Nutrition and Diet Therapy: Proteins
Proteins are the basic material of every body cell. By the age of 4 years, body protein content reaches the adult level of about 18% of body weight. An adequate supply of proteins in the daily diet is essential for normal growth and development and for the maintenance of health. Proteins are appro-priately named. The word protein is of Greek derivation and means “of first importance.”
Functions of Proteins
Functions
Proteins build and repair body tissue, play major roles in regulating various body functions, and provide energy if there is insufficient carbohydrate and fat in the diet.
Building and Repairing Body Tissue
The primary function of proteins is to build and repair body tissues. This is made possible by the provision of the correct type and number of amino acids in the diet. Also, as cells are broken down during metabolism (catabolism), some amino acids released into the blood are recycled to build new and repair other tissue (anabolism). The body uses the recycled amino acids as efficiently as those obtained from the diet.
Regulating Body Functions
Proteins are important components of hormones and enzymes that are essen-tial for the regulation of metabolism and digestion. Proteins help maintain fluid and electrolyte balances in the body and thus prevent edema (abnormal reten-tion of body fluids). Proteins also are essential for the development of antibodies and, consequently, for a healthy immune system.
Providing Energy
Proteins can provide energy if and when the supply of carbohydrates and fats in the diet is insufficient. Each gram of protein provides 4 calories. This is not a good use of proteins, however. In general, they are more expensive than carbohydrates, and most of the complete proteins also contain saturated fats and cholesterol.
Food Sources of Proteins
Food Sources
Proteins are found in both animal and plant foods (Table 6-1). The animal food sources provide the highest quality of complete proteins. They include meats, fish, poultry, eggs, milk, and cheese.
Despite the high biologic value of proteins from animal food sources, they also provide saturated fats and cholesterol. Consequently, complete proteins should be carefully selected from low-fat animal foods such as fish, lean meats, and low-fat dairy products. Whole eggs should be limited to two or three a week if hyperlipidemia is a problem.
Proteins found in plant foods are incomplete proteins and are of a lower biologic quality than those found in animal foods. Even so, plant foods are important sources of protein. Examples of plant foods containing protein are corn, grains, nuts, sunflower seeds, sesame seeds, and legumes such as soy-beans, navy beans, pinto beans, split peas, chickpeas, and peanuts.
Plant proteins can be used to produce textured soy protein and tofu, also called analogues. Meat alternatives (analogues) made from soybeans contain soy protein and other ingredients mixed together to simulate various kinds of meat. Meat alternatives may be canned, dried, or frozen. Analogues are excel-lent sources of protein, iron, and B vitamins.
Tofu is a soft cheeselike food made from soy milk. Tofu is a bland product that easily absorbs the flavors of other ingredients with which it is cooked. Tofu is rich in high-quality proteins and B vitamins, and it is low in sodium. Textured soy protein and tofu are both economical and nutritious meat replacements.
Because of their inclusion of either dairy products and eggs or dairy prod-ucts alone, most individuals who follow lacto-ovo vegetarian or lacto-vegetarian diets will be able to meet their protein requirements through a balanced diet that includes milk and milk products, enriched grains, nuts, and legumes. Strict veg-etarians who consume no animal products will need to be more careful to include other protein-rich food sources such as soybeans, soy milk, and tofu.
Classification of Proteins
Classification
The classification and quality of a protein depends on the number and types of amino acids it contains. There are 20 amino acids, but only 10 are considered essential to humans (Table 6-2). Two additional amino acids are sometimes incorporated into proteins during translation: selenocyteine and pyrrolysine. Essential amino acids are necessary for normal growth and development and must be provided in the diet. Proteins containing all the essential amino acids are of high biologic value; these proteins are called complete proteins and are extremely bioavailable. The nonessential amino acids can be produced in the body from the essential amino acids, vitamins, and minerals.
Incomplete proteins are those that lack one or more of the essentialamino acids. Consequently, incomplete proteins cannot build tissue without the help of other proteins. The value of each is increased when it is eaten in combination with another incomplete protein, not necessarily at the same meal but during the same day. In this way, one incomplete protein food can provide the essential amino acids the other lacks. The combination may thereby pro-vide all the essential amino acids (Figure 6-1). When this occurs, the proteins are called complementary proteins (Table 6-3). Gelatin is the only protein from an animal source that is an incomplete protein.
Composition of Proteins
Composition
Like carbohydrates and fats, proteins contain carbon, hydrogen, and oxygen, but in different proportions. In addition, and most important, they are the only nutrient group that contains nitrogen, and some contain sulfur. Figure 6-1 is an example of an amino acid with a nitrogen (N) molecule.
Proteins are composed of chemical compounds called amino acids (Figure 6-2). Amino acids are sometimes called the building blocks of protein because they are combined to form the thousands of proteins in the human body. Heredity determines the specific types of proteins within each person.
Digestion and Absorption of Proteins
Digestion and Absorption
The mechanical digestion of protein begins in the mouth, where the teeth grind the food into small pieces. Chemical digestion begins in the stomach. Hydrochloric acid prepares the stomach so that the enzyme pepsin can begin its task of reducing proteins to polypeptides.
After the polypeptides reach the small intestine, three pancreatic en-zymes (trypsin, chymotrypsin, and carboxypeptidase) continue chemical digestion. Intestinal peptidases finally reduce the proteins to amino acids.
After digestion, the amino acids in the small intestine are absorbed by the villi and are carried by the blood to all body tissues. There, they are used to form needed proteins.
Metabolism and Elimination of Proteins
Metabolism and Elimination
All essential amino acids must be present to build and repair the cells as needed. When amino acids are broken down, the nitrogen-containing amine group is stripped off. This process is called deamination. Deamination pro-duces ammonia, which is released into the bloodstream by the cells. The liver picks up the ammonia, converts it to urea, and returns it to the bloodstream for the kidneys to filter out and excrete. The remaining parts are used for energy or are converted to carbohydrate or fat and stored as glycogen or adipose tissue.
Dietary Requirements of Proteins
Dietary Requirements
One’s protein requirement is determined by size, age, sex, and physical and emotional conditions. A large person has more body cells to maintain than a small person. A growing child, a pregnant woman, or a woman who is breastfeeding needs more protein for each pound of body weight than the average adult. When digestion is inefficient, fewer amino acids are absorbed by the body; consequently the protein requirement is higher. This is sometimes thought to be the case with the elderly. Extra proteins are usually required after surgery, severe burns, or during infections in order to replace lost tissue and to manufacture antibodies. In addition, emotional trauma can cause the body to excrete more nitrogen than it normally does, thus increasing the need for protein foods.
The National Research Council of the National Academy of Sciences con-siders the average adult’s daily requirement to be 0.8 gram of protein for each kilogram of body weight. To determine your requirement, do the following:
· Divide body weight by 2.2 (the number of pounds per kilogram).
· Multiply the answer obtained in step 1 by 0.8 (gram of protein per kilogram of body weight).
In 2002 the Dietary Reference Intakes (DRIs) for protein were published by the National Academy of Sciences (see Table 6-4). An Adequate Intake (AI) was established for infants 0 to 6 months, with all other recommendations based on 0.8 g/kg of body weight. Table 6-5 provides an idea of the amount of protein in an average day’s diet. (For specific amounts of protein in other foods, refer to Appendix D.)
Protein Excess
It is easy for people living in the developed parts of the world to ingest more protein than the body requires. There are a number of reasons why this should be avoided. The saturated fats and cholesterol common to complete protein foods may contribute to heart disease and provide more calories than are desirable. Some studies seem to indicate a connection between long-term high-protein diets and colon cancer and high calcium excretion, which de-pletes the bones of calcium and may contribute to osteoporosis. People who eat excessive amounts of protein-rich foods may ignore the also essential fruits and vegetables, and excess protein intake may put more demands on the liver (which converts nitrogen to urea) and the kidneys to excrete excess urea than they are prepared to handle. Therefore, the National Research Council recommends that protein intake represent no more than 15% to 20% of one’s daily calorie intake and not exceed double the amount given in the table of DRIs (Table 6-4).
Protein and Amino Acid Supplements
Protein and amino acid supplements are taken for a number of reasons, such as “bulking up” by athletes, growing fingernails, and sparing body protein in weight loss. It is weight lifting, not protein bars or protein supplements, that builds muscles. Fingernails have never been affected by extra protein, and dieters need a balanced diet using the guidelines of MyPyramid.
High-quality protein foods are more bioavailable than expensive sup-plements. Single amino acids can be harmful to the body and never occur naturally in food. The body was designed to handle food, not supplements. If a single amino acid has been recommended, it is very important that a physician be consulted before the amino acid is used.
Nitrogen Balance
Protein requirements may be discussed in terms of nitrogen balance. This occurs when nitrogen intake equals the amount of nitrogen excreted. Posi-tive nitrogen balance exists when nitrogen intake exceeds the amountexcreted. This indicates that new tissue is being formed, and it occurs dur-ing pregnancy, during children’s growing years, when athletes develop additional muscle tissue, and when tissues are rebuilt after physical trauma such as illness or injury. Negative nitrogen balance indicates that protein is being lost. It may be caused by fevers, injury, surgery, burns, starvation, or immobilization.
Protein Deficiency
When people are unable to obtain an adequate supply of protein for an extended period, muscle wasting will occur, and arms and legs become very thin. At the same time, albumin (protein in blood plasma) deficiency will cause edema, resulting in an extremely swollen appearance. The water is excreted when sufficient protein is eaten. People may lose appetite, strength, and weight, and wounds may heal very slowly. Patients suffering from edema become lethar-gic and depressed. These signs are seen in grossly neglected children or in the elderly, poor, or incapacitated. It is essential that people following vegetarian diets, especially vegans, carefully calculate the types and amount of protein in their diets so as to avoid protein deficiency.
Protein Energy Malnutrition (PEM)
People suffering from protein energy malnutrition (PEM) lack both protein and energy-rich foods. Such a condition is not uncommon in developing coun-tries where there are long-term shortages of both protein and energy foods. Children who lack sufficient protein do not grow to their potential size. Infants born to mothers eating insufficient protein during pregnancy can have perma-nently impaired mental capacities.
Two deficiency diseases that affect children are caused by a grossly inad-equate supply of protein or energy or both. Marasmus, a condition resulting from severe malnutrition, afflicts very young children who lack both energy and protein foods as well as vitamins and minerals. The infant with marasmus appears emaciated but does not have edema. Hair is dull and dry, and the skin is thin and wrinkled (Figure 6-3). The other protein deficiency disease that affects children as well as adults is kwashiorkor (Figure 6-4). Kwashiorkor appears when there is a sudden or recent lack of protein-containing food (such as during a famine). This disease causes fat to accumulate in the liver, and the lack of protein and hormones results in edema, painful skin lesions, and changes in the pigmentation of skin and hair. The mortality rate for kwashiorkor patients is high.
0 Comments