“Dietary Taurine Requirement of Cats Is Determined by Microbial Degradation of Taurine in the Gut”, 1994 (; backlinks):
Over half the dietary taurine requirements of adult cats is required to replace the taurine degraded by the intestinal microbes.
The quantity of taurine degraded in the gut is primarily dependent on the source and amount of protein in the diet and the method by which the protein is processed. In purified diets, casein maintains higher concentrations of taurine in plasma than isonitrogenous levels of isolated soy protein.
Addition of oral antibiotics to a taurine-depleting diet decreased taurine loss in feces, increased plasma and whole blood concentrations of taurine, and increased urinary taurine excretion compared to the diet without antibiotics.
Current knowledge suggests that thermal processing in the presence of reducing sugars produces Maillard products which increases microbial degradation of taurine in the intestine. Maillard products may either provide an environment that favors higher numbers of taurine-degrading bacteria, or increase taurine exposure to bacteria. Maillard products may bind proteolytic enzymes in the intestine which permits monitor peptide to stimulate cholecystokinin-containing cells to release cholecystokinin, which in tum increases bile entry into the small intestine, and exposure of taurine to microbial degradation.
Taurine is an essential dietary constituent for cats because in vivo synthesis is limited, and cats have an obligatory requirement for taurine for the conjugation of bile salts. Dogs and cats use only taurine to conjugate bile acids, but dogs unlike cats, appear to have a rate of synthesis adequate to meet their needs.
The basis for the lower synthetic capacity in cats than in dogs and other animals has not been fully elucidated. The activity of cysteine dioxygenase is low in the liver of cats which results in limited production of cysteine sulfinate and synthesis of taurine.
Other contributing factors are the low activity of cysteine sulfinate decarboxylase and the high activity of aspartate aminotransferase which transaminates cysteine sulfinate to pyruvate, rather than allowing it to be decarboxylated to hypotaurine and hence to taurine (29).
…When taurine balances measurements are taken on cats given diets containing fixed amounts of taurine for long periods, less than half the dietary taurine is recovered in the feces and urine.
…When a pulse dose of C labeled taurine was given to cats and the expired CO2 collected and analyzed, it was found that 100× more label was recovered in the CO2 from cats given a thermally processed canned diet (that caused taurine depletion) than from cats given the same diet in the unprocessed state16. These observation indicated that processing a diet had a marked effect on the extent of taurine degradation in the gut. It was not possible from these observations to quantify the taurine degraded.14 O02 could have come from CO2 produced by the microbes directly or from oxidation of products of taurine degradation such as acetate which enters the body pool and may only be partially oxidized in the period of observation.
Similar overall recoveries of taurine from purified and cooked and frozen diets are presented in Table 1b. However, these diets supported very different blood concentrations of taurine. The whole blood taurine concentration of cats given the 4 diets are presented in Table 2. While 3 of the diets (two purified and frozen commercial) produced only a slight fall in taurine concentration in the blood, the cooked diet resulted in marked depletion in whole blood taurine concentration. These results indicate that the cooked diet, relative to uncooked or purified diets, was associated with a greater degree of degradation of taurine in the gut.