Essential Amino Acids
Essential amino acids are also known as indispensable amino acid. It is an amino acid that the body cannot synthesize on its own as such it must be obtained from the diet. Since there are different types of amino acids and they have their own physiology, the list of essential amino acids needed by humans is different from those needed by other organisms. Amino acids considered essential for humans include the following:
- Valine
- Threonine
- Leucine
- Phenylalanine
- Serine
- Tryptophan
- Histidine
- Glycine
- Arginine
- Proline
Additionally, tyrosine, arginine and cysteine are always needed by growing children and infants. These acids are ‘essential’ not because they are extremely important for synthesis of protein or general homeostatis than others but for the simple reason the body does not need to synthesize them. If these are not taken up through the diet, they will not be available for synthesis.
What is more, the amino acids cysteine, arginine, proline, tyrosine, glutamine, glycine and serine are deemed conditionally essentially. This means normally, they are required in the diet though they should be exogenously supplied to certain populations that don’t synthesize them in amounts that are adequate.
A good example is the disease phenylketonuria (PKU). Individuals who have PKU should always keep their phenylalanine intake low in order to prevent the possibility of mental retardation as well as other metabolic complications. However since they are unable to synthesize tyrosine from phenylalanine, tyrosine becomes an essential amino acid in the diet of patients with PKU disease.
The estimation of daily requirement for indispensable amino acids has proven to be extremely difficult. Over the last 20 years, the numbers have gone through numerous revisions. World Health Organization usually recommends the daily use of essential amount acids in adults together with the standard abbreviations.
This makes it easier for people to consume the right amounts on a daily basis. Note that the recommended intake for children daily is 10 to 20 percent higher than that of adult levels. For infants, it is usually 150% higher during the first year of life.
William Cumming Rose led experiments that helped establish the essential amino acids that were essential in the human diet. The experiments included elemental diets to male graduate healthy students.
The diets were comprised of sucrose, cornstarch, butterfat without any protein, corn oil, inorganic salts, corn oil, large brown candy made from liver extract and flavored with peppermint oil, mixtures of purified individual amino acids and the known vitamins. The major outcome measure was the balance of nitrogen and Rose noted that deprivation of an essential amino acid caused lesser or lesser extent symptoms of exhaustion, dizziness and nervousness.
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Sources
http://en.wikipedia.org/wiki/Essential_amino_acid
http://chemistry.about.com/od/lecturenoteslab1/a/Essential-Amino-Acids.htm