Tannins are substances resulting from secondary, or special, metabolism of plants, and for a long time, it was seen as an anti-nutritional factor present in plants as a defense mechanism against herbivores, bacteria, fungi, parasites, etc. However, with the focus of much research on finding alternative additives to growth-promoting antibiotics, tannins have come to be viewed favorably by animal nutrition. They can precipitate alkaloids and proteins by hydrophobic bonds or hydrogen bonds among other interactions, which is the basis for controlling the invaders mentioned above.
Tannins can be condensed and hydrolyzable, the latter consisting of sugars and an acid that can be gallic or ellagic. They are hydrolyzable by acids or enzymes, releasing sugar and one of the acids, either gallic or ellagic, which can later be excreted.
However, condensed tannins are not absorbed or excreted as easily, which in monogastric ends up causing lesions in the mucosa of the gastrointestinal tract – TGI reducing the absorption of nutrients, among other side effects.
Hydrolyzable tannins have shown positive effects on animal nutrition, being used as antimicrobials, anti-inflammatory, and healing agents.
Antimicrobial activities are linked to the inhibition of microbial enzymes, deprivation of substrates such as proteins and polysaccharides necessary for the proliferation of microorganisms, in addition to the deprivation of metal ions. However, the effect of tannins is greater on Gram-positive bacteria, since Gram-negative bacteria have a more complex layer of proteins and lipids.
It also assists in the anti-inflammatory process and healing of epithelial tissues with the formation of a tannic-protein or tannin-polysaccharide protective layer. The antioxidant activity is due to the scavenging of free radicals.
Therefore, like any other ingredient, hydrolyzable tannins must be used in the correct form and quantity, to obtain the expected results without the occurrence of disturbances or intoxication.
