Basic Nutrients in Food: Protein, Peptides and Amino Acids (Part 2)
How well nutrition is used depends on the nutrient your pet lacks most.
What we usually call protein is made of 50 or more amino acids, often together with functional molecules — lipids, metal ions — that give proteins more functions and variety.
Protein-based tissues are everywhere in the body: from enzymes and blood cells to muscle, organs, skin and hair. Excluding water, more than 60% of an animal's tissue is essentially protein. Protein-based enzymes also let the body break down and synthesise substances — indispensable to biological function.
Quality matters more than quantity
We know dietary protein is made of different amino acids (see Part 1), and that protein is broken back into amino acids for the body to use — so the source and type of protein matter a great deal.
In general, the amino-acid profiles of animal proteins resemble each other, so animal protein meets an animal's nutritional needs better than plant protein.
By-products (bone, meat scraps, skin) are not a recommended protein source. Beyond doubts about whether they nourish dogs and cats properly, there are hygiene concerns: by-products are usually the unwanted parts, hard to quality-assure in storage and transport, and at high risk of contamination. The authoritative Whole Dog Journal explicitly advises owners to avoid foods made with by-products.
Nutrient use is limited by the scarcest nutrient
Nutrition science uses the "barrel theory": a barrel holds only as much water as its shortest plank allows. If the shortest plank is 10 cm, the barrel holds 10 cm of water however tall the other planks are.
Nutrition works the same way: how much an animal can absorb and use depends on the nutrient it lacks most. However many supplements we add, if the overall nutrient picture isn't covered, blindly topping up a single nutrient does no real good — it just makes one plank taller — and metabolising the excess adds load to the liver and kidneys.
Nutrient needs vary with species, size, life stage and individual condition, so supplement doses need careful calculation. When adding supplements, follow the recommended dose and consult your vet with the ingredient list of the food, judging by your pet's individual condition whether supplementation is needed, so every nutrient is taken in balance.
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.
