What is Protein?

Protein is an essential macronutrient that plays a role in maintaining the optimal health and function of your body. In this overview, we discuss all aspects of protein, including what it is, the function of protein in your body, deficiency symptoms, sources of protein, and how to determine your protein needs.

What is protein?

Protein is a component of every cell, tissue, and organ in your body and is an essential part of a healthy diet.

Proteins are large molecules consisting of amino acids linked together in different combinations. Your body can break apart the protein molecules to access the amino acids.

There are twenty amino acids, each of which has various bodily functions. Your body makes eleven of the amino acids, called nonessential amino acids (NEAAs). The other nine amino acids must be obtained from food sources and are called essential amino acids (EAAs).

Since you don’t store amino acids, you must consume all nine essential amino acids daily in your diet.

Dietary protein

Protein is one of the three macronutrients, along with carbohydrates and fat, and is a primary source of energy in the diet, providing four calories per gram. Every food source of protein has a different amount of amino acids, so consuming a variety of protein sources helps to ensure an adequate intake of amino acids.

Some protein sources include meat, poultry, fish, eggs, dairy, and legumes. Later in this article, you’ll find more examples of the best protein foods.

What happens to the proteins that we eat?

When you eat protein, the food components move through the gastrointestinal tract to be digested, absorbed, and used by your body.

Stomach acid, called hydrochloric acid, and enzymes, called proteases, break down the large protein molecules into amino acids which are then absorbed in your blood and carried to tissues and organs.

Some amino acids are used for energy, while others are reassembled into new proteins used to make enzymes, hormones, blood, skin, bones, muscle, and more.


Protein consists of amino acids which perform many bodily functions. While your body can make the eleven non-essential amino acids, there are nine essential amino acids that you must get from food sources. The best dietary protein sources include meat, poultry, fish, eggs, dairy, and legumes.

When you eat foods with protein, your body breaks the protein down into amino acids which are then absorbed in the blood for use throughout your body.

Primary functions of protein

Growth, repair, & maintenance of body tissues

Protein is an essential part of growing healthy organs, bones, muscles, and cells, especially during childhood and adolescence. Pregnancy is another time when growth occurs rapidly and protein needs increase.

Protein also helps the body recover from injuries, illnesses, and environmental stress and maintains proper function by repairing damaged organs, cells, and other tissues.

Enzyme & hormone production

Enzymes and hormones both consist of amino acids.

Enzymes are responsible for most of the chemical reactions that occur in cells and contribute to digestion, liver function, breathing, building muscle, nerve function, and toxin removal. (1)

Hormones are important for sending signals through your body to coordinate biological processes between cells, tissues, and organs. Some processes include growth and development, metabolism, blood sugar regulation, reproduction, and bone mineral balance.

Immune function & disease prevention

Amino acids are a component of antibodies that bind to viruses and bacteria to help protect your body from and fight off infections.

Structural roles

Collagen protein is present in connective tissue which helps to bind body structures together, support and protect organs and cells, transport nutrients and waste, defend against pathogens, store fat, and repair damaged tissues. Keratin is another type of structural protein and is found in hair, skin, and nails. (2)

Benefits of protein

Muscle growth & strength

The amino acids from protein help build, repair, and strengthen muscle tissue. A regular strength training regimen and adequate calorie intake are also necessary to increase muscle mass and strength. More frequent weightlifting warrants increased protein intake to ensure adequate rebuilding of damaged muscle tissue.

Weight management & appetite control

By consuming a high-protein meal at regular intervals throughout the day, you can prevent excessive hunger and cravings between meals. High-protein meals can induce greater satiety (the feeling of fullness) and blood sugar balance, so you are less likely to overconsume calories. This is beneficial for both maintaining and losing weight.

Metabolism & energy expenditure

Increased protein intake can enhance your metabolic rate since protein requires more energy to digest compared to fat and carbohydrates. (3, 4, 5, 6)

Another factor that impacts your metabolism and energy expenditure is your muscle mass. Muscle is the most metabolically-active tissue in the body, so preserving it will prevent your metabolism from decreasing significantly during a phase of calorie restriction, increasing your chances of long-term weight loss. By consuming adequate protein, you are more likely to maintain your lean body mass, especially if engaging in resistance training.

Bone health & osteoporosis prevention

Eating an adequate amount of protein for your age and activity level along with incorporating regular strength training and weight-bearing physical activity can help prevent the loss of bone and reduce the risk for bone fractures, especially as you age.

Healthy hair, skin, & nails

Dietary protein supplies your body with the amino acids required to make keratin, the primary type of protein in your hair, skin, and nails. Without enough protein in your diet, your nails may become thin and fragile, your hair may break or fall out more easily, and your skin may not heal properly from injuries like cuts, scrapes, and wounds. (7)

Other essential nutrients for healthy hair, skin, and nails include vitamin A, vitamin B7 (biotin), vitamin C, vitamin E, copper, iron, and zinc.

Deficiency symptoms & risks of excess intake

Signs & symptoms of protein deficiency

Some signs and symptoms of protein deficiency include muscle wasting, decreased muscle strength, weight gain, inadequate growth (in children), low energy, frequent illness, anemia, brittle hair and nails, and chronic disease.

Long-term implications of inadequate protein intake

Long-term protein deficiency can lead to significant loss of muscle mass, longer muscle recovery, decreased metabolism, insulin resistance, increased risk of infection, slow wound healing, and increased risk of bone fracture.

These issues can further increase your risk of falls, injuries, and hospitalizations and decrease your independence.

Vulnerable groups at higher risk of protein deficiency

People experiencing homelessness, living below the poverty line, living in rural areas, and those of older age are groups who are at higher risk of protein deficiency.

Homelessness, poverty, and rural communities

People without a home or personal vehicle or who have a low income may not be able to afford, transport, or properly store their food, so they may rely on fast food or convenience foods that are easy to access and don’t need to be cooked or refrigerated. These foods often provide adequate calories but are low in protein.

Rural areas have fewer traditional grocery stores and more convenience stores, so people living in these areas may have less access to a wide variety of protein sources.

Older adults

Older adults are also at risk of protein deficiency due to inadequate nutrition related to inconsistent meal times, decreased appetite, and less variety in food intake.

Additionally, muscle mass and strength begin to decline around the age of 40 due to decreased physical activity, cellular DNA damage, and decreased nerve-muscle connections. Because of this, older adults have higher protein needs which may be difficult to meet without carefully planning their meals and snacks.

This series of events that causes the gradual loss of muscle mass, strength, and function is referred to as sarcopenia. The condition can affect people at any age, but is more common in older adults and is associated with a lower quality of life due to decreased independence and ability to perform daily tasks. (8, 9)

Maintaining physical activity, especially strength training, in addition to eating enough calories and protein throughout your life can prevent this age-related decline.


Inadequate protein intake is associated with poor variety in food intake, inadequate calorie intake, and older age. Loss of muscle mass and strength is a common long-term effect and can lead to an increased risk of falls, bone fractures, limited mobility, and decreased independence.

Health risks associated with excessive protein consumption

Usually, your body will flush out excess protein in the urine, but in some cases, excess protein can lead to unwanted side effects and increase your risk for some diseases.

If you eat greater than 2 g/kg protein per day, you may experience constipation, dehydration, hyperinsulinemia, kidney failure, weight gain (due to excess calorie intake), or bad breath (if your body is in ketosis).

Long-term excess protein intake can increase the risk of kidney stones, cancer, diabetes, osteoporosis, and heart disease. This is not a concern for most people and the benefits of consuming adequate protein outweigh the risks of overdoing it.

Excessive protein consumption is unlikely for most people, but you may want to limit your intake to less than 2 g/kg per day if you are experiencing unpleasant side effects. If you have high protein needs, be sure to visit your healthcare provider annually for a check-up.


Excessive protein consumption is unlikely for most people, but you may want to limit your intake to less than 2 g/kg per day if you are experiencing unpleasant side effects. If you have high protein needs, be sure to visit your healthcare provider annually for a check-up.

Best protein sources

Animal-based protein:

  • Lean fish: barramundi, cod, haddock, halibut, pollock, red snapper, sole, tilapia, tuna, whiting
  • Shellfish: clams, crab, lobster, mussels, oysters, scallops, shrimp
  • Meat: beef, buffalo, deer, elk, moose, pastrami, pork, veal
  • Poultry: chicken, turkey, duck
  • Dairy: cottage cheese, Greek yogurt, milk
  • Eggs

Plant-based protein:

  • Soy products: soybeans, tempeh, textured vegetable protein (TVP), tofu (firm)
  • Legumes: beans, lentils, peas, peanut butter powder
  • Meat alternatives: mycoprotein, seitan (vital wheat gluten)
  • Nuts & seeds

The protein content of various foods

The table below includes the protein content of meat, poultry, fish, vegetarian foods, and vegan foods. (Nutrition data obtained from: My Food Data)

CategoryProtein SourceServing SizeProtein (g)Calories
Meat, Poultry, & FishBeef (ground, 93% lean, raw)4 oz24172
Chicken Breast (boneless, skinless, raw)4 oz25136
Pork Loin Chop (boneless, raw)4 oz25170
Tuna (Skipjack, canned)4.5 oz36150
Turkey (ground, 93%, raw)4 oz21170
Vegetarian FoodsCheese, Part-Skim Mozzarella1 oz772
Cottage Cheese (2%)4 oz1390
Eggs1 large672
Greek Yogurt (non-fat, plain)5.3 oz18110
Milk (non-fat)8 oz880
Vegan FoodsBuckwheat (Soba) Noodles (uncooked)2 oz7200
Lentils (cooked)1/2 cup9101
Pinto Beans (cooked)1/2 cup11197
Seitan (Vital Wheat Gluten)4 oz37185
Soybeans (cooked)1/2 cup15149
Tempeh4 oz24213
Textured Vegetable Protein4 oz26180
Tofu (firm)4 oz870

Combining different protein sources for optimal nutrition

All foods have a different amino acid profile with a combination of essential and non-essential amino acids. Some foods, primarily animal-based proteins, are complete proteins, meaning they contain all nine of the EAAs in high enough amounts for optimal use by the body.

People eating animal-based proteins on a daily basis will usually always meet their amino acid needs by simply eating enough total protein each day.

On the other hand, people eating only or mostly plant-based proteins benefit from eating a wide variety of protein sources to increase the likelihood of consuming enough of all EAAs. Combining protein sources at meals can help keep you on track to meet your EAA needs and optimize muscle protein synthesis (MPS).

Muscle protein synthesis is the process of building new muscle tissue, and it occurs when protein (or amino acid) consumption exceeds muscle protein breakdown. (10)

Leucine is the most important amino acid for stimulating MPS, so people on a vegan diet wishing to maintain or increase their muscle mass benefit from consuming high-leucine foods each meal, including beans, corn, lentils, peas, seitan, and soy.

Regardless of your dietary preferences, eating a variety of protein sources throughout the day is essential for meeting your vitamin and mineral needs. Just like all protein sources vary in amino acid amounts, they also vary in their micronutrient amounts, so eating many different foods daily to weekly can help prevent deficiencies.


Consuming enough total protein is the easiest way to meet your essential amino acid needs, but you benefit even more by consuming a wide variety of protein sources since every food has different amounts of amino acids, vitamins, and minerals.

Protein needs

You can refer to our overview of recommended protein needs for a complete review of the recommendations, but we have provided a summary of them below.

Determining your individual protein requirements

Most adults can benefit from a daily protein intake of 1.2-2.0 grams per kilogram (g/kg) body weight. Your age, sex, body composition, activity level, overall health, and goals can all impact your individual protein needs. Below, we provide recommendations addressing some of these factors.

Protein intake recommendations for different goals & life stages

While the protein recommendation for most adults is 1.2-2.0 g/kg/day, here are recommendations for various goals and life stages:

  • Sedentary adults
    • Maintain weight: 1.2-1.8 g/kg
  • Active adults
    • Maintain weight: 1.4-2.0 g/kg
    • Build muscle: 1.6-2.4 g/kg
    • Lose fat: 1.6-2.4 g/kg
  • Adults with BMI ≥ 30
    • Lose fat: 1.2-1.5 g/kg
  • Adults ≥ 50 years old
    • Maintain weight: 1.2-1.5 g/kg
    • Build muscle: 1.7-2.0 g/kg
    • Lose fat: 1.5-2.2 g/kg
  • Pregnancy: 1.7-2.0 g/kg
  • Lactation: 1.5-1.9 g/kg

Here are recommendations for infants and children, though you should be sure to consult your child’s doctor or dietitian for individual guidelines:

  • Infants (preterm): 3.0-4.0 g/kg
  • Infants (full-term, 0-6 months): ≥ 1.5 g/kg
  • Infants (full-term, 7-12 months): 1.6 g/kg
  • Toddlers (1-3 years): 3.0-4.0 g/kg
  • Children (4-14 years): 1.5-2.9 g/kg

Meeting protein needs on vegan and vegetarian diets

Vegan and vegetarian diets have fewer high-protein options, making it more difficult to meet protein needs. Plant-based foods can also be more difficult for the body to digest (due to the antinutrients found in fiber) which means the proteins may not be fully broken down for absorption into the blood.

Additionally, most plant-based foods are low in one or more of the essential amino acids which can inhibit necessary bodily functions and impair muscle growth and repair if the missing amino acids aren’t consumed from other foods.

To make up for those potential downfalls, people following a diet that limits animal products should consume a wide variety of plant-based foods each day to get an adequate amount of all essential amino acids.

You may also want to increase your protein intake slightly to account for decreased protein absorption from high-fiber foods. Incorporating a bit more protein with meals or adding in one to two high-protein snacks can be helpful.


People following a primarily plant-based diet will benefit from consuming a variety of protein sources and potentially additional total protein to ensure adequate essential amino acid intake.

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