The utilisation of proteins refers to the amount of of ingested protein that can be digested and used in the body for protein synthesis. To maintain adequate levels regular intake of protein is essential. Protein provides a source of nitrogen and supports essential metabolism.
When a meal containing protein is consumed, the dietary proteins are broken down into amino acids by digestive enzymes, after this point they are available for the building of proteins.
There is no way for the body to store proteins or amino acids, there is a tightly regulated pool of around 100 grams in cells and the blood stream. This pool is maintained by the continuous breaking down and building of proteins within cells. This pool can respond quickly to changes in demands of the cell, if the cell needs more protein than is available in the cell.
The cell can also respond to a limited availability of specific amino acids for the production of all non essential proteins like the keratin needed for hair growth. Keratin is a unique protein due to its high sulphur content, if sulphur containing amino acids are low in availability will cease.
It is likely there is an inherant hierarchy that allows reduction of keratin formation in the hair fibre before there is a reduction in the keratin fibre that make up the skin and nails. If the cell does not have enough amino acids to make the protein it need it will instantly halt production until that amino acid is available.
Sulphur is also essential for a process known as sulphonation and sulphurylation, this is a process that allows the addition of a sulphur group to another molecule. This process allows for transport of hormones in the blood to target tissue, the removal of toxins and the conjugation of bile for the uptake of fat-soluble vitamins.