Nutrition 331 Nutrition for Health

Study Guide

Unit 7: Protein and Amino Acids

Introduction

Protein was the first substance recognized as vital to living cells. Proteins have therefore been highly regarded, being seen as supplying strength and power to the body. Proteins are an important nutrient required by all the body’s cells: they are constituents of muscle tissues, soft tissues, bones, teeth, blood and other body fluids, and enzymes. However, dietary protein is often overvalued and treated as a nutrient 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 will examine the functions of proteins in the body, the health effects of protein deficiency or excess, and the issue of protein quality for vegetarian diets. Finally, we will look at patterns of protein consumption and at intake recommendations.

Objectives

After completing this unit, you should be able to

1. identify the chemical nature of amino acids and proteins.
2. discuss the digestion and absorption of proteins.
3. list seven functions of protein.
4. explain how an amino acid is broken down if it is not needed for a protein-specific role.
5. explain three factors influencing protein quality.
6. discuss the importance of complementary proteins and mutual supplementation and provide examples of each.
7. define nitrogen balance and explain in what situations it will be positive, negative, or balanced.
8. list the major dietary sources of protein.
9. discuss the RDAs and recommended percent calories for protein and compare the protein intakes of Canadian adults to the recommendations.

Section 1: Chemistry of Proteins

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

Proteins are larger and more complex than carbohydrates and lipids in their chemical makeup. Amino acids, the building blocks of proteins, are arranged in various sequences and geometric patterns, some based on the genetic code unique to each of us. Such combinations of amino acids determine the physiological function of each protein.

The term amino means containing nitrogen. Therefore, one of the three common chemical groups of an amino acid is the nitrogen-containing amino group.

Human adults must derive nine essential amino acids from dietary sources. The distinction between essential and non-essential amino acids is not clear cut. In fact, it has been proposed that some amino acids should be termed conditionally essential rather than non-essential. Strictly speaking, non-essential amino acids are produced endogenously (within the body) from other dietary amino acids that have not been used for protein synthesis, or from carbohydrates, provided a source of nitrogen is available. This 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.

Section 2: Digestion and Absorption of Proteins

Protein digestion is more complex than the digestion of carbohydrates because it involves the activation of proenzymes. It begins in the stomach with the denaturation of protein by gastric hydrochloric acid; however, most of the digestive process occurs in the small intestine.

Since the stomach and small intestine are themselves made of proteins, they secrete only proenzymes—the inactive forms of proteases (protein-digesting enzymes). Proenzymes are then activated to form functioning enzymes that hydrolyze the proteins. In this way, digestive organs are protected from auto-digestion. In Unit 4, we briefly reviewed the enzymes involved in protein digestion. Pepsin is secreted as pepsinogen, which is activated by stomach acids. Similarly, the pancreas secretes trypsinogen, which is activated (becomes trypsin) in the small intestine.

The end products of trypsin digestion include dipeptides and tripeptides. Peptidases from the cells lining the small intestine then hydrolyze these substances to free amino acids. Only free amino acids normally enter the general blood circulation. Free amino acids travel via the portal vein to the liver (as do other water-soluble products of digestion). When a complete protein or a protein fragment enters the blood, an allergic reaction may occur; 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. However, when the closure is not totally effective, a small amount of protein may still enter the blood, resulting in an allergic reaction.

Section 3: Protein Functions

Their diverse and complex nature makes proteins capable of serving many vital roles in the body, both structurally and functionally. In times of energy deficiency, they can 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 the primary cause of malnutrition, which is commonly seen in children of developing countries.

The immense variety of protein functions is impossible to describe completely. The eText presents several 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, outlined below.

1. Structural growth and maintenance of body tissues is the primary function of proteins because they constitute the building blocks of cells. Examples are given in Table 7.1.

   Protein	Functions
   collagen	formation of scars, tendons, ligaments, bones, and teeth
   contractile proteins	formation of muscle tissues
   cell membrane proteins	formation of a barrier around cells

   Table 7.1: Functions of structural proteins

2. Regulation of body functions is a second major activity of proteins. The protein-based compounds that regulate body functions include a variety of body substances; some of the most important are highlighted in Table 7.2.

   Protein	Functions
   enzymes	catalysts for metabolic reactions
   proteins in blood	fluid, acid-base, and lymph balance
   antibodies	immunity
   some hormones
   – insulin	regulation of blood glucose
   – thyroid hormone	regulation of metabolic rate
   transport proteins
   – hemoglobin	oxygen transport in blood
   – lipoproteins	fat transport
   – transferrin	iron transport
   prothrombin, fibrin	clotting of blood

   Table 7.2: Functions of regulatory proteins

3. Both food proteins and body proteins can be broken down to supply energy. This is not the primary function of proteins but does occur in instances of carbohydrate and fat shortages. The nitrogen part of amino acids is removed and excreted as urea, and the remaining, carbon-containing fragment is metabolized to produce carbohydrate or fat. Protein provides 4 calories of energy per gram. The eText (p. 224) lists the circumstances under which amino acids are “wasted.”

Section 4: Protein Quality

As pointed out in the reading, factors determining protein quality include

• protein digestibility – Is the protein in the food available to be absorbed?
• essential amino acid completeness and adequacy – Does the protein have all the essential amino acids? At adequate levels?
• the balance of essential amino acids to each other

Too much of a certain amino acid in relation to the others can cause competition between similar amino acids for sites and carriers for absorption. Proteins that occur naturally in foods are balanced in amino acids. Because these proteins are biologically similar to human proteins, competition for absorption is not significant. However, the use of amino acid or liquid protein supplements can upset the body’s amino acid balance and disrupt absorption, hindering protein synthesis. Protein supplements are often used merely for energy by the body since protein intakes are usually more than adequate.

Generally, plant proteins are of lower quality than animal proteins. Plants tend to have less digestible protein, smaller amounts of protein, and incomplete essential amino acids (EAA). Plant proteins vary in quality. Soybeans, potatoes, and rice are among the higher quality plant protein sources.

In a balanced diet, incomplete proteins (proteins missing one or more EAA) can be completed by other proteins. The eText presents the concept of mutual supplementation on page 227. Examples of complementary proteins include the following:

• an animal protein (e.g., meat, milk, eggs) combined with any plant protein
• grains such as wheat, oats, corn, and rice combined with legumes such as soybeans, dried peas, dried beans, or peanuts
• legumes combined with seeds such as sunflower or sesame seeds

Complementary amino acid profiles (in foods) need not be precise and combined at exactly the same meal (Young & Pellett, 1994), although many cultures do combine foods at the same meal (e.g., lentils and rice).

As we discuss later, the potential problem of inadequate protein intake concerns primarily strict vegetarians. People who eat meat, fish, poultry, and/or milk products are seldom short of protein.

Section 5: Protein Requirement and Nitrogen Balance

The human body requires a constant intake of protein to replace daily losses. One method used in determining protein requirements is nitrogen balance studies. Since protein is the only major nutrient that contains significant amounts of nitrogen (16%), by measuring and comparing nitrogen consumption and nitrogen loss, researchers can estimate 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 nitrogen losses through urine, feces, skin, hair, nails, sweat, 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 nitrogen losses are being replaced by dietary intake.

Healthy adults who are neither gaining nor losing weight and are not pregnant are in nitrogen equilibrium. Provided there is an adequate energy intake, the minimum amount of protein needed to maintain nitrogen equilibrium is a person’s protein requirement. If energy intake is inadequate, dietary proteins will be used by the body to produce energy. Nitrogen equilibrium is possible at a wide range of protein intakes above the minimum requirement. Excess protein cannot be stored; therefore, when protein intake is higher than necessary, the body achieves equilibrium by excreting more nitrogen.

The eText defines positive nitrogen balance and negative nitrogen balance on pages 229–230. 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 breakdown or injury (such as cancer, burns, surgery, or infection). Severe emotional stress, prolonged immobilization or bed rest, and diets containing proteins of poor quality or quantity can also result in negative nitrogen balance.

While people in developing countries rely heavily on inexpensive, high-carbohydrate foods, many people in the industrialized world 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. Protein from animal sources is the major type of protein in the Canadian diet.

Protein requirements can be expressed in three ways: as grams per kilogram body weight per day, as total grams per day, or as percentage of total energy intake. The percentage of energy coming from protein shifts depending on the amount of carbohydrates or fats in the diet. If a diet is very low fat (e.g., 15% of calories), the diet may look high in protein (e.g., 25% of calories).

Unlike the requirements for carbohydrates and lipids, the protein requirement can be influenced by several factors. The following factors are considered when RDAs for protein are established:

Age or physiological state: Protein needs based on body weight are high during the first year of life and during adolescence, pregnancy, and lactation when extra protein is required for tissue growth. The RDA for adults is 0.8 grams of protein per kilogram of body weight per day. On page 211, the eText describes how you can individualize your protein RDA by using your appropriate weight. If you are substantially overweight or underweight at the moment, use the Body Mass Index Nomogram at the back of your eText to establish an appropriate weight range for your height.

Lean body mass: Protein need increases as muscle mass increases because protein is needed to maintain muscle tissues. RDAs for protein are based on individuals of average weight in each age group, with proportional lean body mass. Athletes in heavy training may require slightly more protein than non-athletes (1.0–1.5 g protein per kilogram body weight). This increased need can easily be met through food intake, without protein supplements, because athletes normally increase their food intake to meet their increased demand for energy.

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. Vegetarian diets do not have a distinctly different amino acid composition from mixed diets and can provide adequate protein. However, children eating strictly vegetarian diets based primarily on cereals should consume extra protein to ensure an adequate intake of essential amino acids. Vegetarian diets are examined in Unit 15.

Energy intake: Protein can be used efficiently only when enough energy is available from carbohydrates and lipids. The ideal protein intake for adults is 10 to 12 percent of total energy intake, based on average adult energy requirements, rather than on actual energy intake. This recommendation may sometimes be misleading. While protein requirements do not change beyond early adulthood, activity level can vary considerably. Therefore, percentage of calories from protein should actually be increased in the senior years. A recommendation of 15 to 20 percent of total energy intake has been made for seniors, especially women, to ensure adequate protein intake. In general, recommendations for protein intakes expressed as percent calories should always be compared to grams per kilogram body weight.

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

Individual variation: Like vitamins and minerals, RDAs for proteins are established with an extra margin of 30 percent that allows for individual variation in the healthy population.

Section 6: Health Effects

Protein deficiency often occurs in developing countries. Some cases are also reported in Indigenous communities and in poor, inner-city slums. Protein energy undernutrition (PEU) has also been recognized in hospital patients with such diseases as tuberculosis, cancer, diseases of the gastrointestinal tract, and with anorexia nervosa (a psychiatric problem of food refusal).

The eText identifies several problems with excessive protein intake. The risk to infants is greater than to adults because the immature kidneys of infants cannot concentrate urine; water is excreted with urea, creating severe dehydration.

There is some evidence that a high intake of protein, especially from protein supplements such as whey-based beverages or amino acid supplements may increase calcium excretion, contributing to osteoporosis.

The eText discusses AMDR on pages 33 and 34. These state that for people older than 18 years, protein should provide 10 to 35 percent of energy. Consuming 35 percent protein may boost fat and saturated fat intakes to well over their respective 35 percent and 10 percent of calories maximums. Also, a diet this high in protein may not provide adequate fibre.

Section 7: Protein in Foods

Protein-rich foods are familiar to many people. Animal products such as meat, fish, poultry, and milk products are important protein sources. Figure 6-16: Finding the Protein in Food (p. 235) shows the relative amounts of protein in some plant sources. Note the protein contributions of lentils, beans, peas, peanut butter, and almonds in the Meat and Alternatives group as well as the protein contribution of many grain products.

Many Canadians would benefit if their diet had a reduced content of meat and an increased content of foods of plant origin. For that reason, legumes can make a valuable contribution to a healthy diet but, unfortunately, most Canadians rarely consume them. Page 236 describes some of the advantages of eating legumes. One advantage of legumes that is not mentioned in the eText is that their production causes much less environmental damage (such as the generation of carbon dioxide and use of energy and water) than does the production of meat. Increasingly, food manufacturers are creating vegetarian alternatives to meat, substituting primarily soy protein. In Canada, many of these alternatives contain some of the B vitamins and iron typical of meats and poultry. However, this iron may not be readily absorbed by the body. Factors influencing iron absorption will be discussed in Unit 10.

As a result of the high protein content of foods usually provided by the Canadian diet, average consumption of protein exceeds the recommended levels (based on grams per kilogram body weight). Canadian adult women consume 60 to 80 grams of protein per day; adult men consume 85 to 105 grams daily. This represents about 15 to 17 percent of energy intake.

Review Questions

See eText Chapter 5, Self-Check:

• questions 1–10 (pp. 243–244)