Microbial digestion in ruminants and other herbivores

Vitamin A is more readily absorbed from the digestive tract than its precursor carotene, although it is thought that vitamin A esters must first be hydrolysed by an esterase to the alcohol form before being absorbed. Phytosterols are poorly absorbed, and it is generally considered that unless ergosterol has been irradiated to vitamin D2 before ingestion it cannot be absorbed from the tract in any quantity. 

Water-soluble vitamins are believed to be absorbed both by simple diffusion and by carrier-mediated transport, which is sodium-dependent, dtamin 65 is absorbed by passive diffusion, mainly in the small intestine, and the amount absorbed is related linearly to the amount in the digesta.The importance of a carrier glycoprotein (intrinsic factor) for the absorption of vitamin B12 has already been stressed (see p. 98). 

Many food constituents are potentially toxic to the animal consuming them. Microbial contaminants are an obvious example the digestive enzymes kill many bacteria, but some organisms may damage the gut, which allows them or the toxins they produce to invade the animal s tissues. Foreign proteins, especially those with endocrine activity, could harm the animal if absorbed, but the gut provides an effective barrier to prevent their absorption before they are hydrolysed. The same is true of nucleic acids (whose breakdown is a matter of concern, as some animal foods may now be derived from genetically modified plants). Some of the toxic constituents of pasture plants are broken down in the rumen of cattle, sheep and goats (see p. 494). As mentioned above, the animal body avoids excessive intake of the mineral elements calcium and iron by selective absorption. 

Some lai er molecules are able to bypass the barrier presented by the gut. This may be desirable in some instances (e.g. absorption of protein antibodies in the newborn animal and absorption of antibiotics administered orally), but undesirable in others (e.g. apparent absorption of the proteins known as prions, which are responsible for bovine spongiform encephalopathy see p. 579). 

The stomach of the ruminant is divided into four compartments (Fig. 8.5). In the young suckhng, the first two compartments, the rumen and its continuation the reticulum, are relatively tmdeveloped, and milk, on reaching the stomach, is channelled 

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Microbial digestion in ruminants and other herbivores The overall equation for the rumen fermentation of hexoses is ... 

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