Proteins are large molecules that our cells need to function properly. They consist of amino acids. The structure and function of our bodies depend on proteins. The regulation of the body’s cells, tissues, and organs cannot happen without them.

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.

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Protein molecules are essential for the functioning of every cell in the body. The body synthesizes some proteins foods we eat.

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 consists of around 100 trillion cells. Each cell has thousands of different proteins. Together, these cause each cell to do its job. The proteins are like tiny machines inside the cell.

Amino acids and proteins

Protein consists 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, each with a specific function in the body. The structures differ according to the sequence in which the amino acids combine.

The 20 different amino acids that the body uses to synthesize proteins are: Alanine, arginine, asparagine, 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 consist 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.

Types of protein

We sometimes hear that there are three types of protein foods:

Complete proteins: These foods contain all the essential amino acids. They mostly occur in animal foods, such as meat, dairy, and eggs.

Incomplete proteins: These foods contain at least one essential amino acid, so there is a lack of balance in the proteins. Plant foods, such as peas, beans, and grains mostly contain incomplete protein.

Complementary proteins: These refer to two or more foods containing incomplete proteins that people can combine to supply complete protein. Examples include rice and beans or bread with peanut butter.

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.

They can be:

  • structural, like collagen
  • hormonal, like insulin
  • carriers, for example, hemoglobin
  • enzymes, such as amylase

All of these are proteins.

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.

Most enzymes are proteins and are catalysts, which means they speed up chemical reactions. They are necessary for respiration in human cells, for example, or photosynthesis in plants.

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Rice and beans together provide complete protein.

Protein is one of the essential nutrients, or macronutrients, in the human diet, but not all the protein we eat converts into proteins in our body.

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.

All food proteins contain some of each amino acid, but in different proportions.

Gelatin is special in that it contains a high proportion of some amino acids but not the whole range.

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.

The body does not need all the essential amino acids at each meal, because it can utilize amino acids from recent meals to form complete proteins. If you have enough protein throughout the day, there is no risk of a deficiency.

In other words, the recommended nutrient is protein, but what we really need is amino acids.

Protein deficiency due to a low intake of protein in the diet is unusual as an isolated condition in the U.S.

The 2015–2020 Dietary Guidelines for Americans recommend that between 10 and 35 percent of an adult’s daily calories should come from protein. For children, it is 10 to 30 percent.

According to the Centers for Disease Control and Prevention (CDC), men in the United States get 16.1 percent of their calories from protein on average, and women 15.6 percent.

Worldwide, however, a lack of protein in the diet is a matter of concern, especially when it affects children. It can lead to problems of malnutrition, such as kwashiorkor and marasmus. These can be life-threatening.

A deficiency can also arise if a person has a health condition, such as:

Very low protein intake can lead to:

  • weak muscle tone
  • edema, which is swelling due to fluid retention
  • thin and brittle hair
  • skin lesions
  • in adults, loss of muscle mass
  • in children, stunted growth

Biochemical tests may show low serum albumin and hormone imbalances.

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Protein foods do not have to be meat. Seafood, eggs, pulses, and beans provide protein.

Exactly how much protein a person needs remains a matter of debate.

The FDA recommend that adults consume 50 grams of protein a day, as part of a 2,000-calorie diet. A person’s daily value may be higher or lower depending on their calorie needs.

However, specifying 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 1 ounce of protein per serving listed below:

  • one ounce lean meat, poultry, seafood
  • one ounce of meat, poultry, or seafood
  • one egg
  • one tablespoon of peanut butter
  • half an ounce of nuts or seeds
  • one fourth of a cup of cooked beans or peas

The USDA recommend consuming between 5 and 7 ounces of protein foods a day for most people over the age of 9 years.

They provide a calculator to make it easy to find out how much protein and other nutrients a person needs.

Protein and calories

Protein provides calories. One gram of protein contains 4 calories. One gram of fat has 9 calories.

The average American consumes around 16 percent of their calories from protein, whether of animal or plant origin.

It has been suggested that Americans obtain too many calories from protein, but now some experts are calling this a “misperception.”

Protein and weight loss

Some diets recommend eating more protein in order to lose weight.

Results of a review published in 2015 suggest that following a particular type of high-protein diet may encourage weight loss, but more work is needed to establish how to implement such a diet effectively.

Adding protein to an existing diet is unlikely to lead to weight loss, but replacing fat and sugar with protein might help. Replacing high-fiber foods — such as fruit, vegetables, and whole grains — with protein foods could have a negative effect.

People should consider their overall consumption and dietary habits when making this kind of change, and speak to a doctor before going ahead.

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Protein shakes and supplements are popular with athletes, but people should use them with care.

Eating more protein may boost muscle strength and encourage a lean, fat-burning physique. However, this depends on the person’s total food intake and activity levels.

Athletes and bodybuilders need to ensure they have enough protein to build and repair muscle, and this may be more than the minimum amount.

A wide range of protein supplements is currently available, many claiming to encourage weight loss and increase muscle mass and strength.

However, most athletes can get enough protein from a balanced diet without needing supplements.

Some supplements may also contain banned or unhealthy substances.

There is some evidence that too much protein may increase the risk of osteoporosis or kidney problems.

One study has indicated that whey protein may affect glucose metabolism and muscle protein synthesis. 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 (UOM), 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, although this may also be due to other factors.

The UOM note that anyone using whey protein should not consume more than 1.2 grams for each 2.2 pounds of body weight.

In addition, as supplements, whey protein and similar products do not have FDA approval. This means is little or no control over their contents.

Anyone who is considering taking protein supplements for fitness purposes should speak to a doctor who specializes in sports medicine.

For most people, a varied and healthful diet will provide enough protein.

Increasing protein intake does not necessarily mean eating more steak. There are other choices that can help you ensure a healthful protein intake.

Here are some suggestions:

  • Eat a variety of protein foods, choosing from fish, meat, soy, beans, tofu, nuts, seeds, and so on.
  • Choose low-fat meat, poultry, and dairy products, and trim the fat from the meat. Opt for smaller portions and avoid processed meats, as they have added sodium.
  • Use cooking methods that do not add extra fat, such as grilling.
  • Check the ingredients in “protein bars,”as they can also be high in sugar.
  • Opt for healthier versions of your usual favorites, for example, wholemeal rather than white bread and unsweetened peanut butter.
  • Experiment with plant-based proteins, such as beans, lentils, and soy products.
  • Choose nutrient-rich foods that provide other benefits, such as fiber.


Is it dangerous to use protein shakes and whey protein in a weight-loss diet?


Protein shakes and whey protein are acceptable to incorporate in a healthy weight-loss diet plan, as long as the total daily protein intake does not consistently exceed a person’s recommended daily allowance for protein, and as long as a person is replacing other sources of calories with proteins, and not simply adding extra calories to their day.

Greatly exceeding protein needs can be damaging to a person’s health, including kidney damage and dehydration.

Katherine Marengo LDN, RD Answers represent the opinions of our medical experts. All content is strictly informational and should not be considered medical advice.