Protein

Protein was the first substance that gained recognition for its role as vital constituent of living tissue. The word protein is derived from the Greek word proteos, which means taking first place or primary, reflecting this nutrient\'s importance in supporting the bodys functioning. Protein accounts for 20 percent of body weight, with proteins performing a wide variety of functions. Not only do proteins compose body tissues but they are also vital components of enzymes and immune system cells.

Proteins are complex molecules that are comprised of combinations of different amino acids, compounds that contain carbon, oxygen, hydrogen, nitrogen and sometimes sulfur. Proteins differ in structure and function depending upon how the different amino acids link together in specific numbers and unique combinations.

Protein constitutes an essential component of the diet since the body needs the amino acids that it provides in order to synthesize its own proteins. In general terms, there are three types of amino acids, classified as either essential, non-essential or conditionally essential.

Essential amino acids must be obtained from the diet since the body cannot synthesize them on its own. Amino acids that fall in this category include: arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. The nonessential amino acids are those that the body can manufacture so therefore it is not necessary to obtain them from the diet. These amino acids include alanine, aspartate, glutamate and glutamine. Conditionally essential amino acids are those the body cannot manufacture when faced with certain physiological circumstance and which therefore then must be obtained through the diet. Amino acids falling in this category include cysteine, glycine, proline, serine, taurine and tyrosine.

The body can only make proteins that it needs when there is a sufficient quantity of all needed amino acids in the amino acid pool. If there is a deficiency of essential amino acids, the body will break down muscle proteins in order to obtain the amino acids that it needs to manufacture necessary proteins. Consequently, it is imperative that our daily diets feature foods that contain the essential amino acids. This is easily accomplished by eating a variety of whole foods such as vegetables, beans, whole grains, nuts, seeds and meat and animal products if desired.

Physiological functions of protein

• Supports immune system function
• Maintains healthy skin, hair and nails
• Manufacturing source for enzymes

Physiological events that may signal a need for greater protein intake

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

Functions of protein

Providing 4 calories per gram, protein serves as an important energy source for the body when carbohydrates and fats are not available. Additionally, the body uses the amino acids supplied by dietary protein to synthesize its own proteins, which perform a variety of important physiological functions:

• Production and maintenance of structural proteins: Myosin, actin, collagen, elastin and keratin are some of the structural proteins manufactured by the body that maintain the integrity and strength of muscles, connective tissues, hair, nails, and skin.
• Production of enzymes and hormones: Enzymes, molecules that catalyze chemical reactions in the body, are made from proteins. Insulin and glucagon, two hormones involved in blood sugar regulation, and the thyroid hormones are manufactured from proteins.
• Production of transport proteins and lipoproteins: Transport proteins carry various substances throughout the body to the different tissues. These molecules include hemoglobin (carries oxygen), transferrin (carries iron), ceruloplasmin (carries copper), retinol-binding protein (carries vitamin A), and albumin and transthyretin (which both carry other proteins). Fat and cholesterol are transported through the body by lipoproteins.
• Production of antibodies: Antibodies are protein-based compounds that play a critical role in the immune system. They attach to antigens such as bacteria, viruses or other foreign invader, inactivate them and make them more visible to macrophages (a special type of immune cell) that can destroy the antigen.
• Maintenance of proper fluid balance: Proteins are involved in the regulation of osmotic pressure, controlling the amount of water that is inside of cells.
• Maintenance of proper acid-base balance: since proteins can combine with both acidic and basic substances, they help to maintain the bodys normal acid-base balance.

Causes and symptoms of protein deficiency

Since the stomach, pancreas and liver are all involved in protein digestion and metabolism, any medical condition that compromises the function of any of these organs can negatively impact protein status. Additionally, hypochlorhydria can also have a negative impact on protein status since hydrochloric acid is necessary for the initial digestion of protein. Since vitamin B6 is involved in the manufacture of non-essential amino acids, protein status may be affected by inadequate intake of this nutrient. Since bacterial or viral infections as well as severe physical trauma can rapidly use up protein stores, individuals who experience these conditions may need to increase their protein intake.

Assuming that a sufficient amount of calories and all of the essential amino acids are present in the diet, both adults and children can live healthfully on a low dietary intake of protein. Yet, those who consume both a low intake of both protein and calories can be at risk for protein-energy malnutrition. In addition to being a condition that commonly affects impoverished people, especially children, who have limited access to food, protein-energy malnutrition may be experienced by other groups that are at risk, including the elderly and those with a medical or psychological condition that reduces their desire or ability to eat. Additionally, individuals who have suffered severe physical traumas, such as extensive skin burns, that increase their protein needs may also be affected by protein-energy malnutrition. Incidentally, while many nutritionists caution that following a vegetarian or vegan diet may put an individual at risk for protein deficiency, it is easy for people who eat a variety of vegetables, grains and legumes to meet or exceed current protein requirements.

Protein-energy malnutrition can manifest as either marasmus or kwashiorkor. The symptoms of marasmus, which can occur in individuals of all ages, include muscle wasting, loss of visible fat stores, weight loss, weakness and fatigue and frequent infections due to compromised immunity system activity. Kwashiorkor typically occurs in children younger than 4 years old fed a protein deficient, carbohydrate-rich diet and involves symptoms including edema (fluid retention), muscle wasting, and and enlarged and fatty liver. Visible fat stores are preserved in individuals experiencing Kwashiorkor.

Causes and symptoms of protein toxicity

Kidney problems as well as accelerated bone loss leading to osteoporosis may develop after years of excessive protein intake. Individuals who have end-stage kidney disease must carefully monitor their protein intake since the kidney play a primary role in its metabolism.

The recommended upper intake limit of protein is double the Recommended Daily Allowance (RDA).

Effects of cooking, storage and processing on protein

Proteins undergo physical changes such as denaturation and coagulation when they are cooked or agitated (an example of agitation is when egg whites are beaten). Denaturation diminishes the protein molecules solubility since it changes its shape. Coagulated proteins clump together. An example of coagulation is the scrambling of eggs. Additionally, when protein-containing foods are overcooked, the heat can destroy sensitive amino acids (e.g. lysine) or make the protein resistant to catabolism by digestive enzymes.

Interactions between medications and protein

Since protein digestion is dependent upon hydrochloric acid any medication that decreases the secretion, or neutralizes the action, of hydrochloric acid in the stomach may compromise protein digestion and potentially, protein status. Medications with this action include prescription and over-the-counter antacids (for example, Tums and Rolaids) and histamine blockers (for examples, Tagamet and Pepcid).

Since steroidal anti-inflammatory medications (e.g. prednisone) can cause muscle wasting, physicians sometimes recommend that patients taking these medications increase their intake of protein.

Interactions that occur between protein and other nutrients

Inadequate protein intake may impair the function of nutrients including iron, calcium, copper, vitamin A and vitamin D since various proteins bind and transport these nutrients throughout the body.

Health conditions that require special emphasis on protein

Although adequate protein intake is necessary for health, protein is usually not used therapeutically. However, for certain individuals, such as those who have experiences severe physical trauma as well as those who regularly participate in athletic activities, high dietary intake of protein is beneficial. Additionally, the amino acids glutamine, lysine, phenylalanine, tyrosine, arginine, and cysteine are commonly used therapeutically.

Forms in which protein is found in dietary supplements.

There is a wide array of protein powders available. Since soy has received recognition for its potential to prevent conditions such as cardiovascular disease and cancer, soy protein powders are currently very popular.

Foods that are concentrates sources of protein

Excellent sources of protein include halibut, shrimp, snapper, tuna and venison. Very good sources include chicken, lamb, low-fat cheeses, mustard greens, salmon, scallops, tempeh, tofu and turkey while good sources include black beans, cauliflower, collard greens, eggs, kidney beans, lentils, low-fat milk and split peas.

Nutritionists often speak of food protein sources in terms of \"complete\" and \"incomplete\" proteins. Those that provide complete proteins are foods that contain all of the essential amino acids while foods that either provide some or none of the essential amino acids are referred to as incomplete.

Complete proteins are typically considered to include animal foods such as dairy products, eggs, fish, meat and poultry. Although vegetarians, notably vegans, often do not consume foods that are complete proteins, all of the essential amino acids can be obtained by eating a diet that features a variety of beans, grains, nuts, seeds and vegetables.