Volatile Fatty Acids Are Important Energy Substrates for the Host Animal
One can appreciate the elegance and beauty of the symbiotic relationship represented by fermentative digestion by considering the metabolism of VFAs. These molecules are the end products, indeed, the waste products, of anaerobic microbial metabolism, just as carbon dioxide is the waste product of aerobic metabolism.
If the VFAs were allowed to accumulate, they would suppress or alter the fermentative process by lowering the pH of the gut or forestomach. However, the host animal maintains conditions for fermentation both by buffering pH changes and by removing VFAs from the gut by absorption. The benefit derived by the host is from the chemical energy that is contained in the VFAs. These bacterial “waste products” represent spent compounds within the framework of the anaerobic fermentation system, but they still contain considerable energy that can be derived from aerobic metabolism. In ruminants and other large herbivores, the VFAs are the major energy fuels, to a large extent serving the role played by glucose in omnivorous monogastric animals. The metabolic fates of the VFAs are discussed further in Chapter 32.
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