**3. Models used to study parenteral nutrition**

Animal have become a valuable preclinical model for nutritional studies. This includes mice, rats, rabbits, guinea pigs, dogs, and pigs [20–22]. As with all models, some species are considered to be more physiologically relevant to humans, and hence better models for understanding mechanistic pathways influenced by PN in pediatric and adult patients.

Rodents (mice and rats) have many similarities to humans including stages of development, anatomical features, immune responses, and associated physiology. Mice share many common genes, many of which can be knocked out or modified, as well as similar metabolic pathways compared with humans. Rodents are also cost effective and easily manipulated in a controlled environment, with a relatively short gestational period (19–22 days). These animals also offer a valuable tool for easy

genetic/transgenic manipulation. These practical considerations make rodents suitable for many studies. One important difference to note is that the poorly developed guts and brains of pups at birth mature gradually during the early lactational period before weaning, whereas human infants show mature guts at full gestational birth. This feature of mice pups offers a great model to compare rodent guts with the guts of premature human infants. This allows the modeling of PN under the premature setting following a short time protocol [23]. Neonatal dogs are also used to determine the effect of enteral and parenteral feeding on GI growth and maturation because of immature intestines at birth relative to mature newborn human intestines [24–26].

Rabbit models have been used to characterize the nutritional value of different combinations to determine the effect of different TPN components on hepatic cholestasis and bacterial translocation. This includes characterization of nutrition supplements using different solutions such as carbohydrate-based solution, lipidbased solution, enriched amino acid-based solution, and protein deficient solution on newly born rabbits. Rabbits with protein deficient calories developed cholestasis after 7 days of administration [27]. Additionally, newborn rabbits provided with protein deficient solution had increased bacterial translocation [28]. Guinea pigs have been used to study multivitamin mixed lipid emulsion vs. amino-acid and dextrose based mixed solution [29]. Additionally, guinea pigs have been used to study the effect of light exposure on multivitamin mixed lipid emulsion which generates peroxide free radicals and induces oxidative stress in the lung [30].

Extensive research has demonstrated that premature neonatal piglets are a preferred model over rodents to study long term PN related chronic and acute effects on organ size and developmental outcome. This includes effects on tissue components such as immune cells, hepatocytes, neurons, and other metabolic tissues. In particular, the neonatal pig, unlike rodents, rabbits, guinea pigs, and dogs, has been shown to be highly homologous with the human neonate regarding the function of numerous organ systems, especially the liver and the gastrointestinal tract, several aspects of metabolism, and stages of development [22, 31]. Although the piglet has a slightly immature digestive system and shorter gestational length (~115 days) compared with humans, it offers a very good animal model to study the effect of enteral/parenteral nutrition in early life on postnatal growth and development [20]. In early postnatal days, the rapid intestinal growth, adaptation to food, bacterial colonization and improved nutrient absorption provides an elegant model to study PN related issues in premature children. Newborn piglets have been used to study gut maturation and functional changes in preterm piglets (107 day of gestation) and full-term piglets (115 day of gestation) because they are physiologically similar to human preterm infants [20, 22, 31]. Further, preterm neonatal piglets are a wellestablished model to study PN associated PNALD including hepatic cholestasis and brain related disorders [32, 33]. In late postnatal days, the growth of the intestine is gradual, reflecting the transition from milk-feeding to solid-food feeding. This may provide a model for TPN in immature children/neonates and adults. The piglet model is also favored over other model organisms because of body size, which allows for extensive surgical manipulation.
