Chapter Six: Proteins


What Are Proteins?

  • macronutrients found in each cell in the body

  • participate in every cellular activity that helps the body to function

  • all enzymes are made from protein

  • some hormones are made from protein

Proteins Differ Structurally from Carbohydrates and Lipids

  • made up of chains of amino acids that are made based on an individual’s DNA

  • excess dietary protein cannot be stored in the body

  • proteins contain nitrogen, in addition to carbon, hydrogen, and oxygen

  • some protein foods contain sulfur, which is not found in carbohydrates or lipids

The Building Blocks of Proteins are Amino Acids

  • there are 20 amino acids that are used to make proteins

  • the proteins are classified by the number of amino acids in the chain

    • peptides: fewer than 50 amino acids

      • dipeptides: 2 amino acids

      • tripeptides: 3 amino acids

      • polypeptides: more than 10 amino acids

    • proteins: more than 50 amino acids

      • typically 100 to 100,000 amino acids linked together


amino acid anatomy

  • anatomy of an amino acid

    • contains an amine group (NH2), a carboxylic acid group (COOH), and a side chain (R group)

    • the side chains make each amino acid unique and influence the function of each amino acid


  • peptide bonds

    • form when the carbon of the acid group of one amino acid joins with the nitrogen atom of the amine group of a second amino acid

    • formed through condensation

    • broken through hydrolysis

Essential, Nonessential, and Conditional Amino Acids

  • essential

    • must be consumed in the diet

    • nine essential amino acids

  • nonessential

    • can be synthesized in the body

    • eleven essential amino acids

  • conditionally essential

    • nonessential amino acids that become essential because the body cannot make them

    • ex: tyrosine and glycine

The Organization and Shape of Proteins Affect Their Function

  • four levels of structure

    • primary structure: amino acids are linked together to form a simple linear chain

    • secondary structure: geometric shape of the protein that is folded and twisted

    • tertiary structure: 3-dimensional globular shape of the protein

    • quaternary structure: two or more polypeptide chains bond together

What are the Key Steps in Digesting and Absorbing Protein?

  • protein digestion begins in the stomach

    • the bolus enters the stomach

    • gastrin stimulates the release of HCl

    • HCl denatures the protein and converts pepsinogen to pepsin

    • pepsin breaks polypeptides into shorter chains

  • digestion continues in the small intestine

    • polypeptides enter the small intestine and are broken down into smaller peptides

      • cholecystokinin stimulates the release of proteases by the pancreas

      • proteases break apart the polypeptides to tripeptides and dipeptides

      • dipeptidases and tripeptidases break the dipeptides and tripeptides into amino acids


  • amino acid absorption

    • amino acids are absorbed in the small intestine

    • amino acids are transported to the liver from the intestines via the portal vein

    • in the liver, amino acids are:

      • used to synthesize new proteins

      • converted to energy, glucose, or fat

      • released to the bloodstream and transported to cells throughout the body


protein digestion and absorption

How are Amino Acids Metabolized?

  • the liver metabolizes amino acids, depending on bodily needs

    • most amino acids are sent into the blood to be picked up and used by the cells

    • if the diet is low in carbohydrate, the amino acids are converted into glucose

  • amino acid pools supply the body’s ongoing need for protein synthesis

    • the body breaks down and makes proteins daily to maintain body structures such as tissues and organs

    • amino acid pools provide a ready supply of amino acids the body uses to create proteins on demand

    • extra protein is used to help the body heal if a person has extensive wounds

    • protein turnover is the process of degrading and synthesizing proteins

      • more than 200 grams of protein is turned over daily


Protein Synthesis is Regulated by Your Genes

  • 3 steps of protein synthesis

    • transcription: information about building or repairing old proteins is stored in DNA. When a new polypeptide chain is needed, an inverse copy of the DNA is generates forming messenger RNA (mRNA)

    • translation: mRNA binds to a ribosome. the code on the mRNA is ‘read’ and a transfer RNA (tRNA) brings a specific amino acids to the ribosome, based on the code

    • elongation: the translation process continues as amino acids are collected by transfer RNA (tRNA) and brought to the ribosomes to build a chain in the proper sequence, continuing until the sequence is finished and the new protein is released

  • sickle-cell anemia is a common blood disorder that involves abnormal or flawed protein formation


protein synthesis

Deamination Removes the Amine Group from Amino Acids

  • when the amino acid pool reaches capacity, the amino acids are broken down to their component parts for other uses

  • deamination occurs when the amine group is removed from the amino acid. ammonia is formed

  • ammonia is converted to urea in the liver. urea is subsequently excreted in the urine

  • carbon-containing remnants are:

    • converted to glucose, if they are glucogenic amino acids, through gluconeogenesis

    • converted to fatty acids and stored as triglycerides in adipose tissue

    • oxidized for energy via metabolic pathways


Nonessential Amino Acids are Synthesized Through Transamination

  • transamination is the process of forming nonessential amino acids by transferring the nitrogen from one amino acid to a keto acid to form a new nonessential amino acid

How is Protein Metabolized?

  • protein can be used for gluconeogenesis

    • if too few carbohydrates are consumed, the body converts glucogenic amino acids into glucose

  • excess protein is converted to fatty acids and stored as triglycerides in adipose tissue

  • if too few kilocalories are consumed, protein will be oxidized for energy

What is the Function of Protein in the Body?

  • provide structural support and enable movement

  • act as a catalyst

    • enzymes speed up reactions

  • act as a chemical messenger

    • hormones regulate cell actions

  • regulare fluid balance

  • help maintain acid-base balance

  • transport substances throughout the body

    • transport proteins shuttle oxygen, waste products, lipids, some vitamins and sodium and potassium through the blood and cell membranes

  • contribute to a healthy immune system

    • antibodies are proteins that bind and neutralize pathogens that would harm the body

  • provide energy

    • 4kcals/grams

  • improve satiety and appetite control

roles of proteins

How Much Protein Do You Need Daily?

  • healthy adults should be in nitrogen balance

    • should consume enough to replace what is used everyday

  • individuals in positive nitrogen balance

    • pregnant woman, people recovering from surgery or injury, and growing children

    • should consume enough to build new tissue

  • individuals in negative nitrogen balance

    • immediately after surgery, fighting an infection, or severe emotional trauma

    • need to consume enough kilocalories and protein to meet demand


nitrogen balance and imbalance

You Can Determine Your Own Protein Needs

  • protein intake recommendations

    • RDA: 0.8 g/kg daily for adults over 18

    • AMDR: 10 to 35% of total daily kilocalories

    • overweight individuals’ needs are not much greater than normal-weight individuals of similar height

    • the American College of Sports Medicine, the Academy of Nutrition and Dietetics, and other experts advocate:

      • 50 to 100 percent more protein for competitive athletes participating in endurance exercise or resistance exercise (typically this population eats more and therefore gets additional protein)

What are the Best Food Sources of Protein

  • not all protein is created equal

    • high-quality protein:

      • is digestible

      • contains all essential amino acids

      • provides sufficient protein to synthesize nonessential amino acids

  • several methods have been developed to determine the protein quality of foods

    • amino acid score

      • composition of essential amino acids of a protein compared with a standard, usually egg protein

    • protein digestibility corrected aminoa cid score (PDCAAS)

      • includes the digestibility of the protein and the amino acid score

      • used by the FDA to determine the DV% of proteins in a serving of food

    • biological value

      • how quickly the nitrogen from the absorbed protein is synthesized into body protein

What are the Best Food Sources of Protein?

  • complementary and complete proteins

    • complete proteins

      • contain all nine essential amino acids

      • usually, animal sources are complete proteins

        • exception: soy protein

      • are considered higher quality

  • incomplete proteins

    • low in one or more essential amino acids, referred to as limiting amino acids

    • usually plant sources

  • complementary proteins

    • combining an incomplete protein with a food that supplies the limiting amino acids (e.g., combining grains and legumes)


  • eggs, meat, fish, soy, and dairy contain significant amounts of proteins

  • a 3-ounce serving of cooked meat, poultry, or fish:

    • provides 21 to 25 grams of protein

    • provides about 7 grams of protein per ounce

    • is about the size of a deck of cards

    • is an adequate amount for one meal

  • eating a wide variety of foods is the best approach to meeting protein needs

  • taking protein supplements is unnecessary and generally not recommended

What Happens if you Eat Too Much or Too Little Protein

  • too much protein:

    • may increase risk for heart disease

      • high intake of protein sources with high saturated fat

      • choose a variety of plant sources of protein to reduce risk for heart disease

    • increases risk for kidney stones

      • a diet high in animal protein and low in carbohydrates lowers urine pH, which can raise the risk of developing kidney stones

  • increases risk for osteoporosis

    • high protein intake with too low calcium intake can lead to increased urinary calcium losses

    • high-protein diets with adequate calcium (especially from dairy sources) protect bone

    • too low protein intake can lead to bone loss in elderly men and women

  • too much emphasis on protein in a diet can crowd out or displace other nourishing, equally important food choices from the diet such as whole grains, fruit, and vegetables


  • too little protein

    • protein-energy malnutrition (PEM)

      • protein is used for energy rather than for its other functions in the body

      • other important nutrients are in short supply

      • more prevalent in infants and children


  • inadequate protein

    • cells lining the GI tract are not sufficiently replaced as they slough off

    • digestive function is inhibited

    • absorption of food is reduced

    • intestinal bacteria get into the blood ad cause septicemia

    • the immune system is compromised due to malnutrition and cannot fight infection


  • eating too little protein can lead to protein-energy malnutrition (PEM)

    • one in eight people in the world (most are children) do not have adequate protein intake, kcal intake, or both

    • the available protein is used as an energy source rather than for its other roles in the body

    • lack of protein damages the GI tract lining which inhibits the cells ability to absorb nutrients

    • malnourished individuals have compromised immune systems

    • 3 types of PEM: Kwashiorkor, marasmus, marasmic kwashiorkor


Types of PEM: Kwashiorkor

  • severe protein deficiency

    • generally the result of a diet high in grains and deficient in protein

  • symptoms include:

    • edema in legs, feet, and stomach

    • diminished muscle tone and strength

    • brittle hair that is easy to pull out

    • a pale, sad, and apathetic appearance

    • being prone to infection, rapid heart rate, excess fluid in lungs, pneumonia, septicemia, and water and electrolyte imbalances

Types of PEM: Marasmus

  • results from a severe deficiency in kilocalories

    • frail, emaciated appearance

    • weakened and appear apathetic

    • often cannot stand without support

    • appears old beyond their years

    • hair thin, dry, and lacks sheen

    • low body temperature and blood pressure

    • prone to dehydration, infections and unnecessary blood clotting

Types of PEM: Marasmic Kwashiorkor

  • chronic deficiency in kilocalories and protein

    • edema in legs and arms

    • a ‘skin and bones’ appearance

    • with treatment, the edema subsides and appearance becomes more like someone with marasmus

Treatment for PEM

  • medical and nutritional treatment can dramatically reduce the mortality rate

  • should be implemented carefully and slowly

    • step 1: address life-threatening factors

      • severe dehydration

      • fluid and nutrient imbalances

    • step 2: restore depleted tissue

      • gradually provide nutritionally dense kilocalories and high-quality protein

    • step 3: transition to foods and introduce physical activity

What is a Vegetarian Diet?

  • people choose vegetarian diets for a variety of reasons

    • ethical

    • religious

    • environmental

    • health

  • vegetarians must consume adequate amounts of a variety of foods and should plan meals well


  • benefits of a healthy vegetarian diet include reduced risks of:

    • heart disease

    • high blood pressure

    • diabetes

    • cancer

    • stroke

    • obesity

  • potential risks of a vegetarian diet include low intake of key nutrients

    • protein

    • calcium

    • iron

    • vitamin B12

    • zinc

    • vitamins A and D

    • omega-3 fatty acids