Contrary to what is generally believed, germ-free animals require a higher dietary caloric intake than their conventional counterparts. The main reason is very simple. A normal microbiota will break down indigestible dietary substances to compounds that can be absorbed by the host. That is most prominent in ruminants, i.e., the microbiota digest cellulose into short chain fatty acids (SCFAs).
Also contrary to what is generally believed, germ-free animals require a higher intake of nitrogen than their conventional counterparts. The main reason for this is most probably the great loss of non-degraded material from expelled enterocytes that are found in germ-free animals. In conventional animals, the microbiota converts the expelled material into absorbable compounds.
In many germ-free macroorganisms, there might be a demand for an increased dietary intake of some vitamins. Broadly speaking, the gastrointestinal microbiota, placed between the ingesta and the host, may utilize dietary vitamins or produce vitamins themselves.
Among the earliest evidence that the vitamin synthesis is connected to functions by the intestinal microbes was the demonstration that germ-free rats reared without a dietary source of vitamin K developed hemorrhages and hypoprotothrombinemia soon, whereas their conventional controls had normal prothrombin levels and no bleeding tendencies (9).
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