Protein
While the nutritional label lists ‘protein’ as though it is one substance, there are thousands of different proteins in our food, and thousands in ourselves. We need to eat protein in our food mainly so that we can make our own proteins.
In our digestive system, we break down the proteins in the food we eat into their component molecules, called amino acids, and then we join the amino acids back up again, using different numbers of them (from less than ten to several thousand) in a different – but very definite – order, to make our own proteins. Each amino acid molecule has a sort of chemical ‘hook’ on each end, so that it can be attached to any other one, to make the proteins we need, like a series of carriages on a very long train. The code that determines the sequence of amino acids (the order of the ‘carriages’) is in our DNA.
There are about 20 different amino acids. Eight of them are essential in the diet, but the others we can make from those eight, if necessary. Most people take in a sufficient amount and good mix of protein that they do not need to worry about getting enough of the essential amino acids. Those on a strict vegan diet may need to take more care to eat a mix of different plant proteins (particularly beans) to ensure that they have enough. The amount of protein estimated to be needed by an adult each day (the Reference Intake or RI) is 50 g and an average western diet contains much more that.
Essential and non-essential amino acids (with their 3-letter abbreviations)
Essential amino acids Amino acids synthesised from essential amino acids non-essential amino acids
lysine (Lys) tyrosine (Tyr)* glycine (Gly)
methionine (Met) cysteine (Cys)* alanine (Ala)
threonine (Thr) serine (Ser)
leucine (Leu) proline (Pro)
isoleucine (IIe) glutamate (Glu)
valine (Val) glutamine (Gln)
phenylalanine (Phe) asparate (Asp)
tryptophan (Try) asparagine (Asn)
histidine (His) - made only in very small amounts in the body
arginine (Arg) - for young children
*Synthesised from phenylalanine.
+Synthesised from methionine.
Proteins are an essential component of our muscles and other structures in our body, such as hair, nails and tendons. As well as these structural proteins, there are proteins found in our blood that function as hormones, and others that form part of our immune system, which protects us against infection. The proteins in the food that we eat are digested by enzymes that are themselves proteins.
The excess amino acids in our diet cannot be stored for long in the body, if they are not needed for building new proteins. They are taken in the blood stream to the liver, where they are broken down. The part of the amino acid that contains the nitrogen is converted into urea, which circulates in the blood to the kidneys, which provide an amazing biological filtering system. They filter out the urea and send it down tubes to the bladder, along with other substances not needed by the body, and excess water, forming urine. So, when you wee, you are getting rid of surplus amino acids. Clever, isn’t it?
ProteinWhile the nutritional label lists ‘protein’ as though it is one substance, there are thousands of different proteins in our food, and thousands in ourselves. We need to eat protein in our food mainly so that we can make our own proteins.In our digestive system, we break down the proteins in the food we eat into their component molecules, called amino acids, and then we join the amino acids back up again, using different numbers of them (from less than ten to several thousand) in a different – but very definite – order, to make our own proteins. Each amino acid molecule has a sort of chemical ‘hook’ on each end, so that it can be attached to any other one, to make the proteins we need, like a series of carriages on a very long train. The code that determines the sequence of amino acids (the order of the ‘carriages’) is in our DNA.There are about 20 different amino acids. Eight of them are essential in the diet, but the others we can make from those eight, if necessary. Most people take in a sufficient amount and good mix of protein that they do not need to worry about getting enough of the essential amino acids. Those on a strict vegan diet may need to take more care to eat a mix of different plant proteins (particularly beans) to ensure that they have enough. The amount of protein estimated to be needed by an adult each day (the Reference Intake or RI) is 50 g and an average western diet contains much more that.Essential and non-essential amino acids (with their 3-letter abbreviations)Essential amino acids Amino acids synthesised from essential amino acids non-essential amino acidslysine (Lys) tyrosine (Tyr)* glycine (Gly)methionine (Met) cysteine (Cys)* alanine (Ala)threonine (Thr) serine (Ser)leucine (Leu) proline (Pro)isoleucine (IIe) glutamate (Glu)valine (Val) glutamine (Gln)phenylalanine (Phe) asparate (Asp)tryptophan (Try) asparagine (Asn)histidine (His) - made only in very small amounts in the body arginine (Arg) - for young children *Synthesised from phenylalanine.+Synthesised from methionine. Proteins are an essential component of our muscles and other structures in our body, such as hair, nails and tendons. As well as these structural proteins, there are proteins found in our blood that function as hormones, and others that form part of our immune system, which protects us against infection. The proteins in the food that we eat are digested by enzymes that are themselves proteins.The excess amino acids in our diet cannot be stored for long in the body, if they are not needed for building new proteins. They are taken in the blood stream to the liver, where they are broken down. The part of the amino acid that contains the nitrogen is converted into urea, which circulates in the blood to the kidneys, which provide an amazing biological filtering system. They filter out the urea and send it down tubes to the bladder, along with other substances not needed by the body, and excess water, forming urine. So, when you wee, you are getting rid of surplus amino acids. Clever, isn’t it?
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