Carrageenan is a polysaccharide (a linear galactan sugar with alternative 1-3 and 1-4 linkages, to be exact), that is extracted from several species of red seaweed. In pet foods, it is used to make canned foods hold together and come out of the can nicely.
Polysaccharides like this can serve as 'food' for intestinal bacteria, which ferment it to produce short chain fatty acids. These fatty acids are good in that they create an optimal environment for good bacteria in the gut and can help suppress the "bad" ones, and these fatty acids can also be used as fuel by the cells that line the gut.
These bacteria also degrade taurine, which is secreted into the gut as part of bile, and re-absorbed from the gut further down the line. But if these bacteria are extremely active, it's possible that they can degrade too much taurine. This is one theory to explain why cats need more taurine when they eat canned food.
Anantharaman-Barr et al. Fecal Bile Acid Excretion and Taurine Status in Cats Fed
Canned and Dry Diets. J. Nutr. 124: 2546S-2551S, 1994.
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