Feed amino acids

Animals do not require protein. They require amino acids, in the proportions their tissues need. Feed amino acids exist because the gap between what a grain-and-oilseed diet supplies and what the animal needs is concentrated in two or three specific molecules.

The limiting amino acid

Protein synthesis needs all the essential amino acids present together. Whichever is in shortest supply relative to requirement caps the use of all the rest — the surplus is deaminated, its nitrogen excreted as urea or uric acid, and its carbon burned. This is the barrel-stave logic: the shortest stave sets the level.

In cereal-based pig diets, lysine is normally first-limiting; in poultry diets, methionine. The classical response was to raise total protein until enough of the limiting amino acid arrived, which oversupplied everything else.

Supplementing the limiting amino acid directly reverses that. Once lysine is added, the diet’s crude protein can be cut while performance is maintained, because the constraint has been removed rather than overwhelmed. The environmental consequence follows arithmetically: less surplus protein means less nitrogen deaminated and excreted, so less ammonia volatilised from manure and less nitrate leached. Roughly, each percentage point of dietary crude protein removed cuts nitrogen excretion by a substantial fraction — the precise figure depending on species and diet. This is the strongest environmental argument in animal nutrition that rests on straightforward mass balance rather than modelling.

The sequence continues: with lysine added, threonine usually becomes limiting, then tryptophan, then valine — which is why the commercial set expanded in that order.

How they are made

Lysine is produced by fermentation, and its history is a foundational example of metabolic engineering. Corynebacterium glutamicum regulates lysine synthesis by feedback inhibition: the end product inhibits aspartokinase, the committed enzyme. Strains carrying a feedback-resistant aspartokinase cannot sense that their lysine is abundant, and so overproduce and excrete it. Threonine and tryptophan follow similar logic.

Methionine is the exception. It is made chemically, and the synthesis is not stereoselective, so the product is a racemic mixture of D- and L-methionine. This is tolerable only because animals can use the D form: D-amino acid oxidase converts it to the keto acid, which is then transaminated to L-methionine. Methionine is nearly unique among essential amino acids in this respect, and it is the reason a chemical route remains competitive where fermentation dominates elsewhere.

The limit worth knowing

A free crystalline amino acid is absorbed faster than one released by digesting protein. If a large share of an amino acid arrives as the free form while the rest of the profile arrives slowly from protein, the peaks do not coincide, and the early-arriving amino acid is catabolised rather than used. This absorption asynchrony sets a practical ceiling on how far crude protein can be lowered, and it is why very low-protein diets need careful attention to feeding pattern and to which amino acids are supplemented together.

Nutritional requirements are also expressed on a digestible rather than total basis, since not all of a feedstuff’s amino acids are absorbed — a distinction that matters when comparing formulations.