Animals acquire immunity against gastrointestinal (GI) nematode infection depending on their age and continual exposure to larvae, however, expression of this acquired immunity is often penalized during pregnancy and lactating period. This is described as periparturient relaxation in immunity (PPRI), and suggested to have nutritional basis. Although dietary protein has positive effect on immunity against GI nematode infection in mammalian host, we have not fully achieved to characterise the detailed interaction between PPRI and dietary protein. Therefore, this PhD aimed to further investigate this interaction in a well-established Nippostrongylus brasiliensis re-infected lactating rat model. Feeding high protein diet (HP) as opposed to low protein diet (LP) during pregnancy was necessary in maintaining sufficient maternal performances and systemic immune response (Chapter 2 and 3). Accumulation of host’s body protein reserve during pregnancy was significantly higher in HP fed animals compared to LP fed counterparts, which led to improvement in both maternal performances and immunity during the early stage of lactation. However, as lactation period progressed and re-infection of N. brasiliensis took place, importance of current dietary status, rather than the accumulated protein reserve, became evident for maternal performances and immunity. Indeed, animals fed HP during lactation showed significantly heavier pup weight compared to LP fed animals; HP animals showed higher serum immunoglobulin levels and reduced worm burden compared to LP. N. brasiliensis, however, goes through systemic migration, entering host’s skin and migrating to lung parenchyma through blood vessels before reaching the intestine. In Chapter 4, a detailed lung pathology study following N. brasiliensis infection was performed to generate data on the effects of the nematode in the lung of the rat host as such data were scarce. Rat host showed similar lung pathology to that of mice; up-regulation of genes related to type 2 immunity and development of emphysema-like pathology were observed following N. brasiliensis re-infection. In Chapter 5, the effect of dietary protein supplementation on lung and intestinal histology and gene expression analysis was investigated. It was shown that HP fed animals showed higher expression of genes related to type 2 immunity compared to LP in the lung. This effect of protein supplementation in the lung may have contributed to fewer worm burdens in HP fed rats compared to LP in the intestine. Dietary protein supplementation significantly affected the expression of genes related to goblet cells; it resulted in up-regulation of the expression of Retnlb and down-regulation of Agr2 and Tff3 in HP fed animals compared to LP. It is evident that dietary protein is modulating intestinal immunity, and this may be targeted towards specific pathways. In addition, the effect of dietary protein supplementation on immune cell populations of secondary lymphoid organs was analysed. Marked increase in the percentage of macrophage in the spleen and T cell in the mesenteric lymph node was observed following protein supplementation, highlighting the importance of dietary protein on systemic immunity during the parasite infection. These results demonstrate that dietary protein supplementation is effective for improving both maternal performances and immune responses, not only at the intestinal phase but also at the migrating phase, when animal is undergoing PPRI during N. brasiliensis infection. Such information is expected to define strategic utilisation of nutrient supply and to result in development of sustainable parasite control strategies in mammals.

Dietary protein quality depends on two key measures: true ileal protein digestibility and the metabolic utilization of absorbed amino acids (AA). The objectives of this study were to determine the influence of two dietary factors (antinutritional factors and peptides) on ileal endogenous protein flows; to validate the intubation technique used in humans for ileal digesta sampling; to determine the postprandial metabolic utilization of dietary AA depending on their delivery form and to assess the validity of the growing pig for predicting true ileal protein digestibility in the adult human. Investigations were undertaken in the growing rat, growing pig and adult human. Ileal digesta were collected from euthanised rats, post valve T-caecum cannulated pigs, and naso-ileal intubated conscious adult humans. Ileal endogenous nitrogen (N) and AA were measured using a protein-free (PF) diet, diets containing 15N-labelled casein in the intact (C) or hydrolysed (HC) form, or a diet based on free AA (diet A), for which some dispensable AA were omitted to allow a direct determination of their endogenous flows. Digesta centrifugation and ultrafiltration (diet HC) allowed for the determination of ileal endogenous protein flows and the extent of tracer (15N) recycling. Antinutritional factors from a crude extract of kidney beans (Phaseolus vulgaris), when given at amounts commonly ingested in practice, enhanced ileal endogenous protein flows (rats, PF diet). After adaptation to the diet, body N balance per se did not influence ileal endogenous protein flows (rats, diets PF and A) but dietary peptides led to greater ileal endogenous AA and N flows compared with a protein-free diet. Dietary peptides (HC), compared with peptides naturally released in the gut during protein digestion (C), did not enhance ileal endogenous protein flows (rats, pigs, and humans). The extent of tracer recycling, however, was maximal in frequently-fed rats, lower in meal-fed pigs and minimal in meal-fed humans (65, 21, and 11% of 15N-labelled ileal endogenous proteins, respectively). Naso-ileal intubation for ileal digesta sampling in humans was shown to be an accurate method and evidence was obtained supporting the growing pig as a valid model for predicting true ileal protein digestibility in the adult human. Finally, the form of delivery of dietary AA (from HC or C) influenced the postprandial metabolic fate of dietary AA, especially in terms of AA catabolism kinetics. However, the overall nutritional value of C and HC were similar.

Em função de sua disponibilidade no mercado e qualidade nutricional, derivados da soja são importantes fontes de proteína na alimentação animal. Apresentam perfil de aminoácidos semelhante ao de fontes animais, o que estimula seu uso como potencial substituto da farinha de peixe nas rações para aquicultura. No entanto, são deficientes em aminoácidos sulfurados e apresentam fatores antinutricionais que prejudicam o crescimento de peixes. O processamento da soja permite obter produtos mais refinados e altamente digestíveis, como concentrados proteicos (CPS), que têm elevado teor proteico e baixa quantidade de fatores antinutricionais, extraídos no processo de fabricação. Neste contexto, foi determinado o Coeficiente de Digestibilidade Aparente (CDA) do CPS em dietas para juvenis de duas espécies de Characidae autóctones: pacu (Piaractus mesopotamicus) e dourado (Salminus brasiliensis). Também foram avaliados os efeitos do uso deste ingrediente em níveis crescentes de substituição da farinha de peixe na dieta (ensaios com seis tratamentos e quatro repetições), com o objetivo de determinar o nível seguro de substituição sem prejudicar o ganho de peso. O pacu apresentou CDA para proteína (95,33 %) e energia (84,29 %) mais altos que o dourado (CDAproteína = 82,6 %; CDAenergia = 66,6 %), indicando maior plasticidade da espécie para o aproveitamento de fontes vegetais. Baseado em ensaios de desempenho, nas dietas para pacu foi possível substituir até 78,05 % da farinha de peixe das dietas experimentais, enquanto que para dourados, até 37,2%. Acima destes níveis, houve redução no crescimento. A substituição crescente da farinha de peixe pelo CPS exerceu efeito regulatório sobre a atividade de enzimas pancreáticas nos primeiros segmentos do intestino das duas espécies, mensurada através da atividade da protease inespecífica, lipase inespecífica e α-amilase. Também promoveu redução na altura das dobras intestinais quando a substituição foi total, detectada através da morfometria da parede intestinal do intestino anterior. Estes efeitos podem ser consequência da ação conjunta de fatores antinutricionais da soja, principalmente inibidores de enzimas, potencializados quando a farinha de peixe foi totalmente substituída pelo CPS; e da habilidade com a qual cada espécie digere / absorve os nutrientes. Com base nos resultados obtidos, foi possível concluir que o CPS pode ser utilizado em substituições parciais da farinha de peixe em dietas para peixes onívoros e carnívoros. / Because of market availability and nutritional quality, soybean and its by-products are important sources of protein for the animal feed industry. The amino acid profile of soy products is similar to that of animal sources, encouraging their use as surrogate protein source to fishmeal (FM) in aquafeeds. However, soy products and by-products are deficient in sulfur amino acids and contain anti-nutritional factors that hamper fish growth. Secondary processing of soybean meals yield more refined and highly digestible protein ingredients such as the soy protein concentrate (SPC), which have higher protein contents and lower concentration of anti-nutritional factors, partially extracted or inactivated during the manufacturing process. In such a context, this work determined SPC\'s Apparent Digestibility Coefficient (ADC) in diets for two juveniles neotropical, native Characins species: pacu (Piaractus mesopotamicus), and dourado (Salminus brasiliensis), and also studied the effects of replacing graded levels of FM by SPC (six treatment levels and four repetions) in the aim to determine the optimum substitution level for better growth. Pacu presented higher ADC for protein (95,33 %) and energy (84,29 %) than dourado (ADCprotein = 82,6 %; ADCenergy = 66,6 %), indicating the plasticity of the specie to utilize plant sources. Based on growth performance, for pacu it was possible to substitute until 78,05% of FM, whereas for dourado, until 37,20%. Above these levels, there were reduction on growth performance. The increasing replacement of FM by SPC had regulatory effect on the activity of pancreatic enzymes in the first segment of the intestine of both species, measured through enzymatic assays for unspecific protease, unspecific lipase and α-amylase. It also caused a reduction on intestinal fold height of proximal intestine. All of these effects can be attributed to soybean anti-nutritional factors, especially enzyme inhibitors, whose action was enhanced when FM was completely replaced by the SPC; and the specific ability of each specie to digest / absorb plant nutrients. It is safe to infer that SPC can only be used as partial replacement of FM in diets for omnivore and carnivore fish.

<p>Obesity has ascended to become the primary modifiable cause of death in the United States. New evidence has called into question the utility of BMI – the typical index of obesity – in predicting cardiometabolic disturbances. The distribution of body fatness may be just as important as the total quantity. Intermuscular adipose tissue (IMAT) has emerged as a distinct subset of adipose in skeletal muscle that may be particularly metabolically deleterious. Typically, sections of either the calf or thigh are used as proxy measurements for whole-body IMAT in investigations. However, IMAT dispersion may not be consistent across tissues, instead infiltrating specific muscle or muscle compartments, and these have may have different metabolic consequences. The study described in Chapter 2 was designed to address this possibility and investigate and compare associations among thigh and calf IMAT stores with indices of cardiometabolic health. The strength of the relationship between IMAT and glucose control-related indices of cardiometabolic health was dependent upon anatomic location. Specifically, thigh IMAT is a better predictor of cardiometabolic risk that calf IMAT. </p> <p>Skeletal muscle has gained increased recognition in recent years for its importance in promotion of health and wellness throughout the life course. While treatment models addressing issues of declining muscle mass and strength with age previously focused on older adults, the importance of utilizing a life course model to promote skeletal muscle health at all ages was more recently recognized. There is consistent evidence that higher-protein diets modestly improve body composition. However, women are at greater risk for not meeting protein requirements and seem to be less willing to adopt strategies to achieve greater protein intake, such as protein supplementation, for fear that it may cause ‘bulkiness’. Therefore, the study described in Chapter 3 was designed to critically evaluate the effect of whey protein supplementation on body composition changes in women via a systematic review & meta-analysis of published randomized controlled trials. It was hypothesized that whey protein supplementation would moderately improve body composition but would not cause excessive muscle hypertrophy. Consistent with our hypothesis, whey protein supplementation improved body composition by modestly (<1%) increasing lean mass, without influencing fat mass.</p> <p>Dietary protein and skeletal muscle are conceptually inseparable; protein is often only considered in terms of how it impacts skeletal muscle-related outcomes. However, it is of interest to determine if the proposed beneficial effects of increased dietary protein consumption extend beyond skeletal muscle. Consumption of higher protein diets result in lower resting blood pressure, but the potential for protein to attenuate acute exercise blood pressure responses is unclear. The study described in Chapter 4 was designed to investigate the effects of meals with different amounts of protein on blood pressure responses to exercise in a randomized, cross-over trial. We hypothesized that consuming the higher-protein meal would attenuate the blood pressure responses to exercise and result in a more robust post-exercise hypotensive response. Contrary to our hypothesis, a higher-protein meal does not attenuate exercise-induced blood pressure responses compared to a lower-protein meal. These findings build upon previous research suggesting that the beneficial effect of chronically elevated protein intake on blood pressure is typically not observed in an acute setting by extending these findings to encompass blood pressure responses to acute responses to exercise.</p> <p>The three studies packaged herein utilize different techniques and report on different outcomes, but conceptual threads unite these works which augment the collective findings. Future researchers investigating the effects of protein on skeletal muscle anabolism can: 1) learn of the importance of proper reflection on surrogate measures and potential for anatomic-specific effects from the IMAT findings (Chapter 2), 2) appreciate the relevance of energy and training states in modulating responses from the WP meta-analysis (Chapter 3), and 3) recognize the importance of holistic approaches and employing challenges to reveal heterogeneity from the protein and BP trial (Chapter 4). Taken together, the research presented in this dissertation forwards our understanding of the relations and effects of dietary protein with different components of body composition on cardiometabolic health. </p>

Résumé : Les régimes hyperprotéiques sont couramment consommés mais les conséquences au niveau du gros intestin sont peu connues. L’objectif de la thèse était d’étudier les effets des régimes hyperprotéiques et des métabolites bactériens dérivés des acides aminés sur la muqueuse du gros intestinUne série d’expérimentations animales et in vitro a permis de montrer que deux métabolites bactériens dérivés des acides aminés (le sulfure d’hydrogène et le p-cresol) sont toxiques pour l’épithélium lorsqu’ils sont présents en concentration élevée. Les résultats obtenus lors d’une étude clinique montrent que la quantité et la qualité des protéines alimentaires n’ont pas d’effets marqués sur la composition du microbiote fécal mais modifient les concentrations fécales et urinaires en métabolites bactériens.Ces modifications de l’environnement luminal du gros intestin n’étaient pas associées à une augmentation de la cytotoxicité des eaux fécales in vitro. Néanmoins, dans la muqueuse rectale, l’augmentation de l’apport en protéines a régulé l’expression de gènes impliqués dans le maintien de l’homéostasie et ces effets étaient distincts en fonction de la source de protéines utilisée. Toutefois, le niveau d’apport en protéines n’avait pas d’effet sur les paramètres inflammatoires et histologiques dans la muqueuse. Ces résultats ont été complétés par une étude chez le rat montrant qu’un régime hyperprotéique modifie le transcriptome dans les colonocytes mais n’a pas d’effets délétères en termes d’intégrité de l’ADN, de renouvellement de l’épithélium et de fonction barrière. / Abstract: High-protein diets are frequently consumed but the consequences for the large intestine are not well described. The objective of this thesis was to evaluate the effects of high-protein diets and of amino-acid derived bacterial metabolites on the large intestine mucosa. Animal and in vitro studies showed that two amino acid derived bacterial metabolites (hydrogen sulfide and p-cresol) are toxic for the epithelium when present at high concentration. The results obtained in a clinical trial indicate that quantity and quality of dietary protein do not have major effects on the fecal microbiota composition but modify the fecal and urinary concentration of bacterial metabolites.These changes in luminal environment were not associated with an increase in fecal water cytotoxicity in vitro. Nevertheless, in the rectal mucosa, the increase in protein intake regulated the expression of genes implicated in homeostatic processes and these effects were modulated by the source of protein. However, the level of protein intake had no effect on immune and histological parameters in the mucosa. These results were completed with a study in rats showing a clear transcriptome profile in colonocytes induced by a high-protein diet but that was not associated with detrimental effects in terms of DNA integrity, epithelial renewal and barrier function.

Osteoporosis, a major public health problem, likely has its origins in childhood. During periods of rapid skeletal growth, diet may influence accrual of bone mineral density (BMD) and adult bone health. This study used novel approaches in bone imaging to further characterize optimal skeletal development and enhance our understanding of key dietary components that influence attainment of peak bone mass (PBM) and contribute to determinants of peak bone strength in peri-pubertal females. The use of a validated food-frequency questionnaire (FFQ) enabled the influence of usual dietary intake on bone parameters to be examined.This study examined the relationship of dietary intake of micronutrients and bone macro-architectural structure in peri-pubertal girls. This study suggested that vitamin C and zinc intake are associated with objective measures of bone status in 4th, but not 6th grade girls. This indicates potential differences in micronutrient and bone associations at various age-associated stages of bone maturation.The impact of dietary fat on peri-pubertal skeletal growth is not well characterized. This study examined relationships of select dietary fatty acid (FA) intakes and measures of bone status in peri-pubertal girls. This study suggested that MUFA, total PUFA, n-6 and linoleic acid (LA) are inversely associated with bone status prior to menarche, but composition of dietary fat may be more important during the early-pubertal years. Decreased intakes of n-6 PUFA may benefit bone health in young girls.The impact of a dietary protein on volumetric bone mineral density (vBMD), bone mineral content (BMC) and bone strength throughout maturation remains controversial. Given evidence of both anabolic and catabolic affects of protein on bone health, this study examined relations of dietary protein from different sources with bone parameters in peri-pubertal girls. This study showed that dietary protein intake is related to higher trabecular but not cortical vBMD, BMC and BSI, and accounts for 2-4% of their variability in peri-pubertal girls. The relationship seems to vary by the source of dietary protein and calcium intake. However, a negative impact of animal protein on bone health is not supported. Large scale observational and intervention studies are needed to establish causality.

A series of experiments were conducted to investigate how dietary and ruminal factors regulate urea-N recycling in ruminants. In Experiments 1, 2, and 3, urea-N kinetics were measured using 4-d intra-jugular infusions of [15N15N]-urea. In Experiment 1, the objective was to determine how interactions between dietary ruminally-degradable protein (RDP) level and ruminally-fermentable carbohydrate (RFC) may alter urea-N transfer to the gastrointestinal tract (GIT) and the utilization of this recycled urea-N in rapidly-growing lambs fed high N diets. The dietary factors were: 1) dry-rolled barley (DRB) vs. pelleted barley (PB) as the principal source of RFC; and 2) dietary levels of RDP of 60 vs. 70% (% of CP). Nitrogen intake, fecal and urinary N excretion increased as dietary RDP level increased; however, method of barley processing had no effect on N use. Dietary treatment had no effect on urea-N kinetics; however, endogenous production of urea-N (UER) exceeded N intake. For all diets, 0.669 to 0.742 of UER was recycled to the GIT; however, 0.636 to 0.756 of the GER was returned to the ornithine cycle. In Experiment 2, the objective was to delineate the effects of partial defaunation of the rumen on urea-N kinetics in lambs fed low or high N diets. Treatments were: 1) partial defaunation (PDFAUN) vs. faunation (FAUN); and 2) low (10%, LOW) vs. high (15%, HIGH) dietary CP. Linoleic acid-rich sunflower oil was fed as a partially-defaunating agent. Partial defaunation decreased ruminal NH3-N concentrations. The UER and urinary urea-N excretion (UUE) were lower, and the GER tended to be lower in PDFAUN as compared to FAUN lambs; however, as a proportion of UER, GER was higher and the proportion of recycled urea-N that was utilized for anabolism (i.e., UUA) tended to be higher in PDFAUN lambs. The UER, GER and UUE were higher in lambs fed diet HIGH; however, as a proportion of UER, GER and its anabolic use were higher in lambs fed diet LOW. In Experiment 3, the objective was to delineate how, at similar N intakes, interactions between ruminal partial defaunation and altering dietary RFC may alter urea-N kinetics and N metabolism in lambs. Treatments were: 1) PDFAUN vs. FAUN; and 2) DRB vs. PB. Urinary N excretion was lower and retained N was higher in PDFAUN compared to FAUN lambs. The UER was similar across treatments; however, the GER, expressed as absolute amounts or as a proportion of UER, UUA, and microbial N supply were higher in PDFAUN compared to FAUN lambs. As a proportion of UER, GER was higher, whereas UUE was lower in lambs fed PB compared to those fed DRB. In Experiment 4, the objective was to determine the effects of feeding oscillating dietary CP compared to static dietary CP concentration on N retention and in vitro urea flux across ruminal epithelia. Dietary treatments consisted of a medium CP diet (MEDIUM; 12.8% CP) or diets with oscillating CP content (OSC) fed in two different sequences i.e., 2 d of low CP (9.7% CP) followed by 2 d of high CP (16.1% CP; OSC-HIGH) or vice-versa (OSC-LOW). Ruminal epithelial tissues were collected and mounted in Ussing chambers under short-circuit conditions and the serosal-to-mucosal urea flux (Jsm-urea) was measured using 14C-urea. Although N intake was similar, retained N and microbial N supply were greater in lambs fed the OSC diets compared to those fed the MEDIUM diet. The total Jsm-urea was higher in lambs fed the OSC-LOW compared to those fed the OSC-HIGH diet. Across diets, the addition of phloretin (a known specific inhibitor of facilitative urea transporter-B; UT-B) reduced Jsm-urea; however, phloretin-insensitive Jsm-urea was the predominant route for transepithelial urea transfer. In summary, data presented in this thesis provide new insights that the improved N retention typically observed in defaunated ruminants and in ruminants fed oscillating dietary CP concentrations is partly mediated via increased urea-N recycling to the GIT and utilization of recycled urea-N for anabolic purposes.

A series of experiments were conducted to investigate how dietary and ruminal factors regulate urea-N recycling in ruminants. In Experiments 1, 2, and 3, urea-N kinetics were measured using 4-d intra-jugular infusions of [15N15N]-urea. In Experiment 1, the objective was to determine how interactions between dietary ruminally-degradable protein (RDP) level and ruminally-fermentable carbohydrate (RFC) may alter urea-N transfer to the gastrointestinal tract (GIT) and the utilization of this recycled urea-N in rapidly-growing lambs fed high N diets. The dietary factors were: 1) dry-rolled barley (DRB) vs. pelleted barley (PB) as the principal source of RFC; and 2) dietary levels of RDP of 60 vs. 70% (% of CP). Nitrogen intake, fecal and urinary N excretion increased as dietary RDP level increased; however, method of barley processing had no effect on N use. Dietary treatment had no effect on urea-N kinetics; however, endogenous production of urea-N (UER) exceeded N intake. For all diets, 0.669 to 0.742 of UER was recycled to the GIT; however, 0.636 to 0.756 of the GER was returned to the ornithine cycle. In Experiment 2, the objective was to delineate the effects of partial defaunation of the rumen on urea-N kinetics in lambs fed low or high N diets. Treatments were: 1) partial defaunation (PDFAUN) vs. faunation (FAUN); and 2) low (10%, LOW) vs. high (15%, HIGH) dietary CP. Linoleic acid-rich sunflower oil was fed as a partially-defaunating agent. Partial defaunation decreased ruminal NH3-N concentrations. The UER and urinary urea-N excretion (UUE) were lower, and the GER tended to be lower in PDFAUN as compared to FAUN lambs; however, as a proportion of UER, GER was higher and the proportion of recycled urea-N that was utilized for anabolism (i.e., UUA) tended to be higher in PDFAUN lambs. The UER, GER and UUE were higher in lambs fed diet HIGH; however, as a proportion of UER, GER and its anabolic use were higher in lambs fed diet LOW. In Experiment 3, the objective was to delineate how, at similar N intakes, interactions between ruminal partial defaunation and altering dietary RFC may alter urea-N kinetics and N metabolism in lambs. Treatments were: 1) PDFAUN vs. FAUN; and 2) DRB vs. PB. Urinary N excretion was lower and retained N was higher in PDFAUN compared to FAUN lambs. The UER was similar across treatments; however, the GER, expressed as absolute amounts or as a proportion of UER, UUA, and microbial N supply were higher in PDFAUN compared to FAUN lambs. As a proportion of UER, GER was higher, whereas UUE was lower in lambs fed PB compared to those fed DRB. In Experiment 4, the objective was to determine the effects of feeding oscillating dietary CP compared to static dietary CP concentration on N retention and in vitro urea flux across ruminal epithelia. Dietary treatments consisted of a medium CP diet (MEDIUM; 12.8% CP) or diets with oscillating CP content (OSC) fed in two different sequences i.e., 2 d of low CP (9.7% CP) followed by 2 d of high CP (16.1% CP; OSC-HIGH) or vice-versa (OSC-LOW). Ruminal epithelial tissues were collected and mounted in Ussing chambers under short-circuit conditions and the serosal-to-mucosal urea flux (Jsm-urea) was measured using 14C-urea. Although N intake was similar, retained N and microbial N supply were greater in lambs fed the OSC diets compared to those fed the MEDIUM diet. The total Jsm-urea was higher in lambs fed the OSC-LOW compared to those fed the OSC-HIGH diet. Across diets, the addition of phloretin (a known specific inhibitor of facilitative urea transporter-B; UT-B) reduced Jsm-urea; however, phloretin-insensitive Jsm-urea was the predominant route for transepithelial urea transfer. In summary, data presented in this thesis provide new insights that the improved N retention typically observed in defaunated ruminants and in ruminants fed oscillating dietary CP concentrations is partly mediated via increased urea-N recycling to the GIT and utilization of recycled urea-N for anabolic purposes.

In the human gut, bacterial fermentation of dietary fibers and proteins produces metabolites, primarily as short-chain fatty acids (SCFA), that are highly beneficial for host health. However, unlike dietary fiber, bacterial fermentation of protein additionally generates potentially toxic substances such as ammonia, hydrogen sulfide, amines, and indoles. It is believed that most gut bacteria favor utilization of dietary fiber over that of protein for energy. Therefore, when fermentable dietary fiber is readily available to colonic bacteria, protein fermentation, and its subsequent potentially toxic metabolites, remains relatively low. Dietary intake primarily determines the quantity of dietary fiber and protein substrate available to the gut microbiota and the resulting profile of metabolites produced. Increased protein consumption is associated with deleterious health outcomes such as higher risk of colorectal cancer and type II diabetes. Conversely, diets following US dietary recommendations are high in fiber, which promote a healthy microbiome and are protective against disease. Diets following the recommendation are also moderate in protein intake so that, ultimately, far more fiber than protein is available for colonic bacterial fermentation. On the contrary, dietary fiber intake is chronically low in a standard Western diet, while protein consumption is above dietary recommendations, which results in nearly equal amounts of dietary fiber and protein available for gut microbial fermentation. Furthermore, the popularity of high-protein diets for athletes, as well as that of high-protein low-carbohydrate diets for weight loss, may flip fiber and protein substrate proportions upside down, resulting in more protein than fiber available in the gut for fermentation. The objective of this study was to elucidate how substrate ratios in protein-fiber mixtures affect protein fermentation and metabolites, as well as examine the degree to which fiber source may influence these outcomes. Each dietary fiber source [fructooligosaccharides (FOS), apple pectin (Pectin), a wheat bran and raw potato starch mixture (WB+PS), and an even mixture of the three aforementioned fibers (Even Mix)] and protein were combined in three ratios and provided as substrate for in vitro fecal fermentation to understand how low, medium, and high fiber inclusion levels influence fermentation outcomes. They were compared to 100% protein and fiber (each different fiber) controls. Branched-chain fatty acids (BCFAs), metabolites produced exclusively from protein fermentation, were used as a measure of protein fermentation; the data were normalized based on the initial quantity of protein within the substrate. In protein-fiber substrate mixtures, only FOS and Even Mix inhibited BCFAs (mM/g protein basis) and only when they made up at least half of the substrate. Unexpectedly, the rate of protein fermentation was increased when the protein-fiber substrate contained 25% WB+PS fiber, possibly due to the starch component of the fiber. There was evidence that when pH drops during fermentation, as was the case for protein-FOS mixtures, it played a significant role in suppressing protein fermentation. Ammonia production was not largely affected by increasing the proportion of dietary fiber. A significant reduction did not occur until FOS made up at least 50% of the protein-fiber substrate; for Pectin, WB+PS, and Even Mix fibers, 75% inclusion was required for a significant decrease in ammonia. Interestingly, protein was butyrogenic. Protein as the sole substrate produced more butyrate than either Pectin or Even Mix as the sole substrates, and in fact, addition of Pectin to protein significantly reduced butyrate concentrations. However, the possible benefits of butyrate produced via protein fermentation needs to be tempered by the production of potentially toxic compounds and the association between protein fermentation and colorectal cancer. Overall, the thesis findings showed protein fermentation to be relatively stable and not easily influenced by increasing the availability of dietary fiber, and no clear evidence of microbial preference for carbohydrates over protein was found.

La cicatrisation complète de la muqueuse, définie comme l'absence de lésions visibles par endoscopie, est considérée comme un objectif thérapeutique dans la prévention des complications associées aux Maladies Inflammatoires Chroniques de l’Intestin (MICI).Dans ce contexte, le rôle de l’apport protéique alimentaire et les besoins protéiques nécessaires à la cicatrisation ont été peu étudiés. L’objectif de cette thèse était d’évaluer l’effet du niveau d’apport protéique alimentaire sur la réparation épithéliale après un épisode inflammatoire intestinal dans un modèle murin de colite chimio-induite. Dans un premier temps, l’analyse de la progression de certains modulateurs impliqués dans le processus cicatriciel a mis en évidence que la réparation colique s’initiait et se consolidait avant que l’inflammation ne soit résolue et cela, dans un contexte où la composition du microbiote adhérent à la muqueuse était altérée de manière persistante. Les effets de trois régimes alimentaires ayant un niveau d'apport protéique différent (moyen, modérément élevé et élevé) ont ensuite été évalués sur la réparation de la muqueuse colique, ce qui a permis de montrer qu’au-delà d’un certain seuil, le niveau d’apport protéique aggravait et perpétuait l’inflammation colique. En revanche, un apport modérément élevé en protéines était bénéfique par rapport à un apport moyen, de par ses effets sur la perméabilité colique, l'hyper-prolifération cryptique, l’expression de plusieurs gènes codant pour des facteurs de réparation et sur la modulation de la composition du microbiote adhérent. Enfin, ces travaux ont montré que l’inflammation et le niveau d’apport en protéines affectaient le métabolisme protéique dans des organes non-cibles de l’inflammation colique en association avec une endotoxémie persistante.Ce travail a ainsi permis de mieux comprendre les événements locaux et périphériques impliqués dans la cicatrisation de la muqueuse colique et leur modulation par un apport majoré en protéines suite à un épisode inflammatoire aigu. / Advanced mucosal healing, defined by endoscopy as the absence of visible lesions, is considered as a therapeutic goal in the prevention of complications associated with IBD.In this context, the role of dietary protein intake and the protein requirements for mucosal healing have been poorly studied. The aim of this thesis was to evaluate the effect of dietary protein intake level on epithelial repair after an acute intestinal inflammatory episode in a murine model of colitis. Firstly, the progression analysis of several modulators involved in the repairing process showed that colonic repair can be initiated and consolidated in the context of inflamed mucosa, associated with persistent alterations of the colonic luminal environment. The effect of three diets with different levels of protein intake (average, moderately high and high) on colon mucosa repair were evaluated in the same model. This study showed that, beyond a threshold, the level of protein intake aggravated and perpetuated colitis. However, a moderately high protein intake was beneficial due to its effect on colonic permeability, cryptic hyper-proliferation, expression of multiple genes encoding repair factors, and composition modulation of the mucosal-adherent microbiota. Finally, both inflammation and dietary protein intake levels altered protein metabolism of other organs at the periphery of the inflammation in association with persistent endotoxemia.This work deepened the understanding of the events involved in the epithelial repair process and their modulation by an increase in the dietary protein intake after an acute episode of colitis.