Chapter 4 Protein

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Objectives

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Identify the:nature of proteinfunctions of proteinidentify the food sources of proteinbody needs of proteindigestion of proteindietary protein requirements

Key concepts

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Food proteins provide the amino acids necessary for building and maintaining body tissue

Protein balance, both within the body and in the diet, is essential to life and health

The quality of a protein food, and its ability to meet the body’s needs, is determined by the composition of amino acids

Definitions

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Amino Acids: Nitrogen-bearing compounds that form the structural units of protein

Digested proteins yield their specific amino acids

Conditionally indispensable amino acids: The body can make these. However, under certain circumstances such as illness, the body cannot make them in high enough quantities and they become indispensible in the diet.

Nature of Proteins

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Amino Acids are the basic building material

Joined in unique chain sequences to form specific proteins

When digested, the various food proteins yield their constituent amino acids, which are then available for use by cells

Amino acids: basic building material

Role as building units- All protein is made up of amino acidsEach amino acid is joined by a peptide bond

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Amino Acid(s) cont.

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Two amino acids joined together are called a dipeptide.

Polypeptides are chains of up to 100 amino acids

Hundreds of amino acids are linked together to form a single protein.

Amino Acids: Basic building material

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Dietary importance- Amino acids are named for their chemical nature.Amino refers to the compound containing nitrogen

Fats and carbohydrates have carbon, hydrogen and oxygen but no nitrogen

Protein is 16% nitrogen – the primary source of nitrogen in the body

Amino acids

Classes of amino acids

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There are 20 common amino acids which are vital to human life and health:

indispensabledispensableconditionally indispensable

Indispensable amino acids- the body cannot manufacture them in sufficient quantity or at allIndispensable = necessary in the diet; cannot be left out

Nine of them

Classes of Amino Acids

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Dispensable Amino Acids:Note: This term “dispensable” can be misleading. All amino acids have essential tissue building and metabolic functions.

The five amino acids can be synthesized from other amino acids and thus, are not necessary in the diet.

Classes of Amino Acids

Conditionally Indispensable:Under certain physiological conditions, the 6 amino acids, which are normally synthesized in the body like the dispensable amino acids, must be consumed in the diet.

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Balance

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Concept:Refers to the relative intake and output of substances in the body to maintain equilibrium

Necessary for health throughout life

Can be applied to the life-sustaining protein and the nitrogen that it supplies

Balance

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Protein Balance- Proteins being broken down into amino acids is called catabolism, and then

Resynthesized into tissue proteins as needed. This process is called anabolism

The body also maintains a balance between tissue protein and plasma protein, which are further balanced with dietary protein intake

Balance

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Nitrogen balance – indicates how well its tissues are being maintained- intake of protein in diet vs amount of nitrogen excreted in urine –also known as state of equilibrium or protein balance

1g urinary protein (nitrogen) = digestion and metabolism of 6.25 g of dietary protein

At different times in life or states of illness or malnutrition, the balance may shift to + or – nitrogen balance

Balance

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Positive nitrogen balance: takes in more protein than

excretes- storing extra protein in tissuesE.g. during periods of rapid growth, pregnancy, lactation as well as in individuals who have been ill and are “building back up” with increased nourishment.

Balance

Negative nitrogen balance: takes in less protein than excreted- In

children protein deficiency can cause growth retardationCan be due to inadequate protein intake

Losing Nitrogen by breaking down more tissue than it is building up.E.g. Illness, malnutrition [Kwashiorkor]

Positive nitrogen balance

Functions of Protein

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Primarily tissue building- Largest portion of the body (except for water) is made up of protein.

Body protein accounts for about ¾ of the dry matter in most tissue (exceptions: bone, adipose)

Primary functions are to repair worn-out, wasted, or damaged tissue and build up new tissue

Functions of Protein

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Additional body functions of protein:

Energy system- back up source for energy when there is an insufficient supply of CHO & fat

Water balance – Plasma protein, especially albumin, help maintain water balance

Functions of Protein

Metabolism- Protein aids metabolic function through enzymes, transport agents, and hormones

Body defense system – Protein helps build special WBCs (lymphocytes) and antibodies as part of the body’s immune system to defend against disease and infections

Food Sources of Protein

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Types of food proteins – Food contains a mixture of proteins that complement each other- Key to balanced diet is variety: animal and plant sources

Complete proteins = Protein foods that contain all 9 essential amino acids; found primarily in animal source, eggs, milk and cheese.Note: Soy products are the only plant source of

complete protein

Incomplete proteins – Protein food that are missing one or more of the nine indispensable amino acids. These proteins are generally of plant origin – e.g. seeds, vegetables, fruits

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Vegetarian Diets

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Complementary Protein:A mixture of plant proteins can provide

adequate amounts of amino acids when the basic use of various grains is expanded to include soy protein and other dried legume proteins (e.g. beans, peas)

Matching plant foods so that the amino acids missing in one food are supplied in another

This is the art of combining plant protein foods so that they “complement” one another and supply all 9 indispensable amino acids

Vegetarian Diets

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Types of vegetarian dietsLacto-ovo vegetarians – Eggs and dairy ok

Lacto-vegetarians – Dairy ok, no eggs

Ovo-vegetarians – Eggs ok, no dairyVegans- No eggs or dairy, no animal products at all – only plant foods

Vegetarian Diets

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The most recent position paper from the American Dietetic Association and Dieticians of Canada states that a vegetarian diet, including vegan, can meet the current recommendations for all essential nutrients, including protein and that vegetarian diets are appropriate throughout all stages of life.

Health benefits and risk

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Lower levels of dietary saturated fat and cholesterol from animal fat

Higher levels of CHO, fiber, Mg+, Boron, folate, phytochemicals, Vitamin C, E

Lower body mass index and prevalence of obesityLower death rates from heart disease,

hypercholesterolemia, and hypertensionLower risk of Type II DMLower risk of some forms of cancer (prostate,

colon)Lower risk of dementia

Vegetarian Diet

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For the most part this diet is considered an inexpensive and safe approach to prevent modern lifestyle diseases

Digestion of Proteins

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Mouth – Mechanical part of digestion is chewingParticles are mixed with saliva and passed on to the stomach as a semisolid mass

Stomach - chemical digestion begins hereProteins are large and complex- All enzymes needed for break down of protein are stored in an inactive form called proenzymes or zymogens [which are activated upon need]. Can not store in active form or will break down walls of the stomach and intestines.

Digestion of Protein

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Stomach – 3 agents in gastric secretion help with chemical digestion:

Hydrochloric acid – converts pepsinogen to pepsin and also begins unfolding and denaturing of the complex protein chains. When they are unfolded it makes them more available for enzymatic action

Digestion of Protein

(stomach cont’d)Pepsin – Main gastric enzyme, stored as pepsinogen hydrochloric acid converts pepsinogen to pepsin which begins splitting the links of proteins and amino acids into short chains called peptides.

Digestion of Protein

(stomach cont’d)

Rennin – Gastric enzyme in infancy and childhood- Important to digestion of milk. Rennin with Ca act on the casein of milk to produce a curd. Rennin prevents milk from moving too fast from stomach to small intestines by creating this curd

Digestion of Protein

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Small intestine – protein digestion starts in the acid environment of the stomach and is finished in the alkaline environment of the intestines.Pancreatic secretions break down protein to simpler and simpler substances:Trypsin, Enterokinase, Chymotrypsin, Carboxypeptidase

Digestion of Protein

Intestinal secretions – Glands in the intestinal wall produce two more protein-splitting enzymes to complete the digestion processAminopeptidase and Dipeptidase

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1. Partial protein digestion by the enzyme pepsin and stomach acid

2. Further protein digestion by enzymes released by pancreas

3. Final digestion of protein to amino acids take place mostly inside cells of small intestine

4. Amino acids absorbed into the portal vein and transported to the liver. From here they enter the general blood stream.

5. Little dietary protein is present in the feces.

Recommendations for Dietary Protein

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Protein requirements: influencing factorsTissue growth needs – fetal growth, infant

growth, lactation, adolescent growth and development

Dietary protein quality – Four basic measures to determine protein quality (p. 55)Chemical score (CSBiological Value (BV) Net Protein Utilization (NPU)Protein efficiency ratio (PER) intake

Illness or disease – fever and catabolism (increased tissue breakdown), increases body’s need for protein

Dietary Deficiency

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Protein-energy malnutrition (PEM) – Most severe cases are found in less industrialized countries where not only protein-rich foods are in short supply but all foods are.

Children are at highest risk because of elevated needs during rapid growth and development

Other persons at risk:Elderly Poor nutritionDisease or infectionAIDSCancerLiver Failure

Dietary Deficiency

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Two severe forms:Kwashiorkor – Common 18 – 24 month old-

taken off breast because of new baby and put on high carbohydrate diet. Kwashiorkor is a Ghanian word that refers to a disease that takes over the child when the second child is born. A child may receive adequate total kcal but lack enough kcals from protein

Marasmus – Very emaciated, low body fat, Chronic form of protein deficiency – basic starvation. Affects individuals of all ages; usually due to inadequate food sources

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Excess Dietary Protein

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The body has a finite need for protein. Additional protein is stored as fat or used as energy after nitrogen is removed

Problems with high-protein diets can include:May be high in saturated fats if

unbalancedMay increase risk for cardiovascular

diseaseKidneys may have to work overtime to rid

the body of excess nitrogenExcess protein may increase calcium

losses in the bone when dietary calcium is inadequate over the long term

May risk nutritional deficiency if other food groups are limited or eliminated

Dietary Guidelines

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10% - 35 % of total caloric intake from protein

Example: To calculate the protein needs of an individual consuming 2000 kcal per day based on the DRI of 10-35% of total kcal: 2000 Kcal x .10 - .35 = 200 – 700 Kcal per day200 – 700 /4 Kcal/g = 50 – 175 g protein per day

Example: To calculate the protein needs of a female with a body weight of 125 lbs.:[ 0.8g of high quality protein/kg body wt./day(2.2 lbs = 1 kg)125 lbs / 2.2lbs/kg = 56.81 kg.56.81 x 0.8g/kg = 45.45 g protein /day