Protein digestibility of the same protein preparations by human and rat assays and by in vitro enzymic digestion methods.

The apparent and true digestibilities of the same preparations of six proteins (spray dried whole egg, cottage cheese, canned tuna, peanut flour, soy isolate, and wheat gluten) were estimated in four to five men and in rats and compared to estimates of digestibility from three different in vitro enzymic digestion procedures. For all six proteins, the correlation coefficient was 0.46 between true digestibility in humans and in rats; with values for tuna excluded, r = 0.96. With all six proteins, none of the in vitro values was significantly correlated with values from humans or rats. However, with either the three animal proteins alone or the three plant proteins alone, correlations were high (r greater than 0.90) between one or more of the in vitro estimates and the observed true or apparent human and rat digestibilities. The differences in the relationship between enzymic digestion estimates and the human digestibility estimates for plant or animal proteins suggest that for accurate prediction of protein digestibility in humans by these enzymic methods, different equations would have to be used for plant and animal proteins. For protein sources containing both plant and animal protein, use of the in vitro enzymic procedures would give only an approximate estimate of digestibility in humans.