Basic Nutrients in Food: Protein, Peptides and Amino Acids (Part 1)
Peptides and amino acids are the small molecules protein breaks down into — and the form an animal's gut absorbs best.
Protein, peptides and amino acids belong to the same family of nutrients. All protein is made of amino acids and peptides, with amino acids as the smallest unit. (From small to large: amino acids → form peptides → form proteins.)
Animals cannot use dietary protein directly. It must be digested into small peptides and amino acids, absorbed through the gut, and only then reassembled in the body into usable protein.
▲ Figure: the relationship between amino acids, peptides and proteins.
The best-absorbed form — peptides
A peptide is a chain of 2 to 50 amino acids linked by "peptide bonds" — hence the name. Peptide bonds can be broken by reaction with water (hydrolysis), so peptides can be obtained both by hydrolysing protein and by synthesis from amino acids.
Of protein, peptides and amino acids, peptides are absorbed most efficiently — di- and tri-peptides best of all. In fact, the digestive tract of dogs and cats uses enzymes to break protein into peptides before absorbing it.
Nutrients the body can't make — essential amino acids
As the smallest unit of protein, there are only some 20 kinds of amino acid, in all animals. Some cannot be synthesised by the animal itself; these are the "essential amino acids". They can only come from daily food, and amino acids are not stored long-term in the body, so a dietary shortfall soon affects body function.
▲ Figure: essential amino acids compared (left to right: human, dog, cat).
Fortunately, with a well-managed diet most essential amino acids are available from food. But note that essential amino acids differ between species, so animal needs can't be judged from a human-nutrition viewpoint. When choosing pet food, prefer foods with a calculated amino-acid ratio (reputable kibble and complete-diet cans) — no extra amino-acid supplements are then needed.
Continue reading: Protein, Peptides and Amino Acids (Part 2)
References
- Nelson DL, Cox MM. Lehninger Principles of Biochemistry, 4th ed.
- Rogers QR, Morris JG. Essentiality of amino acids for the growing kitten. J Nutr. 1979;109(4):718-23.
