While soy products have long been included in animal diets for their macronutrient fractions, more recent work has focused on the immunomodulatory potential of bioactive components of this feedstuff. This comprehensive review aims to identify the current state of knowledge on minor soy fractions and their impact on the health and growth of pigs and broiler chickens to better direct future research. A total of 7,683 publications were screened, yet only 151 were included in the review after exclusion criteria were applied, with the majority (n = 87) of these studies conducted in pigs. In both species, antinutritional factors and carbohydrates, like stachyose and raffinose, were the most frequently studied categories of bioactive components. For both categories, most publications were evaluating ways to decrease the prevalence of the examined components in soy products, especially when fed at earlier ages. Overall, most studies evaluated the effect of the bioactive component on performance-related outcomes (n = 137), followed by microbial analysis (n = 38) and intestinal structure and integrity measures (n = 37). As they were analyzed in the majority of publications, antinutritional factors were also the most frequently investigated category in relation to each specific outcome. This trend did not hold true for microbiota- or antioxidant-associated outcomes, which were most often studied with carbohydrates or polyphenols, respectively. Changes to the host microbiota have the potential to modulate the immune system, feed intake, and social behaviors through the microbiota-gut-brain axis, though few publications measured behavior and brain characteristics as an outcome. Other identified gaps in research included the study of soy saponins, as most research focused on saponins derived from other plants, the study of phytosterols outside of their role in cardiovascular or reproductive outcomes, and the general examination of bioactive peptides. Overall, given soy's popularity as a current constituent of animal feed, additional research into these bioactive components may serve to define the value of soy products through their potential ability to support the productivity, health, and well-being of animals. A comprehensive review of the current literature in which pigs and broiler chickens were fed minor soy bioactive components determined that antinutritional factors and carbohydrates are historically the most frequently analyzed categories in relation to performance and health outcomes. Other categories of bioactive components such as soy phytosterols, peptides, and saponins were under-studied in relation to these same outcomes associated with animal well-being. For all soy-derived components, there is a paucity of research focused on outcomes related to animal behavior or brain characteristics. Soybeans are commonly included in animal diets, often in the form of soybean meal, as a source of energy and high-quality protein. Aside from these macronutrient fractions, soy also includes many minor components that have the potential to both positively and negatively impact the growth and health of nonruminant animal species used in production agriculture. The purpose of this comprehensive review was to summarize research on the effects of soy bioactive components on performance and immune-related outcomes in pigs and broiler chickens as soybean-derived products are used heavily in diets fed to these species. After screening over 7,600 publications, a total of 151 met the inclusion criteria and were thus included in this review. Most research focused on ways to eliminate deleterious bioactive components that can affect the digestibility of nutrients and intestinal health of animals. Most publications evaluated the effect of bioactive components on performance, the microbiome, or intestinal characteristics, while few studied changes in animal behavior or brain outcomes. Very few publications that investigated soy saponins, peptides, or phytosterols as feed additives met the criteria for inclusion. Future studies should be considered to fill the gaps highlighted in this review to improve the understanding of how soy products can influence animal health and productivity.
