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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.

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. Simply speaking, there are two types of amino acids: essential amino acids and non-essential amino acids. Essential amino acids are those that our body cannot synthesize on its own, so we must obtain them in our diet. The essential amino acids are isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine, arginine, and histidine.

Conversely, the nonessential amino acids are those that the body can manufacture, so it is not necessary to obtain them in the diet. The nonessential amino acids include glutamate, alanine, aspartate, and glutamine. Certain amino acids, such as proline, serine, tyrosine, cysteine, taurine, and glycine, are considered to be conditionally essential, meaning that under certain physiological circumstances the body is unable to manufacture them, so they must be obtained through the diet.

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. The recommended upper intake limit of protein is set at twice the Recommended Dietary Allowance.

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

Introduction to Nutrient Rating System Chart

The following chart shows the foods which are either excellent, very good or good sources of this nutrient. Next to each food name you will find the following information: the serving size of the food; the number of calories in one serving; DV% (percent daily value) of the nutrient contained in one serving; the nutrient density rating; and the food's World's Healthiest Foods Rating. Not all of our Daily Value standards are obtained from the FDA. In most instances, we used FDA Daily Values when available because they are widely recognized and apply to both men and women. However, when unavailable, we've used other science-based research to establish nutritional standards. Underneath the chart is a table that summarizes how the ratings were devised. Read more about our Nutrient Rating System.

Foods Ranked as quality sources of:
protein
Food Serving
Size
Cals Amount
(g)
DV
(%)
Nutrient
Density
World's
Healthiest
Foods Rating
Cod, Pacific, Fillet, 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, MixedSpecies, Steamed, Boiled 4 oz-wt 112.3 23.71 47.4 7.6 excellent
Snapper, Baked 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
Greens, Mustard, 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
Liver, Calf 4 oz-wt 187.1 24.53 49.1 4.7 very good
Spinach (boiled, with salt) 1 cup 41.4 5.35 10.7 4.7 very good
Lettuce, Romaine 2 cup 15.7 1.81 3.6 4.2 good
Mushrooms, Crimini, Raw 5 oz-wt 31.2 3.54 7.1 4.1 very good
Chinook Salmon Fillet-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 (pieces, 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
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, Cow Milk, Low Fat 1 cup 155.1 12.86 25.7 3.0 good
Egg, Hen, Whole, Boiled 1 each 68.2 5.54 11.1 2.9 good
Collard Greens, Boiled, Drained 1 cup 49.4 4.01 8.0 2.9 good
Cauliflower (boiled, drained) 1 cup 28.5 2.28 4.6 2.9 good
Lentils, Boiled 1 cup 229.7 17.86 35.7 2.8 good
Split Peas, Boiled 1 cup 231.3 16.35 32.7 2.5 good
Beans, Kidney, Cooked 1 cup 224.8 15.35 30.7 2.5 good
Kale, Fresh, Boiled 1 cup 36.4 2.47 4.9 2.4 good
Beans, Lima, Cooked 1 cup 216.2 14.66 29.3 2.4 good
Beans, Black, Boiled 1 cup 227.0 15.24 30.5 2.4 good
Milk, Cow, 2% 1 cup 121.2 8.13 16.3 2.4 good
Brussels 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
Beans, Navy, Cooked 1 cup 258.4 15.83 31.7 2.2 good
Beans, Pinto, Cooked 1 cup 234.3 14.04 28.1 2.2 good
Miso (Soybean) 1 oz 70.8 4.06 8.1 2.1 good
Mushrooms, Shiitake, Raw 8 oz-wt 87.2 4.98 10.0 2.1 good
Greens, Turnip, Cooked 1 cup 28.8 1.64 3.3 2.0 good
Beans, Garbanzo, Cooked 1 cup 269.0 14.53 29.1 1.9 good
Green Snap/String Beans, Boiled 1 cup 43.8 2.36 4.7 1.9 good
Seeds, Mustard 2 tsp 35.0 1.88 3.8 1.9 good
Milk, Goat 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
Squash, Summer, All Varieties 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, Dried 0.25 cup 186.7 8.47 16.9 1.6 good
Garlic 1 oz-wt 42.2 1.80 3.6 1.5 good
Oats, Whole Grain 1 cup 145.1 6.08 12.2 1.5 good
Tomato, Red, Raw, 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.

Recommendations for protein intake are given as amounts of protein needed per kilogram (1 kilogram equals 2.2 pounds)of body weight and the number of grams of protein needed each day. These recommendations appear below:

  • Boys and girls aged 0-6 months: 2.2 grams/kg or 13 grams/day
  • Boys and girls aged 6 months to 1 year: 1.6 grams/kg or 14 grams/day
  • Boys and girls aged 1-3 years: 1.2 grams/kg or 16 grams/day
  • Boys and girls aged 4-6 years: 1.1 grams/kg or 24 grams/day
  • Boys and girls aged 7-10 years: 1.0 grams/kg or 28 grams/day
  • Males aged 11-14 years: 1.0 grams/kg or 45 grams/day
  • Males aged 15-18 years: .9 grams/kg or 59 grams/day
  • Males aged 19-24 years: .8 grams/kg or 58 grams/day
  • Males aged 25-50 years: .8 grams/kg or 63 grams/day
  • Males aged 51+ years: .8 grams/kg or 63 grams/day
  • Females aged 11-14 years: 1.0 grams/kg or 46 grams/day
  • Females aged 15-18 years: .8 grams/kg or 44 grams/day
  • Females aged 19-24 years: .8 grams/kg or 46 grams/day
  • Females aged 25-50 years: .8 grams/kg or 50 grams/day
  • Females aged 51+ years: .8 grams/kg or 50 grams/day

Pregnant women require an additional 10 grams of protein per day above the protein requirement for their age. Lactating women require an additional 15 grams of protein per day during the first six months of lactation and an additional 12 grams of protein per day during the second six months of lactation.

References

  • Calder PC and Kew S. The immune system: a target for functional foods. Br J Nutr. 2002 Nov;88 Suppl 2:S165-77.
  • Denke MA. Metabolic effects of high-protein, low-carbohydrate diets. Am J Cardiol 2001 Jul 1;88(1):59-61.
  • Fernstrom JD, Fernstrom MH. Monoamines and protein intake: are control mechanisms designed to monitor a threshold intake or a set point. Nutr Rev 2001 Aug;59(8 Pt 2):S60-5; discussion S66-8.
  • Groff JL, Gropper SS, Hunt SM. Advanced Nutrition and Human Metabolism. West Publishing Company, New York, 1995.
  • Lininger SW, et al. A-Z guide to drug-herb-vitamin interactions. Prima Health, Rocklin, CA, 2000.
  • Mahan K, Escott-Stump S. Krause's Food, Nutrition, and Diet Therapy. WB Saunders Company; Philadelphia, 1996.
  • McWilliams M. Foods. Prentice Hall Publishing Company: New Jersey, 1989.
  • Schaafsma G. The protein digestibility-corrected amino acid score. J Nutr 2000 Jul;130(7):1865S-7S.
  • Stein K. High-protein, low-carbohydrate diets: do they work. J Am Diet Assoc 2000 Jul;100(7):760-1.
  • Tipton KD, Wolfe RR. Exercise, protein metabolism, and muscle growth. Int J Sport Nutr Exerc Metab 2001 Mar;11(1):109-32.
  • Wolfe RR. Protein supplements and exercise. Am J Clin Nutr 2000 Aug;72(2 Suppl):551S-7S.
  • Wu GH, Zhang YW, Wu ZH et al. Modulation of postoperative immune and inflammatory response by immune-enhancing enteral diet in gastrointestinal cancer patients. World J Gastroenterol. 2001 Jun;7(3):357-62.

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This page was updated on: 2004-11-21 12:00:22

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