Isoleucine, leucine and valine are the three branched chain amino
acids. Their metabolism is unique, sharing the same catabolism
pathway and therefore interfere with each other. Leucine is a strong
regulator of the branched chain amino acid catabolism. As a consequence,
minimum supply of valine and isoleucine must be ensured in
the diet and an excess of leucine should be avoided.
Protein synthesis is an indispensable process in the animal. For
protein to be synthesised, the required essential and non-essential
amino acids must be present at the site of synthesis according to the
requirements of the animal. When the supply of one of this essential
amino acid does not meet the animal requirements for this amino
acid, it is said to be limiting.
Hence a limiting amino acid must be present in the diet in sufficient
quantity to meet the total requirements of the animal. The first amino
acid to limit protein synthesis is termed the first limiting amino acid.
Once this amino acid has been supplied the next to limit synthesis is
the second limiting amino acid and so on.
In typical diets offered to poultry and ruminants it is usually methionine
which is first limiting, lysine usually being second limiting. In pigs
lysine is the first limiting amino acid followed by threonine, methionine,
tryptophan and valine. A sufficient quantity of the limiting amino
acids in the diet to meet requirements also governs whether the other
amino acids are efficiently utilised for protein synthesis.
This principle can be illustrated by the “Liebig barrel” where the level
of fill in the barrel represents the capacity for protein synthesis of the
animal (Figure 8). The capacity of the barrel is “limited” by the shortest
stave (the first limiting amino acid). However, if the shortest stave
is lengthened (dietary supplementation with the first limiting amino
acid) then the capacity increases to the level of the “second-limiting”
stave. This repeats for the next limiting amino acids.
In practical diets the “staves” can be lengthened through targeted
supplementation of amino acids. This is demonstrated in Figure 8
for a typical European piglet feed with 5 limiting amino acids covered
by supplementation.
Building upon the concept of Liebig barrel, the idea developed that
there is one ratio of amino acids that is “ideal”. Since in pigs, lysine
tended to be the first limiting amino acid it was decided that lysine
would be taken as a reference value and the remaining essential
amino acids would be referred to lysine, taking lysine as one hundred
percent. For pigs and poultry, these ratios of essential amino acids
compared to lysine are designated as ‘the ideal protein’ or the ‘ideal
amino acid profile’