Tannins digestion

Some of the best investigated anti-nutrients are the enzyme inhibitors present in legumes and other plants. The Bowman-Birk and the Kunitz inhibitors of trypsin and other proteases are among the best characterized. In contrast to the non-specific and widespread influences of tannins and lectins (Carmona, 1996), the Bowman-Birk, Kunitz and other such inhibitors target specific enzymes. Corresponding with this, proteases and other digestive enzymes vary in sensitivity to the different inhibitors. 

CARMONA A (1996) Tannins thermostable pigments which complex dietary proteins and inhibit digestive enzymes. Latinoam Nutr. 44 31S-35S. 

LONGSTAFF M, MCNAB J M (1991) The inhibitory effects of hull polysaccharides and tannins of field beans (Vida faba L.) on the digestion of amino acids, starch and hpid and on digestive enzyme activities in young chicks. Br. TNutr. 65 199-216. 

Makkar HPS, Blummel M, Becker K. Formation of complexes between polyvinyl pyr-rolidones or polyethylene glycols and tannins, and their implication in gas production and tme digestibility in in vitro techniques. British Journal of Nutrition.1995 73 ... 

Herbivores that commonly feed on tannin-rich plants have evolved interesting methods to lessen the effect of ingested tannins on their digestive systems. For example, the salivary proteins of rabbits and other rodents are high in the amino acid proline, which has a very high affinity for tannins. Eating food high in tannins stimulates the secretion of these proteins and diminishes the toxic effect of the tannins. 

Tannins derive their name from their ability to tan (i.e. they combine with protein). They render plants less palatable and impair digestion by binding with the buccal mucosa, dietaiy proteins, and digestive enzymes of the animal. Tannins are thought to bind to proteins upon destruction of plant tissue by herbivores. This reduces the nutritive value of the plant to the herbivore. Some tannins such as oak gallotannins, are even toxic to livestock and rabbits (Meyer and Karasov, 1991). However, Martin and Martin (1983) have questioned the role of tannins as plant defense against herbivores. 

The flavonoid quercetin suppresses growth somewhat in prairie voles, M. ochrogaster, while tannic acid, a hydrolyzable tannin, has an even stronger effect (Lindroth and Batzli, 1984). Quercetin is toxic, but does not bind with protein and, therefore, does not affect protein digestibility. Tannic acid has some effect on protein digestibility but is also toxic. 

Blytt, H. J., Guscar, T. K., and Butler, L. G. (1988). Antinutritional effects and ecological significance of dietary condensed tannins may not be due to binding and inhibiting digestive enzymes. Journal ofChemicalEcology 14,1455. 

Dietz, B. A., Hagerman, A. E., and Barrett, G. W. (1994). Role of condensed tannin on salivary tannin-binding proteins, bioenergetics and nitrogen digestibility in Micro-tuspennsylvanicus. Journal of Mammalogy 75,880-889. 

Tannins have have been widely considered to reduce the digestibility and thus nutritional value of plant imterial to herbivores. Evidence from vivo studies exploring the potential allelochemical effects of tannins is reviewed together with infonnation regarding the probable irechanisms leading to observed effects. 

Recent publications on tannins and their effects on herbivores liave tended to deal specifically with only vertebrate (2,3) or invertebrate (, ) herbivores, and have drawn slightly different conclusions for the two groups. Sirtplistically, these are tliat tannins do not act as effective digestibility - reducers in insects, and that they have sotb, but not completely effective, deterrent action against maitmals. 

Drawing on this and the reported specificity of tannin-protein interactions ( ) leads to the conclusion that any useful in vitro modelling of the impact of tannins on digestion must consider more than pH and the concentrations of the buffer, enzyme, substrate, and tannin. The actual enzyme-substrate system must be nutritionally realistic to control for specificities of the reaction of tannins with proteins (including enzymes) gastrointestinal mucoproteins should perhaps also be included on the same grounds. Besides all this, misleading results nay still be obtained if bile surfactants are omitted from the equation. 

Various excellent reviews are available on phenolic compounds, their chemistry and analysis, content in foods and nutritional significance (Bravo, 1998 Dykes and Rooney, 2006 Manach et al., 2004 Naczk and Shahidi, 2006 Robbins, 2003). From a nutritional perspective, phenolic compoimds (especially tannins) are regarded as antinutritional factors due to their ability to form complexes with dietary proteins and minerals and digestive enzymes (Bravo, 1998). However, lately there has been increasing focus on the positive aspects of phenolics due to their ability to act as antioxidants which may offer potential health benefits such as prevention of diseases such as cancer and cardiovascular disease. 

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