Chapter 6: Proteins and Amino Acids
EXCI 204 — Food for Sport · Chapter Study Notes (with formulas & calculations)
Chapter Overview
The word protein comes from the Greek proteios, meaning “of primary importance.” Proteins are
essential to every cellular function: they catalyze chemical reactions, are fundamental components of
skeletal and muscular tissue, and are vital to immune response. Developed Western nations meet most
protein needs from animal products, while less affluent societies rely on plant proteins; as affluence
rises, societies eat more beef, placing greater demands on agriculture to grow grain for cattle. Current
recommendations encourage plant-based diets: soy (a traditional Asian staple) is the second-most-
planted crop in the U.S., and production of quinoa (a staple of the ancient Inca) has increased fourfold
over the past decade.
The chapter covers: (6.1) plant and animal protein sources; (6.2) amino acid and protein structure; (6.3)
protein digestion and absorption; (6.4) protein synthesis and functions, including protein as energy; (6.5)
protein deficiency, high-protein diets, allergies and intolerances; (6.6) nitrogen balance, protein RDAs,
protein quality and complementation; and (6.7) vegetarian diets. Math in this chapter: protein needs in
g/kg, pounds → kilograms, % of calories from protein, nitrogen balance, and MyPlate protein
equivalents — each is boxed where it appears and collected at the end.
KEY CONCEPT ⭐
Numbers you need for this chapter
• Protein = 4 Calories/gram.
• Adult RDA = 0.8 g protein per kg of body weight.
• Weight in kg = weight in pounds × 0.45 (the textbook’s conversion).
6.1 Proteins in Our Food
Learning objectives: List plant and animal sources of protein · Compare the nutrients in plant vs. animal
protein sources.
Animal foods (steak, eggs, milk) are the most concentrated sources of protein, but plant foods —
grains, nuts, legumes — are also important. Plant proteins contain different combinations of amino
acids, so most are not used as efficiently as animal proteins to build body proteins. However, a diet with
a variety of plant proteins can provide an amino acid combination similar to animal proteins and easily
meet most people’s needs.
KEY DEFINITION ⭐
legume — The starchy seed of a plant that produces bean pods; includes peas, peanuts, beans,
soybeans, and lentils.
KEY DEFINITION ⭐
amino acids — The building blocks of proteins. Each amino acid contains an amino group, an acid
group, and a unique side chain.
EXCI 204 · Chapter 6 · Page 1
,Protein content of common foods ⭐
Food (Figure 6.1b) Protein
1 cup milk 8g
1 egg 7g
3 ounces meat Over 20 g
½ cup legumes 6–10 g
¼ cup nuts or seeds 5–10 g
1 slice bread About 2 g
½ cup rice, pasta, or cereal 2–3 g
Figure 6.1a: meat, fish, poultry, eggs, and dairy are the most concentrated sources; legumes, nuts, and
seeds are also excellent. Grains are lower in protein but make an important contribution because we
eat multiple servings daily.
Animal vs. Plant Protein Sources ⭐
Animal sources of protein Plant sources of protein
Protein quality Amino acid mix matches our needs Different amino acid mix; less efficiently
used unless varied
Good for B vitamins; readily absorbable iron, Fiber, phytochemicals, unsaturated
zinc, and calcium (mono- and polyunsaturated) fats; B
vitamins except vitamin B₁₂; iron, zinc,
calcium but often less absorbable
Drawbacks Low in fiber; often high in saturated fat —
(and cholesterol) → increases heart
disease risk
Ask Yourself: a diet high in animal protein may raise heart disease risk because animal sources add
saturated fat and cholesterol and are low in fiber.
Current recommendations promote patterns based on whole grains, vegetables, and fruits with smaller
amounts of lean meats and low-fat dairy — plenty of protein from a mix of sources. A plant-based diet
that excludes some or all animal products is one of the patterns recommended by the 2020–2025
Dietary Guidelines, and can have positive environmental impacts.
EXCI 204 · Chapter 6 · Page 2
, DEBATE — EXAM EXAMPLE
Will Going Vegan Save the Planet?
For eliminating animal foods: producing 1 kg of protein from beef vs. kidney beans takes 18× more
land, 10× more water, 9× more fuel, 12× more fertilizer, and 10× more pesticide. About one-third
of cereal crops grown globally feed animals; humans get back only a fraction of the grain’s energy.
Animal waste on large farms pollutes waterways and releases gases contributing to acid rain and
global warming.
• Moderate view: the problem is how animals are managed. Animals grazing on land unfit for
crops and eating crop wastes, scraps, and grasses convert inedible material into food; small-
farm manure can fertilize fields.
• Plant foods have costs too: rain forests are cut to grow avocados (and graze cattle); rice
cultivation (staple of half the world) is a major greenhouse-gas source; processing, packaging,
and transport use water and fuel. U.S. produce travels 1300–2000 miles on average; local
produce has a much smaller carbon footprint. A vegan diet of highly processed foods (frozen
soy nuggets) and nonlocal produce may have a large environmental cost, and food waste is
greater for fruits and vegetables than for meat and dairy.
• Nutrition: replacing animal foods with plants improves health and environmental outcomes in
high-income countries but has little impact — and may harm health — in low-income
countries, where small amounts of meat and milk can mean survival. Animal foods provide
vitamin B₁₂, calcium, and highly absorbable iron and zinc.
• Conclusion: whatever we eat must support health and be produced in environmentally friendly
ways.
Concept Check (6.1):
1. Which has more protein: an egg or a cup of rice?
2. How do the nutrients provided by meat and milk differ from those in grains and legumes?
6.2 The Structure of Amino Acids and Proteins
Learning objectives: Describe the general structure of an amino acid · Distinguish essential from
nonessential amino acids · Discuss how amino acid order affects a protein’s shape and function · Explain
how structure relates to function.
Proteins in a lamb chop, a kidney bean, and your thigh muscle are all made of amino acids linked into
one or more folded, chainlike strands. Twenty amino acids are commonly found in proteins; each
protein has a different number, combination, and sequence of amino acids, which gives it its specific
function and characteristics.
Amino Acid Structure ⭐
Structure of an amino acid
Part of an amino acid (Figure 6.2) Notes
Central carbon atom The core to which the other parts are bound
EXCI 204 · Chapter 6 · Page 3
, Part of an amino acid (Figure 6.2) Notes
Hydrogen atom Bound to the central carbon
Amino group Contains nitrogen — the “amino” part of the name
Acid group The “acid” part of the name
Side chain Unique to each amino acid; varies in size and structure and
gives each its properties
KEY CONCEPT ⭐
Nitrogen is what makes protein different
Carbohydrate, fat, and protein all contain carbon, hydrogen, and oxygen, but only protein contains
nitrogen.
The 20 amino acids (Figure 6.2) ⭐
Essential (9) Nonessential (11) — * = conditionally essential
• Histidine • Alanine
• Isoleucine • Arginine*
• Leucine • Asparagine
• Lysine • Aspartic acid
• Methionine • Cysteine*
• Phenylalanine • Glutamic acid
• Threonine • Glutamine*
• Tryptophan • Glycine*
• Valine • Proline*
• Serine
• Tyrosine*
KEY DEFINITION ⭐
essential, or indispensable, amino acid — An amino acid that cannot be synthesized by the body in
sufficient amounts to meet its needs and therefore must be included in the diet.
KEY DEFINITION ⭐
nonessential, or dispensable, amino acid — (Described in text) One of the 11 amino acids commonly
found in protein that can be made in the body.
EXCI 204 · Chapter 6 · Page 4
EXCI 204 — Food for Sport · Chapter Study Notes (with formulas & calculations)
Chapter Overview
The word protein comes from the Greek proteios, meaning “of primary importance.” Proteins are
essential to every cellular function: they catalyze chemical reactions, are fundamental components of
skeletal and muscular tissue, and are vital to immune response. Developed Western nations meet most
protein needs from animal products, while less affluent societies rely on plant proteins; as affluence
rises, societies eat more beef, placing greater demands on agriculture to grow grain for cattle. Current
recommendations encourage plant-based diets: soy (a traditional Asian staple) is the second-most-
planted crop in the U.S., and production of quinoa (a staple of the ancient Inca) has increased fourfold
over the past decade.
The chapter covers: (6.1) plant and animal protein sources; (6.2) amino acid and protein structure; (6.3)
protein digestion and absorption; (6.4) protein synthesis and functions, including protein as energy; (6.5)
protein deficiency, high-protein diets, allergies and intolerances; (6.6) nitrogen balance, protein RDAs,
protein quality and complementation; and (6.7) vegetarian diets. Math in this chapter: protein needs in
g/kg, pounds → kilograms, % of calories from protein, nitrogen balance, and MyPlate protein
equivalents — each is boxed where it appears and collected at the end.
KEY CONCEPT ⭐
Numbers you need for this chapter
• Protein = 4 Calories/gram.
• Adult RDA = 0.8 g protein per kg of body weight.
• Weight in kg = weight in pounds × 0.45 (the textbook’s conversion).
6.1 Proteins in Our Food
Learning objectives: List plant and animal sources of protein · Compare the nutrients in plant vs. animal
protein sources.
Animal foods (steak, eggs, milk) are the most concentrated sources of protein, but plant foods —
grains, nuts, legumes — are also important. Plant proteins contain different combinations of amino
acids, so most are not used as efficiently as animal proteins to build body proteins. However, a diet with
a variety of plant proteins can provide an amino acid combination similar to animal proteins and easily
meet most people’s needs.
KEY DEFINITION ⭐
legume — The starchy seed of a plant that produces bean pods; includes peas, peanuts, beans,
soybeans, and lentils.
KEY DEFINITION ⭐
amino acids — The building blocks of proteins. Each amino acid contains an amino group, an acid
group, and a unique side chain.
EXCI 204 · Chapter 6 · Page 1
,Protein content of common foods ⭐
Food (Figure 6.1b) Protein
1 cup milk 8g
1 egg 7g
3 ounces meat Over 20 g
½ cup legumes 6–10 g
¼ cup nuts or seeds 5–10 g
1 slice bread About 2 g
½ cup rice, pasta, or cereal 2–3 g
Figure 6.1a: meat, fish, poultry, eggs, and dairy are the most concentrated sources; legumes, nuts, and
seeds are also excellent. Grains are lower in protein but make an important contribution because we
eat multiple servings daily.
Animal vs. Plant Protein Sources ⭐
Animal sources of protein Plant sources of protein
Protein quality Amino acid mix matches our needs Different amino acid mix; less efficiently
used unless varied
Good for B vitamins; readily absorbable iron, Fiber, phytochemicals, unsaturated
zinc, and calcium (mono- and polyunsaturated) fats; B
vitamins except vitamin B₁₂; iron, zinc,
calcium but often less absorbable
Drawbacks Low in fiber; often high in saturated fat —
(and cholesterol) → increases heart
disease risk
Ask Yourself: a diet high in animal protein may raise heart disease risk because animal sources add
saturated fat and cholesterol and are low in fiber.
Current recommendations promote patterns based on whole grains, vegetables, and fruits with smaller
amounts of lean meats and low-fat dairy — plenty of protein from a mix of sources. A plant-based diet
that excludes some or all animal products is one of the patterns recommended by the 2020–2025
Dietary Guidelines, and can have positive environmental impacts.
EXCI 204 · Chapter 6 · Page 2
, DEBATE — EXAM EXAMPLE
Will Going Vegan Save the Planet?
For eliminating animal foods: producing 1 kg of protein from beef vs. kidney beans takes 18× more
land, 10× more water, 9× more fuel, 12× more fertilizer, and 10× more pesticide. About one-third
of cereal crops grown globally feed animals; humans get back only a fraction of the grain’s energy.
Animal waste on large farms pollutes waterways and releases gases contributing to acid rain and
global warming.
• Moderate view: the problem is how animals are managed. Animals grazing on land unfit for
crops and eating crop wastes, scraps, and grasses convert inedible material into food; small-
farm manure can fertilize fields.
• Plant foods have costs too: rain forests are cut to grow avocados (and graze cattle); rice
cultivation (staple of half the world) is a major greenhouse-gas source; processing, packaging,
and transport use water and fuel. U.S. produce travels 1300–2000 miles on average; local
produce has a much smaller carbon footprint. A vegan diet of highly processed foods (frozen
soy nuggets) and nonlocal produce may have a large environmental cost, and food waste is
greater for fruits and vegetables than for meat and dairy.
• Nutrition: replacing animal foods with plants improves health and environmental outcomes in
high-income countries but has little impact — and may harm health — in low-income
countries, where small amounts of meat and milk can mean survival. Animal foods provide
vitamin B₁₂, calcium, and highly absorbable iron and zinc.
• Conclusion: whatever we eat must support health and be produced in environmentally friendly
ways.
Concept Check (6.1):
1. Which has more protein: an egg or a cup of rice?
2. How do the nutrients provided by meat and milk differ from those in grains and legumes?
6.2 The Structure of Amino Acids and Proteins
Learning objectives: Describe the general structure of an amino acid · Distinguish essential from
nonessential amino acids · Discuss how amino acid order affects a protein’s shape and function · Explain
how structure relates to function.
Proteins in a lamb chop, a kidney bean, and your thigh muscle are all made of amino acids linked into
one or more folded, chainlike strands. Twenty amino acids are commonly found in proteins; each
protein has a different number, combination, and sequence of amino acids, which gives it its specific
function and characteristics.
Amino Acid Structure ⭐
Structure of an amino acid
Part of an amino acid (Figure 6.2) Notes
Central carbon atom The core to which the other parts are bound
EXCI 204 · Chapter 6 · Page 3
, Part of an amino acid (Figure 6.2) Notes
Hydrogen atom Bound to the central carbon
Amino group Contains nitrogen — the “amino” part of the name
Acid group The “acid” part of the name
Side chain Unique to each amino acid; varies in size and structure and
gives each its properties
KEY CONCEPT ⭐
Nitrogen is what makes protein different
Carbohydrate, fat, and protein all contain carbon, hydrogen, and oxygen, but only protein contains
nitrogen.
The 20 amino acids (Figure 6.2) ⭐
Essential (9) Nonessential (11) — * = conditionally essential
• Histidine • Alanine
• Isoleucine • Arginine*
• Leucine • Asparagine
• Lysine • Aspartic acid
• Methionine • Cysteine*
• Phenylalanine • Glutamic acid
• Threonine • Glutamine*
• Tryptophan • Glycine*
• Valine • Proline*
• Serine
• Tyrosine*
KEY DEFINITION ⭐
essential, or indispensable, amino acid — An amino acid that cannot be synthesized by the body in
sufficient amounts to meet its needs and therefore must be included in the diet.
KEY DEFINITION ⭐
nonessential, or dispensable, amino acid — (Described in text) One of the 11 amino acids commonly
found in protein that can be made in the body.
EXCI 204 · Chapter 6 · Page 4
