Voluntary feed intake and leptin sensitivity in ad libitum fed obese ponies following a period of restricted feeding: a pilot study.

The relation between plasma leptin and daily ad libitum roughage intake was evaluated during a 14-day period in eight obese Shetland ponies. When the feeding strategy was changed from maintenance feeding to ad libitum feeding, feed intake increased the first day, decreased the second day and increased again during the following days to reach a constant level after 8 days. Plasma leptin concentration increased during the first 2 days, but remained constant afterwards. Although the same pattern was found in all ponies, the magnitude of the increase in leptin on day 1 and the resulting decrease in feed intake on day 2 differed between ponies. A lower anorectic effect was seen in ponies with higher initial leptin concentration, suggesting the presence of different degrees in leptin sensitivity in obese ponies. High leptin production in a attempt to compensate for the decrease in leptin sensitivity might explain large variations in plasma leptin among obese ponies with similar body condition score. Further research is necessary to clarify whether the reduced leptin sensitivity precedes obesity in equines or vice versa.

Effects of dietary supplementation of methionine and its hydroxy analog DL-2-hydroxy-4-methylthiobutanoic acid on growth performance, plasma hormone levels, and the redox status of broiler chickens exposed to high temperatures.

Heat stress is known to impair performance and to induce oxidative stress in poultry. The aim of the present study was to compare the effects of dietary supplementation of dl-methionine (dl-M) or the synthetic analog 2-hydroxy-4-methylthiobutanoic acid (dl-HMTBA) on broiler growth performance, plasma hormone levels, and some oxidative stress-related parameters under conditions of chronic exposure to high temperatures (HT). From 2 to 6 wk of age, male broiler chickens were reared under either a constant temperature of 32°C until 6 wk of age or a normal temperature scheme (gradual decrease to 18°C at 5 wk of age). Chicks in both the normal and HT treatments were provided with a commercial grower diet supplemented with either 1.0 or 1.2 g/kg of dl-M or 1.0 or 1.2 g/kg of dl-HMTBA. Because there were no effects of supplement dose, data were pooled over both doses within each temperature treatment. The chronic HT treatment impaired feed intake and BW gain, but these negative effects were less pronounced when the chickens received dl-HMTBA. Exposure to HT was also associated with decreased (P < 0.001) plasma thyroid hormones and increased (P < 0.0001) plasma corticosterone levels. At 4 wk of age, and irrespective of the supplemental source, chickens subjected to HT were characterized by significantly lower plasma TBA-reactive substance levels. In contrast, at 6 wk of age, plasma TBA-reactive substance levels were significantly increased by HT, but this effect was observed only for the chickens receiving dl-M and not for those receiving dl-HMTBA. High temperatures induced a significant increase in hepatic total glutathione (GSH) and oxidized GSH levels, regardless of the supplemental source. However, the hepatic ratios of reduced GSH to total GSH and reduced GSH to oxidized GSH were highest in chickens supplemented with dl-HMTBA. In conclusion, dl-HMTBA supplementation partially prevented the growth-depressing effects of chronic heat exposure compared with dl-M supplementation. It can be inferred that dl-HMTBA is more efficient in alleviating HT-induced oxidative damage because of a more favorable reduced GSH-to-total GSH ratio.

Scheduled feeding results in adipogenesis and increased acylated ghrelin.

Ghrelin, known to stimulate adipogenesis, displays an endogenous secretory rhythmicity closely related to meal patterns. Therefore, a chronic imposed feeding schedule might induce modified ghrelin levels and consequently adiposity. Growing Wistar rats were schedule-fed by imposing a particular fixed feeding schedule of 3 meals/day without caloric restriction compared with total daily control intake. After 14 days, their body composition was measured by DEXA and compared with ad libitum-fed controls and to rats daily intraperitoneal injection with ghrelin. Feeding patterns, circadian activity, and pulsatile acylated ghrelin variations were monitored. After 14 days, rats on the imposed feeding schedule displayed, despite an equal daily calorie intake, a slower growth rate compared with ad libitum-fed controls. Moreover, schedule-fed rats exhibiting a feeding pattern with intermittent fasting periods had a higher fat/lean ratio compared with ad libitum-fed controls. Interestingly, ghrelin-treated rats also showed an increase in fat mass, but the fat/lean ratio was not significantly increased compared with controls. In the schedule-fed rats, spontaneous activity and acylated ghrelin levels were increased and associated with the scheduled meals, indicating anticipatory effects. Our results suggest that scheduled feeding, associated with intermittent fasting periods, even without nutrient/calorie restriction on a daily basis, results in adipogenesis. This repartitioning effect is associated with increased endogenous acylated ghrelin levels. This schedule-fed model points out the delicate role of meal frequency in adipogenesis and provides an investigative tool to clarify any effects of endogenous ghrelin without the need for ghrelin administration.

Effect of macronutrient ratio of the pre-starter diet on broiler performance and intermediary metabolism.

The aim of this study was to investigate the influence of isoenergetic substitution between the three energy delivering macronutrients in pre-starter diets on performance and intermediary nutrient metabolism in broiler chickens. From hatch until 5 days of age, 600 chicks, collected during peak of hatch, were fed one of the three experimental pre-starter diets with isoenergetic (13 MJ metabolisable energy/kg) substitutions between fat (43 vs. 108 g/kg), protein (126 vs. 240 g/kg) and carbohydrates (391 vs. 510 g/kg). After 5 days, commercial grower and finisher diets were provided. Pre-starter composition influenced body weight until slaughter age, although not statistically verifiable. Broilers fed the low protein (LP) pre-starter had the lowest body weight in relation to chickens on the low carbohydrate or low fat pre-starter diet. After hatch, chicks on the LP pre-starter diet were able to use the residual yolk sac more rapidly to fulfil their protein requirement, which is reflected in small intestine and liver development. Also, plasma metabolite levels were influenced mostly by the LP pre-starter, indicating that the main focus for the requirements of newly hatched chicks should be on proteins. Furthermore, optimal nutrition during the first day's post-hatch should take into account the contribution of the yolk.

The impact of nutrient density in terms of energy and/or protein on live performance, metabolism and carcass composition of female and male broiler chickens of two commercial broiler strains.

The objective of this study was to investigate the effects of diet composition on performance, slaughter yield and plasma metabolites, as different modern broiler strains show different responses to feed intake. Broilers of two commercial strains and of both sexes received one of three diets being different in energy and/or protein level [control diet, low energy/low protein diet (LM/LP) and low protein diet (LP)]. Low energy/low protein diet chickens were characterized by significantly lower body weights and feed intake compared with their LP and control counterparts. Broilers of the Cobb strain or broilers that were fed the control diet were most efficient in converting energy to body weight. No significant differences in plasma metabolites were detected due to diet composition or genotype. The diet with the lower energy and crude protein levels reached the lowest slaughter yield but the highest drumstick and wing percentages. The lowest mortality percentages were observed for broilers fed the LM/LP diet, and Cobb birds appeared to be more sensitive for metabolic disorders resulting in death. It is obvious from this study that different genotypes respond differently to changes in diet composition and therefore have adjusted nutritional requirements.

The beta-adrenergic system is involved in the regulation of the expression of avian uncoupling protein in the chicken.

Avian uncoupling protein (avUCP) is orthologous to UCP3, which is suggested to be involved in fatty acid metabolism and to limit the mitochondrial production of reactive oxygen species in mammals. In the chicken, the role and regulation of avUCP remain to be clarified. The aim of this study was to explore the control of avUCP expression by the beta-adrenergic system, known to be involved in avian thermoregulation and lipid utilization, and in UCP expression in mammals. Therefore, we measured the expression of avUCP mRNA and protein in the Pectoralis major muscle of chickens injected with the beta(2) agonist isoproterenol, and we investigated the potential pathways involved in the regulation of avUCP mRNA expression. Avian UCP mRNA expression was increased 7-fold 4h after isoproterenol injection, leading to a tendency to a 40% increase in avUCP protein 24h post-injection. This increase was preceded, 30 min after isoproterenol injection, by changes in the chicken thyroid status and in the muscular expression of PPARalpha, PPARbeta/delta, and PPARgamma coactivator-1alpha (PGC-1alpha). Moreover, the analysis of the avUCP promoter sequence suggested potential binding sites for PPARs and for thyroid hormone receptors. We also detected the activation of AMP-activated protein kinase, which has recently been reported to be involved in UCP3 regulation in mammals. This study presents for the first time evidence of beta-adrenergic control on avUCP messenger expression in chicken muscle and suggests the potential involvement of AMPK and several transcription factors in this regulation.

Regulation of fatty acid oxidation in chicken (Gallus gallus): interactions between genotype and diet composition.

To explore the mechanisms leading to excessive adiposity in chicken, we investigated the regulation of fatty acid oxidation depending on genotype-related body fatness and diet composition. mRNA expression and/or activity of proteins involved in mitochondrial energy metabolism were measured in liver and gastrocnemius muscle of genetically lean or fat chickens reared on a low-fat/high-protein diet or an isoenergetic high-fat/low-protein diet (HF/LP). Muscle expressions of the muscle isoform of carnitine-palmitoyltransferase 1 (M-CPT1) and PPARbeta/delta were higher in fat than in lean chickens. This was also observed in liver, although only with the HF/LP diet for M-CPT1. This could stimulate mitochondrial fatty acid oxidation in fat chickens. Up-regulations of liver and muscle CPT-1 hepatic isoform, and muscle cytochrome-c-oxidase mRNA expressions, and of beta-hydroxyacyl-CoA-dehydrogenase activities suggest higher fatty acid utilization with the HF/LP diet. PPARbeta/delta and PGC-1alpha could control fatty acid oxidation in muscle and liver, respectively. Regulation of avian uncoupling protein (avUCP) mRNA was tissue-dependent. Predominantly expressed in muscle, it was stimulated in fat and in HF/LP-fed chickens, where it could be associated to the special need in muscle anti-oxidant pathways of fatter animals. In liver it was lower in fat than in lean chickens, and its potential function remains to be clarified.

Dietary ß-hydroxy-ß-methylbutyrate supplementation influences performance differently after immunization in broiler chickens.

Dietary addition of the leucine metabolite ß-hydroxy-ß-methylbutyrate (HMB) promotes growth in various species. In addition, HMB is described to enhance immune responses which might be associated with metabolic costs. We elaborated further on the role of HMB in growth, metabolism and immunity of meat-type chickens using the following parameters: zootechnical performance, blood chemistry and a specific immune responses after immunization with a human serum albumin (HSA)/Freund's (in) complete adjuvant combination. The chickens received commercial feeds either unsupplemented or supplemented with 300 mg HMB/kg feed. ß-hydroxy-ß-methylbutyrate-supplemented chickens were significantly heavier at 2 weeks of age but this difference was attenuated at later ages. Compared with their unsupplemented controls, cumulative feed conversion was significantly lower in HMB-supplemented chickens. There were no differences in blood chemistry between both dietary treatments. After immunization, HMB significantly attenuated the acute phase protein response at day 1 of post-immunization compared with that of their unsupplemented counterparts. After day 7 post-immunization, body weight gain of the immuno-challenged HMB-supplemented chickens was significantly depressed, but their specific anti-HSA IgG response was significantly enhanced compared with that of their immuno-challenged unsupplemented counterparts. The underlying mechanisms and signalling pathways for these phenomena need to be elucidated. Nevertheless, we are able to conclude that HMB is beneficial for performance under normal circumstances. On the other hand, HMB stimulates the immune response after an immunological challenge, though at the cost of reduced growth.

PEGylated cholecystokinin is more potent in inducing anorexia than conditioned taste aversion in rats.

BACKGROUND AND PURPOSE: The physiological involvement of endogenous cholecystokinin (CCK) in the termination of feeding has been challenged by evidence of aversive effects of exogenous CCK8. We previously prolonged the anorectic effect of CCK by conjugation to polyethylene glycol (PEGylation) to produce PEG-CCK9. In this study, we investigated the ability of different doses of PEG-CCK9 to induce conditioned taste aversion (CTA) and satiety and identified the receptors involved in CTA induction. EXPERIMENTAL APPROACH: Induction of CTA, measured by the saccharin preference ratio determined in a two-bottle CTA procedure, and of satiety in adult male Wistar rats after intraperitoneal (i.p.) injection of different doses of PEG-CCK9 (1, 2, 4, 8, 16 or 32 microg kg(-1)) was compared. Devazepide (100 microg kg(-1)) and 2-NAP (3 mg kg(-1)), two selective CCK1-receptor antagonists, were co-administered i.p. with PEG-CCK9 (8 microg kg(-1)) and the CTA effects monitored. KEY RESULTS: PEG-CCK9 dose-dependently induced CTA, with a minimal effective dose of 8 microg kg(-1), whereas the minimal effective dose to induce satiety was 1 microg kg(-1). The CTA effects of PEG-CCK9 were completely abolished by i.p. administration of devazepide prior to PEG-CCK9 treatment and only partially abolished by administration of 2-NAP. CONCLUSIONS AND IMPLICATIONS: Although PEG-CCK9-induced satiety and PEG-CCK9-induced CTA both increased with dose, the conjugate was more potent in inducing satiety, suggesting that the anorexia could not be completely attributed to the aversiveness of the drug. As observed with induction of satiety, PEG-CCK9-induced CTA was mediated by CCK1-receptors.

Further investigations on the role of diet-induced thermogenesis in the regulation of feed intake in chickens: comparison of adult cockerels of lines selected for high or low residual feed intake.

The main objective of this study was to investigate the role of diet-induced thermogenesis (DIT) in feed intake regulation in cockerels selected for high (R+) or low (R-) residual feed intake. The selection criterion was defined as the difference between observed feed intake and feed intake predicted by regression between feed intake and BW, BW gain, and egg mass production. Furthermore, the effect of genotype on postprandial oxidation of U-(13)C(6)-glucose, decarboxylation of 1-(13)C(1)-Leu, and key metabolites and hormones was analyzed. Thirty 24-wk-old cockerels of both lines were kept in battery cages under standard conditions on a commercial diet. Three cockerels per genotype were examined twice weekly from wk 30 through 34 in open-circuit respiratory cells. After adaptation, cockerels were feed deprived for 24 h and heat production was measured. During the subsequent 7-h refeeding period, DIT and feed intake, as well as glucose oxidation and Leu decarboxylation were assessed by using breath tests. Blood samples were collected after fasting and refeeding. Finally, 10 animals per genotype were killed to record abdominal fat weight. Body composition of 6 different chickens per genotype was determined by using dual-energy x-ray absorptiometry. During feed deprivation, the R+ cockerels had a significantly higher heat production than their R- counterparts, which was even more pronounced during refeeding. Consequently, the R+ cockerels had a significantly increased DIT and a higher feed intake than the R- cockerels. Thus, no evidence of a feedback effect of DIT on feed intake was observed. The oxidation of U-(13)C(6)-glucose was significantly higher in the R+ cockerels, confirming their higher respiratory quotient values and the augmented fat deposition in the R- chickens, as assessed by abdominal fat weight and dual-energy x-ray absorptiometry measurements. No significant genotype effect on 1-(13)C(1)-Leu decarboxylation was observed, despite increased circulating uric acid levels in the R+ chickens. Genotype did not influence plasma levels of triglycerides, free fatty acids, glucose, triiodothyronine, or thyroxine after refeeding, whereas plasma leptin levels were significantly higher in the R+ cockerels.

Rate of metabolic decarboxylation of leucine as assessed by a l[1-(13)C(1)]leucine breath test combined with indirect calorimetry of broiler chickens fed isocaloric diets with different protein:fat ratio.

Research has shown that broiler chickens reared on a low-protein diet have a more efficient protein digestion. However, information on the fate of absorbed amino acids in relation to the dietary crude protein level in poultry is sparse. Therefore, this study aimed at developing a methodology for a 1-(13)C(1)-leucine breath test combined with indirect calorimetry, and to apply this technique using broiler diets known to induce differences in protein retention. From 14 days of age onwards, broiler chickens were reared on one of two isocaloric diets with substitutions between fat and protein [low-protein (LP) vs. high-protein (HP) diet: 130.4 vs. 269 g protein/kg; and 101.8 vs. 27.9 g fat/kg]. Every 4 or 5 days, three chickens per diet were placed in the respiratory cells for 48 h. The broilers were intubated with 40 mg 1-(13)C(1)-leucine/kg body weight, followed by breath sampling for 4 h at 15-min intervals and mass spectrometric analysis of the (13)C:(12)C ratio in the samples. The CO(2) level in the respiratory cell air was monitored and excreta samples were collected. The methodology to study l[1-(13)C(1)]leucine decarboxyation in chickens using a breath test combined with indirect calorimetry was accomplished. Results of the nitrogen balance test indicated that the LP broilers had an improved dietary protein retention compared with the HP animals. Moreover, LP chickens decarboxylated a significantly lower percentage of l[1-(13)C(1)]leucine, demonstrating several 'protein- or amino acid-sparing' mechanisms in animals reared on a diet with lower protein level, both at the digestive and at the postabsorptive level.

The effect of feed withdrawal and crating density in transit on metabolism and meat quality of broilers at slaughter weight.

Commercial broilers are exposed to a number of stressors prior to slaughter, including feed deprivation, crating density (high vs. low), and transportation. Hence, the individual and additive or overruling effects of these stressors on welfare and energy metabolism were examined. Live weight gain, rectal temperature, physiological responses, and meat quality of broilers were determined. The fasting of broilers before being transported resulted in a decrease of triglycerides, uric acid, and triiodothyronine concentrations, indicating a negative energy balance. Feed withdrawal was also associated with a reduction in body weight, and highest body weight losses were observed after being fasted for 13 h. For some parameters there was a combined effect of feed withdrawal and crating density, whereas for others the crating density overruled the effect of previous feed withdrawal: broilers that had no access to feed before being transported had higher thyroxine and lower lactate concentrations (only at high crating density) compared with their fed counterparts before the transport process, indicating the combined effect of both actions. The distinction due to the feeding pattern could no longer be observed for the plasma uric acid, nonesterified fatty acids, triglycerides, and triiodothyronine concentrations because it was overruled by the transport effect, especially if broilers were transported at high crating density. Plasma corticosterone concentrations increased as a consequence of the procedure of transportation and peaked if broilers were crated at high density. In our study, no significant effect of preslaughter stressors on meat quality, plasma creatine kinase activity, or lipid peroxidation levels were noticed. It can be concluded that transportation at high stocking densities should be avoided to reduce economic losses and stress to broilers. Plasma hormone as well as metabolites, rectal temperature, and heat shock protein 70 mRNA all indicated the high stress level of broilers. Furthermore, this effect often overruled the feed withdrawal and transport effect, indicating the importance of crating density.

Further investigations on the role of diet-induced thermogenesis in the regulation of feed intake in chickens: comparison of age-matched broiler versus layer cockerels.

The aim of this study was to investigate the role of diet-induced thermogenesis (DIT) in the regulation of feed intake in age-matched broiler and layer cockerels. In addition, the effect of genotype on endocrine functioning and key metabolites of the intermediary metabolism as well as on the expression of muscular uncoupling protein (avUCP) was explored. One-day-old male broiler (Ross) and layer (ISA Brown) chicks were reared under standard conditions on commercial broiler starter and finisher diets. From 22 d of age, twice per week, 3 broiler and 6 layer cockerels were placed in open circuit respiratory chambers. After adaptation, the animals were feed-deprived for 24 h, and heat production was measured by indirect calorimetry. During the subsequent 7-h refeeding period, feed intake and DIT were measured. Blood samples were taken after feed deprivation and re-feeding. Muscle samples were taken after refeeding for determination of avUCP expression. A significantly higher heat production per metabolic BW (MBW) in the layer compared with the broiler cockerels, independent of nutritional state, suggests that the broilers used a greater proportion of the metabolizable energy intake for growth. The DIT per MBW and per gram of feed intake was higher for the layer than for the broiler cockerels. However, feed intake per MBW was also significantly higher in the layer cockerels. Thus, no feedback effect of DIT on feed intake per MBW was observed, and the model formulated for adult mammals relating feed intake to DIT could not be corroborated. The muscular expression of avUCP was not different between genotypes, which does not support the hypothesis of an involvement of avUCP in the higher DIT measured in layer cockerels. Circulating uric acid, glucose, triglyceride, and free fatty acid levels were significantly elevated in the layer compared with the broiler cockerels. As the diet was formulated according to broiler requirements, the higher metabolite levels of the layer cockerels might reflect a relative oversupply of dietary nutrients.

Diet-induced thermogenesis and glucose oxidation in broiler chickens: influence of genotype and diet composition.

The main objectives of this study were to explore the role of diet-induced thermogenesis in the regulation of voluntary feed intake and to determine the glucose oxidation of broiler chicken strains, known to differ in glucose-insulin balance. From 2 to 7 wk of age, male broiler chickens of a fat and a lean line were reared on 1 of 2 isoenergetic diets with constant gross energy and carbohydrate levels but with substitutions between fat and protein. The low protein (LP/HF) diet contained 126 g of protein/kg and 106 g of fat/kg, whereas the low fat (LF/HP) diet contained 242 g of protein/kg and 43 g of fat/kg. There was no significant effect of the genetic background of the broilers on the glucose oxidation rate (as measured by stable isotope breath test) or protein oxidation (as measured by plasma uric acid levels). Considering the difference in carcass composition (fat content) of both lines, this leads to the hypothesis that the lines differ predominantly in fat metabolism. Although there was no line effect on plasma triglyceride and free fatty acid concentrations, it was hypothesized that there might be differences in fat oxidation or de novo lipogenesis, or both, between the genotypes. Diet-induced thermogenesis per metabolic body weight (kg of BW0.75) per 24 h, expressed per gram of feed intake, was not significantly influenced by genetic background or by diet composition. Therefore, a model linking feed intake to diet-induced thermogenesis, as postulated for adult mammals, could not be corroborated for growing broiler chickens.

Peripheral ghrelin reduces food intake and respiratory quotient in chicken.

Ghrelin injection, either centrally or peripherally strongly stimulates feeding in human and rodents. In contrast, centrally injected ghrelin inhibits food intake in neonatal chickens. No information is available about the mechanism and its relationship with energy homeostasis in chicken. Since ghrelin is predominantly produced in the stomach, we investigated the effect of peripherally injected ghrelin (1 nmol/100g body weight) on food intake and energy expenditure as measured in respiratory cells by indirect calorimetry for 24h in one-week-old chickens. Plasma glucose, triglycerides, free fatty acids, total protein and T(3) were measured in a separate experiment until 60 min after injection. Food intake decreased until at least 1h after intravenous ghrelin administration. The respiratory quotient (RQ) in ghrelin-injected chickens was reduced until 14 h after administration whereas plasma glucose and triglycerides concentrations were not altered. Free fatty acids and total protein levels also remained unchanged. Ghrelin did not influence heat production and this was supported by the absence of changes in plasma T(3) levels when compared to the control values. In conclusion, peripheral ghrelin reduces food intake as well as RQ and might influence the type of substrate (macronutrient) that is used as metabolic fuel.

Energy homeostasis and gastric emptying in ghrelin knockout mice.

To elucidate the role of endogenous ghrelin in the regulation of energy homeostasis and gastric emptying, ghrelin knockout mice (ghrelin(-/-)) were generated. Body weight, food intake, respiratory quotient, and heat production (indirect calorimetry), and gastric emptying ((14)C breath test) were compared between ghrelin(+/+) and ghrelin(-/-) mice. In both strains, the effect of exogenous ghrelin on gastric emptying and food intake was determined. Ghrelin(-/-) mice showed some subtle phenotypic changes. Body weight gain and 24-h food intake were not affected, but interruption of the normal light/dark cycle triggered additional food intake in old ghrelin(+/+) but not in ghrelin(-/-) mice. Exogenous ghrelin increased food intake in both genotypes with a bell-shaped dose-response curve that was shifted to the left in ghrelin(-/-) mice. During the dark period, young ghrelin(-/-) mice had a lower respiratory quotient, whereas their heat production was higher than that of the wild-type littermates, inferring a leaner body composition of the ghrelin(-/-) mice. Absence of ghrelin did not affect gastric emptying, and the bell-shaped dose-response curves of the acceleration of gastric emptying by exogenous ghrelin were not shifted between both strains. In conclusion, ghrelin is not an essential regulator of food intake and gastric emptying, but its loss may be compensated by other redundant inputs. In old mice, meal initiation triggered by the light/dark cue may be related to ghrelin. In young animals, ghrelin seems to be involved in the selection of energy stores and in the partitioning of metabolizable energy between storage and dissipation as heat.

Effects of substitution between fat and protein on feed intake and its regulatory mechanisms in broiler chickens: endocrine functioning and intermediary metabolism.

The aim of this study was to investigate the influence of dietary macronutrient ratio on postprandial (5 or 48 h after refeeding) endocrine functioning and metabolites of the intermediary metabolism and to relate these parameters with dietary-induced alterations in energy, protein, and lipid metabolism. Male broilers were reared from 1 to 7 wk of age on isoenergetic diets with substitutions between fat and protein but similar carbohydrate contents [low protein (LP): 126 vs. 242 g protein/ kg; low fat (LF): 43 vs. 106 g fat/kg]. The LP chickens had significantly increased postprandial plasma triglyceride levels. This was likely the result of stimulated hepatic lipogenesis, as corroborated by their significantly higher respiratory quotients. Plasma free fatty acid concentrations were higher in LP broilers, whereas glucose levels were unaffected by dietary composition, suggesting that these chickens preferred carbohydrates as an energy source over free fatty acids. Plasma uric acid levels were lower in LP compared with LF chickens, indicating a more efficient protein retention in the former group. LP birds that were fasted and refed at 48 h had higher plasma 3,5,3'-triiodothyronine (T3) levels, corroborating their increased heat production. The postprandial T3 increase was more pronounced in the LF chickens, possibly induced by their higher protein consumption. In conclusion, diet-induced changes in heat production and energy partitioning are reflected in circulating levels of intermediary metabolites and hormones. Furthermore, nutritional studies should consider the ability of organisms to habituate to changed diet compositions and that alterations in feeding status follow higher-order responses.

Effects of substitution between fat and protein on feed intake and its regulatory mechanisms in broiler chickens: energy and protein metabolism and diet-induced thermogenesis.

The objective of this study was to investigate the effect of dietary macronutrient ratio on energy, protein, and lipid metabolism and on the involvement of diet-induced thermogenesis in feed intake regulation of broiler chickens. Male broilers were reared from 1 to 7 wk on isoenergetic diets with substitutions between fat and protein and similar carbohydrate content [low protein (LP): 126 vs. 242 g of protein/kg; low fat (LF): 43 vs. 106 g of fat/kg]. Every week from 21 d onward, 3 chickens per group were placed in open-circuit respiratory cells to measure energy and protein metabolism in fasting, short-term refeeding (5 h) and ad libitum conditions. As LP chickens had a significantly lower BW from 2 wk onward, all parameters were expressed per kilograms of metabolic BW. Feed intake, gross energy intake, and apparent metabolizable energy intake were significantly higher in LP than LF birds. The excessive energy relative to protein intake resulted in significantly increased heat production and energy retention as fat. The latter effect and a significantly increased respiratory quotient indicated higher de novo lipogenesis in the LP chickens. Furthermore, the efficiency of protein retention was significantly better in LP broilers. Neither diet-induced thermogenesis nor feed intake during a 5-h refeeding period was affected by diet composition. Our results indicate that isoenergetic substitution of fat for protein has a strong effect on growth and on energy and protein balance in broilers. The theory linking diet-induced thermogenesis to feed intake could not be corroborated or countered, and further research is warranted.

Validation of dual-energy x-ray absorptiometry for determining in vivo body composition of chickens.

The purpose of this study was to validate dual-energy x-ray absorptiometry (DXA) for measuring the body composition of chickens in vivo. Four trials were conducted with broiler chickens using a DXA instrument (Lunar, DPX-L) and small animal total body scan software (version 4.7a). In the first 2 trials, the effects of scan mode (high resolution or detail slow), scanning position (ventral or dorsal), and the use of attenuating materials (AM) (2-mm polyvinyl chloride or 4-mm polystyrene) on the precision and values of the DXA parameters body mass, lean tissue mass, fat tissue mass and percentage, bone mineral content (BMC), and bone mineral density (BMD) were evaluated. The precision was highest for body mass and lean tissue mass, followed by BMC and BMD, and was lowest for fat tissue mass and percentage. The precision of the measurements was not influenced by scan mode, position, or type of AM. In contrast, the values for all DXA parameters except body mass were significantly influenced by the scan mode but not by the position. The high resolution mode gave significantly higher estimates of fat mass and BMC but significantly lower measures of lean tissue mass and BMD compared with the detail slow mode. A significant difference between AM was only observed for the DXA estimates of fat tissue mass and fat percentage. In trial 3, the accuracy of the DXA measurements was tested by comparison with chemical body composition analysis. Linear regression equations between the respective DXA and chemical parameters were established. High correlations (r > 0.9; P < 0.0001) were obtained for all parameters, except for fat percentage (r = 0.593; P < 0.05). The purpose of the validation trial was to compare the predicted body composition based on the DXA measurements with established equations and the chemical body composition. There was extremely good agreement for body mass, lean tissue mass, and fat tissue mass and percentage, but not for ash weight. It is concluded that, after proper methodological standardization and application of specifically determined regression equations, DXA can be used for estimating the body composition of chickens in vivo. However, the regression equations are strictly limited to one particular instrument, software version, and applied methodology.