The obligatory fecal intestinal protein losses have been variably estimated to range from 20mg/kg/day in young infant and preschool children to approximately 12mg/kg/day in adults. These losses may result in some difficulty in interpreting digestibility findings. The estimated value of true digestibility of food and feed proteins is dependent on the excretion of metabolic fecal nitrogen (MFN). Results of many studies show that a high-fiber content of the diet increases MFN excretion and lowers the true digestibility of the diet protein. The exact estimation of MFN is only possible with isotopic methods. Experimental studies indicate that for human subjects the fecal digestibility values are significantly higher than the ileal values for Arg, Asp, Gly, Phe, Pro, Ser, Thr, and Trp, with the exception of faecal digestibility of Met, which is significantly lower than the ileal value.

Table 4 gives the true digestibility values for several common foods and diets. In general, milk and eggs have the highest true digestibility values of approximately 97%, followed by meats, fish, and

Table 4 Illustrative values of protein digestibility in humans

Protein sources


Digestibility relative

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