Protein

Proteins are made up of amino acids (peptides are short chains of amino acids). When we eat foods containing protein, the protein is broken down into amino acids during digestion so they can be absorbed and then used in the synthesis of our own proteins or for other functions (see below).  (Amino acids that can be incorporated into proteins are called proteinogenic amino acids)

There are both essential and non-essential amino acids. In this context ‘essential’ means we can’t synthesise it and so must get it from the diet. Although in practice, the synthesis of the non-essential amino acids is quite limited and dietary sources supply plenty of non-essential amino acids, so it’s largely an academic point.  Below are the essential and non-essential amino acids and some of their main functions (the ‘W’ hyperlink goes to Wikipedia and ‘E’ goes to Examine.com)

Essential AAs	Main Functions
Histidine (W)	Converted to histamine for immune response
Isoleucine (W,E)	Muscle recovery, protein synthesis, increase glucose uptake
Leucine (W,E)	Activate mTOR and sirtuins, muscle recovery, protein synthesis
Lysine (W)	Calcium absorption, tissue repair, carnitine precursor
Methionine (W,E)	Methyl donor, cysteine precursor, carnitine precursor
Phenylalanine (W)	Tyrosine precursor
Threonine (W)	Protein balance, collagen and elastin synthesis, antibody formation
Tryptophan (W,E)	Serotonin, melatonin and niacin (vitamin B3) precursor
Valine (W,E)	Muscle recovery, tissue repair, protein synthesis

Non-Essential AAs	Main Functions
Alanine (W,E)	Modest effect on performance and body composition, carnosine precursor
Arginine (W,E)	Nitric oxide precursor, growth hormone secretion
Asparagine (W)	Nervous system development
Aspartate (W,E)	Weak excitatory neurotransmitter, helps convert glutamine to glutamate (and is converted to asparagine in the process)
Cysteine (W,E)	Glutathione and taurine precursor
Glutamate (W)	Main excitatory neurotransmitter, GABA (main inhibitory neurotransmitter) precursor
Glutamine (W,E)	Refills TCA cycle, an energy source for rapidly dividing cells, may improve intestinal permeability
Glycine (W,E)	Inhibitory neurotransmitter outside cortex and co-agonist on NMDA receptors (excitatory) inside cortex the main amino acid in collagen
Proline (W)	Hydroxyproline is an abundant amino acid in collagen
Serine (W,E)	Similar neural function as glycine, but in glial cells
Tyrosine (W,E)	Dopamine, noradrenaline and adrenaline precursor

Some other things to note: isoleucine, leucine and valine are the branched chain amino acids (BCAAs) and methionine and cysteine are the only amino acids that contain sulphur.

You don’t really need to worry about content of those tables much because simply eating protein will simply all the amino acids, but there are some exceptions

·         Many plant proteins are ‘incomplete’, meaning they have a lower than ideal proportion of the essential amino acids.  Generally plant proteins have a lower proportion of the sulphur amino acids (methionine and cysteine), the BCAAs and/or lysine

·         Most protein sources are low in glycine, except connective tissue, skin and bones [1] [2]

Also, some amino acids are commonly supplemented for various benefits, such as:

·         Leucine to increase protein synthesis [3]

·         BCAAs to enhance exercise performance and/or protein synthesis [4]

·         Tryptophan to increase serotonin synthesis

·         Beta-alanine to increase carnosine [5]

·         Arginine (or nuts as they’re high in arginine) to increase nitric oxide

·         N-Acetylcysteine (NAC) for a number of effects including glutathione synthesis[6]

·         Glycine (or gelatin as it’s high in glycine) for a number of effects [2]

The metabolism of amino acids is more complex than fat and carbohydrate.  After being deaminated (removal of nitrogen), amino acids can also enter the TCA cycle in certain positions (see here) and then depending on the amino acid can either be broken down for ATP synthesis, converted into glucose or converted into ketones.  The removal of nitrogen as urea/uric acid is a limiting factor of protein intake and exceeding that capacity with a very high protein intake coupled with low fat and carbohydrate induces rabbit starvation.