Amino acids are the miracle of the universe. It was the appearance of amino acids on earth 3.5 billion years ago that led to the start of life and the start of evolution. Amino acids have a central role in all cellular metabolism. Scientists have now discovered more than 500 amino acids in nature, but only 20 participate in DNA translation or human protein synthesis. Aside from their role in making all proteins and all enzymes, amino acids have many other biologically important functions. As discussed in this chapter, amino acids are readily metabolized to sugars and fatty acids to make NADH and ATP the energy molecules.

Amino acids and peptides are absorbed in the small intestine by different mechanisms. The most convincing evidence for the presence of a distinct transport mechanism for small peptides is that in the amino acid transport defects Hartnup disease and cystinuria, the affected amino acids are poorly absorbed when presented in free form, but the absorption is normal when these amino acids are presented as small peptides. Furthermore, there is no competition between amino acids and small peptides for intestinal absorption and also when compared on the basis of equivalent amino acid composition, small peptides are absorbed more efficiently than free amino acids. Maximal capacity to absorb free amino acids versus small peptides also differs along the longitudinal axis of the small intestine. These findings clearly indicate that amino acid absorption and peptide absorption are two totally different processes.