Protein



Protein




Role in Wellness


imagehttp://evolve.elsevier.com/Grodner/foundations/ imageNutrition Concepts Online


In 1928 a political slogan promised “a chicken in every pot!” At that time, being able to afford animal protein on a daily basis was the mark of a high standard of living and an assurance of good health. Today the phrase might be “Rice and beans for all of us!” We now know that there are many sources of protein available in our food supply. Some offer advantages over others by being lower in fat and higher in other nutrients such as complex carbohydrates and fiber.


Protein in food is our only source of amino acids, which are absolutely necessary to make the thousands of proteins that form every aspect of the human body. No wonder protein, which is plentiful in our food supply, has gained the status of a super nutrient for Americans. A common but inaccurate belief is that we expect that the more protein we eat, the stronger our immune system will be, the less we will weigh, and the more muscles we will develop.


Although proteins formed by our bodies do have a role in those functions, the amounts we consume are often greater than we need. Awareness of protein sources and portion sizes is important as we work toward achieving health promotion goals to decrease our risk of diet-related diseases (Box 6-1).



BOX 6-1   MyPlate


Proten Foods


The protein group includes not only meat and dry beans or peas but also poultry, fish, eggs, nuts, and seeds, all of which provide protein. Choose lean or low-fat cuts of meat and poultry, trimming fat and removing skin. Some fish (such as tuna, salmon, and trout), nuts, and seeds (walnuts and flax) contain healthy oils and are good sources of omega-3 fatty acids, which may reduce the risk for cardiovascular disease. Other nuts and seeds (almonds, hazelnuts, and sunflower seeds) provide vitamin E. Eating an assortment of protein sources means consuming other valuable nutrients as well.



What Counts as an Ounce?*


The focus of this MyPlate box is on portions of the protein foods group.


In general, 1 ounce of meat, poultry, or fish; image cup of cooked dry beans; 1 egg; 1 tbsp of peanut butter; or image ounce of nuts or seeds can be considered as 1 ounce-equivalent from the meat and beans group.










































PROTEIN SOURCE AMOUNT THAT COUNTS AS 1 OUNCE-EQUIVALENT IN THE MEAT AND BEANS GROUP COMMON PORTIONS AND OUNCE-EQUIVALENTS
Meats 1 ounce cooked lean beef
1 ounce cooked lean pork or ham		 1 small steak (eye of round, filet) = image to 4 ounce-equivalents
1 small lean hamburger = 2 to 3 ounce-equivalents
Poultry 1 ounce cooked chicken or turkey, without skin
1 sandwich slice of turkey (image × image × image inches)		 1 small chicken breast half = 3 ounce-equivalents
image Cornish game hen = 4 ounce-equivalents
Fish 1 ounce cooked fish or shellfish 1 can of tuna, drained = 3 to 4 ounce-equivalents
1 salmon steak = 4 to 6 ounce-equivalents
1 small trout = 3 ounce-equivalents
Eggs 1 egg  
Nuts and seeds image ounce of nuts (12 almonds, 24 pistachios, 7 walnut halves)
image ounce of seeds (pumpkin, sunflower, or squash seeds, hulled, roasted)
1 Tbsp of peanut butter or almond butter		 1 ounce of nuts or seeds = 2 ounce-equivalents
Dry beans and peas image cup of cooked dry beans (such as black, kidney, pinto, or white beans)
image cup of cooked dry peas (such as chickpeas, cowpeas, lentils, or split peas)
image cup of baked beans, refried beans
image cup (∼2 ounces) of tofu
1 ounce tempeh, cooked
image cup of roasted soybeans
1 falafel patty (image inches, 4 ounces)
2 Tbsp hummus		 1 cup split pea soup = 2 ounce-equivalents
1 cup lentil soup = 2 ounce-equivalents
1 cup bean soup = 2 ounce-equivalents
1 soy or bean burger patty = 2 ounce-equivalents
 


image


*Accessed June 14, 2012, from www.choosemyplate.gov/food-groups/protein-foods-counts.html.


The five dimensions of health provide ways to think about the effects of protein consumption. Our overall physical health and well-being depend on our eating enough essential amino acids for body protein synthesis. The ability to comprehend and apply new approaches to protein consumption by adapting to different protein sources (e.g., legumes and grains) and reducing portion sizes depends on our intellectual health capacity to implement change. Protein is a super-status food for some Americans; favorite sources may provide emotional health security. Clients needing to make dietary changes, such as changing to lower-fat sources of protein (e.g., cutting back on sausages), may need our advice on coping strategies. Because we and our clients follow different eating patterns, such as practicing vegetarianism or reducing consumption of animal protein, family and social dynamics may be affected when one member changes and thereby tests our level of social health. Religious and spiritual health beliefs lead individuals to nourish their bodies through a harmless philosophy that views humans as civilized enough to nourish their bodies without taking life.



Structure of Protein


Proteins are organic compounds formed by the linking of many smaller molecules of amino acids. Amino acids, like glucose, are organic compounds made of carbon, hydrogen, and oxygen. However, amino acids also contain nitrogen, which clearly distinguishes protein from other nutrients.


There are 20 amino acids from which all the proteins that are required by plants and animals are made. The human body is able to manufacture some of the amino acids for its own protein-building function; however, 9 amino acids cannot be made by the cells of the body. Therefore, these essential amino acids (EAAs) must be eaten in food, digested, absorbed, and then brought to cells by circulating blood. The remaining 11 are non-essential amino acids (NEAAs) (Box 6-2). The liver can create NEAAs as long as structural components, including nitrogen, from other amino acids are available.



BOX 6-2   Amino Acids









































ESSENTIAL AMINO ACIDS NONESSENTIAL AMINO ACIDS
Histidine Alanine
Isoleucine Arginine
Leucine Aspartic acid
Lysine Cysteine
Methionine Cystine
Phenylalanine Glutamic acid
Threonine Glutamine
Tryptophan Glycine
Valine Proline
  Serine
  Tyrosine


Each cell constructs or synthesizes the proteins it needs. To build proteins, the cell must have access to all 20 amino acids. This available supply of amino acids is in the metabolic amino acid pool. The amino acid pool is a collection of amino acids that is constantly resupplied with EAAs (from dietary intake) and NEAAs (synthesized in the liver). The pool allows the cell to build proteins easily.



Protein Composition


The functions of proteins are closely related to their structures. The complex composition of proteins is best understood through four structural levels: primary, secondary, tertiary, and quaternary1 (Figure 6-1).


image
FIG 6-1 Structural levels of protein. A, Primary structure: determined by number, kind, and sequence of amino acids in the chain. B, Secondary structure: hydrogen bonds stabilize folds of helical spirals. C, Tertiary structure: globular shape maintained by strong intramolecular bonding and by stabilizing hydrogen bonds. D, Quaternary structure: results from bonding between more than one polypeptide unit. (Courtesy Bill Ober. In Thibodeau GA, Patton KT: Anatomy & physiology, ed 4, St Louis, 1999, Mosby.)

The primary structure of protein composition is determined by the number, assortment, and sequence of amino acids in polypeptide chains. Amino acids are linked together by peptide bonds to form a practically unlimited number of proteins. The peptide bond occurs at the point at which the carboxyl group of one amino acid is bound to the amino group of another amino acid (Figure 6-2).


image
FIG 6-2 Peptide bonds.

The 20 amino acids form chains that may contain any combination or assortment of amino acids. This allows for thousands of different proteins to be formed. Two proteins may contain the same assortment and number of amino acids yet still have different functions because of the sequencing or order of the amino acids.


The secondary structure level of proteins affects the shape of the chain of amino acids; they may be straight, folded, or coiled. The tertiary structure results when the polypeptide chain is so coiled that the loops of the coil touch, forming strong bonds within the chain itself. The quaternary structural level is proteins containing more than one polypeptide chain.


A protein may not be able to perform its original function if its structure or shape changes. The shape may be changed by heat (cooking), ultraviolet light (exposure to sunlight), acids (vinegar), alcohol, and mechanical action. A protein has been denatured and physically changed when the shape of a protein is affected (e.g., a folded chain unfolding).


An example of denaturing a food protein is the change that occurs when the white of an uncooked egg (a clear liquid) is beaten. The clear liquid turns white, foamy, and stiff. Although the protein in the egg has been denatured, it is still a valuable source of amino acids. The amino acids are not affected; only the shape of the chain has been changed.


Inside the body, denaturing of proteins is controlled by mechanisms that keep the internal body environment from getting too basic or too acidic. Either extreme can lead to the denaturation of vital proteins within the body. Body temperature also affects the protein structure of the body. High fevers can become lethal when protein structures within the body become denatured. When body proteins are denatured, they cannot perform their original functions.


Although uncontrolled denaturation can be dangerous, it is helpful for digestion. Denaturing changes the three-dimensional structure of a protein, providing more surface area on which digestive juices act to release the amino acids of the food proteins.



Protein as a Nutrient in the Body


The proteins we consume in foods are not the same proteins used by our bodies. Actually, the only nutrient role protein in foods serves is to provide amino acids, the building blocks of all proteins.



Digestion and Absorption


Because of the complex structure of proteins, a number of protein enzymes, or proteases, produced by the stomach and pancreas are required to hydrolyze proteins into smaller and smaller peptides until individual amino acids are ready for absorption (Figure 6-3).


image
FIG 6-3 Summary of protein digestion and absorption. (From Rolin Graphics.)




Small Intestine


In the small intestine, pancreatic and intestinal proteases continue the hydrolysis of polypeptides. As these smaller peptides touch the intestinal walls, peptidases are released that complete the hydrolysis of protein into absorbable units of individual amino acids and dipeptides.


The primary pancreatic enzyme is trypsin. It is first secreted as trypsinogen, an inactive form. The intestinal hormone enteropeptidase activates trypsinogen into trypsin, which continues the hydrolysis of polypeptides. Two other pancreatic enzymes assist in the hydrolysis process: chymotrypsin hydrolyzes polypeptides into dipeptides, and carboxypeptidase breaks polypeptides and dipeptides into amino acids. Two intestinal peptidases are aminopeptidase, which releases free amino acids from the amino end of short-chain peptides, and dipeptidase, which completes the hydrolysis of proteins to amino acids.


Absorption of amino acids occurs through the intestinal walls by means of competitive active transport that requires vitamin B6 (pyridoxine) as a carrier. Because amino acids are water soluble, they easily pass into the bloodstream.



Metabolism


To understand the importance of protein metabolism in the growth and maintenance of the body, consider that most protein functions are a result of protein anabolism (synthesis) in cells. Hormones have a major role in the regulation of protein metabolism. Anabolism is enhanced by the effect of growth hormone (from the pituitary gland) and the male hormone testosterone. Hormones affecting the catabolism (break down) of proteins are the glucocorticoids that are enhanced by adrenocorticotropic hormone (ACTH); these hormones are secreted from the adrenal cortex. This process releases proteins in the cells to break down to amino acids, and then the amino acids travel in the bloodstream, contributing to an available pool of amino acids (Figure 6-4).


image
FIG 6-4 The body’s equilibrium depends on a balance between the rates of protein breakdown (catabolism) and protein synthesis (anabolism). (Modified from Williams SR: Essentials of nutrition and diet therapy, ed 7, St Louis, 1999, Mosby.)

The liver cells begin the process of catabolism through deamination. Deamination results in an amino acid (NH2) group breaking off from an amino acid molecule, resulting in one molecule each of ammonia (NH3) and a keto acid. Liver cells convert most of the ammonia to urea, which is later excreted in urine. The keto acid may enter the tricarboxylic acid (TCA) cycle to be used for energy (see Figure 9-2) or, through gluconeogenesis and lipogenesis, be converted to glucose and fat1 (see Figure 6-4).



Protein Excess


An excessive intake of protein results in increased deamination by the liver. The increased deamination may result in high levels of keto acids, possibly putting the body into a state of ketosis. The increased urea is excreted by the kidneys. Because the liver and kidneys are involved with the deamination process, the increased stress on the organs could initiate an underlying disorder of these organs. Because there are no definitive benefits of excessive protein intake, the general recommendation is to consume no more than twice the Recommended Dietary Allowance (RDA) for protein.


In fact, the source of excess protein may be a health concern. Animal-derived protein sources such as meats may also be high in saturated fat and cholesterol. This may increase the risk of coronary artery disease (CAD) and some cancers. The relationship between protein intake and osteoporosis also has been considered. When protein intake is high, there is a slight increase of calcium excretion from the body, but calcium absorption is not affected. Studies have yielded mixed results about this effect on the risk of osteoporosis. Because osteoporosis is multifactorial, this specific relationship is difficult to determine. Recommendations to consume moderate amounts of protein and to meet the new Dietary Reference Intake (DRI) levels for calcium are the best dietary approaches to decrease the risk of CAD and cancer. (See Chapter 8 for an in-depth discussion of osteoporosis.)



Nitrogen Balance


Nitrogen-balance studies are used to determine the protein requirements of the body throughout the life cycle and to assign value to the protein quality of foods to determine their biologic value.2 Because nitrogen (N) is a primary component of protein, the body’s use of protein can be determined by nitrogen-balance studies that compare the amount of nitrogen entering the body in food protein with the nitrogen lost from the body in feces and urine.


Nitrogen lost or excreted from the body may be endogenous nitrogen (from catabolism of body protein), metabolic nitrogen (from intestinal cells), or exogenous nitrogen (from dietary proteins). Nitrogen in feces may be metabolic and exogenous (from cells and dietary proteins) and in urine may be endogenous (catabolism of body protein) and exogenous (from dietary proteins).


imageAn individual is in nitrogen equilibrium or zero nitrogen balance if the amount of nitrogen consumed in foods equals the amount excreted. This occurs in normal, healthy adults when nitrogen in food protein entering (input) the body equals the nitrogen leaving the body (output). Because adults are no longer growing, the nitrogen that enters the body is not needed to build new tissue but is used simply to maintain the body.


imagePositive nitrogen balance occurs when more nitrogen is retained in the body than excreted. The nitrogen is used to form new cells for growth or healing. This occurs in growing children and in pregnant women who require additional nitrogen (and protein) for the growth of the fetus. Individuals recovering from illness or injury may be in positive nitrogen balance as the body heals. Negative nitrogen balance happens when more nitrogen is excreted from the body than is retained from dietary protein sources. This occurs when there is a breakdown of proteins within the body, such as in muscles and organs. Negative nitrogen balance may be caused by aging, physical illness, extreme stress, starvation, surgery, or eating disorders.



Functions


Proteins created in our bodies perform numerous functions, including the following:




Growth and Maintenance


Each body cell contains proteins. All growth depends on a sufficient supply of amino acids. The amino acids are needed to make the proteins required to support muscle, tissue, bone formation, and the cells themselves.


Maintaining our bodies also requires a constant supply of amino acids. There is a continual turnover of body cells, which are composed of protein. The cells break down and must immediately be replaced. Each replacement cell requires the formation of additional protein.


Also needed for growth and maintenance is the protein collagen, found throughout the body. Collagen forms connective tissues such as ligaments and tendons and acts as a glue to keep the walls of the arteries intact. In addition, collagen has a role in bone and tooth formation by forming the framework structure that is then filled with minerals such as calcium and phosphorus. Synthesis of scar tissue also depends on collagen. Other structures such as hair, nails, and skin are composed of similar protein substances.






Acid-Base Balance


Some reactions occurring within the body lead to the release of acidic substances; others cause basic matter to enter the fluids of the body. Blood proteins can buffer the effects of fluids to maintain a safe acidic level in body fluids. The ability of protein to regulate the balance between the acidic and base characteristics of fluids is called the buffering effect of protein. Because the chemical structure of amino acids combines an acid (the carboxyl group [COOH]) and base (amine), an amino acid can function either as an acid or a base depending on the pH of its medium. This is why the buffering effect of blood proteins is possible. This function is crucial to protect all proteins in the body. If fluids become either too acidic or too basic, the shape of proteins is altered or denatured. Denatured proteins are not able to perform their usual functions.


Many of the constituents of blood are protein based, and if protein functions are affected, the result can be lethal. Therefore proteins maintain a delicate pH level to ensure the proper functioning of all body systems (Box 6-3).



BOX 6-3   Genetic Disorders


Phenylketonuria (PKU) is a genetic disorder with a protein link. This disorder is characterized by the inability to use or break down excess phenylalanine, an essential amino acid. The excess phenylalanine circulating inside the body can cause various health problems. Infants with this disorder consume low-phenylalanine formulas, whereas children and adults follow a limited protein diet to control the intake of phenylalanine.


Another genetic protein disorder is sickle cell disease, which affects the shape of red blood cells. Because of abnormalities of the hemoglobin molecule, the red blood cell is curved or sickle shaped rather than round. The sickle shape can cause these blood cells to clog small blood vessels. This can be painful, may cause damage to internal organs such as the kidneys and heart, and may lead to frequent infections throughout the body. Early screening followed by long-term penicillin treatment can prevent secondary infections.


Having the sickle cell disease trait is not the same as having the disease itself. Both parents have to have the trait for a child to be at risk. Even then, there is only a 25% chance of developing the disorder. Sickle cell disease may occur in any ethnic group, but it is more common among Africans and African Americans; some states screen all infants to determine susceptibility.




Food Sources


Quality of Protein Foods


The proteins in foods are categorized by the EAAs they contain. Complete protein contains all nine EAAs in sufficient quantities that best support growth and maintenance of our bodies. Animal-derived foods, including meat, poultry, fish, eggs, and most dairy products, contain complete protein. (A notable exception is gelatin, which is incomplete.) Soybeans are the only plant source that provide all nine essential amino acids. Foods that contribute the best balance of EAAs and the best assortment of NEAAs for protein synthesis and are easily digestible are high-quality protein foods. The two highest-quality protein foods are eggs and human milk. The egg is of high quality because it contains all the necessary nutrients to support life. Human breast milk is the perfect food; its nutrient profile is ideal for human growth.


Incomplete protein lacks one or more of the nine essential amino acids. These proteins will not provide a sufficient supply of amino acids and will not support life (Box 6-4). Many plant foods contain considerable amounts of incomplete proteins. Some of the better sources are grains and legumes.



BOX 6-4   Sources of Complete and Incomplete Proteins


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Feb 9, 2017 | Posted by in NURSING | Comments Off on Protein

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