The effect of a low-protein diet and a cold environment on calorie intake and body composition in the rat

Biology of heat stress; the nexus between intestinal hyperpermeability and swine reproduction

Unfavorable weather conditions are one of the largest constraints to maximizing farm animal productivity. Heat stress (HS), in particular, compromises almost every metric of profitability and this is especially apparent in the grow-finish and reproductive aspects of the swine industry. Suboptimal production during HS was traditionally thought to result from hypophagia. However, independent of inadequate nutrient consumption, HS affects a plethora of endocrine, physiological, metabolic, circulatory, and immunological variables. Whether these changes are homeorhetic strategies to survive the heat load or are pathological remains unclear, nor is it understood if they temporally occur by coincidence or if they are chronologically causal. However, mounting evidence suggest that the origin of the aforementioned changes lie at the gastrointestinal tract. Heat stress compromises intestinal barrier integrity, and increased appearance of luminal contents in circulation causes local and systemic inflammatory responses. The resulting immune activation is seemingly the epicenter to many, if not most of the negative consequences HS has on reproduction, growth, and lactation. Interestingly, thermoregulatory and production responses to HS are only marginally related. In other words, increased body temperature indices poorly predict decreases in productivity. Further, HS induced malnutrition is also a surprisingly inaccurate predictor of productivity. Thus, selecting animals with a "heat tolerant" phenotype based solely or separately on thermoregulatory capacity or production may not ultimately increase resilience. Describing the physiology and mechanisms that underpin how HS jeopardizes animal performance is critical for developing approaches to ameliorate current production issues and requisite for generating future strategies (genetic, managerial, nutritional, and pharmaceutical) aimed at optimizing animal well-being, and improving the sustainable production of high-quality protein for human consumption.

Metabolomics of heat stress response in pig adipose tissue reveals alteration of phospholipid and fatty acid composition during heat stress

To determine the effect of heat stress (HS) on adipose tissue metabolome, a combination of liquid chromatography-mass spectrometry-based metabolomics profiling approaches was applied to characterize changes of metabolite classes in adipocytes differentiated in culture (in vitro) and mesenteric adipose tissue of pigs exposed to HS (in vivo). Effect of HS on the composition of individual fatty acids in cultured adipocytes, mesenteric adipose tissue, and serum of animals was also investigated using gas chromatography analysis. In vitro, preadipocytes were differentiated either under control (37 °C) or HS (41.5 °C) temperature for 9 d. For the animal experiment, pigs were kept either in control (Con) environment (20 °C) with ad libitum feed intake, HS (35 °C) temperature with ad libitum feed intake (HS), or at 20 °C with pair feeding to the HS pigs. In cultured cells, HS increased triglyceride and decreased monoacylglycerol (P < 0.05) species accumulation compared with control. Phosphatidylinositol and phosphatidylserine were increased by HS, whereas phosphatidylcholine, phosphatidylethanolamine, and phosphatidylglycerol were decreased relative to control (P < 0.05). Heat-stressed adipocytes in culture also had higher concentrations of saturated (SFA) and monounsaturated fatty acids (P < 0.05) relative to control. Pathways of proline and biotin metabolism were elevated (P < 0.05) by HS in adipocytes. The metabolomics signatures in adipocytes cultured under HS indicates that pathways centered around diacylglycerol metabolism are impacted by HS. In adipose tissue from animals in the HS treatment, there was increased (P < 0.05) abundance of 4,8 dimethylnonanoyl carnitine (P < 0.05). Heat-stressed animals also had higher (P < 005) serum linoleic, total polyunsaturated fatty acids, and decreased total SFA than PF (P < 0.05). These results indicate that HS elevates lipogenic pathways while suppressing fatty acid oxidation and demonstrate the usefulness of metabolomics analysis as a tool for determining the impact of HS in pig tissues.

PHYSIOLOGY AND ENDOCRINOLOGY SYMPOSIUM: Roles for insulin-supported skeletal muscle growth

Basic principles governing skeletal muscle growth and development, from a cellular point of view, have been realized for several decades. Skeletal muscle is marked by the capacity for rapid hypertrophy and increases in protein content. Ultimately, skeletal muscle growth is controlled by 2 basic means: 1) myonuclear accumulation stemming from satellite cell (myoblast) proliferation and 2) the balance of protein synthesis and degradation. Each process underlies the rapid changes in lean tissue accretion evident during fetal and neonatal growth and is particularly sensitive to nutritional manipulation. Although multiple signals converge to alter skeletal muscle mass, postprandial changes in the anabolic hormone insulin link feed intake with enhanced rates of protein synthesis in the neonate. Indeed, a consequence of insulin-deficient states such as malnutrition is reduced myoblast activity and a net loss of body protein. A well-characterized mechanism mediating the anabolic effect of insulin involves the phosphatidylinositol 3-kinase (PI3K)-mammalian target of rapamycin (mTOR) signaling pathway. Activation of mTOR leads to translation initiation control via the phosphorylation of downstream targets. Modulation of this pathway by insulin, as well as by other hormones and nutrients, accounts for enhanced protein synthesis leading to efficient lean tissue accretion and rapid skeletal muscle gain in the growing animal. Dysfunctional insulin activity during fetal and neonatal stages likely alters growth through cellular and protein synthetic capacities.

Physiological consequences of heat stress in pigs

Heat stress negatively influences the global pork industry and undermines genetic, nutritional, management and pharmaceutical advances in management, feed and reproductive efficiency. Specifically, heat stress-induced economic losses result from poor sow performance, reduced and inconsistent growth, decreased carcass quality, mortality, morbidity, and processing issues caused by less rigid adipose tissue (also known as flimsy fat). When environmental conditions exceed the pig’s thermal neutral zone, nutrients are diverted from product synthesis (meat, fetus, milk) to body temperature maintenance thereby compromising efficiency. Unfortunately, genetic selection for both increased litter size and leaner phenotypes decreases pigs’ tolerance to heat, as enhanced fetal development and protein accretion results in increased basal heat production. Additionally, research has demonstrated that in utero heat stress negatively and permanently alters post-natal body temperature and body composition and both variables represent an underappreciated consequence of heat stress. Advances in management (i.e. cooling systems) have partially alleviated the negative impacts of heat stress, but productivity continues to decline during the warm summer months. The detrimental effects of heat stress on animal welfare and production will likely become more of an issue in regions most affected by continued predictions for climate change, with some models forecasting extreme summer conditions in key animal-producing areas of the globe. Therefore, heat stress is likely one of the primary factors limiting profitable animal protein production and will certainly continue to compromise food security (especially in emerging countries) and regionalise pork production in developed countries. Thus, there is an urgent need to have a better understanding of how heat stress reduces animal productivity. Defining the biology of how heat stress jeopardises animal performance is critical in developing approaches (genetic, managerial, nutritional and pharmaceutical) to ameliorate current production issues and improve animal wellbeing and performance.

Nutritional interventions to alleviate the negative consequences of heat stress

Energy metabolism is a highly coordinated process, and preferred fuel(s) differ among tissues. The hierarchy of substrate use can be affected by physiological status and environmental factors including high ambient temperature. Unabated heat eventually overwhelms homeothermic mechanisms resulting in heat stress, which compromises animal health, farm animal production, and human performance. Various aspects of heat stress physiology have been extensively studied, yet a clear understanding of the metabolic changes occurring at the cellular, tissue, and whole-body levels in response to an environmental heat load remains ill-defined. For reasons not yet clarified, circulating nonesterified fatty acid levels are reduced during heat stress, even in the presence of elevated stress hormones (epinephrine, glucagon, and cortisol), and heat-stressed animals often have a blunted lipolytic response to catabolic signals. Either directly because of or in coordination with this, animals experiencing environmental hyperthermia exhibit a shift toward carbohydrate use. These metabolic alterations occur coincident with increased circulating basal and stimulated plasma insulin concentrations. Limited data indicate that proper insulin action is necessary to effectively mount a response to heat stress and minimize heat-induced damage. Consistent with this idea, nutritional interventions targeting increased insulin action may improve tolerance and productivity during heat stress. Further research is warranted to uncover the effects of heat on parameters associated with energy metabolism so that more appropriate and effective treatment methodologies can be designed.

2011 and 2012 Early Careers Achievement Awards: metabolic priorities during heat stress with an emphasis on skeletal muscle

Environmental heat stress undermines efficient animal production resulting in a significant financial burden to agricultural producers. The reduction in performance during heat stress is traditionally thought to result from reduced nutrient intake. Recently, this notion has been challenged with observations indicating that heat-stressed animals may exploit novel homeorhetic strategies to direct metabolic and fuel selection priorities independent of nutrient intake or energy balance. Alterations in systemic physiology support a shift in metabolism, stemming from coordinated interactions at whole-body and tissue-specific levels. Such changes are characterized by increased basal and stimulated circulating insulin concentration in addition to the ostensible lack of basal adipose tissue lipid mobilization coupled with reduced adipocyte responsiveness to lipolytic stimuli. Hepatic and skeletal muscle cellular bioenergetics also exhibit clear differences in carbohydrate production and use, respectively, due to heat stress. The apparent dichotomy in intermediary metabolism between the 2 tissue types may stem from factors such as tricarboxylic acid cycle substrate flux and mitochondrial respiration. Thus, the heat stress response markedly alters postabsorptive carbohydrate, lipid, and protein metabolism through coordinated changes in fuel supply and use across tissues in a manner that is distinct from commonly recognizable changes that occur in animals on a reduced plane of nutrition. Perhaps most intriguing is that the coordinated systemic, cellular, and molecular changes appear conserved across physiological states and among different ruminant and monogastric species. Ultimately, these changes result in the reprioritization of skeletal muscle fuel selection during heat stress, which may be important for whole-body metabolism and overall physiological adaptation to hyperthermia.

Ruminant Nutrition Symposium: ruminant production and metabolic responses to heat stress

Heat stress compromises efficient animal production by marginalizing nutrition, management, and genetic selection efforts to maximize performance endpoints. Modifying farm infrastructure has yielded modest success in mitigating heat stress-related losses, yet poor production during the summer remains arguably the costliest issue facing livestock producers. Reduced output (e.g., milk yield and muscle growth) during heat stress was traditionally thought to result from decreased nutrient intake (i.e., a classic biological response shared by all animals during environmental-induced hyperthermia). Our recent observations have begun to challenge this belief and indicate heat-stressed animals employ novel homeorhetic strategies to direct metabolic and fuel selection priorities independently of nutrient intake or energy balance. Alterations in systemic physiology support a shift in carbohydrate metabolism, evident by increased basal and stimulated circulating insulin concentrations. Perhaps most intriguing given the energetic shortfall of the heat-stressed animal is the apparent lack of basal adipose tissue mobilization coupled with a reduced responsiveness to lipolytic stimuli. Thus, the heat stress response markedly alters postabsorptive carbohydrate, lipid, and protein metabolism independently of reduced feed intake through coordinated changes in fuel supply and utilization by multiple tissues. Interestingly, the systemic, cellular, and molecular changes appear conserved amongst different species and physiological states. Ultimately, these changes result in the reprioritization of fuel selection during heat stress, which appears to be primarily responsible for reduced ruminant animal productivity during the warm summer months.

The effects of rearing-temperature on body conformation and organ size in young pigs

Piglets were weaned at 14 days of age and subsequently reared for 23, 35, 42 or 59 days in a cold (10 degrees C) or a warm (35 degrees C) environment. They were fed to grow at the same rate by feeding cold-reared pigs more than warm-reared pigs. The external surface area, the surface area of the nasal cavity and the length of extremities were greater in warm- than in cold-reared pigs. Likewise, the mass of the skin with subcutaneous fat was greater in warm-reared pigs. The mass of the heart, liver, kidneys, stomach and small intestines was greater in cold- than in warm-reared pigs. There was no difference in the mass of the muscle, spleen, lungs, and large intestine between the two groups.

The effects of rearing-temperature on body composition in young pigs

Pigs were reared for 23, 35, 42, or 59 days in a cold (10 degrees C) or a warm (35 degrees C) environment. They were fed to grow at the same rate. The total amount of fat was greater in warm-reared than in cold-reared pigs and fat was distributed differently in the two groups. Warm-reared pigs had more fat in the subcutaneous layer while cold-reared pigs had more fat in their abdominal tissues and skeletal muscle. The total amount of nitrogen was greater in cold-reared than in warm-reared pigs and no difference was found in its distribution.