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Key Questions
What are proteins made of? What are the different kinds of
amino acids? What are the three important
functions of protein? What does the body need to make protein tissues? Can protein be used for energy?
Protein gets a
positive press
People know protein is important
Nearly all in U.S. get enough protein
15% of total calories in U.S. adult diet supplied by protein, far above what we need.
Image versus Reality
Term protein is derived from the Greek word protos for “first”
Indicates importance Essential structural component
of all life Involved in most biological
processes
Four Functions of Protein
Integral structural component of skeletal muscle, bone, connective tissues, organs, red blood cells and hemoglobin, hair, and fingernails Collagen is most common protein here
Basic substance that makes up digestive and other enzymes in the human body
Major component of hormones such as insulin
Also serves as an energy source
Protein Dynamics
A 154-pound man has 24 lbs of protein
All protein is broken down and re-built
Protein turnover uses 9 oz each day
We only consume 2-3 oz per day
Protein Dynamics
Most protein is recycled from protein substances being ‘turned over’
Proteins play key roles in repair of tissues and replacing tissue proteins damaged by illness or injury
Source of Energy
Protein also serves as a source of energy
Protein contains nitrogen, not stored For energy, amino acids first lose
nitrogen Free nitrogen in excess excreted in
urine Nitrogen excretion increases need for
water Amino acids converted to glucose or
fat, then used to form energy
Amino Acids
•The “building blocks” of protein are amino acids
•Protein in food is broken down by digestive enzymes and absorbed into the bloodstream as amino acids
•20 common amino acids form proteins when linked together
•Each type of protein in the body is a unique combination of amino acids linked together in chains
•DNA is genetic material that directs protein synthesis and organizes amino acids into chains
Protein Structures: Insulin
For each protein, its chain of amino acids fold up into a unique, complex shape
Insulin proteins function because of their unique shapes.
Ras protein: 3-D Structure
This molecular switch governs cell growth
Knowing its structure may enable interventions to shut off switch in cancer cells
Red blood cells and fibrin are mostly protein
Exactly which amino acids and their arrangement determine whether the protein is an enzyme, a component of red blood cells, a muscle fiber, or another tissue made from protein
Fig. 15-3, p. 3
Proteins rarely ‘solo’
Proteins usually work by assembling into protein complexes
Function like complicated machines
Execute protein synthesis and degradation, cell-to-cell signaling, and many other operations.
Essential Amino Acids
Nine of the 20 common amino acids are considered essential
11 are nonessential All 20 are required to build and
maintain protein tissues Body cannot produce (or produce
enough) essential amino acids, so they must be provided by the diet
Food proteins contain essential and nonessential amino acids
Proteins Differ in Quality
How proteins support tissue construction is protein “quality”
High quality have a balance of essential amino acids to support tissue needs
If essential amino acids are missing, proteins can not be built
Proteins Differ in Quality
When required level of essential amino acid is lacking, remaining amino acids are used for energy
Amino acids are not stored in the body, so we need essential amino acids daily
We need foods that provide enough of all essential amino acids EVERY DAY.
Complete Proteins
Food sources of high-quality protein are called complete proteins
Complete proteins include those found in animal products and soy products
Incomplete proteins are deficient in one or more essential amino acids
Most plant proteins are “incomplete” Vegetarians “complement” essential
amino acid composition of plant protein by combining them to form a “complete” source of protein
Amino Acid Supplements
Amino acids are natural, so people think they’re harmless
But high intakes of amino acids can harm health
High amounts disrupt protein synthesis by causing a surplus of some amino acids and a deficit of others
Amino Acid Supplements Can
Cause loss of appetite, diarrhea, and gastrointestinal upsets
Increase the workload of the liver and kidneys
Increase risk of dehydration Only known benefit of supplements:
Big Money-maker for Supplement Companies!
Build Muscle?
Amino acids and protein powders do NOT cause muscles to grow
No amino acids are delivered directly to our muscles and cause muscles to enlarge
Only exercise and a good diet (and, too frequently, steroids!) build muscles Most athletes get more than enough protein from
their usual diet
Food as a Source of Protein
Average intake in the U.S. is 75 grams per day RDA: 56 grams for men and 46 grams for women 70% comes from meats, milk, and other animal
products Dried beans and grains are good sources Most protein provides vitamins, minerals, and fat
as well Beef and pork are good sources of iron Calories from lean sirloin steak are 37% fat and
63% from protein
Food Sources of Protein
Tuna 3 oz. 24 grams Beef steak 3 oz. 26 grams Milk 16 oz. 16 grams Cereal and bread 12 grams (6 servings) ____________ Total 78 grams RDA for protein, men: 56 grams
Food Sources of Protein
Milk, 16 oz. 16 grams 1 cup oatmeal 6 grams Peanut butter sandwich 16 grams 2 bean burritos 28 grams 1 cup Spanish rice 4 grams_________________________________ 70 grams
Too Little Protein
Protein deficiency occurs alone or with deficiency of calories and other nutrients
Protein foods contain nutrients (iron, zinc, vitamin B12, niacin)
Body protein will be used for energy during starvation
Body will take protein from liver, intestines, heart, muscles, other organs and tissues
Loss of 30% of body protein causes reduced body strength for breathing, susceptibility to infection, abnormal organ functions, and death
Marasmus
Protein deficiency and lack of calories cause marasmus, or protein-energy malnutrition
Frequent where hunger and famine are common Occurs among people with anorexia nervosa and
diseases such as cancer and AIDS Individuals with marasmus look starved They have little body fat Must utilize protein from muscles, liver, and
tissues as an energy source
Marasmus Protein-calorie Malnutrition
Very susceptible to infection, have dry skin and brittle hair
Use of the fat pads in the cheeks (last fat used for energy during starvation)
Accompanied by vitamin and mineral deficiencies
Conditions are complicated by infection, diarrhea, and
dehydration
Fig. 15-12, p. 8
Is There Enough Protein?
Mammals: Expensive Scarce
Poultry & Fish Plant sources Insects
Abundant Traditional in many
cultures
Too Much Protein?
Adults can consume 35% of total calories without immediate ill effects
Consumption of 45% of total calories from protein is too high
This much protein causes nausea, weakness, and diarrhea; diets very high in protein result in death after several weeks
Disease from excess protein intake termed “rabbit fever”
High Protein Intakes
Generally accompanied by high-fat and low-fiber intakes
Foods high in protein (hamburger, cheese, eggs) are often high in fat and contain little or no fiber
Lean meats provide a large part of their total calories as fat
Too Much Protein?
High protein diets implicated in weak bones, kidney stones, cancer, heart disease, and obesity.
Tolerable Upper Intake Level for protein has not been established
People are cautioned not to consume high levels of protein from foods or supplements
Trends
2/3 of U.S. protein comes from animal products
Wealth leads to meat protein increase
Increased consumption of fat, too Rates of heart disease and some
cancers increase Adequate dietary intakes without
excess fat have beans and grains mixed with lean meats and low-fat dairy products