For most of us when we think of dietary protein our minds may go to muscle growth & repair, thinking of all the protein supplements, bars, powders, shakes and more with promises to ‘bulk us up’, to get jacked, if you will – but, the functions of protein is much more than just getting them gains…
Let’s start with the structure of protein itself. A long polypeptide chain comprised of multiple amino acids, held together by covalent peptide bonds. There are a multitude of many different amino acids, and these are commonly referred to either essential amino acids, non-essential amino acids and conditionally essential amino acids.
Our essential amino acids cannot be synthesised by the body and must therefore be obtained through the diet, these include histidine, isoleucine, leucine, lysine, methionine, phenylalanine, tryptophan, threonine and valine. All essential amino acids can be found together within animal derived sources of protein, however for vegetarians and vegans they are limited to protein sources such as grains, seeds, nuts, beans and legumes which do not contain all essential amino acids together and is why the term ‘protein combining’ was coined, meaning that two of the vegan/vego proteins are eaten together for example, hummus (chickpeas + tahini) to ensure that adequate protein is obtained.
Conditionally essential amino acids can be synthesised within the body, however during times of increased demands (such as illness or stress) may require these to be increased through dietary sources to help support the body with the workload, these include arginine, tyrosine, cysteine glutamine, glycine, ornithine, proline, taurine, carnitine and serine (Lourenco et al, 2002; Kelly, 1998; Riedijk et al, 2007; Reeds, 2000).
Each amino acid serves many different functions both individually and cumulatively. Some of the main functions of protein is highlighted below, however amino acids are known to be involved in multiple other functions that are not listed within this article…
1. Digestion
Protein is required for optimal digestive function. Amino acids are needed to facilitate carbohydrate, protein and fat digestion via production enzymes needed to breakdown as well as pancreatic functioning. Glutamine is also an amino acid that is beneficial for the gastrointestinal tract lining and its integrity, helping to heal and seal any damage inflicted upon the supporting cells (Wang et al, 2009).
2. Mental health –
The amino acids derived from protein are required for neurotransmitter synthesis; serotonin & melatonin from tryptophan, GABA from glutamine, norepinephrine & epinephrine from tyrosine and histamine from glutamic acid
3. Hormones –
A conditionally essential amino acid, tyrosine, is required for the configuration of our thyroid hormones. Our ‘T3’ and T4’ thyroid hormones are essentially a double tyrosine with either 3 or 4 iodine atoms.
4. Gene expression –
Amino acids are required for the function and translation of mRNA in the process of gene expression and cell replication. Leucine in particular is required for this, through a process of modulating the proteins involved in mRNA translation and the selection of which mRNA for translation (Kimball & Jefferson, 2006).
5. Immunity –
There is of course too much of a good thing, and protein in excess can inflict damage upon kidneys, contribute to the pathogenesis of neurological disorders, oxidative stress and cardiovascular disease (Wu, 2009).
As always, consult with your health professional if unsure about what your optimal protein needs are, or if you’re having trouble including complete protein sources in your diet daily.
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References
Kelly, G. S. (1998). L-Carnitine: therapeutic applications of a conditionally-essential amino acid. Alternative medicine review: a journal of clinical therapeutic, 3(5), 345-360.
Kimball, S. R., & Jefferson, L. S. (2006). New functions for amino acids: effects on gene transcription and translation. The American journal of clinical nutrition, 83(2), 500S-507S.
Lourenco, R., & Camilo, M. E. (2002). Taurine: a conditionally essential amino acid in humans? An overview in health and disease. Nutr Hosp, 17(6), 262-270.
Reeds, P. J. (2000). Dispensable and indispensable amino acids for humans. The Journal of Nutrition, 130(7), 1835S-1840S.
Riedijk, M. A., van Beek, R. H., Voortman, G., de Bie, H. M., Dassel, A. C., & van Goudoever, J. B. (2007). Cysteine: a conditionally essential amino acid in low-birth-weight preterm infants?. The American journal of clinical nutrition, 86(4), 1120-1125.
Wang, W. W., Qiao, S. Y., & Li, D. F. (2009). Amino acids and gut function. Amino acids, 37(1), 105-110.
Wu, G. (2009). Amino acids: metabolism, functions, and nutrition. Amino acids, 37(1), 1-17.