satiety was investigated by preceding the 80% fat infusate with CCK and/or 5-HT3 receptor antagonists Devazepide (Dev) and Tropisetron (Trop). A CCK releaser, trypsin inhibitor (TI), was added to the 20% fat infusate to enhance satiety. Pretreatment with Dev or Trop alone attenuated the inhibitory effects of the 80% solution on IMI, whereas reversal of the inhibitory effects on MS and TFI were sensitive only to Dev at the doses provided. Both antagonists together completely blocked the satiating effects of the 80% fat infusate on all feeding variables measured. Addition of TI to the 20% fat infusate lengthened the IMI but did not affect MS or TFI. These results provide evidence for the participation of both endogenous CCK and 5-HT in the satiety response to fat in the intestine.

Postprandial lipid, glucose, insulin, and cholecystokinin responses in men fed barley pasta enriched with beta-glucan

BACKGROUND

Fiber regulates the rate and site of lipid and carbohydrate digestion and absorption and thus can modify the alimentary responses to a meal. When fiber sources containing viscous polysaccharides are included in a meal, a slower rate of carbohydrate and lipid absorption will modify the alimentary hormone and lipid responses.

OBJECTIVE

We investigated in 11 healthy men the response of insulin, glucose, cholecystokinin, and lipid to 2 test meals containing beta-glucan.

DESIGN

One of the meals was high in fiber (15.7 g) and the other meal was low in fiber (5.0 g). The low-fiber meal contained pasta made with wheat flour. The high-fiber meals contained pasta prepared by replacing 40% of the wheat with 2 types of barley flour: barley naturally high in beta-glucan and the other a flour enriched in beta-glucan during processing.

RESULTS

Plasma glucose and insulin concentrations increased significantly after all meals but the insulin response was more blunted after the barley-containing meals. The test meals were low in fat (25% of energy) but elicited an increase in plasma triacylglycerol and cholecystokinin. Cholecystokinin remained elevated for a longer time after the barley-containing meals. After the low-fiber meal, plasma cholesterol concentrations did not change significantly; however, 4 h after the barley-containing meals, the cholesterol concentration dropped below the fasting concentration and was significantly lower than that after the low-fiber meal.

CONCLUSIONS

Carbohydrate was more slowly absorbed from the 2 high-fiber meals. Consumption of the barley-containing meals appeared to stimulate reverse cholesterol transport, which may contribute to the cholesterol-lowering ability of barley.

Reduced oxidative susceptibility of LDL from patients participating in an intensive atherosclerosis treatment program

The goal of this investigation was to determine whether participation in an atherosclerosis treatment program would reduce the oxidative susceptibility of LDL from patients with coronary artery disease. The treatment program included intensive exercise therapy, stress management, and consumption of a diet containing 10% fat. The size and antioxidant and lipid contents of LDL particles from 25 patients were analyzed at baseline and after 3 mo of therapy. The susceptibility of LDL to copper-mediated oxidation was measured by a conjugated diene assay and headspace gas chromatography (HSGC). Atherosclerosis treatment significantly reduced plasma total cholesterol and apolipoprotein B concentrations and the molar ratio of LDL cholesterol ester to apolipoprotein B (P < 0.01). The LDL content of alpha-tocopherol and beta-carotene was increased (27% and 17%, respectively, P < 0.04) and the molar ratio of LDL cholesterol ester the sum of LDL alpha-tocopherol and LDL beta-carotene decreased from 159 at baseline to 122 at 3 mo (P < 0.01). The lag phase of LDL conjugated diene formation increased 24%, whereas the maximum rate of oxidation slowed 29% (P < 0.01). As assessed by HSGC, copper-catalyzed formation of volatile lipid oxidation products was reduced 15% (P < 0.007); the reduction in volatiles was correlated with an increase in the alpha-tocopherol content of LDL (r=-0.48, P < 0.01). The principal determinants of reduced LDL oxidative susceptibility were the particle contents of alpha-tocopherol and beta-carotene. To our knowledge, this is the first report to document a reduction in LDL oxidation in coronary artery disease patients undergoing atherosclerosis-reversal therapy.

Alimentary lipemia is enhanced in fiber-fed rats

The effect of dietary fiber on the pattern of postprandial lipemia was examined in two studies with male Wistar rats. In the first study, groups of rats were killed after food deprivation (0 h) or 1, 4.5 or 8.5 h after a high fat test meal containing either cellulose (CL) or oat bran (OB). Plasma triglycerides (TG) were higher in the OB group at 4.5 h compared with both the 0-h and the CL-groups at 4. 5 h. In both groups, LDL and TG-rich lipoprotein cholesterol (TRL-C) concentrations were higher at 8.5 h than at 0 h; HDL cholesterol was significantly lower at 8.5 h than at 0 h for the OB group only. The enhanced lipemia when OB was fed may stimulate cholesterol movement from HDL to LDL and TRL. To examine whether TRL secretion rates were responsible for the enhanced lipemia, a second study was conducted. Rats were fitted with jugular catheters and allowed to recover. Two groups were fed either CL or OB and infused with Triton-1339 (400 mg/kg). Two control groups were not fed and were infused with either Triton or saline. Rats were killed 2.5 h after infusion. Plasma TG was 10-fold higher in the Triton group than in the saline group, but did not differ between the OB and CL groups. The relative contribution of TRL-C to total cholesterol was significantly greater in the Triton control than in the OB and CL groups. Enhanced secretion of TRL was not responsible for the lipemia observed in the first study. Rather, alterations in clearance rate were responsible.

Meal pattern analysis to investigate the satiating potential of fat, carbohydrate, and protein in rats

We examined meal patterns after isocaloric duodenal infusions of fat, carbohydrate (CHO), and protein by measuring meal size, intermeal interval (IMI) and total food intake (TFI). Wistar rats were adapted to normal feeding 6 h/day, with continuous computer monitoring of feeding patterns. One of five solutions (10 ml of 1 kcal/ml at 0.45 ml/min; 0, 20, 50, 80, or 100% of energy from fat) or saline (control) was infused 10 min after initiation of eating. Separate rats received casein or casein hydrolysate at 18.5 or 37% energy. Equivalent energy loads varying in fat, CHO, and protein content compared with saline resulted in similar reductions in first meal intakes. The second meal did not differ among fat and CHO treatments including saline; however, infusion with a protein-containing solution increased the size of meal 2. The IMI was doubled by protein infusion independently of dose or source but extended dose dependently by fat. TFI was lower after high fat and higher after protein than after saline infusion. The results indicate that the concentrations of fat, CHO, and protein differentially affect the qualitative and quantitative aspects of feeding in rats.

Lipid infused into the duodenum of rats at varied rates influences food intake and body weight gain

We investigated the influence of luminal fat, including intestinal exposure time to fat, on food intake and body weight (BW) gain under conditions that resemble normal feeding. Male Wistar rats were divided into three groups: two groups received 30 mL Intralipid and one group received 30 mL saline, in addition to having access to a nonfat diet. The saline-infused and one of the fat-infused groups were schedule fed (SF), i.e., allowed to eat for 7 h of the dark cycle; the other fat-infused group had diet available for 24 h. The three groups of rats were subdivided further by rate of infusion, i.e., 0.13 mL/min or 1.0 mL/min for a total of 6 groups. Rats infused with fat had lower BW gain than saline controls, except when diet was available for 24 h and rate of fat infusion was 1.0 mL/min (BW gain was similar to saline group). Daily energy intake corresponded to BW gain data. Pancreata and intestinal mucosa were examined (after 2 wk of treatment) for hypertrophic changes that may be related to specific factors stimulated by luminal fat; these may also be involved in the control of food intake. Rats infused with fat had heavier pancreata than saline-infused rats, and infusion of fat at 0.13 mL/min resulted in heavier pancreata than infusion at the faster rate. DNA and protein analysis indicated that hypertrophy rather than hyperplasia was responsible for this effect. Thus, prolonged fat exposure to intraluminal receptors resulted in reduced total daily energy intake, suppressed BW gain and heavier pancreata. The combined data support a connection among intraluminal fat, negative feedback signals that inhibit food intake and factor(s) associated with pancreatic stimulation.

Rat plasma triglycerides and hepatic fatty acid synthetase mRNA, but not apolipoprotein B and A-IV mRNA, respond to dietary fat content

The objective was to determine whether apolipoprotein B and A-IV mRNA abundance or plasma lipid concentrations would be altered by chronic or acute consumption of diets that differed in fat content. Forty Wistar male rats were fed either a low fat (5 g/ 100 g) or high fat (20 g/100 g) diet for 4 wk. Animals were killed unfed or 3 h after consumption of a test meal of the diet to which they had been adapted (n = 8). In addition, a low fat diet-adapted group was fed a high fat test meal and killed 3 h after the meal. Adaptation to the high or low fat diets did not result in differences in triglyceride or cholesterol concentrations in the plasma of unfed rats. In fed animals, plasma, VLDL, and LDL triglyceride concentrations were significantly higher in those fed the high fat test meal than in those fed the low fat test meal. Feeding did not alter plasma cholesterol concentrations; however, LDL cholesterol concentrations in the groups fed the high fat meals were significantly higher than in the group fed the low fat meal. There were no differences in plasma apolipoproteins B, A-IV, E, and A-I nor in the liver or intestinal apolipoprotein B and A-IV mRNA contents. Fatty acid synthetase (FAS) activity was significantly higher in rats adapted to the low fat diet, and no increase in activity due to feeding was observed. Hepatic FAS mRNA was higher in fed than unfed rats, and the low fat test meal resulted in a higher level than the high fat test meal. Plasma lipid concentrations were affected by the fat content of test meals rather than by the adaptation diet fat content. Apolipoprotein B and A-IV mRNA do not seem to respond to dietary fat or meal feeding.

Use of the Food Guide Pyramid and US Dietary Guidelines to improve dietary intake and reduce cardiovascular risk in active-duty Air Force members

OBJECTIVE

To determine whether adoption of dietary patterns consistent with the US Dietary Guidelines for Americans and the Food Guide Pyramid, combined with exercise training, result in significant reductions in cardiovascular risk compared with a regimen of exercise therapy alone.

DESIGN

A randomized trial to compare the effects of exercise alone (n = 17) with the effects of exercise and dietary intervention (n = 15).

SETTING

McClellan Air Force Base medical clinic (Sacramento, Calif).

SUBJECTS

Thirty-two members of the Air Force (20 men and 12 women) were recruited at the time they entered a 90-day fitness improvement program. Mean age was 32 years.

INTERVENTION

All subjects participated in a 90-day fitness program. Half of the group received individualized dietary counseling using the Food Guide Pyramid as a primary educational tool.

MAIN OUTCOME MEASURES

Changes in body mass index, plasma lipids and lipoprotein levels, aerobic capacity, and dietary intake were selected to evaluate the effectiveness of the intervention.

STATISTICAL ANALYSES PERFORMED

Outcome measures were evaluated by analysis of variance. A paired t test was performed to compare changes in food-group servings and food-group fat intake from baseline values for the exercise-plus-diet group.

RESULTS

Percentage of energy from fat decreased from 39% to 23% for the exercise-plus-diet group, and servings from each of the food groups changed to reflect current guidelines. This group also had significant reductions in body mass index, total cholesterol level, and low-density lipoprotein level: 2% (P = .0001), 9% (P = .003), and 13% (P = .005), respectively. No change was observed for the exercise-only group. Additionally, a significant improvement in maximum oxygen consumption (P = .01) of 38% (vs 14% for the control group) was achieved.

CONCLUSIONS

Dietary modification in accordance with the Food Guide Pyramid and the US Dietary Guidelines results in significant reductions in known cardiovascular risk factors and improves the response to exercise training.

Dietary fiber

Most of our understanding of the physiologic effects of dietary fiber are derived from studies in adults. These investigations have indicated the potential problems with fiber consumption are most likely to occur if isolated polysaccharides or nonpurified fiber supplements are consumed excessively. Such problems include intestinal obstruction and significant reductions in nutrient availability. Inclusion of foods that contain dietary fiber such as fruits, vegetables, beans, and whole-grain products is unlikely to result in complications for most individuals. Young children may need to consume smaller servings of these foods than adults; however, their inclusion in the diet is an important part of meeting dietary recommendation for fiber intake. Excess consumption of fiber-supplemented foods (e.g., bran cereals) should be monitored to avoid appetite suppression that could limit selection of foods to maintain a balanced diet.

Nutrient infusion into the ileum of rats does not lower plasma lipids or alter apolipoprotein mRNA abundance

Viscous polysaccharides enhance the delivery of nutrients to the ileum. Our objective was to determine if the presence of nutrients in the ileum, in the absence of fiber, alters plasma and hepatic lipids. Twenty-four Wistar male rats weighing 150-160 g were fed a fiber-free purified diet (30% of energy as fat) for 7 d. The small intestine was cannulated 30 cm proximal to the ileo-cecal junction. The animals consumed the purified diet ad libitum and the intestine was infused (3 mL/h) with saline, or an enteral solution contributing 10, 20 or 30% total energy for half of a 12-h feeding period. Infusion was performed for 14 d, after which the animals were killed. The animals adjusted their food intake so that each group had an equivalent energy intake and weight gain over the experimental period. The relative ileal mucosa weight was increased with nutrient infusion indicating hypertrophy. Plasma cholesterol in the nutrient-infused animals tended to be higher than in the saline-infused animals (P < 0.06). Apolipoprotein B and A-IV mRNA were not affected by ileal infusion. Thus, enhanced delivery of nutrients to the ileum, without fiber present in the diet, does not lower plasma cholesterol.

Effects of a carotene-deficient diet on measures of oxidative susceptibility and superoxide dismutase activity in adult women

The effect of consuming a low carotene diet (approximately 60 micrograms carotene/day) on oxidative susceptibility and superoxide dismutase (SOD) activity in women living in a metabolic research unit was evaluated. The diet had sufficient vitamins A, E, and C. The women ate the diet supplemented with 1500 micrograms/day beta-carotene for 4 days (baseline), then the unsupplemented diet for 68 days (depletion), followed by the diet supplemented with > 15,000 micrograms/day carotene for 28 days (repletion). Production of hexanal, pentanal, and pentane by copper-oxidized plasma low density lipoproteins from carotene-depleted women was greater than their production of these compounds when repleted with carotene. Erythrocyte SOD activity was depressed in carotene-depleted women; it recovered with repletion. Thiobarbituric acid reactive substances in plasma of carotene-depleted women were elevated and diminished with repletion. Dietary carotene seems to be needed, not only as a precursor of vitamin A, but also to inhibit oxidative damage and decrease oxidation susceptibility.

Carbohydrates: significance for energy balance and gastrointestinal function

The 10th edition of the RDA discusses carbohydrates in the context of dietary fiber and digestible carbohydrates, which provide energy, and recommends that more than half the energy requirement beyond infancy be provided by carbohydrates and that a desirable intake of fiber be achieved by consumption of fruits, vegetables, legumes, and whole-grain cereals. The recommendations on digestible carbohydrates are primarily based on the desirability of limiting the intake of fat. An additional consideration is the suggestion that excess calories from carbohydrate vs. fat are metabolized and stored with different efficiencies. The scientific basis for recommending an appropriate intake of dietary fiber is not clearly evident in the current RDA. The association between a high plant food, and thus fiber intake, and lower incidence of cardiovascular disease, colon cancer, and diabetes is of interest but difficult to use to form the basis of specific recommendations for an appropriate amount of fiber to include in the diet. The importance of fiber for normal gastrointestinal function provides the best scientific basis for developing recommended intake levels. Dietary fiber, as well as non-digested starch, are the only known dietary components that increase stool weight and are the primary substrates for the microflora in the large intestine. Metabolism of this microflora, including production of short chain fatty acids that are used by epithelial cells, appears to be important for intestinal health. However, the challenge we still face is to define the parameters of gastrointestinal function that are compatible with health.

A food-grade silicon dioxide is hypocholesterolemic in the diet of cholesterol-fed rats

Silicon dioxide, commonly referred to as silica, is present in plant cell walls and interstitial spaces, and is often found as a component of dietary fibers that have exhibited hypocholesterolemic activity in animals. The primary objective of this study was to determine the cholesterolemic effects of two different morphological forms of silicon in the diet of cholesterol-fed rats. Male Wistar rats were provided diets containing 1 g cholesterol/100 g diet, and 0.65 g Si/100 g diet as either a sodium salt (silicate group) or silicon dioxide, a synthetic silica polymer (silica group). Cellulose was used as a control (control group). The in vitro bile acid binding capacity of the SiO2 was also measured. After 44 d of diet administration, animals were deprived of food for 24 h and then killed. Plasma total, VLDL, and LDL cholesterol concentrations were 18%, 29%, and 26% lower, respectively, in the silica group than in the control group. However, liver cholesterol concentrations were not different among dietary treatments. During the initial 15 d of the study, average daily total fecal bile acids were 38% higher in the silica group than in the control group, but fecal bile acid outputs were not different for the remainder of the experiment. The silica polymer used in the feeding trial was found to adsorb 5 times more cholate than chenodeoxycholate, at pH 7.5 in vitro. In vivo, the potential for silica to enhance fecal cholic acid excretion, relative to chenodeoxycholic acid during the initial stage of the study, may have contributed to the hypocholesterolemic response to the silica diet.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of n-3 fatty acid-rich fish oil supplementation on the oxidation of low density lipoproteins

This study was aimed at determining the effect of fish oil supplementation on copper-catalyzed oxidation of low density lipoproteins (LDL) from nine hypertriglyceridemic human subjects. A rapid headspace gas chromatographic method was used to measure the volatile oxidation products from LDL. Propanal and hexanal were the major volatile products formed in the oxidation of n-3 and n-6 polyunsaturated fatty acids (PUFA), respectively. Fish oil supplementation resulted in a significant increase in propanal formation from 3.7 to 13.4 nmol/mL LDL (P < 0.01); it also resulted in small decreases in pentanal formation from 14.7 to 11.4 nmol/mL LDL and in hexanal formation from 138 to 108 nmol/mL LDL (P < 0.05). The changes in peroxidation products paralleled the changes in LDL composition, which showed a significant increase in n-3 PUFA from 3.2 to 14.6% (P < 0.01) and a decrease in n-6 PUFA from 43.7 to 35.0% (P < 0.05). Propanal formation was highly and significantly correlated with n-3 PUFA content (r = 0.950, P < 0.001). Since total volatiles remained unchanged, this indicated that the two groups of LDL samples did not differ in overall oxidative susceptibility. Although fish oil intake did not alter the oxidative susceptibility of LDL, the chemically modified LDL particles generated a distinct pattern of volatile oxidation products that reflected changes in their fatty acid composition.

Prune fiber or pectin compared with cellulose lowers plasma and liver lipids in rats with diet-induced hyperlipidemia

Previous research indicated that prunes in the diet of men with hypercholesterolemia lowered plasma and LDL cholesterol concentrations. To further study lipid metabolism in response to ingesting prunes, we conducted an animal study to test the hypotheses that fiber extracted from prunes, compared with purified cellulose, lowers plasma and liver cholesterol in rats with diet-induced hyperlipidemia and that the response is dose dependent. Rats were randomly assigned to one of five experimental diet groups. Four of the diets contained cholesterol and cholic acid to induce hyperlipidemia. The fiber source in the hyperlipidemic diets was 6% cellulose, 3% prune fiber, 6% prune fiber or 3% pectin. The fifth group, the nonhyperlipidemic control, was fed a diet containing 6% cellulose without cholesterol or cholic acid. Rats consumed one of the five diets ad libitum for 28 d, then were killed after 16 h without food. Plasma, LDL and liver cholesterol concentrations were higher in the hyperlipidemic control than the nonhyperlipidemic control and lower in the groups fed diets containing pectin or prune fiber than in the hyperlipidemic control group. No differences in plasma or liver cholesterol concentrations were detected between groups fed either dose level of prune fiber or between groups fed 6% prune fiber and pectin. These results indicate that fiber extracted from prunes lowers plasma and liver cholesterol in hyperlipidemic rats, although a dose-dependent response was not detected.

Changes in plasma and hepatic lipids, small intestinal histology and pancreatic enzyme activity due to aging and dietary fiber in rats

Rats were fed either a control diet or a control diet supplemented with wheat bran, psyllium husk or oat bran to increase intake of fiber. Groups of rats were killed after 3.5, 10, 15, or 18.5 mo of consuming the diets. Plasma cholesterol and triglyceride concentrations were significantly higher in 18.5-mo-old than younger animals. Fiber supplementation did not prevent the age-related increase in lipids. Cecal weight, including contents, was higher in the psyllium husk and oat bran groups than control, and smooth muscle thickness in the ileum of psyllium husk and oat bran animals was greater than control. The score for torn villi in the small intestine was lower than expected in the wheat bran group. Amylase activity in the pancreas declined significantly with age in all groups. In aging animals fiber supplementation may enhance ileal compensation for decreases in proximal intestinal function but does not prevent age-related changes in the gut or in lipid concentrations.

Relationships between the responses of triglyceride-rich lipoproteins in blood plasma containing apolipoproteins B-48 and B-100 to a fat-containing meal in normolipidemic humans

The concentration of triglyceride-rich lipoproteins containing apolipoprotein (apo) B-48 (chylomicrons) and apo B-100 (very low density lipoproteins) was measured in blood plasma of healthy young men after an ordinary meal containing one-third of daily energy and fat. Plasma obtained in the postabsorptive state and at intervals up to 12 hr after the meal was subjected to immunoaffinity chromatography against a monoclonal antibody to apo B-100 that does not bind apo B-48 and a minor fraction of apo B-100 rich in apo E. Measurements of the concentrations of components of the total and unbound triglyceride-rich lipoproteins separated from plasma by ultracentrifugation showed that about 80% of the increase in lipoprotein particle number was in very low density lipoproteins containing apo B-100 and only 20% was in chylomicrons containing apo B-48 that carry dietary fat from the intestine. The maximal increments and the average concentrations of apo B-48 and B-100 during the 12 hr were highly correlated (r2 = 0.80), suggesting that preferential clearance of chylomicron triglycerides by lipoprotein lipase leads to accumulation of hepatogenous very low density lipoproteins during the alimentary period. The composition of the bulk of very low density lipoproteins that were bound to the monoclonal antibody changed little and these particles contained about 90% of the cholesterol and most of the apo E that accumulated in triglyceride-rich lipoproteins. The predominant accumulation of very low density lipoprotein rather than chylomicron particles after ingestion of ordinary meals is relevant to the potential atherogenicity of postprandial lipoproteins.

Modification of triacylglycerides and apolipoprotein B in rats fed diets containing whole milk, skim milk and milk proteins

The objective of this study was to determine the effects of diets containing milk and milk protein fractions on plasma and hepatic lipids, apolipoprotein B mRNA abundance, and plasma apolipoprotein concentrations and lipoprotein composition. Male rats were fed for 6 wk diets that contained (wt/wt) 76% whole milk (WM diet), 55% skim milk (SMFF diet), 22% casein (CAS diet), 22% whey protein isolate (WHY diet) or 55% skim milk-low fat (SMLF diet). The fat concentration in the SMLF diet was 7%. Butter oil (20%) and corn oil (2%) were added to the SMFF, CAS and WHY diets. Plasma and VLDL triacylglycerides in the WM-fed rats were about half of the level in the groups fed the SMFF and SMLF diets, but not significantly different from those of the WHY-fed group. Hepatic triacylglycerides generally were lower in the WM-fed group than in the other groups. Plasma cholesterol concentration did not differ among groups. Plasma apolipoprotein B was significantly lower in the WM-fed group than in rats fed the SMLF, SMFF or WHY diets. However, apolipoprotein B mRNA abundance in the liver and small intestinal mucosa did not differ due to dietary treatment. Thus the lipemic response due to whole milk is not associated with milk protein fractions and may be due to the presence of fat globule membrane in the diet containing whole milk.

Postprandial lipid response following a high fat meal in rats adapted to dietary fiber

Rats were adapted to diets containing 5 g/100 g cellulose (CL), 5 g/100 g oat bran fiber (OB) or 5 g/100 g psyllium husk (Psy) for 4 wk. Following a 12-h fast, animals were either killed at 0 h (baseline) or fed 4.5 g of a test meal that provided 50% energy from fat, then killed at 1, 4 or 6 h postprandially. Fasting plasma and HDL cholesterol concentrations were lower in Psy-fed animals than in rats fed either CL or OB. Plasma triglycerides increased significantly from baseline (0 h) in all groups but did not differ among diet treatments. Increases in triglyceride content of the treatments. Increases in triglyceride content of the chylomicron/VLDL fraction occurred in the CL- and OB-fed groups and in the HDL fraction of the Psy-fed group during the postprandial period. In unfed animals the hepatic and intestinal levels of apolipoprotein A-IV mRNA were higher in the CL-fed group than in the groups fed OB and Psy. Apolipoprotein B mRNA was higher in the intestine of the OB-fed group than in the groups fed CL and Psy and had a significant gradient along the small intestine, increasing in the distal third. The results suggest that chronic consumption of fiber is less likely to modify the acute plams triglyceride response to a fat-containing test meal than if a fiber supplement is incorporated into the meal.

Consumption of prunes as a source of dietary fiber in men with mild hypercholesterolemia

Forty-one free-living adult men with mild hypercholesterolemia (5.2-7.5 mmol/L) voluntarily participated in an 8-wk crossover study designed to determine the effect of prunes as a source of fiber on plasma cholesterol and on fecal output and bile acid concentration. During the prune period, subjects supplemented their usual diets with 12 prunes (100 g; 6 g dietary fiber) daily. Plasma low-density-lipoprotein cholesterol was significantly lower after the prune period (3.9 mmol/L) than after the grape-juice-control period (4.1 mmol/L). Fecal bile acid concentration of lithocholic acid was significantly lower after the prune period (0.95 mg bile acid/g dry wt stool) than after the grape-juice-control period (1.20 mg bile acid/g dry wt stool). Both fecal wet and dry weights were approximately 20% higher after the prune period than after the grape-juice-control period. Total bile acids (mg/72 h) did not significantly differ between experimental periods.

Effects of dietary fibers on nonfasting plasma lipoprotein and apolipoprotein levels in rats

This study was undertaken to determine whether selected dietary fibers had an effect on plasma lipoproteins, apolipoproteins and enzymes involved in cholesterol metabolism in rats. Each experimental diet contained 8% dietary fiber by weight; all animals were killed in a nonfasted state. After 4 wk, final body weight and liver cholesterol were similar in fiber-free controls and in rats fed diets containing cellulose, pectin, oat bran or wheat bran. Pectin-fed animals has significantly lower plasma cholesterol, HDL-cholesterol and apolipoprotein A-I levels, and exhibited significantly higher hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase activity than did the fiber-free control group. In addition, plasma triglyceride concentrations were lowest in pectin-fed animals. These multiple effects on lipid metabolism were not observed when oat bran, containing one-third soluble fiber, was used. Although total plasma cholesterol levels in wheat bran-fed animals were not different from those in the fiber-free controls or the cellulose-oat bran-fed animals, the LDL cholesterol level was significantly higher than in fiber-free controls or pectin-fed animals. This study demonstrate that dietary fibers included in the diet of rats are able to alter nonfasting lipoprotein cholesterol and apolipoproteins and that pectin, a soluble fiber, was most effective in lowering plasma cholesterol levels.

The effect of pyruvate or dihydroxyacetone on parenterally induced liver lipid accumulation in the rat

Orally fed pyruvate (pyr) and dihydroxyacetone (DHA) have been shown to decrease liver lipid accumulation in animal models. These compounds lessen the degree of fatty liver in ethanol-fed rats and in a genetic strain of hens predisposed to fatty liver. Total parenteral nutrition can result in liver dysfunction, including fatty infiltration of the liver. In this study, rats were assigned to either control, pyr, or DHA groups. All rats were fitted with jugular vein catheters, and following a 3-day recovery, were infused continuously for 7 days. The infusate provided adequate nutrition (including 7% kcal as fat) with 5% pyr or 5% DHA (g/liter) substituted for dextrose in the experimental groups. Plasma triglycerides were lower in the pyr groups relative to controls: 62.2 +/- 34.7 (SE) vs 96.8 +/- 44.3 mg/dl, though this was significant only at P less than 0.10. Neither pyr nor DHA decreased liver lipids. Pyr and DHA were administered intravenously in this study, and therefore passed through the heart and to peripheral tissues first. These compounds may need to be fed orally, passing via the portal system, to produce the liver lipid-lowering effects seen in other studies.

Dietary fiber and gender: effect on postprandial lipemia

The ability of guar gum and oat bran to alter postprandial lipemia and lipoprotein composition when added to a test meal (42% total calories as carbohydrate, 16% as protein, and 42% as fat) was examined in six males and six females who consumed both low- (0.4 g) and high- (15.4 g) fiber test meals on separate days. After an overnight fast participants consumed the test meal, and blood samples were obtained hourly for 8 h. Plasma glucose concentrations did not increase markedly after either meal in any of the subjects. In males, postprandial triglyceridemia was unaffected by fiber supplementation. In females, postprandial triglyceridemia was greater for the high- than for the low-fiber meal at 2, 3, and 4 h. After both meals triglyceridemia was higher in males than in females, which may be related to the lower ratio of high-density lipoprotein2 (HDL)2 to HDL3 in males vs females. Fiber supplementation and gender influence postprandial glycemia, lipemia, and lipoprotein composition.

Interaction of bile acids, phospholipids, cholesterol and triglyceride with dietary fibers in the small intestine of rats

Certain dietary fibers have been reported to lower plasma cholesterol by binding bile acids and reducing their recycling through the enterohepatic circulation. In addition, certain fibers may delay the digestion and absorption of fat. In the present study, the interaction of bile acids with guar gum (GG), konjac mannan (KM) and chitosan (CH) was determined. Rats were fed during a 20-min period a test meal containing either 5% cellulose (CE), GG, KM or CH and also containing 14C-labeled triolein and 3H-labeled cholesterol. The group fed CE served as control, since CE does not bind bile acids or phospholipids in vivo. Two hours after presentation of the test meal, rats were killed and the stomach and small and large intestine removed. All four groups ate the same amount of the test meal, about 1.9 g. The aqueous phase of the small intestinal contents was separated by ultracentrifugation, and the amount (mumol) of bile acids and phospholipids in the total intestinal contents and in the aqueous phase was estimated. The ratio of bile acids in the aqueous phase to that in total intestinal contents was significantly higher in the GG and KM groups and significantly lower in the CH group than that in the CE group, demonstrating that the bile acids are bound or trapped by each of these fiber sources. Only CH appeared to bind phospholipids, reducing the proportion in the aqueous phase compared to that in the CE group.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of cardiac rehabilitation on postprandial response to a high fat meal in patients with coronary artery disease

Past studies have shown decreased lipemic responses to a high fat meal in healthy trained vs. untrained subjects. The purpose of this study was to characterize fasting lipid profiles and lipemic responses in 13 male cardiac patients (6 in cardiac rehabilitation (CR) and 7 controls (NONCR]. Body composition and dietary composition were assessed. Plasma total cholesterol (TC), HDL-cholesterol (HDL-C) and triglyceride (TG) were determined after a 12-h fast. Subjects consumed a high fat meal (60% of calories) and the lipemic response to the meal was assessed by determining plasma TG hourly for 8 h following the meal. CR had a lower percent body fat than NONCR (26% vs. 34%, P less than 0.05). CR consumed fewer calories from fat than NONCR (28% vs. 41%, P less than 0.05). There were no significant differences between groups in TC, HDL-C or TG, although the differences approached significance (P less than 0.10). CR had a lower TC/HDL-C ratio than NONCR (5.0 vs. 7.7, P less than 0.05). Lipemic responses between groups were similar. There was no significant difference in peak TG, time to peak TG, or area under the TG curve between CR and NONCR groups. Thus, subjects undergoing cardiac rehabilitation appear to have both a more favorable diet and fasting lipid profile than NONCR; however, both groups had a similar response to a high fat meal.

Reduction of plasma and hepatic triacylglycerides with whole milk-containing diets in rats

Two studies were conducted to determine the effect of diets containing milk on plasma cholesterol, triacylglycerides, apolipoproteins, and lipoprotein composition and hepatic lipid levels in rats. In the first study four groups were fed either a diet containing skim milk (SM) powder or whole milk (WM) powder or one of two control diets with casein as the protein source. The SM diet and one casein diet (SM control) each contained 5% fat by weight as corn oil. The WM diet contained 20% fat by weight, derived from the milk powder, and the second casein diet (WM control) contained 20% fat by weight from corn oil and lard. The SM group had similar plasma and hepatic lipids, as did the SM control group; plasma apolipoprotein AI, AIV and B did not differ between the groups, but the apo E level was lower in the control group. The WM group had lower plasma triacylglycerides and apo B and lower hepatic cholesterol and triacylglycerides than did the WM control group. In the second study the WM diet was fed again, and the two control diets contained casein as the protein source, 20% butter oil (so that the level of dietary fat was comparable to that of the WM diet), and either lactose or sucrose was added. The plasma apo B and triacylglycerides and the hepatic cholesterol and triacylglycerides were significantly lower in the WM group than in the casein/lactose or casein/sucrose groups. The amount of protein, cholesterol and triacylglycerides in the VLDL fraction of the plasma was lower in the WM group than in the casein/lactose or casein/sucrose groups.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of guar gum or wheat bran on the disappearance of 14C-labeled starch from the rat gastrointestinal tract

An animal model was used to test the hypothesis that guar gum, a source of soluble fiber, will slow the disappearance of starch from the gastrointestinal tract. Male Wistar rats were adapted for 1 wk to a purified diet and then divided into three groups and given 30 min to consume a test meal. The test meal contained 5% fiber derived from guar gum (GG) or wheat bran (WB) or was fiber-free (FF), and each contained 2.5 microCi 14C-labeled starch. At 1, 2.5, or 5 h postprandial, a group from each dietary treatment was killed, and stomach, small and large intestine and cecum were removed to determine the distribution of radioactivity. The isotope emptied from the stomach at a similar rate for all three groups. The percent ingested dose in the small intestine (SI) was highest in the upper half at 1 h and highest in the lower half at 2.5 h for all groups. The percent ingested dose of 14C-starch was significantly higher from upper SI segments in the GG group than from those in the FF group at 2.5 and 5 h, whereas percent ingested dose did not differ in the upper SI segments in the WB group compared to the FF group at 1, 2.5 and 5 h. These data indicate that a viscous polysaccharide (GG) can delay the disappearance of starch from the SI whereas a source of insoluble fiber (WB) did not slow starch disappearance.

Intestinal zinc and carboxypeptidase A and B activity in response to consumption of test meals containing various proteins by rats

Rats were fed test meals containing 23% protein as casein (C), soy protein isolate (SPI) or egg white (EW) to assess effects of dietary protein sources on te amount of zinc and carboxypeptidase (CP, CPA, CPB) activity in the small intestinal contents. Groups of six rats each were killed at 1, 2.5 and 5 h postprandial and six rats fasted for 19 h served as a 0 time control. Consumption of a test meal increased the weight of small intestinal contents at 1 and 2.5 h compared with unfed animals and in a similar manner for all three protein sources. However, at various times during the postprandial period differences in the level of zinc and CP activity within the small intestinal contents among the three dietary protein sources were observed. Both the SPI and EW groups had significantly higher levels of CPA and CPB activity at 1 h postprandial than the C group. Only in the SPI group was the intestinal level of zinc increased at 1 and 2.5 h compared with the unfed group. At 1 and 2.5 h postprandial intestinal zinc was significantly higher in the SPI group than in the C and EW groups. The results indicate that the disappearance of zinc from the intestine is delayed in rats fed SPI, probably because of the presence of phytate.

Reduction in plasma apolipoprotein E and HDL1 levels in rats with essential fatty acid deficiency

Rats were fed linoleate- or triolein-supplemented total parenteral solutions by continuous intragastric infusion for 7 or 14 d to characterize plasma lipid and apolipoprotein (apo) levels, and the high density lipoprotein (HDL) profile associated with essential fatty acid deficiency (EFAD). Results indicate that plasma cholesterol and triglyceride levels tend to be lower with EFAD, whereas plasma levels of apolipoproteins E and B are lower and apo A-I levels are higher in EFAD animals. EFAD was also associated with 30% fewer apo E--enriched HDL1 particles and a decrease from 11.4 to 11.1 nm in the mean peak diameter of HDL (P less than 0.05). These observations emphasize the sensitivity of apo E content to alterations in plasma cholesterol level and suggest that cholesterol transport is decreased during EFAD in the rat.

The effect of varying dietary zinc levels on the concentration and localization of zinc in rat bile-pancreatic fluid

To determine if zinc homeostasis occurs by zinc output from bile-pancreatic secretions, the bile-pancreatic duct and intestine of rats were surgically cannulated, and bile-pancreatic fluid was collected 2 h/d from the day of surgery to 8 d after surgery. The rats were fed diets containing 10, 100 or 1000 micrograms Zn/g diet before and after surgery. The effect of surgery itself was significant; food intake was initially lower than presurgery levels, but returned to presurgery levels by d 5. Protein and zinc concentrations, and carboxypeptidase A (CpA) and carboxypeptidase B (CpB) activities in bile-pancreatic fluid increased after surgery and leveled off at approximately d 6. Among the dietary zinc groups, the concentration of zinc in bile-pancreatic fluid varied significantly, whereas concentrations of calcium and copper did not. Zinc concentration in bile-pancreatic fluid for d 1-8 postsurgery averaged 1.8, 3.2 and 4.4 micrograms Zn/g, in the groups fed 10, 100 and 1000 micrograms Zn/g diet, respectively. Because the percent of zinc ingested that was secreted in bile-pancreatic fluid, estimated to be 57, 9.5 and 1.2% for the groups consuming the diets containing 10, 100 and 1000 micrograms Zn/g diet respectively, was not similar in the three groups, the quantity of zinc in bile-pancreatic fluid is not proportionally related to the amount of zinc ingested. Our results therefore suggest that zinc secretion in bile-pancreatic fluid does not play a major role in zinc homeostasis. Molecular localization of zinc in bile-pancreatic fluid and measurement of activity of CpA and CpB indicated that zinc in bile-pancreatic fluid is associated primarily with these digestive enzymes.

Altered high density lipoprotein composition in manganese-deficient Sprague-Dawley and Wistar rats

The objective of this experiment was to determine whether changes in liver cholesterol content, plasma lipids and lipoprotein composition result from dietary manganese (Mn) deficiency in the Sprague-Dawley (SD) and the Wistar (W) rat. Weanling rats of both strains were fed Mn-deficient (1 ppm Mn) or Mn-adequate (45 ppm Mn) diets for 10 wk. After the 10-wk period rats were killed and plasma, liver, kidney and heart were collected. Liver, kidney and heart Mn concentrations were significantly lower in the rats fed the Mn-deficient diet than in controls. Plasma high density lipoprotein (HDL) (density 1.050-1.1963) was isolated by sequential ultracentrifugation. Plasma cholesterol levels were similar for all groups. HDL protein, cholesterol and apo E levels were significantly reduced with Mn deficiency. A shift to a smaller HDL particle was associated with Mn deficiency, based on gradient gel electrophoresis. The shift in size was more pronounced in the SD strain. Liver concentrations of total and free cholesterol were lower with Mn deficiency but more noticeably in the SD group. The results of our study suggest a role for Mn in HDL metabolism in vivo in both SD and W rats with a trend toward a greater effect in the SD strain.

Rapid induction of essential fatty acid deficiency by intragastric infusion of triolein-supplemented total parenteral nutrition solutions: evidence of increased hepatic cholesterol esterification in the rat

Rats were fed linoleic acid from a safflower oil emulsion or triolein-supplemented total parenteral nutrition solutions by continuous intragastric infusion for 7 and 14 d. Biochemical signs of essential fatty acid deficiency (EFAD) developed in rats supplemented with triolein compared with those receiving linoleic acid, and the relationship between hepatic cholesterol esterification and the distribution of free cholesterol in plasma lipoproteins was investigated in the EFAD and control animals. Results indicate that hepatic triglyceride (TG) and cholesterol ester content are greater and plasma levels of TG and cholesterol are lower in triolein-supplemented groups. Hepatic accumulation of cholesterol esters is associated with an increase in hepatic acyl-CoA:cholesterol acyltransferase activity and also with plasma very low density lipoprotein (VLDL) and high-density lipoprotein (HDL), which contain a greater proportion of cholesterol esters. These data suggest that EFAD can be rapidly induced with continuous intragastric feeding and that hepatic accumulation of cholesterol esters and enrichment of VLDL and HDL with cholesterol esters are early indicators of EFAD in the rat.

Pancreatic response to long-term feeding of soy protein isolate, casein or egg white in rats

Rats were fed purified diets which provided 24% protein from casein (C), soy protein isolate (SPI), or egg white (EW) for 18 mo. Groups of rats were killed at 3, 6, 12 and 18 mo; the pancreata were removed and examined histologically for occurrence of atypical nodules. The weight, protein, DNA, trypsin and chymotrypsin concentrations of the pancreas at each period were measured. Over the entire experimental period, body weight did not differ among groups. Pancreatic weight, protein and trypsin activity were highest in the EW group, followed by the SPI group, and lowest in the C group. Chymotrypsin activity was significantly higher in the EW and SPI groups than the C group. DNA content did not differ significantly among groups over the entire experimental period, although it was elevated in the SPI or EW groups compared to the C groups at some of the time periods. Only one microscopic nodule was observed in all of the animals; it was found at 3 mo in the pancreas from an animal fed EW. Overall, the results suggest that the elevation in enzyme activity and pancreatic weight associated with long-term consumption of EW and SPI did not result in development of pancreatic lesions in rats.