Forgot password?

Join NutriDesk

Amino Acids

An overview including BCAAs

Structure

Amino Acids - The Building-Blocks of Proteins


Amino acids are organic molecules that consist of:

  • a basic amino group (−NH2),
  • an acidic carboxyl group (−COOH), and
  • an organic R group (or side chain) that is unique to each amino acid.

Amino Acids: Essential [indispensable], Nonessential [dispensable] or Conditionally Indispensable

  • There are approximately 20 common amino acids in food.
  • 9 of these amino acids are considered essential, or indispensable, for humans.
  • These essential or indispensible amino acids cannot be synthesized in the body or cannot be synthesized in sufficient quantities and thus you must acquire these in your diet.

Essential

[Indispensable]

Nonessential

[Dispensable]

Precursors for Conditionally Indespensable Amino acids73   Conditionally Indispensable73

Primary Limiting Amino Acids73,74

Branched Chain Amino Acids [BCAA's]
Histidine Alanine Glutamine or glutamate, aspartate Arginine Lysine Isoleucine
Isoleucine Asparagine Methionine, Serine Cysteine Methionine Leucine
Leucine Aspartate Glutamate, ammonia Glutamine Tryptophan Valine
Lysine Glutamate Glutamate Proline
Methionine Phenylalanine Tyrosine
Phenylalanine
Threonine
Tryptophan
Valine

Conditionally indispensible amino acids

  • Neonates that are born prematurely are unable to synthesize many nonessential amino acids such a cysteine or proline and will need this in their diet or supplementary feeds73.
  • Metabolism of methionine and phenylalanine becomes impaired due to cirrhosis of the liver and as these are precursors to cysteine and tyrosine respectively [see above], cysteine and tyrosine in this circumstance become conditionally indispensible and the individual will need supplementation of these amino acids if and when normal organ function returns73.

Primary Limiting Amino Acids

  • Foods of animal origin ---meat, fish, eggs and dairy all have complete protein i.e. a full complement of amino acids [except gelatin which lacks tryptophan74, beef has lower methionine levels76.]
  • Lysine is an essential amino acid that is relatively uncommon in the plant kingdom
  • Some grains such as quinoa [actually a pseudocereal] have complete protein with all the essential amino acids including a high lysine value75 and also high in the sulphur containing amino acids, methionine and cysteine75
  • Foods of plant origin [with the exception of soybeans], have poor quality protein i.e. they are incomplete sources of all the amino acids and the primary limiting amino acids are lysine, methionine and tryptophan.
  • In grains and seeds, lysine is the primary limiting amino acid76.
  • In legumes the primary limiting amino acid is methionine76.
  • In corn [maize] the primary limiting amino acid is tryptophan76.
  • Legumes such as beans have a high lysine content but are low in methionine while grains have the opposite profile. This is why mixing legumes and grains such as beans and rice are complementary allowing for an adequate intake of all the essential amino acids to meet human daily nutritional needs for the intake of these amino acids.
  • Protein combining e.g. mixinig beans and rice as described above may not be necessary in the same meal as long as complementary protein sources are consumed in a day76.
  • According to the Vegan Society, combining is 'old fashioned' - they state:
    "Even vegetarians are sometimes advised to combine vegetable proteins with dairy foods. This advice is now very old fashioned. Protein combining may reduce the amount of protein required to keep the body in positive protein balance but several human studies have indicated that this is neither necessary nor even always the case. Diets based solely on plant foods easily supply the recommended amounts of all the indispensable amino acids, and protein combining at each meal is unnecessary. Soya protein is actually equivalent in biological value to animal protein."
  • The statement above does not mean that a vegan should limit their grain intake to one variety only as larger amounts of this grain may need to be consumed to enable an adequate intake of the limiting amino acid of that particular grain. Also keep in mind that consuming large quantities of grains [low GI or high GI] is not a healthy practice and that one needs to keep the glycemic load in check.
  • Vegans should consume grains known to have complete protein such as soya beans, quinoa, amaranth, buckwheat. A good book such as that featured opposite should be read to ensure that you become a healthy vegan.
  • The focus of the NutriDesk diets is to ensure that you not only obtain complete protein but also that you consume carbohydrates with a low glycemic index [low-GI] and low glycemic loads [low-GL] compatible with the energy requirements of an individual. 
Amino_Acids_Metabolic_Pathways.png

Click to enlarge

Amino Acid Catabolism

Overview

Some facts

  • Taurine is also essential in preterm babies.
  • Branched-Chain Amino Acids [BCAA’s] which are Isoleucine, Leucine and Valine, make up 70% of the body’s amino acid pool.
  • All amino acids have the L-configuration except for glycine which is optically inactive.
  • Taurine is conjugated via its terminal group with bile acids [cholic acid and chenodeoxycholic acid] to form bile salts.
  • Taurine also acts as an inhibitory neurotransmitter.
  • The major pathway for mammalian taurine synthesis is via the Methionine-Homocysteine transulfuration pathway.

References:

  1. Fürst P, Stehle P (01 Jun 2004). "What are the essential elements needed for the determination of amino acid requirements in humans?". Journal of Nutrition 134 (6 Suppl): 1558S–1565S.
  2. Groff, J. L. and S. A. S. Gropper (2000). Advanced nutrition and human metabolism, West/Wadsworth Belmont, CA.
  3. "human nutrition." Encyclopædia Britannica. 2009. Encyclopædia Britannica Online. 12 Apr. 2009 <http://www.britannica.com/EBchecked/topic/422896/human-nutrition>.
  4. Greg Schlick and David L. Bubenheim (November 1993). "Quinoa: An Emerging "New" Crop with Potential for CELSS (NASA Technical Paper 3422).
  5. "Position of the American Dietitians Association and Dietitians of Canada: Vegetarian Diets". Dietitians of Canada. 2003. http://www.dietitians.ca/news/downloads/vegetarian_position_paper_2003.pdf. Retrieved on 2008-06-18. "Research indicates that an assortment of plant foods eaten over the course of a day can provide all essential amino acids and ensure adequate nitrogen retention and use in healthy adults; thus complementary proteins do not need to be consumed at the same meal."
  6. http://www.vegansociety.com/food/nutrition/protein.php. Accessed 12th April 2009.
Couple-Faces-xxs.png