Å Krogdahl, G Løkka, T M Kortner
The aims of the present review are to remind and give food for thought regarding the complexity of gastrointestinal functions, impacts of antinutrients and functional ingredients, and consequences of large knowledge gaps for feed formulation. Emphasis is put on organs and mechanisms in and around the intestine, i.e., cells of the epithelium and layers below, immune apparatus, and nerve system, and their interaction and regulations. Examples are picked mainly from studies the authors have been involved in, addressing responses in humans and monogastric production animals, with emphasis on Atlantic salmon. First mentioned is a series of experiments conducted to find the explanation for great differences in protein value of soybean varieties. The differences were found to be related to the variation in content of protease inhibitors. Consequently, the inhibitors became tools for the study of regulation of intestinal functions in humans, revealing very complicated mechanisms, illustrated by inhibitor-induced secretion of an inhibitor-resistant trypsinogen. The new insight stimulated an interest in other soybean antinutrients, enforced in the 1990s by the interest in soybean meal (SBM) as a protein source for salmon. In Atlantic salmon, as well as in rainbow trout, SBM was found to reduce growth and induce intestinal inflammation, whereas alcohol-extracted SBM, i.e. soy protein concentrate, showed high nutritional value. This indicated that the antinutrient(s) causing the inflammation is/are alcohol-water soluble, such as phytoestrogens, phytosterols, and/or saponins. Physiological effects of these antinutrients are discussed briefly as a background for interpretation of the results from the follow-up studies with fish. Saponins were found to be the main inducers of inflammation, affecting a wide range of molecular, biochemical, physiological and structural biomarkers. The results, however, indicated that at very low levels, saponins may induce beneficial, adjuvant effects. The studies also documented that such effects may depend on the developmental stage of the animal, animal breed, and species. Atlantic cod, for example, do not show SBM-induced intestinal inflammation. Likewise, plant feed ingredients show great variation in content of antinutrients between breeds and developmental stages. Some plant breeding programmes seek to reduce the content of specific antinutrients; others may, unnoticed, decrease or increase the content of such components. In conclusion, plant antinutrients and functional components may be beneficial or detrimental for growth and health depending on the animal species and developmental stage and may serve as important tools for the study of physiological and health aspects.