Nutrition 330 Introductory Nutrition

Study Guide: Unit 6

Protein: Amino Acids

Protein (Greek proteios, “of prime importance”) was the first substance recognized as vital to cells. Proteins have therefore been highly regarded, seen as supplying strength and power to the body.

Protein is, of course, an important nutrient required by all the body cells. Proteins are constituents of muscle tissues, soft tissues, bones, teeth, blood and other body fluids, and enzymes. However, dietary proteins are often overemphasized and treated as nutrients with fantastic properties, especially for muscle building.

This unit provides an overview of proteins: their chemical characteristics, the processes of protein digestion and absorption, and protein quality. We examine the functions of proteins in the body, the health effects of protein deficiency or excess, and the issue of vegetarianism. Finally, we look at the patterns of protein consumption and the recommendations for intake.

This unit consists of four sections:

6.1—Chemistry
6.2—Protein Digestion, Absorption, and Quality
6.3—Functions and Health Effects
6.4—Protein in the Diet

Objectives

After completing this unit, you should be able to

1. define amino acid and identify the chemical groups that are common in all amino acids.
2. describe the basic chemical formation of proteins.
3. define denaturation and describe the effect of denaturation on the physical and physiological properties of proteins.
4. outline the steps in the digestion of proteins, and briefly describe how the products of protein digestion are absorbed by the body.
5. distinguish between essential, non‑essential, and conditionally essential amino acids.
6. list and describe the factors that determine protein quality, and define complete protein, incomplete protein, limiting amino acid, and complementary protein, giving examples of each.
7. identify the three general categories of protein functions, giving examples of each.
8. explain the concept of nitrogen balance; define nitrogen equilibrium, positive nitrogen balance, and negative nitrogen balance; and describe examples of each of these states of nitrogen balance.
9. identify some health effects of protein deficiency and protein excess.
10. differentiate among the three major types of vegetarian diets—lactovegetarian, lacto‑ovo vegetarian, and vegan—and discuss the health benefits of the vegetarian way of eating.
11. discuss the possible nutritional concerns posed by vegetarianism and suggest ways to overcome them.
12. identify the factors considered when establishing the RDA for protein and state the current recommendations and the RDA for dietary protein for adults.
13. describe the patterns of protein consumption in Canadians.

6.1 Chemistry

Introduction

Like carbohydrates and lipids, proteins are composed of carbon, hydrogen, and oxygen. Proteins also contain nitrogen, which constitutes about 16% of the molecular weight. Sulphur and sometimes phosphorus, iron, and cobalt, may also be present in lesser amounts.

Proteins are larger and more complex than carbohydrates and lipids. Amino acids, the building blocks of proteins, are arranged in various sequences and geometric patterns. The combination of amino acids determines the physiological function of each protein.

Objectives

After completing this section, you should be able to

• define amino acid and identify the chemical groups that are common in all amino acids.
• describe the basic chemical formation of proteins.
• define denaturation and describe the effect of denaturation on the physical and physiological properties of proteins.

Key Terms

After completing section 6.1, you should be able to define and use the following terms in context:

Reading Assignment

• Chapter 6: Introduction and “The Chemist’s View of Proteins,” pages 177–180

Protein Chemistry

Amino means “containing nitrogen.” One of the three common chemical groups of an amino acid is the nitrogen‑containing amino group. The other two groups are the carboxylic acid group and the hydrogen group. The feature that differentiates one amino acid from another is the side group. Depending on the characteristics of this side group, the amino acid takes on acidic, basic, aromatic, or neutral properties. Figure 6‑1 on page 178 illustrates the general structure of an amino acid.

The textbook explains how amino acids are linked together to produce proteins through the formation of peptide bonds. The formation of a peptide bond involves condensation and results in the creation of a water molecule. Figure 6‑3 on page 179 illustrates the formation of a peptide bond.

Denaturation is the unfolding or rearrangement of a protein shape by heat, acid, or other conditions (such as mechanical agitation—whipping or beating), which results in a loss of functional activity. The textbook gives some examples of protein denaturation, such as the hardening of an egg in the heating process and the curdling of milk by hydrochloric acid in the stomach. Denaturation is, in fact, the first step in protein digestion. Since denatured proteins lose their biological activity as a result of alterations in their structure, ingested proteins such as enzymes and insulin are unable to perform their functions in the body. However, peptide bonds are not broken during denaturation, and the amino acid composition is unchanged, as is the nutritional value of the protein.

Study Questions

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6.2 Protein Digestion, Absorption, and Quality

Introduction

Protein digestion is more complex than carbohydrate digestion, because it involves the activation of proenzymes. Digestion of proteins occurs in the stomach and small intestine, which are made of proteins themselves; therefore, these organs secrete only proenzymes—the inactive forms of proteases (protein‑digesting enzymes). In the presence of foods, proenzymes are activated to become functioning enzymes, which hydrolyze the proteins. In this way, digestive organs are protected from auto‑digestion.

Objectives

After completing this section, you should be able to

• outline the steps in the digestion of proteins and briefly describe how the products of protein digestion are absorbed by the body.
• distinguish between essential, non‑essential, and conditionally essential amino acids.
• list and describe the factors that determine protein quality, and define complete protein, incomplete protein, limiting amino acid, and complementary protein, giving examples of each.

Key Terms

After completing section 6.2, you should be able to define and use the following terms in context:

Reading Assignment

• Chapter 6: “Digestion and Absorption of Protein,” pages 180–182

  Note: You will not be tested on the details of the digestive enzymes (Figure 6‑6).

Protein Digestion

Figure 6‑6 on page 181 outlines the steps of protein digestion in the GI tract. The process begins in the stomach with the denaturation of protein by gastric hydrochloric acid; however, most of the digestion occurs in the small intestine. The end product of intestinal digestion of proteins is a mixture of free amino acids and small peptides, mainly dipeptides and tripeptides.

Only free amino acids normally enter the general blood circulation at the portal vein. When a complete protein or a protein fragment enters the bloodstream, an allergic reaction may occur. However, an exception occurs in newborn infants when intact protein antibodies present in human colostrum enter the mucosa, thereby giving the baby some of the mother’s immunity. A few days after birth, the mucosal cells are closed to entry of whole proteins.

Reading Assignment

• Review Chapter 6: “Amino Acids,” pages 177–179

Amino Acids

Table 6‑1 on page 178 lists the 20 essential and non‑essential amino acids required by the human body. You need not memorize these names, but you should know that a human adult must derive nine essential amino acids from the diet. As the textbook tells us, the distinction between essential and non‑essential amino acids is not clear cut. In fact, it has been proposed that some amino acids, such as arginine, cysteine, tyrosine, and taurine, should be called conditionally essential, rather than non‑essential. Strictly speaking, non‑essential amino acids are produced endogenously from dietary amino acids, which have not been used for protein synthesis. They can also be produced from carbohydrate, provided a source of nitrogen is available. Such nitrogen comes primarily from dietary amino acids that have been used for energy. Although non‑essential amino acids can be produced endogenously, they are also provided by dietary proteins along with essential amino acids. We will discuss the metabolism of proteins in Unit 7.

Reading Assignment

• Chapter 6: “Protein in Foods,”191–193

Protein Quality

As the textbook points out, two factors that determine protein quality are the amino acid composition, especially the essential amino acids (EAA) in the protein, and the digestibility of the protein. A third factor that is less clearly described in the textbook is the balance of EAA. The presence of too much of a particular amino acid relative to the others can cause competition for sites and carriers for absorption between similar amino acids. Proteins that occur naturally in foods are balanced with respect to amino acids, because these proteins are biologically similar to human proteins; thus, competition for absorption is not significant. However, the use of amino acid or liquid protein supplements can upset the amino acid balance and disrupt absorption, thus hindering protein synthesis. Such supplements are often used merely for energy by the body, since protein intakes are usually adequate.

The textbook defines high‑quality proteins and provide examples. The term limiting amino acid is significant here because protein synthesis in the body stops when even just one essential amino acid (EAA) either runs out or is not present. Plant proteins tend to be “limiting” in one or more EAA; therefore, they are typically of lower quality than animal proteins. Some plant proteins are of higher quality than others. Soybean, potato, and rice are among the higher quality plant proteins, but corn, which has a low amount of the EAA lysine, is considered a low-quality protein source.

The text explains the concept of complementary proteins. Figure 6‑16 (p. 192) illustrates how the amino acids in legumes and grains complement each other. Four other examples of complementary proteins are given below.

• an animal protein, such as meat, milk, or eggs, combined with any plant protein (e.g., ham and beans, macaroni and cheese);
• grains, such as wheat, oats, corn, or rice, combined with legumes, such as soybeans, split peas, peanuts, or black beans (e.g., peanut butter sandwich, beans with rice);
• legumes combined with seeds such as sunflower or sesame seeds; and
• leafy vegetables combined with grains.

At one time it was recommended that complementary proteins be consumed at the same meal so that all essential amino acids would be absorbed into the bloodstream at the same time. This theory suggested that malnutrition would result if the complementary proteins were not consumed within a strict time interval. However, as the textbook states, it is not necessary to balance amino acids at each meal as long as protein intake is varied and energy intake is sufficient.

A complete protein is defined as a protein containing all the essential amino acids in amounts and proportions required by humans. An incomplete protein lacks one or more essential amino acids. High‑quality proteins are complete proteins. Combining complementary proteins will also produce a complete protein.

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6.3 Functions and Health Effects

Introduction

The diverse and complex nature of proteins makes them capable of serving many vital structural and functional roles in the body. In times of energy deficiency, they can also serve as a source of energy. Since protein is of such great importance, insufficient intake creates adverse health effects. Protein deficiency, along with energy deficiency, is a primary cause of malnutrition, and is commonly seen in children in developing countries.

Objectives

After completing this section, you should be able to

• identify the three general categories of protein functions and give examples of each.
• explain the concept of nitrogen balance; define nitrogen equilibrium, positive nitrogen balance, and negative nitrogen balance; and describe examples of each of these states of nitrogen balance.
• identify some health effects of protein deficiency and protein excess.
• differentiate among the three major types of vegetarian diets—lactovegetarian, lacto‑ovo vegetarian, and vegan—and discuss the health benefits of the vegetarian way of eating.
• discuss the possible nutritional concerns posed by vegetarianism and suggest ways to overcome them.

Key Terms

After completing section 6.3, you should be able to define and use the following terms in context:

Reading Assignment

• Chapter 6: “Roles of Proteins,” pages 185–188

Protein Functions

Proteins have an immense variety of functions. The textbook presents examples of protein functions, some of which we discuss in connection with related vitamins and minerals in later units. In general, protein functions can be categorized into three main groups:

Structural growth and maintenance of body tissues: This is the primary function of proteins, because they constitute the building blocks of cells.

Examples:

Regulation of body functions

Examples:

Source of energy: Although providing energy is not the primary function of proteins, in instances of carbohydrate and fat shortages, food protein and body protein are broken down for energy. The nitrogen part of amino acids is removed and excreted as urea via the urine, and the remaining carbon‑containing fragment is metabolized to produce energy; it can also be converted to glucose or stored as fat. Amino acid metabolism will be discussed further in the next unit.

Reading Assignment

• Chapter 6: “A Preview of Protein Metabolism,” pages 188–191

Nitrogen Balance

As the textbook describes, the human body requires an ongoing intake of protein to replace daily losses. One method used in determining protein requirements is the study of nitrogen balance. Protein is the only major nutrient that contains significant amounts of nitrogen (16%); therefore, measuring and comparing nitrogen consumption and nitrogen loss allows the estimation of protein use. Nitrogen consumption can be determined by analyzing all the foods consumed by a subject over a period of time. Nitrogen loss can be determined by analyzing the nitrogen lost through urine, feces, skin, hair, nails, sweat, tears, menses, mucus, and so on. Generally, only the urinary and fecal losses of nitrogen are measured. If the amount of nitrogen consumed is equal to the amount of nitrogen lost, one can assume nitrogen equilibrium, which means that body losses are being replaced or balanced by dietary intake. Healthy adults in maintenance (neither gaining or losing weight, and not pregnant) are in nitrogen equilibrium. The minimum amount of protein required to maintain nitrogen equilibrium is the protein requirement, provided there is an adequate energy intake; otherwise, dietary protein would be used by the body to produce energy. Nitrogen equilibrium is possible at a wide range of protein intakes above the minimum requirement. When protein intake is high, the body achieves equilibrium by excreting more nitrogen, because excess protein cannot be stored.

Positive and negative nitrogen balances are defined in the textbook. Growing children, pregnant women, and patients recovering from illness or tissue injury are in positive nitrogen balance. Negative nitrogen balance occurs during fasting or in illness involving tissue break down or injury, such as cancer, burns, surgery, and infection. Severe emotional stress and prolonged immobilization or bedrest can produce negative nitrogen balance. Diets containing proteins of poor quality or quantity can also result in negative nitrogen balance.

Reading Assignment

• Chapter 6: “Health Effects and Recommended Intakes of Protein and Amino Acids,” pages 193–197

• Chapter 6: “Protein and Amino Acid Supplements,” pages 200–201

  Note: The textbook suggests (page 196) that a high intake of coffee may raise the risk of heart disease. There is no evidence of this.

Protein Deficiency

Studies have shown that when protein intake suddenly falls below what the body requires, the body tends to adapt by increasing the efficiency of protein use. That is to say, nitrogen equilibrium is re‑established at a lower level of protein intake. However, there is a point beyond which the body cannot adapt; this level depends on the level of energy intake and on the quality of protein ingested. Below this level, protein deficiency, with such symptoms as wasting of body tissues, weakness, loss of vigour, edema, and increased susceptibility to infections will occur. Protein deficiency is seen more often in children than in adults, because in relation to body weight, children have higher protein requirements.

Protein deficiency occurs in developing countries, but it is also reported on Native reserves and in poor inner‑city slums of North America. Protein‑energy malnutrition (PEM) has also been recognized in hospital patients with tuberculosis, cancer, and diseases of the GI tract, and with anorexia nervosa (a psychiatric disorder involving food refusal).

Protein Excess

The textbook looks at several problems of excessive protein intake, but some have yet to be confirmed. The safe upper limit for protein intake depends largely on the body’s ability to excrete urea, the main nitrogen‑containing end product of protein metabolism. Under normal conditions, the body seems able to metabolize a high protein intake—as much as four or five times the recommended levels—without any detrimental effect (provided there is a sufficient fluid intake). However, long‑term effects on the human body have yet to be established. Generally, infants have a much greater risk of adverse effects because their immature kidneys cannot concentrate urine; therefore, since water is excreted with urea, severe dehydration can occur.

Reading Assignment

• Chapter 2: “Highlight 2: Vegetarian Diets,” pages 60–64

Vegetarian Diets

The textbook provides an interesting discussion of vegetarian and low‑meat diets. A large body of evidence has accumulated demonstrating that the diet eaten by many people in North America (often high in meat) is associated with various chronic diseases, especially obesity, coronary heart disease, and colon cancer. Research shows that well‑planned vegetarian diets offer many health benefits, including an increase in life expectancy. In general, the negative health effects are much stronger for processed meat than for unprocessed red meat.

The text refers to Eating Well with Canada’s Food Guide, which you studied in Unit 2. The healthy eating pattern and recommendations provided in this Guide are suitable for vegetarians over the age of two years. To ensure adequate intakes of protein, iron, zinc, and vitamin B₁₂, vegetarians can choose a variety of meat alternatives such as beans, lentils, eggs, tofu, soy‑based meat substitutes, nuts, nut butters, and seeds. Milk and fortified soy beverages also provide calcium, vitamin B₁₂, vitamin D, and protein.

Vegetarian diets are also superior from an environmental perspective as they require far less energy and water. This topic is looked at in the textbook on pages 730–731.

Study Questions

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6.4 Protein in the Diet

Introduction

While millions of people in developing countries rely heavily on inexpensive carbohydrate foods, people in the Western world tend to eat excessive amounts of animal protein foods. With a great variety of plant and animal proteins readily available, protein consumption is largely determined by personal preference, cultural habits, and income. People living in the Western world seem to prefer diets containing about 14–18% of the energy intake as protein. In recent years the diets of many millions of people living in China and other countries that have seen rapid economic growth have seen increasing amounts of meat.

Objectives

After completing this section, you should be able to

• identify the factors considered when establishing the RDA for protein and state the current recommendations and the RDA for dietary protein for adults.
• describe the patterns of protein consumption in Canadians.

Reading Assignment

• Chapter 6: “Recommended Intakes of Protein and Amino Acids,” pages 197–200

  Note: you will not be tested on the content of Table 6‑4.

Recommendations for Dietary Protein Intake

Protein intake can be expressed as total grams per day, as a percentage of total energy intake, or as grams per kilogram body weight per day.

Unlike carbohydrates and lipids, protein requirement can be influenced by several factors. The factors considered when RDA are established include age or physiological state, lean body mass, protein quality, energy intake, health status, and variations among individuals.

Age or physiological state: Protein needs based on body weight are highest during the first year of life, in adolescence, and during pregnancy and lactation, when extra protein is required for the growth of tissues. The table on the front inside cover of the textbook lists the protein RDA for each age group. You need not memorize the total grams recommended for each age group, but you should remember that the RDA for adults is 0.8 grams of proteins per kilogram of body weight per day. If you are substantially overweight or underweight, base your calculation on an appropriate weight for your height.

Lean body mass: Protein need increases as the amount of muscle increases, because protein is needed to maintain muscle tissues. RDAs for protein are based on individuals of average weight in each age group, assuming proportional lean body mass. Athletes in heavy training may require somewhat more protein than non‑athletes (1.0 to 1.5 grams of protein per kilogram body weight). This need can easily be met by the usual food intake without any protein supplements, because athletes normally increase their food intake to meet their increased demand for energy. Problems may occur, however, with athletes engaged in intense training. Here the requirement may be in the range of 1.5 to 2 grams of protein per kilogram of body weight. As a result, protein malnutrition is possible among some athletes. This can be a serious problem with athletes who try to maintain a low BMI, such as gymnasts.

Protein quality: If dietary protein is of low quality, more protein will be needed to supply the essential amino acids. RDAs for protein are based on a mixed diet containing both plant and animal proteins. The vegetarian diets consumed in Canada do not have a distinctly different amino acid composition from mixed diets, so the protein therein can be consumed at the same rate. However, children eating vegan diets may need to consume higher amounts of protein to ensure an adequate intake of essential amino acids.

Energy intake: Protein can be utilized efficiently only when adequate energy from carbohydrate and fat is available. Protein recommendations are based on adequate energy intakes. When energy intake is low, protein must be used for energy needs rather than for replacing body protein. The AMDR for protein, 10–35% of total energy, has a considerable range to accommodate differences in energy intake and dietary preferences. For a 2000‑kcalorie diet, a reasonable protein intake can range from 50 to 175 grams. For a diet supplying only 1000 kcalories, 10% of energy as protein gives only 25 grams of protein, which does not meet the RDA for protein (46 grams). Therefore, the lower the energy intake, the higher the percentage protein energy becomes to meet the RDA for protein. This may be relevant, for example, for senior citizens. While protein requirements do not change, activity levels, and therefore energy requirements, may decrease considerably. The ADMR for protein for seniors, especially women, may be in the range of 15–20% of total energy. In general, recommendations for protein based on the ADMR should always be compared to the RDA for protein, which gives an absolute amount of protein in grams per day. The textbook provides further discussion of this under Adequate Energy on page 198.

Health status: Any severe physiological or psychological stress, such as burns, fever, or surgical trauma, can dramatically increase protein requirements. RDA for protein are based on healthy individuals and must be adjusted for special metabolic needs.

Individual variation: As with vitamins and minerals, RDAs for proteins are established with a safety margin of 30% above protein requirements, which allows for individual variation in a healthy population.

Protein in the Canadian Diet

As a result of the typically high protein content of foods in the Canadian diet, the average consumption of protein greatly exceeds the recommended levels. This trend in protein consumption has been fairly consistent over the years. The average protein consumption by adults in Canada is about 85–105 grams per day in men and 60–80 grams in women. This is about 15–17% of total energy intake. These average daily protein intakes exceed the RDAs for adults—56 grams for men and 46 grams for women—especially in men.

Protein from animal sources is the major type of protein in the Canadian diet. Canada’s Food Guide puts greater emphasis on vegetables, fruit, and grain products to encourage a shift to eating less meat. The Guide also states, “have meat alternatives such as beans, lentils, and tofu often.” This makes good sense because a great many Canadians would benefit if their diet had a reduced content of meat and an increased content of foods of plant origin.

A segment of the Canadian population that may have an inadequate protein intake is women over age 65. According to the Nutrition Canada Survey data, the mean intake of this group is 0.78 grams of protein per kilogram of body weight, lower than the present recommendation of 0.8 grams per kilogram of body weight. Note that the recommendation assumes that protein requirements remain the same throughout adulthood.

One area in which the high intake of protein may be harmful is in connection with bone loss (see pp. 196–197). Bone loss is also associated with low protein intakes, especially in the elderly. Protein intake should also be moderated in persons with kidney disease (p. 197).

In conclusion, most Canadians consume more than adequate amounts of high-quality protein. Although it is tempting to advocate reducing these intakes, doing so is not necessary, provided that the liver and kidneys are functioning normally. Lowering protein intakes may also inadvertently decrease calcium, iron, and zinc intakes. Following Canada’s Food Guide is the optimal way to achieve the correct amount and balance of protein in the diet.

Study Questions

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References