Human Biochemistry, Volume 2 - Murray R. 1993

Special Topics
Nutrition, Digestion, and Absorption
Putrefaction and Fermentation Processes in the Intestine

Most of the ingested food is absorbed in the Small Intestine, while the remainder passes into the Large Intestine. It is here that significant Water absorption takes place, and the semi-fluid intestinal contents gradually become more solid. Bacterial activity becomes prominent during this period. The Fermentation and putrefaction processes induced by Bacteria produce various gases, such as CO₂, methane, hydrogen, nitrogen, and hydrogen sulfide, as well as acetic, lactic, and butyric acids. Bacterial breakdown of phosphatidylcholine leads to The formation of Choline and related toxic amines, such as neurine.

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Table 53.13. Disorders associated with malabsorption

Fate of amino Acids

Most amino acids undergo Decarboxylation as a result of the action of intestinal bacteria, yielding toxic amines (ptomaines).

Decarboxylation reactions lead to the formation of cadaverine from Lysine, agmatine from Arginine, tyramine from Tyrosine, putrescine from Ornithine, and histamine from Histidine. Many of these amines are potent vasopressor agents.

Through a series of reactions, The amino acid Tryptophan is converted into indole and methylindole (skatole). These are the primary compounds responsible for the characteristic odor of feces.

The sulfur-containing amino acid Cysteine undergoes a series of transformations to form mercaptans, such as ethyl and methyl mercaptans, as well as H₂S.

A significant amount of Ammonia is produced in the large intestine as a product of putrefaction resulting from the action of intestinal bacteria on nitrogenous substrates. Ammonia is absorbed into the portal Circulation, but under normal conditions, it is rapidly cleared from the Blood by the Liver. In liver disease, this hepatic function may be impaired, causing the concentration of ammonia in the peripheral blood to rise to toxic levels. It is believed that ammonium intoxication may play a role in the onset of hepatic coma in certain patients. Oral administration of neomycin has been shown to reduce The amount of ammonia entering the bloodstream from the intestine due to its antibacterial action. In patients with severe liver damage, a high-protein diet, as well as gastrointestinal bleeding, can contribute to The Development of ammonia toxicity. Neomycin is also indicated in these cases.

Intestinal Bacteria

The intestinal flora can constitute a significant portion (up to 25%) of the dry weight of feces. In herbivores, whose diet consists largely of Cellulose, gut or rumen bacteria play a crucial role in digestion by breaking down Polysaccharides and thereby facilitating their absorption. Furthermore, these bacteria synthesize Essential Amino Acids and vitamins. For humans, the intestinal flora is not as vital as it is for herbivores. However, bacterial activity makes a certain beneficial contribution to Human Nutrition, as it is associated with the synthesis of vitamins K and B₁₂, and possibly other B-complex vitamins, which are subsequently utilized by the body.

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