Dietary ratio of protein to carbohydrate induces plastic responses in the gastrointestinal tract of mice

The gut microbiome influences host diet selection behavior

The behavior of diet selection or diet choice can have wide-reaching implications, scaling from individual animals to ecological and evolutionary processes. Previous work in this area has largely ignored the potential for intestinal microbiota to modulate host foraging decisions. The notion that the gut microbiome may influence host foraging behavior has been highly speculated for years but has not yet been explicitly tested. Here, we show that germ-free mice colonized by differential microbiomes from wild rodents with varying natural feeding strategies exhibited significant differences in their voluntary dietary selection. Specifically, colonized mice differed in voluntary carbohydrate selection, and divergent feeding preferences were associated with differences in circulating essential amino acids, bacterial tryptophan metabolism, and intestinal morphology. Together, these results demonstrate a role for the microbiome in host nutritional physiology and foraging behavior.

Diet selection is a fundamental aspect of animal behavior with numerous ecological and evolutionary implications. While the underlying mechanisms are complex, the availability of essential dietary nutrients can strongly influence diet selection behavior. The gut microbiome has been shown to metabolize many of these same nutrients, leading to the untested hypothesis that intestinal microbiota may influence diet selection. Here, we show that germ-free mice colonized by gut microbiota from three rodent species with distinct foraging strategies differentially selected diets that varied in macronutrient composition. Specifically, we found that herbivore-conventionalized mice voluntarily selected a higher protein:carbohydrate (P:C) ratio diet, while omnivore- and carnivore-conventionalized mice selected a lower P:C ratio diet. In support of the long-standing hypothesis that tryptophan—the essential amino acid precursor of serotonin—serves as a peripheral signal regulating diet selection, bacterial genes involved in tryptophan metabolism and plasma tryptophan availability prior to the selection trial were significantly correlated with subsequent voluntary carbohydrate intake. Finally, herbivore-conventionalized mice exhibited larger intestinal compartments associated with microbial fermentation, broadly reflecting the intestinal morphology of their donor species. Together, these results demonstrate that gut microbiome can influence host diet selection behavior, perhaps by mediating the availability of essential amino acids, thereby revealing a mechanism by which the gut microbiota can influence host foraging behavior.

Cardio-Metabolic Effects of High-Fat Diets and Their Underlying Mechanisms-A Narrative Review

The majority of the epidemiological evidence over the past few decades has linked high intake of fats, especially saturated fats, to increased risk of diabetes and cardiovascular disease. However, findings of some recent studies (e.g., the PURE study) have contested this association. High saturated fat diets (HFD) have been widely used in rodent research to study the mechanism of insulin resistance and metabolic syndrome. Two separate but somewhat overlapping models-the diacylglycerol (DAG) model and the ceramide model-have emerged to explain the development of insulin resistance. Studies have shown that lipid deposition in tissues such as muscle and liver inhibit insulin signaling via the toxic molecules DAG and ceramide. DAGs activate protein kinase C that inhibit insulin-PI3K-Akt signaling by phosphorylating serine residues on insulin receptor substrate (IRS). Ceramides are sphingolipids with variable acyl group chain length and activate protein phosphatase 2A that dephosphorylates Akt to block insulin signaling. In adipose tissue, obesity leads to infiltration of macrophages that secrete pro-inflammatory cytokines that inhibit insulin signaling by phosphorylating serine residues of IRS proteins. For cardiovascular disease, studies in humans in the 1950s and 1960s linked high saturated fat intake with atherosclerosis and coronary artery disease. More recently, trials involving Mediterranean diet (e.g., PREDIMED study) have indicated that healthy monounsaturated fats are more effective in preventing cardiovascular mortality and coronary artery disease than are low-fat, low-cholesterol diets. Antioxidant and anti-inflammatory effects of Mediterranean diets are potential mediators of these benefits.

Cardio-metabolic consequences of dietary carbohydrates: reconciling contradictions using nutritional geometry

Carbohydrates are the major source of dietary energy, but their role in health and disease remains controversial. Recent epidemiological evidence suggests that the increased consumption of carbohydrates is associated with obesity and increased risk of mortality and dietary trials show that carbohydrate restriction leads to weight loss and improved glycaemic status in obese and diabetic subjects. In contrast, the diets of populations with long and healthy lifespans (e.g. traditional Okinawans from Japan) are high in carbohydrate and low in protein, and several clinical and preclinical studies have linked low-carbohydrate-high-protein diets with increased mortality risk. In this paper we attempt to reconcile these contradictory findings by moving beyond traditional single-nutrient analyses to consider the interactions between nutrients on health outcomes. We do so using the Geometric Framework (GF), a nutritional modelling platform that explicitly considers the main and interactive effects of multiple nutrients on phenotypic characteristics. Analysis of human data by GF shows that weight loss and improved cardio-metabolic outcomes under carbohydrate restriction derive at least in part from reduced caloric intake due to the concomitantly increased proportion of protein in the diet. This is because, as in many animals, a specific appetite for protein is a major driver of food intake in humans. Conversely, dilution of protein in the diet leverages excess food intake through compensatory feeding for protein ('protein leverage'). When protein is diluted in the diet by readily digestible carbohydrates and fats, as is the case in modern ultra-processed foods, protein leverage results in excess calorie intake, leading to rising levels of obesity and metabolic disease. However, when protein is diluted in the diet by increased quantities of less readily digestible forms of carbohydrate and fibre, energy balance is maintained and health benefits accrue, especially during middle age and early late-life. We argue that other controversies in carbohydrate research can be resolved using the GF methodology in dietary studies.

Compensatory changes in villus morphology of lactating Mus musculus in response to insufficient dietary protein

Energetic challenges match intestinal size to dietary intake but less is known about how the intestine responds to specific macronutrient challenges. We examined how intestinal size responds to insufficient dietary protein at the microscopic level. Villi, enterocytes and surface area were measured across the length of the small intestine in non-reproductive and lactating Mus musculus fed isocaloric control or protein-deficient diets. Lactating mice on the protein-deficient diet exhibited a 24% increase in villus height and a 30% increase in enterocyte width in the proximal small intestine and an overall similar increase in surface area; on the control diet, changes in villus height were localized in the mid region. Flexibility localized to the proximal small intestine suggests that enterokinase, a localized enzyme, may be a candidate enzyme that promotes compensation for a protein-deficient diet. Such flexibility could allow species to persist in the face of anthropogenically induced changing dietary profiles.

Diet composition modulates intestinal hydrolytic enzymes in white-footed mice (Peromyscus leucopus)

We tested whether white-footed mice (Peromyscus leucopus) modulate the activity of three key intestinal digestive enzymes (maltase, sucrase, and aminopeptidase-N [APN]) based on diet composition. To test the adaptive modulation hypothesis (AMH), we fed mice either of three kinds of synthetic diet, high starch (HS, 50% carbohydrate), high protein (HP, 60% protein), and high lipid (HL, 25% lipid), and determined their digestive responses. First, there was no effect of either diet itself, or time eating the diet, on body mass, or mass and length of small intestine. Second, the activity of both disaccharidases summed over the entire small intestine was highest on the HS diet, which was higher than on the HP diet by about 45% and higher than on the HL diet by 400%. This was consistent with our prediction that starch induces disaccharidase activity, and demonstrated induction of disaccharidase activities by high dietary carbohydrate in a wild mammal. Third, both summed and mass-specific activity of maltase and sucrase of HL mice were lower than those of HP mice, even though their diets had the same content of starch, which suggests that lipid in the HL diet inhibited disaccharidase activity. Finally, the summed activity of APN was highest on the HP diet, which was higher than on the HS diet or HL diet by ~100%, consistent with our prediction that high protein content induces peptidase activity. Taken together, our results support the AMH, though they also illustrate that high lipid content in the diet can confound some predicted patterns. Flexibility of digestive enzyme activity is likely important in allowing white-footed mice to cope with fluctuations in the environmental availability of different food types.

Probamos si el ratón de patas blancas (Peromyscus leucopus) modula las actividades de tres enzimas digestivas intestinales claves – maltasa, sacarasa y N-aminopeptidasa- al modificarse la composición de la dieta. Para someter a prueba la hipótesis de la modulación adaptativa, se alimentaron paralelamente ratones con tres tipos de dietas semi-sintéticas, una alta en almidón (HS, 50% carbohidratos), otra alta en proteína (HP, 60% proteínas), y una alta en lípidos (HL, 25% lípidos), y se determinaron sus respuestas digestivas. No se observó un efecto de la dieta o del tiempo que la consumieron sobre la masa corporal o la masa y el largo del intestino delgado (SI). La sumatoria de las actividades de cada una de las disacaridasas a lo largo de todo el intestino delgado fue más alta con la dieta HS que con las dietas HP y HL, un 45% y un 400% mayor, respectivamente. Esto fue consistente con nuestra predicción acerca de que el almidón induce la actividad disacaridásica, constituyendo el primer estudio que demuestra inequívocamente en un animal silvestre, que la inducción de las actividades de las disacaridasas intestinales es mediada por un incremento de los carbohidratos en la dieta. Las actividades hidrolíticas totales y masa-específicas de la maltasa y sacarasa de los ratones HL fueron más bajas que las de los alimentados con dieta HP, aun cuando sus dietas tenían el mismo contenido de almidón, lo que sugiere que los lípidos en la dieta HL inhiben la actividad de las disacaridasas. La actividad hidrolítica total de la N-aminopeptidasa fue mayor con la dieta HP, ~100% más alta que para las dietas HS y HL, de manera consistente con la predicción que propone que la presencia de mayor cantidad de proteína en la dieta induce la actividad peptidásica. En conjunto nuestros resultados dan soporte a la hipótesis de la modulación adaptativa, además de ilustrar que los lípidos en las dietas pueden confundir la predicción de patrones de procesamiento de alimentos. La flexibilidad de la actividad de las enzimas digestivas es probablemente importante para los ratones de patas blancas, ya que les permite adecuarse a las fluctuaciones ambientales de disponibilidad de diferentes tipos de recursos.

Low-carbohydrate diet induces metabolic depression: a possible mechanism to conserve glycogen

Long-term studies have found that low-carbohydrate diets are more effective for weight loss than calorie-restricted diets in the short term but equally or only marginally more effective in the long term. Low-carbohydrate diets have been linked to reduced glycogen stores and increased feelings of fatigue. We propose that reduced physical activity in response to lowered glycogen explains the diminishing weight loss advantage of low-carbohydrate compared with low-calorie diets over longer time periods. We explored this possibility by feeding adult Drosophila melanogaster a standard or a low-carbohydrate diet for 9 days and measured changes in metabolic rate, glycogen stores, activity, and body mass. We hypothesized that a low-carbohydrate diet would cause a reduction in glycogen stores, which recover over time, a reduction in physical activity, and an increase in resting metabolic rate. The low-carbohydrate diet reduced glycogen stores, which recovered over time. Activity was unaffected by diet, but metabolic rate was reduced, in the low-carbohydrate group. We conclude that metabolic depression could explain the decreased effectiveness of low-carbohydrate diets over time and recommend further investigation of long-term metabolic effects of dietary interventions and a greater focus on physiological plasticity within the study of human nutrition.

Diet-Microbiome Interactions in Health Are Controlled by Intestinal Nitrogen Source Constraints

Diet influences health and patterns of disease in populations. How different diets do this and why outcomes of diets vary between individuals are complex and involve interaction with the gut microbiome. A major challenge for predicting health outcomes of the host-microbiome dynamic is reconciling the effects of different aspects of diet (food composition or intake rate) on the system. Here we show that microbial community assembly is fundamentally shaped by a dichotomy in bacterial strategies to access nitrogen in the gut environment. Consequently, the pattern of dietary protein intake constrains the host-microbiome dynamic in ways that are common to a very broad range of diet manipulation strategies. These insights offer a mechanism for the impact of high protein intake on metabolic health and form the basis for a general theory of the impact of different diet strategies on host-microbiome outcomes.

Effect of antibiotic on survival and development of Spodoptera litura (Lepidoptera: Noctuidae) and its gut microbial diversity

Addition of antibiotics to artificial diets of insects is a key component in the rearing of insects in the laboratory. In the present study an antimicrobial agent, streptomycin sulphate was tested for its influence on survival and fitness of Spodoptera litura (Fabricus) (Lepidoptera: Noctuidae) as well as its gut microbial diversity. The antibiotic did not adversely affect the survival of S. litura. Faster growth of larvae was recorded on diet amended with different concentrations of streptomycin sulphate (0.03, 0.07 and 0.15%) as compared to diet without streptomycin sulphate. The overall activity of various digestives enzymes increased on S+ diet while the activity of detoxifying enzymes significantly decreased. In addition, alteration in microbial diversity was found in the gut of S. litura larvae fed on diet supplemented with antibiotic (S+) and without antibiotic (S-).

Macronutrients mediate the functional relationship between Drosophila and Wolbachia

Wolbachia are maternally inherited bacterial endosymbionts that naturally infect a diverse array of arthropods. They are primarily known for their manipulation of host reproductive biology, and recently, infections with Wolbachia have been proposed as a new strategy for controlling insect vectors and subsequent human-transmissible diseases. Yet, Wolbachia abundance has been shown to vary greatly between individuals and the magnitude of the effects of infection on host life-history traits and protection against infection is correlated to within-host Wolbachia abundance. It is therefore essential to better understand the factors that modulate Wolbachia abundance and effects on host fitness. Nutrition is known to be one of the most important mediators of host-symbiont interactions. Here, we used nutritional geometry to quantify the role of macronutrients on insect-Wolbachia relationships in Drosophila melanogaster. Our results show fundamental interactions between diet composition, host diet selection, Wolbachia abundance and effects on host lifespan and fecundity. The results and methods described here open a new avenue in the study of insect-Wolbachia relationships and are of general interest to numerous research disciplines, ranging from nutrition and life-history theory to public health.

Recent advances in the integrative nutrition of arthropods

In this review we highlight recent advances in four areas in which nutrition shapes the relationships between organisms: between plants and herbivores, between hosts and their microbiota, between individuals within groups and societies, and between species within food webs. We demonstrate that taking an explicitly multidimensional view of nutrition and employing the logic of the geometric framework for nutrition provide novel insights and offer a means of integration across different levels of organization, from individuals to ecosystems.

Towards an integrated understanding of gut microbiota using insects as model systems

Metazoans form symbioses with microorganisms that synthesize essential nutritional compounds and increase their efficiency to digest and absorb nutrients. Despite the growing awareness that microbes within the gut play key roles in metabolism, health and development of metazoans, symbiotic relationships within the gut are far from fully understood. Insects, which generally harbor a lower microbial diversity than vertebrates, have recently emerged as potential model systems to study these interactions. In this review, we give a brief overview of the characteristics of the gut microbiota in insects in terms of low diversity but high variability at intra- and interspecific levels and we investigate some of the ecological and methodological factors that might explain such variability. We then emphasize how studies integrating an array of techniques and disciplines have the potential to provide new understanding of the biology of this micro eco-system.

Balancing of specific nutrients and subsequent growth and body composition in the slug Arion lusitanicus

Feeding generalists typically occupy broad ecological niches and so are potentially pre-adapted to a range of novel food objects. In northern Europe, the slug Arion lusitanicus has spread rapidly as an invasive species and a serious horticultural and agricultural pest. We used nutritional geometry to analyze nutrient balancing capabilities and consequences for performance in A. lusitanicus when provided with one of three nutritionally fixed diets or when given dietary choice. The slugs over-ingested high amounts of the most abundant nutrient in order to get more of the limited nutrient. However, they regulated protein intake more tightly than carbohydrate intake resulting in a very high food intake when fed a protein-poor diet. Growth and body composition were highly affected by the nutrient balance of their diet. When given the choice to feed from two nutritionally different diets, the slugs selected an intake balance of protein and carbohydrate with sufficient precision to maximize growth. Nutrient utilization efficiency increased with increasing deficiency of the specific nutrient in the diet. Ingested carbohydrate was more efficiently stored as lipid in slugs fed more carbohydrate-poor diets, and ingested nitrogen was more efficiently incorporated into slug bodies in slugs fed more protein-poor diets. Our experiments suggest that the evolved behavioral and physiological regulatory capacities of A. lusitanicus may explain some of the success that this slug experiences as an invasive species. We furthermore propose that invasive species might be more dependent on high protein availability in the environment than non-invasive species.

Lactating mice (Mus musculus) exhibit compensatory flexibility in gut morphology in response to reduced dietary protein

Protein affects key life-history traits, and deficiencies in this nutrient may have selected for the ability to invoke physiological or morphological mechanisms to aid nutrient assimilation. I examined the effect of dietary protein on gut characters in lactating mice (Mus musculus L., 1758) and predicted that mice, to improve assimilation efficiency, would increase the mass of the stomach and small intestine and (or) increase food retention in these organs. Mice were maintained on isocaloric diets differing in protein and carbohydrate content (P:C) during the reproductive period. The hypothesis that food would be preferentially retained was not supported. However, both the stomach and the small intestine responded to low P:C with increased mass, and the small intestine exhibited increased diameter. This study demonstrates that mammalian gut morphology of lactating mice can respond to nutrient availability under conditions of constant energy intake. Further study is needed to determine if gut flexibility in response to decreasing P:C levels results in improved nitrogen assimilation efficiency and if this response is a general strategy of mammals or is limited to those with particular breeding strategies.

Integrating nutrition and immunology: a new frontier

Nutrition is critical to immune defence and parasite resistance, which not only affects individual organisms, but also has profound ecological and evolutionary consequences. Nutrition and immunity are complex traits that interact via multiple direct and indirect pathways, including the direct effects of nutrition on host immunity but also indirect effects mediated by the host's microbiota and pathogen populations. The challenge remains, however, to capture the complexity of the network of interactions that defines nutritional immunology. The aim of this paper is to discuss the recent findings in nutritional research in the context of immunological studies. By taking examples from the entomological literature, we argue that insects provide a powerful tool for examining the network of interactions between nutrition and immunity due to their tractability, short lifespan and ethical considerations. We describe the relationships between dietary composition, immunity, disease and microbiota in insects, and highlight the importance of adopting an integrative and multi-dimensional approach to nutritional immunology.

Protein-induced weight increase of the gastrointestinal tract of locusts improves net nutrient uptake via larger meals rather than more efficient nutrient absorption

Increasing the tissue biomass and/or volume of the gastrointestinal tract (GIT) is commonly seen when animals feed on poor quality diets. This increase can simply permit larger meal sizes, but may also rebalance nutritionally imbalanced ingesta by allowing selective absorption of limiting nutrients. In an insect herbivore, the migratory locust, a synthetic diet with a high ratio of protein to carbohydrate was found to induce weight enhancement of the GIT. When normalized for sex and overall body size, increases to the mass of the foregut and midgut caeca resulted in higher absorption (20-30%) of both protein and carbohydrate when subsequently feeding on three chemically and structurally different grasses. Greater net absorption of macronutrients occurred because these locusts ate larger meals that transited at the same time and with the same digestive efficiency as locusts in which the GIT was not enlarged. Thus, plasticity of the GIT did not improve nutritional homeostasis, but increased the rate of nutrient uptake.

The nature of nutrition: a unifying framework

We present a graphical approach, which we believe can help to integrate nutrition into the broader biological sciences, and introduce generality into the applied nutritional sciences. This ‘Geometric Framework’ takes account of the fact that animals need multiple nutrients in changing amounts and balance, and that nutrients come packaged in foods that are often hard to find, dangerous to subdue and costly to process. We then show how the Geometric Framework has been used to understand the links between nutrition and relevant aspects of the biology of individual animals. These aspects include the physiological mechanisms that direct the nutritional interactions of the animal with its environment, and the fitness consequences of these interactions. Having considered the implications of diet for individuals, we show that these effects can translate into the collective behaviour of groups and societies, and in turn ramify throughout food webs to influence the structure of ecosystems.

Maternal preconceptional nutrition leads to variable fat deposition and gut dimensions of adult offspring mice (C57BL/6JBom)

BACKGROUND

Maternal nutrition during pregnancy or lactation may affect the chance of offspring becoming obese as adults, but little is known regarding the possible role of maternal nutrition before conception. In this study, we investigate how variable protein and carbohydrate content of the diet consumed before pregnancy affects fat deposition and gut dimensions of offspring mice.

METHODS

Eight-week-old female mice (C57BL/6JBom) were fed isocaloric low protein (8.4% protein; LP), standard protein (21.5% protein; ST) or high protein (44.2% protein; HP) diets. After 8 weeks of feeding, females were mated and fed a standard laboratory chow diet (22.5% protein) throughout periods of mating, gestation, lactation and weaning. Offspring mice were fed the same standard diet up to 46 days of age. Then offspring were killed and measures of dissected fat deposits and of the digestive system were taken.

RESULTS

Fat deposition of the offspring was significantly affected by preconceptional maternal nutrition and the effects differed between sexes. Male offspring deposited most fat when mothers were fed the LP diet, whereas female offspring deposited most fat when mothers were fed the ST diet. The mass and length of the digestive organs were affected by preconceptional maternal nutrition. Total gut from pyloric sphincter to anus was significantly shorter and dry mass was heavier in mice whose mothers were fed LP diets compared with offspring of mothers fed ST diets or HP diets. There was no significant effect of maternal nutrition on dry mass of the stomach or ceca.

CONCLUSION

Our study shows that preconceptional nutrition can have important influence on several body features of offspring in mice, including body composition and dimensions of the digestive system.

The gastrointestinal tract as a nutrient-balancing organ

Failure to provision tissues with an appropriate balance of nutrients engenders fitness costs. Maintaining nutrient balance can be achieved by adjusting the selection and consumption of foods, but this may not be possible when the nutritional environment is limiting. Under such circumstances, rebalancing of an imbalanced nutrient intake requires post-ingestive mechanisms. The first stage at which such post-ingestive rebalancing might occur is within the gastrointestinal tract (GIT), by differential release of digestive enzymes-releasing less of those enzymes for nutrients present in excess while maintaining or boosting levels of enzymes for nutrients in deficit. Here, we use an insect herbivore, the locust, to show for the first time that such compensatory responses occur within the GIT. Furthermore, we show that differential release of proteases and carbohydrases in response to nutritional state translate into differential extraction of macronutrients from host plants. The prevailing view is that physiological and structural plasticity in the GIT serves to maximize the rate of nutrient gain in relation to costs of maintaining the GIT; our findings show that GIT plasticity is integral to the maintenance of nutrient balance.