Proteins

What are Proteins?

Proteins are primarily used as building blocks to form and maintain the structural and functional elements of your body. To be more specific, proteins are building blocks for the construction of muscles, tendons, ligaments, skin, hair, finger nails, bone, cartilage, brains, antibodies, enzymes, hormones, hemoglobin, and many other substances within the body.

Proteins, along with fats and carbohydrates, are macronutrients. That means that proteins are nutrients that are required in relatively large amounts within the body, and that they are capable of providing caloric energy when consumed (as opposed to vitamins and minerals, which are micronutrients and do not provide any caloric energy when consumed). Generally speaking, proteins will primarily be used by the body as building blocks in their structural roles discussed above, rather than as a source of caloric energy. However, when there is a shortage of fat and carbohydrate derived caloric energy within the body, for example during starvation, protein (primarily muscle) can be broken down to become an energy source.

Amino Acids

Proteins are made up of sub-units called amino acids. There are 20 standard amino acids that can be combined in various sequences and quantities to create all the various proteins that exist. When you consume proteins in food, they are broken down within your digestive system into their amino acid units, which can then be reassembled by your body to form the specific proteins that it needs. Your body is even capable of making some amino acids by itself. These types of amino acids are called non-essential amino acids, because they are not considered to be essential in your diet due to the fact that your body is capable of making them. There are also some amino acids that your body is incapable of making on its own. These types of amino acids are called essential amino acids, because you're body cannot make them so it is essential that you consume them in your diet.

Non-Essential Amino Acids

The following amino acids are non-essential amino acids for adults:

• Alanine
• Arginine
• Asparagine
• Aspartic acid
• Cysteine
• Glutamic acid
• Glutamine
• Glycine
• Proline
• Serine
• Tyrosine

Essential Amino Acids

The following amino acids are essential amino acids for adults:

• Histidine
• Isoleucine
• Leucine
• Lysine
• Methionine
• Phenylalanine
• Threonine
• Tryptophan
• Valine

Joint WHO/FAO/UNU Consultation. Protein and amino acid requirements in human nutrition. World Health Organ Tech Rep Ser. 2007; (935): pg. 150, par. 8.1.

Protein in your Diet

Complete Proteins

Dietary sources of protein that contain all of the essential amino acids in sufficient proportions to meet our nutritional requirements are called complete proteins. Generally speaking, proteins from animal sources are likely to be complete proteins. For example, meats, fish, poultry, eggs, cheese, milk, and yogurt are sources of complete proteins.

Incomplete Proteins

Dietary sources of protein that do not contain all of the essential amino acids in sufficient proportions to meet our nutritional requirements are called incomplete proteins. Generally speaking, proteins from plants, grains, legumes, seeds, nuts, fruits, and vegetables tend to be sources of incomplete proteins. It is, however, possible to combine incomplete protein sources to create a complete protein mix. For example, rice or corn combined with beans makes a complete protein source.

Medline Plus Medical Encyclopedia. U.S. National Library of Medicine and National Institute of Health. July 21, 2009. Protein in diet

Protein Dietary Reference Intakes

The table below displays the Dietary Reference Intakes (DRIs) for protein amongst all age groups of both sexes.

Protein Dietary Reference Intakes
Age	Protein (grams/day)
Infants
0 to 6 months	9.1 *
7 to 12 months	11.0
Children
1 to 3 years	13
4 to 8 years	19
Males
9 to 13 years	34
14 to 18 years	52
19 to 30 years	56
31 to 50 years	56
50 to 70 years	56
over 70 years	56
Females
9 to 13 years	34
14 to 18 years	46
19 to 30 years	46
31 to 50 years	46
50 to 70 years	46
over 70 years	46
Pregnant Women
less than 18 years	71
19 to 30 years	71
31 to 50 years	71
Lactating Women
less than 18 years	71
19 to 30 years	71
31 to 50 years	71

* For infants aged 0 to 6 months old, the value provided is an Adequate Intake (AI) value rather than a Recommended Dietary Allowance RDA value. The source of the above table makes the following comments about RDAs and AIs:

RDAs and AIs may both be used as goals for individual intake. RDAs are defined to meet the needs of almost all (97 to 98 percent) individuals in a group. For healthy breastfed infants, the AI is the mean intake.

When broken down into a percentage of total energy intake, your consumption of proteins should follow the general guidelines in the table below:

Age	Protein
1 to 3 years	5% to 20% of energy requirements
4 to 18 years	10% to 30% of energy requirements
19 years and older	10% to 35% of energy requirements

For example, if you are an adult that consumes 2000 calories per day it is recommended that 10% to 35%, or from 200 calories to 700 calories, are derived from proteins.

Trumbo P, Schlicker S, Yates AA, Poos M; Food and Nutrition Board of the Institute of Medicine, The National Academies. Dietary reference intakes for energy, carbohydrate, fiber, fat, fatty acids, cholesterol, protein and amino acids. J Am Diet Assoc. 2002 Nov;102(11):1621-30. Erratum in: J Am Diet Assoc. 2003 May;103(5):563.

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