A high-protein diet enhances satiety without conditioned taste aversion in the rat

Mediobasal hypothalamic leucine sensing regulates food intake through activation of a hypothalamus-brainstem circuit

In response to nutrient stimuli, the mediobasal hypothalamus (MBH) drives multiple neuroendocrine and behavioral mechanisms to regulate energy balance. While central leucine reduces food intake and body weight, the specific neuroanatomical sites of leucine sensing, downstream neural substrates, and neurochemical effectors involved in this regulation remain largely unknown. Here we demonstrate that MBH leucine engages a neural energy regulatory circuit by stimulating POMC (proopiomelanocortin) neurons of the MBH, oxytocin neurons of the paraventricular hypothalamus, and neurons within the brainstem nucleus of the solitary tract to acutely suppress food intake by reducing meal size. We identify central p70 S6 kinase and Erk1/2 pathways as intracellular effectors required for this response. Activation of endogenous leucine intracellular metabolism produced longer-term reductions in meal number. Our data identify a novel, specific hypothalamus-brainstem circuit that links amino acid availability and nutrient sensing to the control of food intake.

Improved glucose regulation on a low carbohydrate diet in diabetic rats transplanted with macroencapsulated porcine islets

Islet xenografts from porcine donors can reverse diabetes in experimental animal models and may be an alternative to human islet transplantation. We have recently reported the ability of porcine islets encapsulated in a double layer of hydrophilic agarose to maintain in vitro functional ability for >6 months. Although beta-cells are capable of adapting their secretory capacity in response to glucose levels, evidence has shown that prolonged hyperglycemia can compromise this ability. The aim of the present study was to determine the effects of diet manipulation on the long-term regulation of blood glucose levels, and the preservation of functional islet in the macrobeads. Twenty-one streptozotocin-induced diabetic Wistar-Furth male rats were randomly assigned to two diets containing 64% carbohydrate (CHO) or 20% CHO. Groups of five to six animals assigned to either diet were implanted with either empty (EM) or porcine islet-containing macrobeads (PIM) and followed for 333 days. Observations included general condition, body weight, blood glucose, and food and water intakes. Monthly blood samples were collected for insulin and C-peptide analysis. The 20% CHO diet significantly lowered blood glucose values when compared with those of the 64% CHO groups for both the empty (14.94 +/- 0.41 vs. 16.26 +/- 0.42 mmol/L, respectively, p < 0.001) and islet macrobead recipients (12.88 +/- 0.39 vs. 15.57 +/-0.85 mmol/L, respectively, p <0.001). The different diets, however, had no statistically significant effects on the preservation of islet mass in the macrobead. Serum porcine C-peptide was detected throughout the experiment in animals receiving porcine islet macrobeads, regardless of diet. Diabetic rats fed a low carbohydrate level diet and transplanted with porcine islet macrobeads had improved total tissue glucose disposal and improved clinical parameters associated with diabetes.

Protein, amino acids and the control of food intake

PURPOSE OF REVIEW

The present review presents recent findings on peripheral and central pathways involved in protein and amino acid-induced satiety.

RECENT FINDINGS

A high-protein load leads to a higher decrease of energy intake at the next meal than carbohydrate and fat. A protein-enriched diet induces satiety, improves body composition and results in weight loss. At the peripheral level, proteins seem to induce the release of anorexigenic gut hormones cholecystokinin, glucagon-like peptide-1 and peptide YY, whereas the involvement of ghrelin remains uncertain. Energy expenditure and glucose are probably involved as metabolic signals in protein-induced satiety. Moreover, there is some evidence that the circulating level of leucine could impact food intake. Leucine has been shown to modulate the activity of the energy and nutrient sensor pathways controlled by AMPK and mTOR in the hypothalamus. Moreover, high-protein diets lead to activation of the noradrenergic/adrenergic neuronal pathway in the nucleus of the solitary tract and in melanocortin neurons in the arcuate nucleus.

SUMMARY

Complex and redundant pathways are involved in protein and amino acid-induced satiety. Significant advances have recently allowed a better understanding of the involved cellular and molecular mechanisms. The involvement of some specific area of the brain including the hypothalamus and the nucleus of the solitary tract has to be further analyzed.

Proteins activate satiety-related neuronal pathways in the brainstem and hypothalamus of rats

Our objective was to study the relationship between the satiety induced by high-protein meals and the activation of brain areas involved in the onset of satiety. In rats, we used immunohistochemistry to monitor brain centers activated by a meal by receiving information from the gastrointestinal tract or via humoral pathways. In the nucleus of the solitary tract (NTS), the acute or chronic intake of high-protein meals led to increased activation of the noradrenergic/adrenergic neurons involved in cholecystokinin-induced satiety. In the arcuate nucleus of the hypothalamus, the melanocortin pathway was also more strongly activated after the acute or chronic intake of high-protein meals. Moreover, the glucagon-like peptide 1 pathway arising from the NTS, which is triggered, among other behaviors, during nonphysiological anorexia, was not activated by high-protein meals, supporting the lack of aversive behavior associated with this diet. Taken together, these results show that the ability of high-protein meals to inhibit food intake occurs alongside the activation, in nutrient-sensitive brain areas, of several specific neuronal populations involved in satiety.

A central role for neuronal AMP-activated protein kinase (AMPK) and mammalian target of rapamycin (mTOR) in high-protein diet-induced weight loss

OBJECTIVE

A high-protein diet (HPD) is known to promote the reduction of body fat, but the mechanisms underlying this change are unclear. AMP-activated protein kinase (AMPK) and mammalian target of rapamycin (mTOR) function as majors regulators of cellular metabolism that respond to changes in energy status, and recent data demonstrated that they also play a critical role in systemic energy balance. Here, we sought to determine whether the response of the AMPK and mTOR pathways could contribute to the molecular effects of an HPD.

RESEARCH DESIGN AND METHODS

Western blotting, confocal microscopy, chromatography, light microscopy, and RT-PCR assays were combined to explore the anorexigenic effects of an HPD.

RESULTS

An HPD reduced food intake and induced weight loss in both normal rats and ob/ob mice. The intracerebroventricular administration of leucine reduced food intake, and the magnitude of weight loss and reduction of food intake in a leucine-supplemented diet are similar to that achieved by HPD in normal rats and in ob/ob mice, suggesting that leucine is a major component of the effects of an HPD. Leucine and HPD decrease AMPK and increase mTOR activity in the hypothalamus, leading to inhibition of neuropeptide Y and stimulation of pro-opiomelanocortin expression. Consistent with a cross-regulation between AMPK and mTOR to control food intake, our data show that the activation of these enzymes occurs in the same specific neuronal subtypes.

CONCLUSIONS

These findings provide support for the hypothesis that AMPK and mTOR interact in the hypothalamus to regulate feeding during HPD in a leucine-dependent manner.

Amino acids inhibit Agrp gene expression via an mTOR-dependent mechanism

Metabolic fuels act on hypothalamic neurons to regulate feeding behavior and energy homeostasis, but the signaling mechanisms mediating these effects are not fully clear. Rats placed on a low-protein diet (10% of calories) exhibited increased food intake (P < 0.05) and hypothalamic Agouti-related protein (Agrp) gene expression (P = 0.002). Direct intracerebroventricular injection of either an amino acid mixture (RPMI 1640) or leucine alone (1 mug) suppressed 24-h food intake (P < 0.05), indicating that increasing amino acid concentrations within the brain is sufficient to suppress food intake. To define a cellular mechanism for these direct effects, GT1-7 hypothalamic cells were exposed to low amino acids for 16 h. Decreasing amino acid availability increased Agrp mRNA levels in GT1-7 cells (P < 0.01), and this effect was attenuated by replacement of the amino acid leucine (P < 0.05). Acute exposure to elevated amino acid concentrations increased ribosomal protein S6 kinase phosphorylation via a rapamycin-sensitive mechanism, suggesting that amino acids directly stimulated mammalian target of rapamycin (mTOR) signaling. To test whether mTOR signaling contributes to amino acid inhibition of Agrp gene expression, GT1-7 cells cultured in either low or high amino acids for 16 h and were also treated with rapamcyin (50 nM). Rapamycin treatment increased Agrp mRNA levels in cells exposed to high amino acids (P = 0.01). Taken together, these observations indicate that amino acids can act within the brain to inhibit food intake and that a direct, mTOR-dependent inhibition of Agrp gene expression may contribute to this effect.

Energy balance and hypothalamic effects of a high-protein/low-carbohydrate diet

Diets high in fat or protein and extremely low in carbohydrate are frequently reported to result in weight loss in humans. We previously reported that rats maintained on a low-carbohydrate-high fat diet (LC-HF) consumed similar kcals/day as chow (CH)-fed rats and did not differ in body weight after 7 weeks. LC-HF rats had a 45% decrease in POMC expression in the ARC, decreased plasma insulin, and increased plasma leptin and ghrelin. In the present study we assessed the effects of a low-carbohydrate-high-protein diet (HP: 30% fat, 65% protein, and 5% CHO) on body weight, caloric intake, plasma hormone levels and hypothalamic gene expression. Male rats (n=16) were maintained on CH or HP for 4 weeks. HP rats gained significantly less weight than CH rats (73.4+/-9.4 and 125.0+/-8.2 g) and consumed significantly less kcals/day (94.8+/-1.5 and 123.6+/-1.1). Insulin was significantly reduced in HP rats (HP: 1.8+/-0.6 vs. CH: 4.12+/-0.8 ng/ml), there were no differences between groups in plasma leptin and plasma ghrelin was significantly elevated in HP rats (HP: 127.5+/-45 vs. CH: 76.9+/-8 pg/ml). Maintenance on HP resulted in significantly increased ARC POMC (HP: 121+/-10.0 vs. 100+/-5.9) and DMH NPY (HP: 297+/-82.1 vs. CH: 100+/-37.7) expression compared to CH controls. These data suggest that the macronutrient content of diets differentially influences hypothalamic gene expression in ways that can affect overall intake.

Protein, amino acids and the control of food intake

The influence of protein and amino acid on the control of food intake and the specific control of protein and amino acid intakes remains incompletely understood. The most commonly accepted conclusions are: (1) the existence of an aversive response to diets deficient in or devoid of protein or deficient in at least one essential amino acid; (2) the existence of a mechanism that enables attainment of the minimum requirement for N and essential amino acids by increasing intake of a low-protein diet; (3) a decrease in the intake of a high-protein diet is associated with different processes, including the high satiating effect of protein. Ingested proteins are believed to generate pre- and post-absorptive signals that contribute to the control of gastric kinetics, pancreatic secretion and food intake. At the brain level, two major afferent pathways are involved in protein and amino acid monitoring: the indirect neuro-mediated (mainly vagus-mediated) pathway and the direct blood pathway. The neuro-mediated pathway transfers pre-absorptive and visceral information. This information is for the main part transferred through the vagus nerve that innervates part of the oro-sensory zone: the stomach, the duodenum and the liver. Other information is directly monitored in the blood. It is likely that the system responds precisely when protein and essential amino acid intake is inadequate, but in contrast allows a large range of adaptive capacities through amino acid degradation and substrate interconversion.

Meal pattern of male rats maintained on histidine-, leucine-, or tyrosine-supplemented diet

OBJECTIVE

Food intake is known to be affected by macronutrient composition of the diet, and protein manipulation has been reported to alter food intake, but the effect of individual amino acids on eating behavior has not been fully studied. This study investigated the effect of diet supplementation with three individual amino acids on meal pattern in male rats.

RESEARCH METHODS AND PROCEDURES

Thirty-two Sprague-Dawley rats were randomly divided into four equal groups and fed control diet or histidine (5%)-, leucine (5%)-, or tyrosine (5%)-supplemented diet for 2 weeks and were monitored for their meal pattern.

RESULTS

Total food intake and feeding rate of the different groups were not affected, although other components of meal pattern were altered. Histidine supplementation reduced diurnal meal size by 42% (p < 0.05), whereas that of leucine increased nocturnal meal size by approximately 35% (p < 0.05). Tyrosine supplementation increased food intake of the nocturnal period and decreased that of the diurnal period. Both histidine and tyrosine supplementation elevated fasting plasma insulin levels and suppressed fasting glucose significantly.

DISCUSSION

Individual amino acids were found to alter meal pattern differently. Further investigations are required to dissect the involvement of central and peripheral factors in these alterations.

Yeast proteins enhance satiety in rats

This study was designed to characterize the suppressant effect of yeast protein and purified peptides on energy intake. For this purpose, 5 experiments were carried out using adult male Wistar rats. Rats that consumed ad libitum a standard yeast protein diet ate significantly less and were leaner over 21 d than rats that consumed ad libitum a standard milk protein diet (Expt. 1). Moreover, rats fed a high yeast protein load reduced their next meal and daily energy intake more than rats fed any other well-balanced, amino acid, high protein load (soy, total milk protein, or wheat gluten) and more than those fed a wheat starch diet (Expt. 2). Purified peptides from the yeast protein extract produced similar effects on subsequent energy intake (Expt. 3). Study of the behavioral satiety sequence showed that rats consuming P14-y or P55-y diets ad libitum did not acquire a conditioned food aversion (Expt. 4). Finally, a preliminary study of gastric emptying in rats fed yeast protein loads showed that yeast protein was emptied more rapidly through the pylorus than total milk protein during a meal, which may induce satiety (Expt. 5). Taken together, these experiments show that yeast proteins enhance satiety in rats more than other proteins.

Critical role for peptide YY in protein-mediated satiation and body-weight regulation

Dietary protein enhances satiety and promotes weight loss, but the mechanisms by which appetite is affected remain unclear. We investigated the role of gut hormones, key regulators of ingestive behavior, in mediating the satiating effects of different macronutrients. In normal-weight and obese human subjects, high-protein intake induced the greatest release of the anorectic hormone peptide YY (PYY) and the most pronounced satiety. Long-term augmentation of dietary protein in mice increased plasma PYY levels, decreased food intake, and reduced adiposity. To directly determine the role of PYY in mediating the satiating effects of protein, we generated Pyy null mice, which were selectively resistant to the satiating and weight-reducing effects of protein and developed marked obesity that was reversed by exogenous PYY treatment. Our findings suggest that modulating the release of endogenous satiety factors, such as PYY, through alteration of specific diet constituents could provide a rational therapy for obesity.

The reduced energy intake of rats fed a high-protein low-carbohydrate diet explains the lower fat deposition, but macronutrient substitution accounts for the improved glycemic control

The metabolic effect of high-protein low-carbohydrate (HP) diets on body composition and glucose homeostasis remains incompletely understood. This study assesses the respective roles of the increased protein:carbohydrate ratio (P:C) and the resulting moderate decrease in energy intake in the metabolic effects of HP diets. Rats had free access to normal (NP; 14%) or high (HP; 53%) total milk protein isoenergetic diets, or were fed the NP diet but restricted to the energy intake of HP rats (NPr), which was 89.1 +/- 9.3% that of NP rats. After 8 wk, body weight was lower in HP and NPr rats than in NP rats. In HP rats, the lower body weight was associated with a lower adipose tissue mass and a reduced proportion of large adipocytes. HP rats also had an improved oral glucose tolerance and insulin sensitivity, as assessed by the homeostatic model assessment index, compared with NPr and NP rats, and these effects were related solely to the increased P:C. These data suggest that the reduced energy intake of rats fed a high-protein, low-carbohydrate diet explains the lower fat deposition but an increased P:C per se improves glucose homeostasis.

Protein intake and body-weight regulation

Body-weight management requires a multi-factorial approach. Recent findings suggest that an elevated protein intake seems to play such a key role in body-weight management, through (i) increased satiety related to increased diet-induced thermogenesis, (ii) its effect on thermogenesis, (iii) body composition, and (iv) decreased energy-efficiency. Supported by these mechanisms a relatively larger weight loss and stronger body-weight maintenance thereafter have been observed.

Fos-positive neurons are increased in the nucleus of the solitary tract and decreased in the ventromedial hypothalamus and amygdala by a high-protein diet in rats

Transition from a normal- (NP) to a high-protein (HP) diet induces a rapid depression in food intake and a progressive but incomplete return to the initial intake during the succeeding days. The aim of this study was to determine which CNS regions are involved in the HP diet-induced satiety in rats. Brains were collected from 3 groups of adult rats after habituation to an NP diet (21 d), during the transition phase to a HP diet (2 d), or after habituation to the HP diet (21 d). Fos expression was measured in several brain areas that are involved in the control of food intake (solitary tract nucleus, anterior piriform cortex, lateral hypothalamus, arcuate nucleus, posterior para ventricular nucleus, medio ventral hypothalamus, dorso medial hypothalamus, amygdala, and accumbens nucleus). Changes occurred in the majority of these regions during the transition period from the NP diet to the HP diet. After habituation to the HP diet, significant changes in Fos expression were restricted to an increase in the nucleus of the solitary tract and a decrease in the ventromedial hypothalamus and the cortex of the amygdala. Considering the functional characteristics of these areas, the present results suggest that the vagus nerve conveys the information relative to the quantity of protein ingested, that hypothalamic sites regulate food intake and may alter sympathetic nervous system activity, and that higher brain functions such as memory processing by the limbic system or food reward system are involved in the HP diet-induced satiety in rats.

Increasing the protein content in a carbohydrate-free diet enhances fat loss during 35% but not 75% energy restriction in rats

The purpose of the present study was to test the influence of the amount of protein in a carbohydrate-free diet during a weight reducing program using severe (75%) or more moderate (35%) energy restriction in rats. In Expt. 1, 3 groups (n = 6) consumed ad libitum a high-carbohydrate, low-fat diet [P21C69L10 containing 21% of energy as protein (P21), 69% carbohydrate (C69) and 10% lipids (L10)], a high-carbohydrate, high-fat diet (P21C34L45), or a carbohydrate-free, high-fat, high-protein diet (P55L45). In Expt. 2, 7 groups (n = 7) were studied. For 20 d, groups 1-4 consumed ad libitum diets containing macronutrients at the proportions indicated in their designations [P14C56L30 (control diet), P30L70, P50L50, and P90L10]. Groups 5-7 were pair-fed the same diets at the level of the spontaneous intake of the P90L10 group on the previous day (35% energy restriction). In Expt. 3, 5 groups (n = 7) were fed 1 of the following diets for 20 d. Group 1 consumed the control diet (P14C56L30) ad libitum. Groups 2-5 were energy restricted to 25% of the daily energy intake of group 1 with diets varying in their protein and lipid concentrations (P14C56L30, P50L50, P70L30, and P90L10). A high-fat content in the diet devoid of carbohydrate did not increase energy intake and body adiposity and neither body weight nor body composition was significantly affected by the protein to lipid ratio when energy restriction was 75%; however, a protein content > 50% preserved lean body mass at the expense of fat mass when energy restriction was 35%. Our results show that the absence of carbohydrates from the diet induces a low energy intake and the preferential deposition of protein.

Anxiety as a predictor of alcohol preference in rats?

Many clinical studies based on retrospective self-reports indicate a relationship between anxiety and increased alcohol consumption or relapse in individuals with alcohol abuse or dependence. However, by these retrospective studies it cannot be definitely concluded whether the alcohol abuse or the anxiety was first. In the present study, alcohol-consuming behaviour was determined in three rat strains showing different anxiety-related behaviour but being not genetically selected for high or low alcohol consumption. The innate anxiety of the three rat strains (Harlan-Fischer, Wistar-BgVV and Wistar-Harlan) was measured by the elevated plus maze test. Thereafter voluntary ethanol intake was measured for 3 months followed by a progressive ratio paradigm, in which the number of responses required to obtain alcohol was successively increased during session. The point at which rats ceased to respond (breaking point) was taken as a measure of their motivation to obtain ethanol. The study revealed that Harlan-Fischer rats showing most anxiety-related behaviour in the elevated plus maze test displayed the lowest ethanol intake [g/kg/d b.w.] and the lowest breaking points in the progressive ratio paradigm. The Wistar-Harlan rats with least anxiety-related behaviour and the Wistar-BgVV rats with medium anxiety-related behaviour drank more alcohol and showed higher breaking points than the Harlan-Fischer rats. Thus, in the present study, a distinct relationship between innate anxiety and alcohol-consuming behaviour in rat strains not genetically selected for high and low ethanol intake could not be shown.

Palatability affects the percentage of metabolizable energy as protein selected by adult beagles

The percentage of protein that dogs voluntarily choose and the effect of palatability on the quantity selected were determined. Six beagles were offered a choice of two isoenergetic purified diets containing 0 vs. 25, 9 vs. 32, 18 vs. 32, 18 vs. 48 and 25 vs. 48% metabolizable energy from protein (MEp). To examine whether palatability modifies the choice, the dogs were offered 0 vs. 25% MEp, with the 0% protein diet containing 2.9 times more sucrose than the diet containing 25% MEp. To determine the effect of concentration of protein in the diet on dietary choice and plasma amino acid concentrations (PAA), dogs were adapted to 9% MEp, followed by a choice of diets containing 9 vs. 32% MEp. The choice was repeated after adaptation to a diet containing 32% MEp. Dogs selected diets to obtain 21-27% of the MEp (mean, 25% MEp; median, 27% MEp) when sucrose was kept at 6.4%. When the protein-free diet contained 25% sucrose, dogs selected 17% of MEp, but increased food intake to ingest about the same amount of protein per day. PAA did not correlate linearly with protein intake. Food intake and total PAA were the lowest after consumption of the 9% MEp diet. We conclude that when fed equally bland diets, dogs select food to ingest approximately 25% MEp. As a palatability enhancer, sucrose increases food intake and selection of the diet containing the higher sucrose concentration.

A very high 70%-protein diet does not induce conditioned taste aversion in rats

This study was designed to assess the effects of transition and adaptation to a very high protein diet on behavioral food responses, energy intake, body weight gain, and body composition in rats. For this purpose, adult male Wistar rats were fed either a diet with 70% of energy as protein (P70 group) or a diet with 14% of energy as protein (P14 group) for 16 d. These two groups were compared with a P14 pair-fed (P14-pf) group. A behavioral satiety sequence was also examined. The P70 group ate 21% less than the P14 rats (P < 0.001) and gained less body weight (P < 0.01). The P70 group gained more carcass weight than either P14 or P14-pf rats (P < 0.05). Behavior and food intake data were affected in P70 rats on d 1 of eating the very high protein diet and then returned to baseline values as early as d 2 of consuming the P70 diet. Rats that adapted to the very high protein diet did not acquire a conditioned taste aversion but rather exhibited satiety and a normal behavioral satiety sequence.

A high-whey-protein diet reduces body weight gain and alters insulin sensitivity relative to red meat in wistar rats

A high-protein diet can reduce body weight and increase insulin sensitivity, but whether the type of dietary protein affects these outcomes is unknown. We hypothesized that feeding insulin-resistant rats a high-protein diet (32%) containing whey protein concentrate (WPC) would reduce body weight and tissue lipid levels and increase insulin sensitivity more than a diet containing red meat (RM). Rats were fed a high-fat diet (300 g fat/kg diet) for 9 wk, then switched to a diet containing either 80 or 320 g protein/kg diet, provided by either WPC or RM, for 6 wk (n = 8). The rats were then killed after overnight food deprivation. High dietary protein reduced energy intake (P < 0.001) and visceral (P < 0.001), subcutaneous (P < 0.001), and carcass fat (P < 0.05). Increasing the dietary density of WPC, but not of RM, reduced body weight gain by 4% (P < 0.001). Dietary WPC also reduced plasma insulin concentration by 40% (P < 0.05) and increased insulin sensitivity, compared to RM (P < 0.05). These findings support the conclusions that a high-protein diet reduces energy intake and adiposity and that whey protein is more effective than red meat in reducing body weight gain and increasing insulin sensitivity.

The significance of protein in food intake and body weight regulation

PURPOSE OF REVIEW

To highlight the underexposed but important role of protein in food intake and body weight regulation.

RECENT FINDINGS

Protein plays a key role in food intake regulation through satiety related to diet-induced thermogenesis. Protein also plays a key role in body weight regulation through its effect on thermogenesis and body composition. A high percentage of energy from dietary protein limits body weight (re)gain through its satiety and energy inefficiency related to the change in body composition.

SUMMARY

Protein is more satiating than carbohydrate and fat in the short term, over 24 h and in the long term. Thermogenesis plays a role in this satiety effect, but the role of satiety hormones still needs to be elucidated. On the short-term 'fast' proteins are more satiating than 'slow' proteins, and animal protein induces a higher thermogenesis than vegetable protein. In the longer term the higher postabsorptive satiety and thermogenesis are sustained irrespective of the protein source. High-protein diets affect body weight loss positively only under ad-libitum energy intake conditions, implying also a decreased energy intake. Body composition and metabolic profile are improved. Additional protein consumption results in a significantly lower body weight regain after weight loss, due to body composition, satiety, thermogenesis, and energy inefficiency, while the metabolic profile improves. Implications from these findings are: for practice, recommendations for increasing the percentage of energy from protein while reducing energy intake; for clinical research, assessment of the paradox of increasing the percentage energy from a highly satiating macronutrient; of the potential roles of protein in a negative and positive energy balance; assessment of possibilities of replacing dietary protein by effective amino acids or peptides that may show a similar impact on body weight regulation.

Total subdiaphragmatic vagotomy does not suppress high protein diet-induced food intake depression in rats

This study was undertaken to determine whether the subdiaphragmatic vagus nerve is involved in the depression of food intake induced by the ingestion of a high protein diet (P50) in rats. After total subdiaphragmatic vagotomy (Vago group) or sham surgery (Sham group), rats consumed the control diet for a 2-wk recovery period and then both groups consumed the high protein diet for 16 d. Daily food intake, meal pattern analysis and behavioral satiety sequence were measured. Total subdiaphragmatic vagotomy did not modify the daily intake of the control diet or suppress the dramatic depression in food intake produced by acute transition to a high protein diet. However, the daily intake of a high protein diet was slightly reduced under acute conditions or even after adaptation (P < 0.005). Analysis of meal parameters and the behavioral satiety sequence after adaptation indicated no major metabolic distress. In conclusion, these results suggest that the subdiaphragmatic vagus nerve does not constitute an obligatory pathway for the transfer of information to the brain, resulting in a depression of high protein diet intake. In contrast, a defect in this visceral regulating system could reinforce the metabolic-associated food intake depression signal.