Muscles, skin, bones, and other parts of the human body contain significant amounts of protein, including enzymes, hormones and antibodies. Proteins also work as neurotransmitters. Hemoglobin, a carrier of oxygen in the blood, is a protein.
A Dutch chemist, Gerhardus Johannes Mulder, first described proteins, and he coined the name in 1838.
The Greek word protos means "first" and the Greek word proteios means "the first quality."
Proteins were believed to be a primary, or first, quality for life.
What are proteins?
Protein is essential for many bodily functions.
Proteins are long chains of amino acids that form the basis of all life. They are like machines that make all living things, whether viruses, bacteria, butterflies, jellyfish, plants, or human function.
The human body is made up of approximately 100 trillion cells. Each cell has thousands of different proteins, which together cause the cell to do its specific job. The proteins are tiny machines within the cell.
Amino acids and proteins
Protein is made up of amino acids, and amino acids are the building blocks of protein. There are around 20 amino acids.
These 20 amino acids can be arranged in millions of different ways to create millions of different proteins, and each protein has a specific function in the body. The structures differ according to the sequence in which the amino acids combine.
The U.S. National Library of Medicine lists the 20 different amino acids as: Alanine, arginine, asparagines, aspartic acid, cysteine, glutamic acid, glutamine, glycine - histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, and valine.
Amino acids are organic molecules that are made of carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur.
It is the amino acids that synthesize proteins and other important compounds in the human body, such as creatine, peptide hormones, and some neurotransmitters.
What do proteins do?
Proteins play a role in nearly every biological process, and their functions vary widely.
The main functions of proteins in the body are to build, strengthen and repair or replace things, such as tissue.
Keratin is a structural protein that strengthens protective coverings, such as hair. Collagen and elastin, too, have a structural function, and they also provide support for connective tissue.
Enzymes are catalysts. This means they speed up chemical reactions. They are needed for respiration in human cells, for example, or photosynthesis in plants.
We know that protein is one of the essential nutrients in the human diet, but the protein in the foods we eat does not all convert into proteins in our body.
Protein foods provide amino acids that our bodies use to synthesize proteins.
When people eat foods that contain amino acids, these amino acids make it possible for the body to create, or synthesize, proteins. If we do not consume some amino acids, we will not synthesize enough proteins for our bodies to function correctly.
There are also nine essential amino acids that the human body does not synthesize, so they must come from the diet.
The nine essential acids that the human body does not synthesize are: Histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
Foods that contain these nine essential acids in roughly equal proportions are called complete proteins. Complete proteins mainly come from animal sources, such as milk, meat, and eggs.
Soy and quinoa are vegetable sources of complete protein. Combining red beans or lentils with wholegrain rice or peanut butter with wholemeal bread also provides complete protein. Research shows that the body does not require all essential amino acids at each meal because it can utilize amino acids from recent meals to form complete proteins.
So, the recommended nutrient is protein, but what we really need is amino acids.
Protein deficiency is unusual as an isolated condition. If a person lacks protein, they normally have a wider lack of nutrients and energy, due to low food intake. This may be due to poverty or illness.
Very low protein intake can lead to weak muscle tone, edema, or swelling, thin and brittle hair, and skin lesions, and, in children, stunted growth. Biochemical tests may show low serum albumin and hormone imbalances.
Eating more protein may boost muscle strength and encourage a lean, fat-burning physique. This, of course, depends on total food intake and activity levels.
The body's need for dietary protein is tied up with nitrogen levels in the body. Not having enough of the right levels of the right amino acids can lead to a nitrogen imbalance.
Exactly how much protein a person should consume remains a matter of debate.
Past studies have shown that a breastfed American infant aged up to 3 months will develop satisfactorily with a protein intake of 1.68 grams per kilogram of body weight per day, but experts have suggested that 1.1 grams per kilogram is probably enough.
The current minimum recommended amount of protein per day is 0.8 grams per kilogram for an average adult to stay healthy, according to Harvard Health.
However, recommending exact amounts is difficult, because a range of factors, such as age, gender, activity level, and status, for example, pregnancy, play a role.
Other variables include the proportion of amino acids available in specific protein foods, and the digestibility of individual amino acids. It also remains unclear how protein metabolism affects the need for protein intake.
According to the U.S Department of Agriculture (USDA), the following foods will provide about 7 grams of protein per serving listed below:
- 1 ounce lean meat, poultry, seafood
- 1 egg
- 1 Tablespoon peanut butter
- Half an ounce nuts or seeds
- A quarter cup cooked beans or peas
The USDA provides a calculator to make it easy to find out how much protein and other nutrients a person needs.
Protein and calories
Consuming 0.8 grams per kilogram of protein each day would provide 10 percent of an average person's calories.
The average American consumes around 16 percent of their calories from protein, whether of animal or plant origin.
The 2015 to 2020 Dietary Guidelines for Americans recommend that for people over the age of 4 years, between 10 percent and 35 percent of their calories should come from protein, depending on their age and gender.
Derek Pendick, former editor of Harvard Men's Health, comments in an article on protein requirements:
"Research on the optimal amount of protein to eat for good health is ongoing and is far from settled. The value of high-protein diets for weight loss or cardiovascular health, for example, remains controversial."
He adds that consuming more protein does not necessarily mean eating more steak, and it may mean eating less of something else, say, carbohydrates, to maintain a healthy weight.
Some diets recommend eating more protein in order to lose weight.
However, nutritionists stress that there is insufficient evidence that adding protein will guarantee weight loss, and that people should consider their overall consumption and dietary consequences when making this kind of change.
If eating more protein results in a lower fiber intake, for example, this may not be beneficial.
What about protein drinks and foods?
A wide range of protein supplements is currently available, many claiming to encourage weight loss and increase muscle mass and strength.
Research has not confirmed that protein supplements are beneficial for weight loss.
Athletes and body builders need to ensure they have enough protein to build and repair muscle, and this may be more than the minimum amount.
However, no evidence has so far proven that high protein supplements are more beneficial than consuming protein as part of a healthy diet. Some supplements may also contain banned or unhealthy substances.
One study has indicated that whey protein may affect glucose metabolism and muscle protein synthesis, but other research concludes that at least one type of whey supplement can reduce body fat and preserve lean muscle when used in a reduced-calorie diet.
According to the University of Michigan, one investigation has found that whey protein enhanced performance in cyclists, and while another has suggested that it may lead to bone loss and osteoporosis.
The online magazine for bodybuilders, Muscle and Fitness points out that whey protein shakes and powders are manufactured from what used to be waste products from dairy production, and cautions that quality can be inconsistent between brands.
Manufacturers add flavorings, colorings and additional amino acids, and they may use low-quality whey, so that instead of 90 percent protein, the product may be just 35 percent protein, they warn.
While no side effects or interactions of whey protein have yet been proven, neither have the benefits of using whey protein supplements been confirmed.
Athletes and others who are thinking about taking protein supplements should first ensure they are eating a healthy diet, then speak to a dietitian or physician for advice about supplements.