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.