protein

What can high-protein foods do for you?

• Keep your immune system functioning properly
• Maintain healthy skin, hair and nails
• Help your body produce enzymes

What events can indicate a need for more high-protein foods?

• Muscle wasting
• Weight loss
• Fatigue and weakness
• Frequent infections
• Severe edema (fluid retention)
• Slow growth and development in children

Excellent sources of protein include tuna, shrimp and cod.

For serving size for specific foods, see Nutrient Rating Chart below at the bottom of this page.

Description Function Deficiency Symptoms Toxicity Symptoms Cooking, storage and processing Factors that affect function

Drug-nutrient interaction Nutrient interaction Health conditions Supplements Food Sources Public Recommendations References

Description

What is protein?

Protein was the first substance to be recognized as a vital part of living tissue. In fact, the word protein comes from the Greek word proteos, which means "primary" or "taking first place," indicating the importance of this nutrient in the function of the body. Accounting for 20 percent of our body weight, proteins perform a wide variety of functions throughout the body as vital components of body tissues, enzymes, and immune cells.

Proteins are complex molecules comprised of a combination of different amino acids, which are compounds that contain carbon, oxygen, hydrogen, nitrogen and sometimes sulfur. Amino acids link together in specific numbers and unique combinations to make each different protein.

Protein is an essential component of the diet, because it provides the amino acids that the body needs to synthesize its own proteins. In traditional nutrition textbooks, there have always been two types of amino acids: essential amino acids and non-essential amino acids. Essential amino acids have been defined as those amino acids that our body cannot synthesize on its own. Essential amino acids must therefore be obtained from our diet. As traditionally defined, the eight essential amino acids are isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. There has been ongoing debate over the status of a ninth amino acid, histidine. Because the body appears routinely unable to make sufficient amounts of histidine during certain periods of development, this amino acid has sometimes been classified as essential and sometimes not.

Nonessential amino acids have traditionally been defined as those that the body can manufacture on its own. It is therefore not necessary to obtain these amino acids from the diet. As traditionally defined, the nonessential amino acids include glutamate, alanine, aspartate, and glutamine, as well as arginine, proline, serine, tyrosine, cysteine, taurine, and glycine.

This traditional separation of amino acids into the categories of "essential" and "non-essential" seems unrealistic. While it is true that the human body has the potential to manufacture all non-essential amino acids, this potential is not the same as actually making them. There are many circumstances in which the body cannot make nearly enough of the non-essential amino acids it needs. For example, when a person is exposed to large amounts of environmental toxins and pollutants, the amount of glycine (a non-essential amino acid) made by the body may be far from adequate. For this reason, it may be more constructive to think about all non-essential amino acids as "conditionally essential." This classification would point out that under certain physiological circumstances, the body would unable to manufacture enough of these amino acids and they would have to be obtained through diet (or supplementation). This concept of "conditionally essential amino acids" tells us that all of the amino acids can be equally important when it comes to our diet, and that it's worthwhile for us to pay attention to all amino acids when thinking about the nourishment we get from our food.

The body is only able to make the proteins it needs when there are sufficient quantities of all the necessary amino acids in the so-called "amino acid pool." If we are deficient in essential amino acids, the body will be unable to make proteins and will have to break down muscle proteins to obtain the amino acids it needs.

As a result, it is imperative that our daily intake of food contains each of the essential amino acids, which is easily accomplished by eating a variety of vegetables, beans, whole grains, nuts, seeds, and meat and animal products if desired.

How it Functions

What is the function of protein?

Protein, providing 4 calories per gram, is an important source of energy for the body, when carbohydrates and fats are not available. In addition to using protein to generate energy for cellular function whenever necessary, the body uses the amino acids contained in the protein we eat to manufacture its own proteins. The proteins synthesized by the body perform a variety of important physiological functions:

• Production and maintenance of structural proteins: The body manufactures several structural proteins, such as myosin, actin, collagen, elastin, and keratin, that maintain the strength and integrity of muscles, connective tissues (ligaments and tendons), hair, skin, and nails.
• Production of enzymes and hormones: All of the enzymes, which are compounds that catalyze chemical reactions in the body, are made from protein. In addition, the hormones involved in blood sugar regulation (insulin and glucagon) as well as the thyroid hormones are synthesized from proteins.
• Production of transport proteins and lipoproteins: Certain proteins are used by the body to carry various substances to body tissues. These transport proteins include hemoglobin (carries oxygen), transferrin (carries iron), ceruloplasmin (carries copper), retinol-binding protein (carries vitamin A), albumin and transthyretin (both carry other proteins). Lipoproteins participate in the transportation of fat and cholesterol.
• Production of antibodies: Antibodies, which are proteins, play an important role in the immune system by attaching to antigens (viruses, bacteria, or other foreign invaders), thereby inactivating the antigens and making them more visible to the immune cells (called macrophages) that destroy antigens.
• Maintenance of proper fluid balance: Proteins participate in the maintenance of osmotic pressure, which controls the amount of water that is found inside of cells.
• Maintenance of proper acid-base balance: Due to their ability to combine with both acidic and basic substances, proteins help to maintain the normal acid-base balance in the body.

Deficiency Symptoms

What are deficiency symptoms for protein?

Both adults and children can live healthfully on a low intake of protein, assuming they eat a sufficient amount of calories and all of the essential amino acids are present in the diet. As a result, the symptoms of protein deficiency are most often seen in impoverished people who have limited access to food.

Protein-energy malnutrition, caused by low intake of both protein and calories, is especially common in children in underdeveloped nations, because children require more protein per kilogram of body weight than adults to support the rapid growth and development that occurs during childhood.

According to the World Health Organization, approximately 300 million children throughout the world suffer from growth retardation due to protein-energy malnutrition. Additionally, children with protein-energy malnutrition have a 40% mortality rate, due to increased susceptibility to infections.

In developed countries, protein-energy malnutrition is most likely to affect people who have suffered severe physical trauma that increases protein needs (for example, extensive skin burns) or those who have a medical condition or psychological problem that impacts their desire or ability to eat. The elderly are also at risk for protein-energy malnutrition.

There are two types of protein-energy malnutrition: marasmus and kwashiorkor. Marasmus is a state of semi-starvation that can occur in people of all ages who have limited access to food, but is most common in non-breastfed children given diluted infant formula. The symptoms of marasmus include weight loss, muscle wasting, loss of visible fat stores, weakness and fatigue, and frequent infections due to diminished activity of the immune system.

Kwashiorkor, a Ghanian word for "the evil spirit that infects the child", was first described in 1933 and typically occurs in children younger than 4 years old fed diets high in carbohydrates with little or no protein. Symptoms of kwashiorkor include muscle wasting, edema (fluid retention), and an enlarged and fatty liver, with the preservation of visible fat stores.

Because meat and dairy foods are a primary source of protein in the American diet, many nutritionists caution that those following a vegetarian or vegan diet may be at risk for protein deficiency. However, vegetarians and vegans who eat a variety of vegetables, grains and legumes can easily meet or exceed current protein requirements.

Toxicity Symptoms

What are toxicity symptoms for protein?

Excessive intake of protein over many years may lead to kidney problems and/or accelerated bone loss eventually leading to osteoporosis. Due to the lack of a dose-response relationship at higher levels of protein intake, the National Academy of Sciences (NAS) decided not to set a Tolerable Upper Limit (UL) for protein in 2002. However, the NAS did note that the building blocks of protein, called amino acids, should not be consumed in amounts significantly above the amounts found in food.

Because the kidneys play a primary role in protein metabolism, individuals with end-stage kidney disease must carefully monitor their intake of protein.

Impact of Cooking, Storage and Processing

How do cooking, storage, or processing affect protein?

When cooked or agitated (as occurs when egg whites are beaten), proteins undergo physical changes called denaturation and coagulation. Denaturation changes the shape of the protein, thereby decreasing the solubility of the protein molecule.

Coagulation causes protein molecules to clump together, as occurs when making scrambled eggs. Overcooking foods containing protein can destroy heat sensitive amino acids (for example, lysine) or make the protein resistant to digestive enzymes.

Factors that Affect Function

What factors might contribute to a deficiency of protein?

Protein digestion and metabolism involves the stomach, pancreas and liver. Hydrochloric acid, secreted by the stomach, is necessary for the initial digestion of protein. Pancreatic enzymes participate in the breakdown of protein. And the liver controls amino acid metabolism.

Consequently, any medical condition that comprises the function of the stomach, pancreas, or liver may negatively impact protein status. In addition, the ability of the body to manufacture non-essential amino acids may be hampered with inadequate intake of vitamin B6.

Individuals with bacterial or viral infections and those who have experienced severe physical trauma use up their protein stores rapidly, and may need to increase their intake of protein.

Drug-Nutrient Interactions

What medications affect protein?

Any medication that decreases the secretion, or neutralizes the action, of hydrochloric acid in the stomach may negatively impact protein digestion. Such medications include prescription and over-the-counter antacids (for example, Tums and Rolaids) and histamine blockers (for examples, Tagamet and Pepcid).

In addition, the steroidal anti-inflammatory medications (for example, prednisone) can cause muscle wasting. Consequently, physicians sometimes recommend that patients taking these medications increase their intake of protein.

Nutrient Interactions

How do other nutrients interact with protein?

Various proteins bind and carry certain vitamins and minerals including iron, copper, calcium, vitamin A, and vitamin D. As a result, inadequate protein intake may impair the function of these nutrients.

Health Conditions

What health conditions require special emphasis on protein?

Although adequate protein intake is necessary for health, protein is not often used therapeutically. However, high dietary intake of protein is beneficial for people who have experienced severe physical trauma and may be helpful for athletes. Additionally, several individual amino acids including glutamine, lysine, phenylalanine, tyrosine, arginine, and cysteine are commonly used therapeutically.

Form in Dietary Supplements

What forms of protein are found in dietary supplements?

A wide range of protein powders are available. Especially popular at present are those that contain soy protein, as soy has received much attention for its potential to prevent cardiovascular disease and cancer.

Food Sources

What foods provide protein?

Excellent sources of protein include tuna, shrimp, and cod.

Very good sources of protein include snapper, venison, halibut, salmon, scallops, turkey, chicken, lamb, beef, calf's liver, spinach, tofu, mustard greens, crimini mushrooms, soybeans, and mozarrella cheese.

Good sources of protein include eggs, milk, collard greens, cauliflower and many legumes including lentils, split peas, kidney beans, black beans, pinto beans and garbanzo beans.

When discussing food sources of protein, nutritionists often speak in terms of "complete" and "incomplete" proteins. Foods that provide complete protein are those that include all of the essential amino acid, while foods that provide some or none of the essential amino acids are said to be incomplete.

Eggs, dairy foods, meat, fish and poultry are typically considered to be complete proteins. Vegetarians, and especially vegans, often do not have a source of complete protein in their diets, but can easily obtain all of the essential amino acids by eating a variety of beans, grains, nuts, seeds, and vegetables.

 

Introduction to Nutrient Rating System Chart

In order to better help you identify foods that feature a high concentration of nutrients for the calories they contain, we created a Food Rating System. This system allows us to highlight the foods that are especially rich in particular nutrients. The following chart shows the World's Healthiest Foods that are either an excellent, very good, or good source of protein. Next to each food name, you'll find the serving size we used to calculate the food's nutrient composition, the calories contained in the serving, the amount of protein contained in one serving size of the food, the percent Daily Value (DV%) that this amount represents, the nutrient density that we calculated for this food and nutrient, and the rating we established in our rating system. For most of our nutrient ratings, we adopted the government standards for food labeling that are found in the U.S. Food and Drug Administration's "Reference Values for Nutrition Labeling." Read more background information and details of our rating system.

World's Healthiest Foods ranked as quality sources of: protein
Food	Serving Size	Cals	Amount (g)	DV (%)	Nutrient Density	World's Healthiest Foods Rating
Cod, baked/broiled	4 oz-wt	119.1	26.03	52.1	7.9	excellent
Tuna, yellowfin, baked/broiled	4 oz-wt	157.6	33.99	68.0	7.8	excellent
Shrimp, steamed/boiled	4 oz-wt	112.3	23.71	47.4	7.6	excellent
Snapper, baked/broiled	4 oz-wt	145.2	29.82	59.6	7.4	very good
Venison	4 oz-wt	179.2	34.25	68.5	6.9	very good
Halibut, baked/broiled	4 oz-wt	158.8	30.27	60.5	6.9	very good
Tamari (Soy Sauce)	1 tbs	10.8	1.89	3.8	6.3	good
Scallops, baked/broiled	4 oz-wt	151.7	23.11	46.2	5.5	very good
Turkey breast, roasted	4 oz-wt	214.3	32.56	65.1	5.5	very good
Chicken breast, roasted	4 oz-wt	223.4	33.79	67.6	5.4	very good
Mustard greens, boiled	1 cup	21.0	3.16	6.3	5.4	very good
Beef tenderloin, lean, broiled	4 oz-wt	240.4	32.04	64.1	4.8	very good
Lamb loin, roasted	4 oz-wt	229.1	30.15	60.3	4.7	very good
Calf's liver, braised	4 oz-wt	187.1	24.53	49.1	4.7	very good
Spinach, boiled	1 cup	41.4	5.35	10.7	4.7	very good
Romaine lettuce	2 cup	15.7	1.81	3.6	4.2	good
Crimini mushrooms, raw	5 oz-wt	31.2	3.54	7.1	4.1	very good
Salmon, chinook, baked/broiled	4 oz-wt	261.9	29.14	58.3	4.0	very good
Asparagus, boiled	1 cup	43.2	4.66	9.3	3.9	very good
Broccoli, steamed	1 cup	43.7	4.66	9.3	3.8	very good
Tofu, raw	4 oz-wt	86.2	9.16	18.3	3.8	very good
Soybeans, cooked	1 cup	297.6	28.62	57.2	3.5	very good
Mozzarella cheese, part-skim, shredded	1 oz-wt	72.1	6.88	13.8	3.4	very good
Swiss chard, boiled	1 cup	35.0	3.29	6.6	3.4	very good
Tempeh, cooked	4 oz-wt	223.4	20.63	41.3	3.3	good
Yogurt, low-fat	1 cup	155.1	12.86	25.7	3.0	good
Egg, whole, boiled	1 each	68.2	5.54	11.1	2.9	good
Collard greens, boiled	1 cup	49.4	4.01	8.0	2.9	good
Cauliflower, boiled	1 cup	28.5	2.28	4.6	2.9	good
Lentils, cooked	1 cup	229.7	17.86	35.7	2.8	good
Split peas, cooked	1 cup	231.3	16.35	32.7	2.5	good
Kidney beans, cooked	1 cup	224.8	15.35	30.7	2.5	good
Kale, boiled	1 cup	36.4	2.47	4.9	2.4	good
Lima beans, cooked	1 cup	216.2	14.66	29.3	2.4	good
Black beans, cooked	1 cup	227.0	15.24	30.5	2.4	good
Cow's milk, 2%	1 cup	121.2	8.13	16.3	2.4	good
Brussel sprouts, boiled	1 cup	60.8	3.98	8.0	2.4	good
Green peas, boiled	1 cup	134.4	8.58	17.2	2.3	good
Navy beans, cooked	1 cup	258.4	15.83	31.7	2.2	good
Pinto beans, cooked	1 cup	234.3	14.04	28.1	2.2	good
Miso	1 oz	70.8	4.06	8.1	2.1	good
Shiitake mushrooms, raw	8 oz-wt	87.2	4.98	10.0	2.1	good
Turnip greens, cooked	1 cup	28.8	1.64	3.3	2.0	good
Garbanzo beans (chickpeas), cooked	1 cup	269.0	14.53	29.1	1.9	good
Green beans, boiled	1 cup	43.8	2.36	4.7	1.9	good
Mustard seeds	2 tsp	35.0	1.88	3.8	1.9	good
Goat's milk	1 cup	167.9	8.69	17.4	1.9	good
Cabbage, shredded, boiled	1 cup	33.0	1.53	3.1	1.7	good
Summer squash, cooked, slices	1 cup	36.0	1.64	3.3	1.6	good
Peanuts, raw	0.25 cup	207.0	9.42	18.8	1.6	good
Pumpkin seeds, raw	0.25 cup	186.7	8.47	16.9	1.6	good
Rye, whole grain, uncooked	0.33 cup	188.7	8.31	16.6	1.6	good
Spelt grains, cooked	4 oz-wt	144.0	6.24	12.5	1.6	good
Garlic	1 oz-wt	42.2	1.80	3.6	1.5	good
Oats, whole grain, cooked	1 cup	147.4	6.08	12.2	1.5	good
Tomato, ripe	1 cup	37.8	1.53	3.1	1.5	good

World's Healthiest Foods Rating	Rule
excellent	DV>=75%	OR	Density>=7.6	AND	DV>=10%
very good	DV>=50%	OR	Density>=3.4	AND	DV>=5%
good	DV>=25%	OR	Density>=1.5	AND	DV>=2.5%

Public Health Recommendations

What are current public health recommendations for protein?

Recommendations for protein intake are based on the concept of "nitrogen balance." Protein contains nitrogen, and as proteins are broken down in the body, nitrogen is excreted. Consequently, nitrogen must be continually replaced through the diet (as protein) so that the body can continue to make proteins.

For most adults, an even nitrogen balance is ideal, meaning that the amount of nitrogen provided in the diet is equivalent to the amount of nitrogen excreted. In contrast, children require a positive nitrogen balance to support growth and development, meaning that more nitrogen is supplied by the diet (as protein) than is eliminated. Pregnant and lactating women also require a positive nitrogen balance.

In 2002, the National Academy of Sciences (NAS) set Recommended Dietary Allowances (RDAs) for protein for all age groups 7 months and older, and Adequate Intake (AI) levels for infants 0-6 months of age. The daily standards set by the NAS were as follows:

• Boys and girls aged 0-6 months: 9.1 grams
• Boys and girls aged 7 months to 1 year: 13.5 grams
• Boys and girls aged 1-3 years: 13 grams
• Boys and girls aged 4-8 years: 19 grams
• Boys and girls aged 9-13 years: 34 grams
• Males aged 14-18 years: 52 grams
• Males aged 19+: 56 grams
• Females aged 14+ years: 46 grams

Pregnant and lactating women require an additional 25 grams of protein per day for a total of 71 grams.

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