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
|
Muscle recovery,
protein synthesis, increase glucose uptake
|
|
Activate mTOR and sirtuins,
muscle recovery, protein synthesis
|
|
Lysine (W)
|
Calcium absorption,
tissue repair, carnitine precursor
|
Methyl donor,
cysteine precursor, carnitine precursor
|
|
Phenylalanine (W)
|
Tyrosine precursor
|
Threonine (W)
|
Protein balance,
collagen and elastin synthesis, antibody formation
|
Serotonin, melatonin
and niacin (vitamin B3) precursor
|
|
Muscle recovery,
tissue repair, protein synthesis
|
Non-Essential AAs
|
Main Functions
|
Modest effect on
performance and body composition, carnosine precursor
|
|
Nitric oxide
precursor, growth hormone secretion
|
|
Asparagine (W)
|
Nervous system
development
|
Weak excitatory
neurotransmitter, helps convert glutamine to glutamate (and is converted to
asparagine in the process)
|
|
Glutathione and
taurine precursor
|
|
Glutamate (W)
|
Main excitatory
neurotransmitter, GABA (main inhibitory neurotransmitter) precursor
|
Refills TCA cycle,
an energy source for rapidly dividing cells, may improve intestinal
permeability
|
|
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
|
Similar neural
function as glycine, but in glial cells
|
|
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
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
·
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.
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