Compounds that interfere with the intake, availability, or metabolism of nutrients in the animal are referred to as anti-nutritional factors. Their biological effects can range from a mild reduction in animal performance to death, even at relatively small intakes. The subject is complicated by the fact that different species and ages react in different ways to the presence of anti-nutritional factors.
The raw grain of soybean contains several anti-nutritional factors in variable amounts. Some of them are not important in monogastric animals, because they are not considered harmful for these species. Others, like phytate, are present in most vegetable materials and their negative effects can be overcome by using the appropriate technology (addition of phytase).
Proper processing of soybeans requires precise control of moisture content, temperature and processing time to destroy the anti-nutritional factors. Both over and under-toasting of soybean meal can result in a meal of lower nutritional quality. Underheating produces incomplete inactivation of the anti-nutritional factors and over-toasting can reduce amino acid availability (lysine). The most important anti-nutritional factors to monogastric animals are:
Protease inhibitors: Protease inhibitors can inhibit the activity of proteolytic enzymes and can cause a decrease in digestive efficiency, inadequacy in dietary sulfur amino acids. As a consequence of inhibition of proteolytic enzymes the animals tend to react to the presence of protease inhibitors by secreting more digestive enzymes, which results in pancreatic hypertrophy. In poultry and swine, trypsin inhibitors significantly reduce the digestibility and utilization of amino acids. At least five trypsin inhibitors have been identified. However, the principal protease inhibitors present in raw or underprocedded soybeans are the Kunitz factor and the Bowman-Birk factor; the latter is more resistant to the action of heat, alkali and acid. Their average levels in raw soybeans are 1.4 and 0.6%, respectively.
Lectins: These are glycoproteins noted for their capability to agglutinate erythrocytes and bind sugar components. Lectin content in beans ranges from one to three percent. Lectins are not broken down in the gut, attach to mucosa cells damaging the intestinal wall and reducing the absorption of nutrients. Heat treatment is very effective and necessary in the inactivation of lectins.
Goitrogenic factors: These, similarly, are glycosides belonging to the isoflavinic group, some of which like genistin; have goitrogenic activity resulting in enlargement of the thyroid gland and a reduction in the activity of thyroxine secreted by the thyroid itself.
Saponins: Although they appear in low levels they can decrease feed palatability.
Rachitogenic factors: These factors are associated principally with genistin (about 0.10% of raw soybeans) which interfere with calcification of bone. Turkeys are particularly sensitive.
Phytic acid: Phytic acid complexes with certain minerals - such as calcium, phosphorus, magnesium, copper, iron and zinc - reducing their bioavailability. Levels of phytate in soybeans range from 1.0 - 2.3 percent.