Parenteral nutrition and oral intake: effect of glucose and fat infusions

Diet composition and energy intake in humans

Absolute energy from fats and carbohydrates and the proportion of carbohydrates in the food supply have increased over 50 years. Dietary energy density (ED) is primarily decreased by the water and increased by the fat content of foods. Protein, carbohydrates and fat exert different effects on satiety or energy intake (EI) in the order protein > carbohydrates > fat. When the ED of different foods is equalized the differences between fat and carbohydrates are modest. Covertly increasing dietary ED with fat, carbohydrate or mixed macronutrients elevates EI, producing weight gain and vice versa. In more naturalistic situations where learning cues are intact, there appears to be greater compensation for the different ED of foods. There is considerable individual variability in response. Macronutrient-specific negative feedback models of EI regulation have limited capacity to explain how availability of cheap, highly palatable, readily assimilated, energy-dense foods lead to obesity in modern environments. Neuropsychological constructs including food reward (liking, wanting and learning), reactive and reflective decision making, in the context of asymmetric energy balance regulation, give more comprehensive explanations of how environmental superabundance of foods containing mixtures of readily assimilated fats and carbohydrates and caloric beverages elevate EI through combined hedonic, affective, cognitive and physiological mechanisms. This article is part of a discussion meeting issue 'Causes of obesity: theories, conjectures and evidence (Part II)'.

The Influence of Ketone Bodies on Circadian Processes Regarding Appetite, Sleep and Hormone Release: A Systematic Review of the Literature

Chrononutrition is an emerging branch of chronobiology focusing on the profound interactions between biological rhythms and metabolism. This framework suggests that, just like all biological processes, even nutrition follows a circadian pattern. Recent findings elucidated the metabolic roles of circadian clocks in the regulation of both hormone release and the daily feeding–fasting cycle. Apart from serving as energy fuel, ketone bodies play pivotal roles as signaling mediators and drivers of gene transcription, promoting food anticipation and loss of appetite. Herein we provide a comprehensive review of the literature on the effects of the ketogenic diets on biological processes that follow circadian rhythms, among them appetite, sleep, and endocrine function.

Hyperphagia in short bowel patients: Fat-free mass is a strong predictor

OBJECTIVES

Some patients with short bowel syndrome (SBS) develop hyperphagic behavior. Such an increase in food intake stimulates intestinal adaptation and limits dependence on parenteral nutrition (PN). The aim of this study was to determine the factors modulating food consumption in patients with SBS.

METHODS

The associations between oral energy intake (OEI) and anthropometric, metabolic, nutritional, and intestinal absorption-related characteristics were determined in a monocentric cohort of patients with SBS on PN with a stable nutritional status. Body composition was assessed by dual x-ray absorptiometry. Data were retrospectively collected from clinical records.

RESULTS

After screening, 38 adult patients with a SBS on PN were included in this study. OEI ranged from 577 to 4054kcal/d. OEI correlated positively with weight, fat-free mass, handgrip strength, and resting energy expenditure (REE) and negatively with free triiodothyronine and C-reactive protein using Spearman correlation. Fat-free mass and thyroid-stimulating hormone remained positively correlated with OEI independently of all other parameters in a multilinear regression model.

CONCLUSIONS

Fat-free mass is a strong predictor of OEI in patients with SBS on PN and without debilitating gastrointestinal symptoms. Increasing fat-free mass could be a way to stimulate OEI in these patients. Further studies are needed to assess this assumption.

Circulating free fatty acids inhibit food intake in an oleate-specific manner in rats

Previous rodent studies showed that when injected into the brain, free fatty acids (FFAs) reduced food intake in an oleate-specific manner. The present study was performed to test whether food intake is regulated by circulating FFAs in an oleate-specific manner. Male Wistar rats received an intravenous infusion of olive, safflower, or coconut oil (100mg/h), together with heparin, to raise circulating oleate, linoleate, or palmitate, respectively, and their effects on overnight food intake were evaluated. Compared to other oils, olive oil infusion showed a significantly greater effect to reduce food intake (P<0.01). Total caloric intake, the sum of the calories from the diet and infused oil, was significantly reduced with olive oil (P<0.01) but not with coconut or safflower oil infusion, suggesting an oleate-specific effect on caloric intake. To further test this idea, different groups of rats received an intravenous infusion of oleate, linoleate, or octanoate (0.5mg/h). Oleate infusion decreased overnight food intake by 26% (P<0.001), but no significant effect was seen with linoleate, octanoate, or vehicle infusion (P>0.05). The effects of olive oil or oleate infusion could not be explained by changes in plasma glucose, insulin, leptin, or total FFA levels. The olive oil effect on food intake was not reduced in vagotomized rats, suggesting that oleate sensing may not involve peripheral sensors. In contrast, olive oil's effect was attenuated in high-fat-fed rats, suggesting that this effect is regulated (or impaired) under physiological (or pathological) conditions. Taken together, the present study provides evidence that circulating oleate is sensed by the brain differentially from other FFAs to control feeding in rats.

Altered metabolic homeostasis is associated with appetite regulation during and following 48-h of severe energy deprivation in adults

BACKGROUND

Military personnel frequently endure intermittent periods of severe energy deficit which can compromise health and performance. Physiologic factors contributing to underconsumption, and the subsequent drive to overeat, are not fully characterized. This study aimed to identify associations between appetite, metabolic homeostasis and endocrine responses during and following severe, short-term energy deprivation.

METHODS

Twenty-three young adults (17M/6F, 21±3years, BMI 25±3kg/m(2)) participated in a randomized, controlled, crossover trial. During separate 48-h periods, participants increased habitual energy expenditure by 1647±345kcal/d (mean±SD) through prescribed exercise at 40-65% VO2peak, and consumed provided isovolumetric diets designed to maintain energy balance at the elevated energy expenditure (EB; 36±93kcal/d energy deficit) or to produce a severe energy deficit (ED; 3681±716kcal/d energy deficit). Appetite, markers of metabolic homeostasis and endocrine mediators of appetite and substrate availability were periodically measured. Ad libitum energy intake was measured over 36h following both experimental periods.

RESULTS

Appetite increased during ED and was greater than during EB despite maintenance of diet volume (P=0.004). Ad libitum energy intake was 907kcal/36h [95% CI: 321, 1493kcal/36h, P=0.004] higher following ED compared to following EB. Serum beta-hydroxybutyrate, free fatty acids, branched-chain amino acids, dehydroepiandrosterone-sulfate (DHEA-S) and cortisol concentrations were higher (P<0.001 for all), whereas whole-body protein balance was more negative (P<0.001), and serum glucose, insulin, and leptin concentrations were lower (P<0.001 for all) during ED relative to during EB. Cortisol concentrations, but not any other hormone or metabolic substrate, were inversely associated with satiety during EB (R(2)=0.23, P=0.04). In contrast, serum glucose and DHEA-S concentrations were inversely associated with satiety during ED (R(2)=0.68, P<0.001). No associations between physiologic variables measured during EB and ad libitum energy intake following EB were observed. However, serum leptin and net protein balance measured during ED were inversely associated with ad libitum energy intake following ED (R(2)=0.48, P=0.01).

CONCLUSION

These findings suggest that changes in metabolic homeostasis during energy deprivation modulate appetite independent of reductions in diet volume. Following energy deprivation, physiologic signals of adipose and lean tissue loss may drive restoration of energy balance.

CLINICAL TRIALS REGISTRATION

www.clinicaltrials.gov #NCT01603550.

Do ketogenic diets really suppress appetite? A systematic review and meta-analysis

Very-low-energy diets (VLEDs) and ketogenic low-carbohydrate diets (KLCDs) are two dietary strategies that have been associated with a suppression of appetite. However, the results of clinical trials investigating the effect of ketogenic diets on appetite are inconsistent. To evaluate quantitatively the effect of ketogenic diets on subjective appetite ratings, we conducted a systematic literature search and meta-analysis of studies that assessed appetite with visual analogue scales before (in energy balance) and during (while in ketosis) adherence to VLED or KLCD. Individuals were less hungry and exhibited greater fullness/satiety while adhering to VLED, and individuals adhering to KLCD were less hungry and had a reduced desire to eat. Although these absolute changes in appetite were small, they occurred within the context of energy restriction, which is known to increase appetite in obese people. Thus, the clinical benefit of a ketogenic diet is in preventing an increase in appetite, despite weight loss, although individuals may indeed feel slightly less hungry (or more full or satisfied). Ketosis appears to provide a plausible explanation for this suppression of appetite. Future studies should investigate the minimum level of ketosis required to achieve appetite suppression during ketogenic weight loss diets, as this could enable inclusion of a greater variety of healthy carbohydrate-containing foods into the diet.

Fat sensing and metabolic syndrome

Overconsumption of dietary fat contributes to the development of obesity and metabolic syndrome. Recent evidence suggests that high dietary fat may promote these metabolic states not only by providing calories but also by inducing impaired control of energy balance. In normal metabolic states, fat interacts with various organs or receptors to generate signals for the regulation of energy balance. Many of these interactions are impaired by high-fat diets or in obesity, contributing to the development or maintenance of obesity. These impairments may arise largely from fundamental alterations in the hypothalamus where all peripheral signals are integrated to regulate energy balance. This review focuses on various mechanisms by which fat is sensed at different stages of ingestion, circulation, storage, and utilization to regulate food intake, and how these individual mechanisms are altered by high-fat diets or in obesity.

Ketosis and appetite-mediating nutrients and hormones after weight loss

BACKGROUND/OBJECTIVES

Diet-induced weight loss is accompanied by compensatory changes, which increase appetite and encourage weight regain. There is some evidence that ketogenic diets suppress appetite. The objective is to examine the effect of ketosis on a number of circulating factors involved in appetite regulation, following diet-induced weight loss.

SUBJECTS/METHODS

Of 50 non-diabetic overweight or obese subjects who began the study, 39 completed an 8-week ketogenic very-low-energy diet (VLED), followed by 2 weeks of reintroduction of foods. Following weight loss, circulating concentrations of glucose, insulin, non-esterified fatty acids (NEFA), β-hydroxybutyrate (BHB), leptin, gastrointestinal hormones and subjective ratings of appetite were compared when subjects were ketotic, and after refeeding.

RESULTS

During the ketogenic VLED, subjects lost 13% of initial weight and fasting BHB increased from (mean±s.e.m.) 0.07±0.00 to 0.48±0.07 mmol/l (P<0.001). BHB fell to 0.19±0.03 mmol/l after 2 weeks of refeeding (P<0.001 compared with week 8). When participants were ketotic, the weight loss induced increase in ghrelin was suppressed. Glucose and NEFA were higher, and amylin, leptin and subjective ratings of appetite were lower at week 8 than after refeeding.

CONCLUSIONS

The circulating concentrations of several hormones and nutrients which influence appetite were altered after weight loss induced by a ketogenic diet, compared with after refeeding. The increase in circulating ghrelin and subjective appetite which accompany dietary weight reduction were mitigated when weight-reduced participants were ketotic.

48-h glucose infusion in humans: effect on hormonal responses, hunger and food intake

Experimentally-induced hyperglycemia by prolonged glucose infusion allows investigation of the effects of sustained stimulation of the pancreatic beta-cell on insulin secretion and sensitivity. Hormonal responses to a meal following prolonged glucose infusions have not been investigated. To determine if a 48-h glucose infusion alters hormonal responses to a test meal as well as food intake and hunger in normal weight individuals, 16 subjects (8 men, 8 women, age 18-30 years, mean BMI=21.7+/-1.6 kg/m2) were infused for 48 h with either saline (50 ml/h) or 15% glucose (200 mg/m2/min). Subjects ingested a 600 kcal mixed nutrient meal 3 h after infusion termination. Blood samples were taken during the 48 h and for 4 h following food ingestion. The 48-h glucose infusion elicited a metabolic profile of a glucose intolerant obese subjects, with increased plasma glucose, insulin and leptin (all P<0.01) and increased HOMA-IR (P<0.001). During meal ingestion, early insulin secretion was increased (P<0.05) but post-prandial glucose (P<0.01) and insulin (P<0.01) excursions were lower following the glucose infusion. Post-prandial plasma triglyceride concentrations were increased after glucose compared with saline. Food intake and hunger ratings were not different between the two conditions. Plasma leptin levels were inversely correlated with hunger (P<0.03) in both conditions and with food intake (P<0.003) during the glucose condition only. Thus, a 48-h glucose infusion does not impair post-prandial hormonal responses, alter food intake or hunger in normal weight subjects. The glucose-induced increases in plasma leptin result in a stronger inverse relationship between plasma leptin and hunger as well as food intake. These data are the first to demonstrate a relationship between leptin and hunger in normal weight, non-calorically restricted human subjects.

The effect of different macronutrient infusions on appetite, ghrelin and peptide YY in parenterally fed patients

BACKGROUND & AIMS

Patients receiving parenteral nutrition (PN) still feel hungry despite adequate provision of calories intravenously. It is not known whether PN or its constituent macronutrients acutely affect appetite and to what degree this may be mediated by ghrelin and peptide YY (PYY).

METHODS

Six medically stable patients (four men) with intestinal failure receiving PN received an isocaloric 200 kcal infusion on three separate occasions following a 12 h fast. The infusions consisted of either carbohydrate (10% dextrose), fat (10% intralipid) or mixed protein/carbohydrate (PN). Changes in ghrelin and peptide YY levels and changes in subjective symptoms of hunger, satiety and nausea during each macronutrient infusion were assessed.

RESULTS

None of the three infusions acutely affected subjective symptoms of hunger, satiety and nausea (P>0.05 ANOVA). Ghrelin levels decreased significantly during dextrose [-19.1 (-35.9, -12.4), regression coefficient (95% CI), P<0.001] and parenteral nutrition infusions [-18.2 (-26.8, -9.6), P<0.001]. Lipid infusion had no effect on ghrelin levels but led to a significant decrease in PYY [-0.076 (-0.0123, -0.028), P=0.004]. Dextrose and PN infusion had no significant effect on PYY levels.

CONCLUSIONS

Dextrose and PN infusions decrease ghrelin levels. Lipid infusion does not affect ghrelin levels but in contrast to oral nutrients leads to a significant decrease in PYY. Despite these changes, in patients receiving PN, macronutrient infusions do no acutely affect appetite.

Parenteral nutrition for the acutely ill

Parenteral nutrition (PN) is one of the most sophisticated forms of intravenous therapy in use today. Intravenous feeding is a life-saving technology for patients unable to maintain their nutritional status using the gastrointestinal tract. Although PN has become an integral component of patient care, the risks associated with this therapy must be weighed against the potential benefits. Comprehensive clinical management includes selection of candidates, implementation and monitoring of therapy, and ensuring a seamless transition when PN is no longer required. Optimal parenteral nutrition demands expertise in caring for vascular access devices. A collaborative approach to care minimizes the risks associated with PN and ensures positive patient outcomes.

Influence of surgical stress and parenteral nutrition on serum leptin concentration

Experimental studies suggest that leptin may be an important metabolic signal for energy regulation.

AIM

To assess whether surgical stress produces changes in serum leptin concentration and to investigate and compare the effect of total parenteral nutrition and hypocaloric parenteral nutrition on serum leptin levels.

PATIENTS AND METHODS

Twenty-two surgical patients (11 male and 11 female) in need of parenteral nutrition were recruited. Parenteral nutrition was always initiated 24 h after surgical procedure. Group I (n=15) received total parenteral nutrition, while Group II (n=7) were treated with hypocaloric parenteral nutrition. Serum leptin concentration was determined before surgical procedure (day -1), after surgery and before parenteral nutrition was started (day +1), and after 5 days of treatment with parenteral nutrition (day +6).

RESULTS

A tendency to increase serum leptin levels was observed after surgical procedure (6.0+/-1.9 vs 9.9+/-2.7 ng/ml;P= 0.07). After starting parenteral nutrition no significant changes on serum leptin concentrations were found in both groups, but a trend to raise serum leptin was observed in Group I (6.2+/-1.7 vs 8.3+/-2.7 ng/ml) whereas a trend to decrease serum leptin was detected in Group II (4.6+/-2.5 vs 1.6+/-0.5 ng/ml). On day +6 an increase of serum leptin and insulin levels was observed in Group I in comparison with Group II (8.3+/-2.7 vs 1.6+/-0.5 ng/ml;P<< 0.05 and 58+/-41 vs 12+/-15 microU/l;P<< 0.05 respectively). Finally, a positive correlation at day +6 between insulin and serum leptin levels was observed (r= 0.66;P<< 0.01).

CONCLUSIONS

a) Surgical stress is associated to an increase of serum leptin concentrations; b) Total and hypocaloric parenteral nutrition produces quite different effects on serum leptin levels that could be related to distinct insulin response.

Control of food intake by fatty acid oxidation and ketogenesis

Fatty acid oxidation seems to provide an important stimulus for metabolic control of food intake, because various inhibitors of fatty acid oxidation (mercaptoacetate, methyl palmoxirate, R-3-amino-4-trimethylaminobutyric acid) stimulated feeding in rats and/or mice, in particular when fed a fat-enriched diet, and long-term intravascular infusion of lipids reduced voluntary food intake in various species, including humans. The feeding response to decreased fatty acid oxidation was due to a shortening of the intermeal interval with meal size remaining unaffected. Thus, energy derived from fatty acid oxidation seems to contribute to control of the duration of postmeal satiety and meal onset. Since inhibition of glucose metabolism by 2-deoxy-D-glucose affects feeding pattern similarly, and spontaneous meals were shown to be preceded by a transient decline in blood glucose in rats and humans, a decrease in energy availability from glucose and fatty acid oxidation seems to be instrumental in eliciting eating. Since the feeding response of rats to inhibition of fatty acid oxidation was abolished by total abdominal vagotomy and pretreatment with capsaicin destroying non-myelinated afferents and attenuated by hepatic branch vagotomy, fatty acid oxidation in abdominal tissues, especially in the liver, apparently is signalled to the brain by vagal afferents to affect eating. Brain lesions and Fos immunohistochemistry were employed to identify pathways within the brain mediating eating in response to decreased fatty acid oxidation. According to these studies, the nucleus tractus solitarii (NTS) of the medulla oblongata represents the gate for central processing of vagally mediated afferent information related to fatty acid oxidation. The lateral parabrachial nucleus of the pons seems to be a major relay for pertinent ascending input from the NTS. In particular the central nucleus of the amygdala, a projection area of the parabrachial nucleus, appears to be crucial for eating in response to decreased fatty acid oxidation. As ketones are products of hepatic fatty acid oxidation that are released into the circulation and peripheral (and central) administration of 3-hydroxybutyrate reduced voluntary food intake in rats, ketones being utilized as fuels by the peripheral and central nervous system might contribute to control of eating by fatty acid oxidation, especially when high levels of circulating ketones occur. Whether a modulation of the hepatic membrane potential resulting from changes in the rate of fatty acid oxidation and/or ketogenesis represent a signal for control of eating transmitted to the brain by vagal afferents remains to be established. Recent in vivo studies investigating the effects of mercaptoacetate on the hepatic membrane potential and on afferent activity of the hepatic vagus branch are consistent with this notion. Further investigations are necessary to delineate the coding mechanisms by which fatty acid oxidation and/or ketogenesis modulate vagal afferent activity.

Peripheral signals affecting food intake

Peripheral signals arise from the sequence involving location, selection, ingestion, digestion, and absorption of food. These signals can be anatomically localized to gastrointestinal signals, circulating factors, metabolic signals, nutrient stores, and the sensory capabilities of the nervous system. Since many of the physiologic signals thought to affect feeding are triggered by nutrient ingestion and feeding behavior is influenced by diet composition, it is useful to consider peripheral appetite signals in the context of energy and nutrient balance. Evidence suggests that nutrient metabolism is (directly or indirectly) related to postingestive satiety. Protein is the most satiating macronutrient and has the greatest detectable effect on qualitative intake. Carbohydrates (CHOs) exert potent effects on satiety. Inhibition of CHO metabolism stimulates intake, as do transient declines in plasma glucose. Inhibition of fat metabolism also stimulates intake, but fat is the least readily metabolized macronutrient, and therefore, joule for joule, is less satiating than CHO or protein. High-fat, energy-dense diets lead to excess energy intakes (EIs) and weight gain relative to lower-fat, less energy-dense diets, and fat intake is a risk factor for subsequent weight gain. Earlier models viewed peripheral control of feeding as due to one or more simple negative feedback loops. More recently research has focused on the multiple signalling systems involved in the maintenance of nutrient and energy balance (EB). While protein influences satiety at several levels, relatively little is known about "aminostatic" mechanisms. CHO status appears to be monitored in both the central nervous system (CNS) and periphery; signals relating to fat status largely appear to arise in the periphery. More progress has been made in identifying peripheral signals and some of their connections to the brain than in understanding their quantitative importance for normal feeding.

The effects of enteral tube feeding and parenteral nutrition on appetite sensations and food intake in health and disease

Enteral tube feeding (ETF) and parenteral nutrition (PN) are unphysiological methods of feeding. They may not elicit the cephalic phase response because part or all of the gastrointestinal tract is bypassed, nutrients are typically given in liquid form by a continuous infusion over many hours and often overnight while patients sleep. Work conducted in animals, healthy subjects and patients suggests that nutrients delivered as ETF or PN are less effective in relieving appetite sensations than food intake. Distressing appetite sensations may even occur despite the provision by artificial nutrition of sufficient nutrients to meet requirements. The energy provided by ETF and PN is largely additional to oral food intake in humans eating ad libitum, although the extent to which this occurs may decrease with time. There is a need to establish ways (e.g. nutritional, pharmacological, psychological) to suppress appetite sensations and food intake when eating is contraindicated, and to enhance them when weaning from artificial nutrition is desirable.

Effect of i.v. amino acids on satiety in humans

BACKGROUND

Parenteral nutrients suppress oral food intake. Separate i.v. infusion of amino acids (IVAA) at high doses affects gastrointestinal motility and secretion. However, little is known on the effects of separate i.v. infusion of amino acids at these high doses on satiety. Therefore, we have studied the effect of two different doses of a commercially available mixed amino acids solution on satiety and food intake.

METHODS

Six healthy volunteers (ages 20 to 34 years) were studied on three separate occasions in random order during (a) i.v. saline (control), (b) low-dose IVAA ([LDA] 125 mg protein/kg/h, Vamin 18EF; Kabi Pharmacia BV, Woerden, The Netherlands), or (c) high-dose IVAA ([HDA] 250 mg protein/kg/h) for 360 minutes. Subjective criteria such as wish to eat, prospective feeding intentions, and feelings of hunger and fullness were scored on 100-mm visual analog scales at 30-minute intervals. Food preference also was measured every 60 minutes with food selection lists. At the end of the experiment a meal was presented.

RESULTS

Feelings of fullness were significantly (p < .05) increased during both LDA and HDA. The wish to eat was significantly (p < .05) decreased during HDA compared with control and LDA. Prospective feeding intentions also tended to be reduced during HDA (not significant). Feelings of hunger were not significantly different between the three experiments. Total food selection was significantly (p < .05) decreased during LDA and HDA, mainly because of a significantly (p < .05) decreased preference for fat-rich items. However, the total amount of food consumed at the end of the experiment was not significantly different between the three experiments.

CONCLUSIONS

The present study shows that in healthy volunteers, IVAA (1) increase satiety ratings, (2) increase feelings of fullness, (3) decrease preprandial food selection, and (4) have no effect on subsequent oral food intake.

Influence of hyperglycemia on the satiating effect of CCK in humans

In the present study the effects of intraduodenal (i.d.) fat (endogenous CCK) and of CCK infusion on satiety were studied during normo-and hyperglycemic conditions. Eight healthy subjects participated in two protocols consisting of two experiments each. First protocol: (a) normoglycemia (control) with i.d. emulsified fat (i.d. fat) infusion, (b) acute hyperglycemia (HG) with plasma glucose levels stabilized at 15 mmol/L and i.d. fat infusion. In the second protocol the effect of exogenous cholecystokinin (CCK) on satiety was studied during normo- and hyperglycemia. Intraduodenal fat (Intralipid 10%) was infused at a dose of 1 g/h via a nasoduodenal tube in the first protocol, whereas in the second protocol CCK-33 was infused intravenously at a dose of 0.5 IDU/kg x h. Satiety was scored using visual analog scales (VAS). Plasma CCK levels were determined at regular intervals. During infusion of i.d. fat and i.v. CCK the VAS scores of wish to eat, hunger, and prospective feeding decreased significantly (p<0.05) in the normoglycemic experiments. During hyperglycemia satiety did not significantly change in the basal period; however, the scores of wish to eat, hunger, and prospective feeding increased significantly (p<0.05) when i.d. fat or i.v. CCK was administered. Plasma CCK levels in the basal and the stimulated period were not significantly different between normo- and hyperglycemia. In summary, the present study shows that in healthy humans volunteers 1) during normoglycemic conditions satiety can be induced by very low dose of i.d. fat and by CCK infusion, 2) during hyperglycemia the effect of i.d. fat and CCK on satiety are reversed, resulting in increased appetite.

Effects of hyperglycemia and hyperinsulinemia on satiety in humans

Hyperglycemia may influence satiety. One mechanism by which glucose could influence food intake is hyperinsulinemia. Therefore, we investigated the short-term effects of acute hyperglycemia and euglycemic hyperinsulinemia on satiety. Six healthy volunteers (aged 20 to 26 years) were studied for 240 minutes on three separate occasions in random order during (1) intravenous (i.v.) saline (control), (2) acute hyperglycemic hyperinsulinemia (HG) with plasma glucose at 15 mmol/L, and (3) euglycemic hyperinsulinemia (HI) with plasma insulin at 80 mU/L and glucose at 4 to 5 mmol/L. Subjective criteria for appetite like the wish to eat, prospective feeding intentions ("How much food do you think you can eat?"), and feelings of hunger and fullness were scored on a 100-mm visual analog scale (VAS) at 30-minute intervals. Appetite was also measured every 60 minutes with the use of a food selection list (FSL). Appetite (prospective feeding intentions, feelings of hunger, and the wish to eat) gradually increased over basal levels during control conditions and HI. In contrast, prospective feeding intentions and feelings of hunger gradually decreased during HG and were significantly (P < .05) reduced versus basal and control levels during the last hour of the experiment. The wish to eat followed the same pattern. Feelings of fullness did not significantly change in all three experiments. Total food selection was not significantly decreased during HG, but the preference for fat-rich or carbohydrate-rich items tended to be reduced. The study suggests that in humans hyperglycemia induces satiety. This effect seems not to be mediated by insulin, since HI had no effect on appetite. However, a potentiating effect of endogenous insulin on the satiating effect of high blood glucose levels cannot be excluded.

Probing the causes of high-fat diet hyperphagia: a mechanistic and behavioral dissection

High-fat diets promote hyperphagia in both rats and humans; however, understanding of the process by which dietary fat increases intake is incomplete. Since altering the fat content of a diet simultaneously changes both its sensory properties and postingestive effects, it is unclear whether high-fat diet hyperphagia is driven by oral influences, postingestive factors, or both. Previous findings from both animal and human studies indicate that relatively "less palatable" high-fat diets are overeaten relative to high-carbohydrate diets, indicating that the postingestive effects of high-fat foods are sufficient to promote hyperphagia. A program of research on rats is described, which isolates and assesses the independent effects of sensory and postingestive influences on intake of liquid high-fat and high-carbohydrate diets. An integrated series of experiments investigates both short-term (meal size, postprandial satiety) and long-term (ad lib intake over weeks) effects of diet composition on intake in order to "dissect" the causes of high-fat diet hyperphagia. Preliminary findings from this approach indicate that the postingestive effects of a high-fat diet promote longer meal size, less postprandial satiety per calorie, and greater daily calorie intake than a high-carbohydrate diet.

Effects of valine, leucine, isoleucine, and a balanced amino acid solution on the seizure threshold to picrotoxin in rats

During infusion of branched-chain amino acids (BCAAs) in humans, changes in ventilatory drive, sleeping pattern, and appetite have been reported. The mechanism by which BCAA exerts their effects on CNS remains unclear. An infusion of a BCAA solution (300 mg/kg) has previously been found to increase the seizure threshold in rats to the proconvulsant drug picrotoxin, an antagonist on the GABA-benzodiazepine receptor complex. In this study, each of the BCAAs given separately (valine, leucine, isoleucine; 300 mg/kg) (n = 10) increased the mean latency time to onset of seizures vs. placebo as an indication of an increased seizure threshold. A balanced amino acid solution (Vamin-Glucose) had no effect on the seizure threshold. Thus, these CNS effects are specific for BCAAs and occur with all three.

Taste and smell sensations enhance the satiating effect of both a high-carbohydrate and a high-fat meal in humans

The effects of meal sensory properties (tasty vs. bland) and nutrient composition [high-CHO (carbohydrate) vs. high-FAT] on hunger ratings, blood glucose and free fatty acids (FFA), taste perception, and subsequent food intake, were studied in human subjects. Aspartame and vanilla were used to augment meal palatability, yielding four isocaloric liquid meals: bland-FAT, tasty-FAT, bland-CHO, tasty-CHO. Normal-weight, nondieting young adults consumed each of the meals for breakfast on separate days. The main finding was that tasty versions of high-FAT and high-CHO meals were more satiating than nutritionally identical bland meals, as indicated by a greater decrease in hunger ratings following the tasty meals. Changes in blood glucose and FFA were related to meal nutrient composition, but not to meal sensory properties. High-CHO meals tended to be more satiating than high-FAT meals. Consumption of each of the meals produced a similar decrease in pleasantness ratings of food-related tastes. Intake of carbohydrates was significantly higher at a self-selected lunch 5.25 h following a tasty breakfast. These findings indicate that hunger is decreased to a greater extent by meals flavored with aspartame and vanilla relative to nutritionally identical, unflavored meals. The satiety-enhancing effect of oral stimulation was found for both high-FAT and high-CHO meals.

Branched-chain amino acids increase the seizure threshold to picrotoxin in rats

During infusion of branched-chain amino acids (BCAAs) in humans, changes in ventilatory drive, appetite, and sleep have been reported. The mechanism by which BCAAs exert their effects on CNS remains unclear. Picrotoxin is a proconvulsant drug, acting as an antagonist on the GABA-benzodiazepine receptor complex. Twenty rats were randomized to receive either an IP injection with 4% BCAAs (300 mg/kg; 8 ml/kg) (n = 10) or placebo (saline 8 ml/kg) (n = 10). The mean latency time from injection to onset of seizures was recorded as an indication of the seizure threshold. Latency time was significantly longer for BCAAs than for placebo, 11.2 (+/- 1.9) vs. 8.3 (+/- 1.8) min. Thus, a BCAA injection increased the seizure threshold to picrotoxin (p < 0.03). This suggests that BCAA infusion may exert effects on the GABA-benzodiazepine receptor complex.

Selection of nutrition support regimens

The selection of a nutrition support regimen should be the product of a logical, stepwise process. After an appropriate candidate is selected, the integrity and function of the gastrointestinal tract must be assessed to determine if nutrients can be administered enterally or must be given by vein. The anticipated length of therapy will help determine the type of feeding access. Long-term nutrition support requires permanent access, such as a percutaneously or surgically placed feeding tube for enteral nutrition or a tunnelled catheter or implanted port for parenteral nutrition. Formula selection for enteral nutrition, providing adequate liver and renal function, depends largely on the patient's ability to assimilate intact nutrients. The primary decision in parenteral formula selection is whether to provide the macronutrients as a mixed fuel system. The ultimate goal of nutrition support is to make a smooth transition to oral feedings while maintaining adequate nutritional intake.

Role of dietary fat in calorie intake and weight gain

This paper reviews the literature on the role of dietary fat in calorie intake and body weight gain in humans and laboratory animals. An overview of 40 animal studies which compared growth on high-fat (HF) and high-carbohydrate (HC) solid/powdered diets indicated that the HF diet elicited greater weight gain in 33 out of 40 studies. Enhanced growth on the HF diet was often, but not exclusively, attributable to greater caloric intake. Additional evidence for the growth-enhancing effect of HF diets emerges from "diet option" and "supermarket" feeding studies in rats, and experimental and epidemiological studies in humans. Three principal factors that contribute to the different responses to HF and HC diets are (a) caloric density, (b) sensory properties and palatability, and (c) postabsorptive processing. It is concluded that both calorie intake and metabolic energy expenditure are biased towards weight gain when a HF diet is consumed, and that the high caloric density of high-fat diets plays a primary role in weight gain. Humans may be biologically predisposed to gain weight when a HF diet is consumed.