Protein

Protein is an essential part of the human diet. We need protein to make muscles, enzymes, and hormones.

Protein is made by combining different small molecules called amino acids to make "chains". All life on earth uses protein made up of the same 22 amino acids [humans use 20 of these]. Our DNA directs how these 20 amino acids are combined to make the many different kinds of protein in our body.

Humans can make 11 of these amino acids, which include:

• arginine, aspartic acid, glutamic acid
• alanine, tyrosine
• cysteine, glycine, proline
• serine, asparagine, glutamine

Humans cannot make 9 of these amino acids, and so these amino acids are called "essential" and we must get these amino acids from our food:

• Branched chain: leucine, isoleucine, valine
• Ring-containing: tryptophan, phenylalanine, histidine
• Other: lysine, threonine, methionine

Human proteins do not contain the amino acids selenocystine or pyrrolysine.

People normally do not eat amino acids (except for medical reasons or sports training), but instead eat proteins derived from animals, plants, fungi, etc. The proteins that we eat must be digested in our stomach to break down the protein to the constituent amino acids, which are then absorbed into our blood.

In order to make any protein, all needed amino acids must be available in our blood at the same time. Some foods, notably plants, do not contain a balanced mix of all 9 essential amino acids. These are called "incomplete" proteins. For example, corn (and other grains) is deficient in the amino acids lysine and tryptophan; beans (and other legumes) are deficient in the amino acid methionine. However, eating a mixture of grains and legumes can produce a complete protein. This is important to vegans and indiginous cultures that have few available animal proteins. There are, however, a few complete plant proteins - notably soy and quinoa.

PhD. Barry Sears [Sears1995, pg 68] recommends each meal consist of 30% of its calories from protein, but notes that there is genetic variation in the optimal amount of protein (and fats and carbohydrates).

Genetic testing can help determine how much protein you need.

The best sources of protein are "complete":

• Animal flesh, eggs, dairy
• Soy, quinoa

All plant products contain some protein, but usually lower amounts of "incomplete" proteins; vegans must carefully balance different sources of protein in order to obtain a balanced blend of amino acids in their meals.

•       [Sears1995]   Bary Sears. Enter The Zone: A Dietary Road map. (1995). Amazon
• 
  [Wiki]
• 
  [WebMD]
• 
  [nutrition.org]
• 
  [medicalnewstoday]