Relationship between in vitro digestion of proteins and in vivo assessment of their nutritional quality.

The relationship between the nature of in vitro digestion products and protein quality evaluation in rats was established. Eleven protein sources of animal and vegetable origin and of various purities were digested by pepsin then by pancreatin in a dialysis bag of 1,000 molecular weight cutoff. Animal proteins were generally better digested in vitro than vegetable proteins. Amino acid release partly reflected enzyme specificity and varied depending on the nature of the protein. Essential amino acids were generally released rapidly at the expense of non-essential amino acids. The composition of protein hydrolysis products was thus markedly different from that of the protein before digestion. Digestibility determination in rats showed much less variation between proteins than the in vitro method; the two digestibility measurements did not correlate. However, by using stepwise multiple regression analysis, the amino acid composition of in vitro digestion products was found to correlate with the protein efficiency ratio (PER) more accurately than protein composition (R2 = 0.981 and 0.934, respectively). A regression analysis with net protein ratio (NPR) gave lower R2 coefficients than with PER (0.941 and 0.921, respectively). When regression equations were employed to predict the PER and digestion products were used instead of protein composition, an improvement was seen for almost all the test proteins, especially beef, rapeseed and wheat gluten. Better evaluation of protein quality by the use of protein digestion products demonstrates the possible impact of amino acid availability on protein quality.