What Are Proteins? What Is A Protein? How Much Protein Do I Need?

Editor's Choice
Main Category: Nutrition / Diet
Also Included In: Biology / Biochemistry
Article Date: 29 Jul 2010 - 14:00 PDT

email to a friend   printer friendly   opinions  

Proteins are large molecules consisting of amino acids which our bodies and the cells in our bodies need to function properly. Our body structures, functions, the regulation of the body's cells, tissues and organs cannot exist without proteins. Our muscles, skin, bones and many other parts of the body contain significant amounts of protein. Protein accounts for 20% of total body weight.

Enzymes, hormones and antibodies are proteins. Proteins also work as neurotransmitters and carriers of oxygen in the blood (hemoglobin).

Imagine proteins as machines; machines which make all living things, from viruses, bacteria, butterflies, jellyfish, plants and humans function. The human body is made up of approximately 100 trillion cells - each one has a specific function. Each cell has thousands of different proteins, which together make the cell do its job - the proteins are tiny machines within the cell. Amino acids and proteins - protein is made up of amino acids; amino acids are the building blocks of protein, there are 20 of them. Imagine there are 20 different types of bricks, and a much larger number of different types of houses which we could name according to the way we combined the bricks (their sequence). The bricks are the amino acids and the houses are the proteins.

The 20 different amino acids are:

• Alanine
• Arginine
• Asparagines
• Aspartic acid
• Cysteine
• Glutamic acid
• Glutamine
• Glycine, histidine
• Isoleucine
• Leucine
• Lysine
• Methionine
• Phenylalanine
• Proline
• Serine
• Threonine
• Tryptophan
• Tyrosine
• Valine

These 20 amino acids can be arranged in millions of different ways to create millions of different proteins, each one with a specific function in the body. Amino acids are organic molecules - they are made out of carbon, hydrogen, oxygen, nitrogen and sometimes sulphur.

There are three types of nutrients that are essential as energy sources for the human body:

• Proteins
• Carbohydrates
• Fats

One gram of protein or carbohydrate contains 4 calories, while one gram of fat has 9 calories.

Jons Jacob Berzelius (1779-1848), a Swedish doctor and chemist, introduced the word protein into science. Dutch chemist Gerhardus Johannes Mulder had already described proteins beforehand, but had not introduced the word as we know it today.

The Greek word protos means "first" and the Greek word proteios means "the first quality". As European scientists believed it was essential to life (a primary/first quality for life, a raw/primary material), they created a new word from the original Greek one.

According to Medilexicon's medical dictionary:

Proteins are Macromolecules consisting of long sequences of α-amino acids [H₂N-CHR-COOH] in peptide (amide) linkage (elimination of H2O between the α-NH2 and α-COOH of successive residues). Protein is three fourths of the dry weight of most cell matter and is involved in structures, hormones, enzymes, muscle contraction, immunologic response, and essential life functions. The amino acids involved are generally the 20 α-amino acids (glycine, l-alanine) recognized by the genetic code. Crosslinks yielding globular forms of protein are often effected through the -SH groups of two l-cysteinyl residues, as well as by noncovalent forces (hydrogen bonds, lipophilic attractions, etc.).

The function of proteins

In virtually every biological process proteins are playing a role. The main functions of proteins in the human body are to:

• Build, strengthen and repair/replace things, such as tissue. Examples include keratin (strengthens protective coverings, such as hair), collagen and elastin (both provide support for connective tissue).
• Make antibodies for our immune system
• Make hormones, which help cells send messages and coordinate bodily activities
• Muscle contractions - actin and myosin, two types of proteins, are involved in muscle contraction and movement.
• Make enzymes. An enzyme facilitates a biochemical reaction.
• Carry things - hemoglobin, a protein, transports oxygen through the blood.
• Mediate cell response - rhodopsin is a protein in the eye which is used for vision
• Storing things - ferritin is a protein which stores iron in the liver

Dietary requirements of protein

Nobody seems to agree on how much protein we can eat; experts from industry, government agencies, diet companies and nutritional organizations have a varying list of assertions.

An individual's daily protein requirement depends on several factors, including:

• Age - a growing child's needs will not be the same as an individual aged 80 years
• Sex - males generally require more protein than (non-pregnant or non-breastfeeding) females
• Weight - an individual who weighs 200lbs will require more protein compared to somebody who weighs 120lbs. In fact, recent studies indicate that weight matters more than age when determining dietary protein requirements.
• Muscular exertion - an individual who earns his living delivering pianos will require more protein than a computer programmer of the same age and height
• Muscle mass - a muscle-bound weight trainer will need more dietary protein than a marathon runner
• Health - a person who is convalescing after an illness or medical procedure may need more dietary protein than other people

The Institute of Medicine, part of the National Academy of Sciences, USA, which created the DRI (Dietary Reference Intake), a system of nutrition, recommends the following for dietary protein intake:

Percentage of energy that should come from protein:
(Source: DRI page7)

• Children aged 1 to 3 years - 5% to 20%
• Children aged 4 to 18 years - 10% to 30%
• Adults - 10% to 35%

According to the US Department of Agriculture, our protein intake should be:

• Infants - 10 grams per day
• Teenage boys - up to 52 grams per day
• Teenage girls - up to 46 grams per day
• Adult men- approximately 56 grams per day
• Adult women - approximately 46 grams per day
• Pregnant or lactating (breastfeeding) women - about 71 grams per day

Protein deficiency

In some developing countries protein deficiency is s major cause of illness and premature death. Protein deficiency can lead to mental retardation and reduced IQ.

In most parts of the world where protein deficiency is common, total food energy consumption is also too low - i.e. people are not getting enough food in general. Protein deficiency can lead to:

• Growth problems
• Wasting and shrinkage of muscle tissue
• Apathy
• Diarrhea
• Fatty liver
• Swollen belly
• Swollen legs
• Anemia
• Weaker immune system, leading to a higher susceptibility to infections and diseases

In several countries where protein deficiency is a serious problem, the leaves and other parts of the Moringa tree can help provide dietary protein.

In developed countries, especially Western Europe where the dietary requirements of poorer people are very carefully monitored and resolved, protein deficiency is quite rare. In developed nations, protein deficiency is more likely to occur among people on crash diets, or among very elderly individuals who do not eat properly. Some studies indicate that protein deficiency and general malnutrition is probably worse than people realize, especially in the USA among poor people, convalescing patients, and the elderly. The difference in the nutritional content of a Japanese/Finnish state school lunch, compared to one in the USA would shock most Finnish or Japanese nutritionists.

Sources of dietary protein

• Meat
• Poultry
• Fish and fish eggs
• Insects
• Dairy products
• Seeds and nuts
• Soy products
• Quorn - a fungus extract, popular in the UK/Ireland. Egg-white is used as a binder, so it is not a vegan source.
• Eggs
• Grains, vegetables and legumes also have protein (less per kilo of total weight)

Articles of interest:

What Are The Eight Most Popular Diets Today?
Atkins Is Best Diet Around, Says Stanford University Study
High Protein Diet Shrinks Brain
Why High-Protein, Low-Fat, And Low-Carbohydrate Diets Suppress Hunger

Written by Christian Nordqvist
Copyright: Medical News Today
Not to be reproduced without permission of Medical News Today

• Additional
• References
• Citations