ABSTRACT
Proteins in foods may be divided into two groups: plant proteins, which might be called “primary proteins,” and animal proteins termed “secondary proteins.” The latter are obtained by conversion of plant N-compounds by an imals. In that chain of events, ruminants play an exceptional role due to syn thesis of different amino acids by the symbiotic microorganisms in the rumen and their incorporation by the animal into milk and meat. Animal proteins are generally rich in exogenic amino acids-essential for humans and monogastric animals: isoleucine, leucine, valine, phenylalanine, metionine, lysine, argi nine, threonine, tryptophan, and histidine. Histidine is not essential for adults, but infants require it due to insufficient synthesis. Cysteine and tyrosine are also semi-essential because they can be manufactured in the organism from methionine and phenylalanine.
