Proteins and Amino Acids

Protein Structure

Proteins are complex macromolecules composed of amino acids linked by peptide bonds. Their structure is organized into four levels:

• Primary structure: Linear sequence of amino acids.

• Secondary structure: Local folding into alpha-helices and beta-sheets.

• Tertiary structure: Three-dimensional folding driven by side chain interactions.

• Quaternary structure: Association of multiple polypeptide chains.

Bond between amino acids: Peptide bonds form between the amino group of one amino acid and the carboxyl group of another.

Protein Functions in the Body

• Structural: Collagen in connective tissues, keratin in hair and nails.

• Enzymatic: Catalyze biochemical reactions (e.g., digestive enzymes).

• Transport: Hemoglobin transports oxygen; albumin transports nutrients.

• Hormonal: Insulin and glucagon regulate blood glucose.

• Immune: Antibodies defend against pathogens.

• Energy: Used as an energy source when necessary (4 kcal/g).

Food Sources of Protein

• Highest sources: Meat, poultry, fish, eggs, dairy products, legumes, nuts, and seeds.

Protein Digestion and Absorption

• Begins in the stomach with pepsin and hydrochloric acid.

• Continues in the small intestine with pancreatic proteases.

• Amino acids are absorbed into the bloodstream via active transport.

Essential vs Nonessential Amino Acids

• Essential amino acids: Cannot be synthesized by the body; must be obtained from diet (9 total).

• Nonessential amino acids: Can be synthesized by the body (11 total).

What Makes Protein Unique?

• Proteins contain nitrogen, unlike carbohydrates and fats.

Complete vs Incomplete Proteins

• Complete proteins: Contain all essential amino acids in adequate amounts (e.g., animal proteins, soy).

• Incomplete proteins: Lack one or more essential amino acids (e.g., most plant proteins).

Complementary Proteins

• Combining two or more incomplete protein sources to provide all essential amino acids (e.g., rice and beans).

Limiting Amino Acids

• The essential amino acid in shortest supply in a food protein, limiting protein synthesis.

Denaturation

• Structural alteration of proteins due to heat, acid, or agitation, resulting in loss of function.

Nitrogen Balance

• Balanced: Nitrogen intake equals output (healthy adults).

• Positive: Intake exceeds output (growth, pregnancy, recovery).

• Negative: Output exceeds intake (illness, injury, malnutrition).

Recent Research on Protein Intake

• Distributing protein intake evenly across meals may optimize muscle protein synthesis.

Protein-Energy Malnutrition

• Marasmus: Severe energy and protein deficiency; symptoms include wasting and stunted growth.

• Kwashiorkor: Protein deficiency with adequate energy; symptoms include edema, fatty liver, and skin changes.

Plant-Based Diets

• Benefits: Lower risk of heart disease, hypertension, type 2 diabetes, and certain cancers.

• Potential deficiencies: Vitamin B12, iron, calcium, zinc, omega-3 fatty acids.

Types of Vegetarians

Vitamins

Water-Soluble vs Fat-Soluble Vitamins

• Water-soluble: B vitamins, vitamin C; not stored extensively, low toxicity risk, excreted in urine.

• Fat-soluble: Vitamins A, D, E, K; stored in fat tissues, higher toxicity risk, require fat for absorption.

Provitamin

• A compound that can be converted into an active vitamin (e.g., beta-carotene to vitamin A).

Bioavailability

• The proportion of a nutrient absorbed and utilized by the body (e.g., heme iron is more bioavailable than non-heme iron).

Free Radicals and Antioxidants

• Free radicals: Unstable molecules that can damage cells.

• Antioxidants: Neutralize free radicals (e.g., vitamins C, E, and beta-carotene).

• Oxidative stress: Imbalance between free radicals and antioxidants, leading to cell damage.

• Excess free radicals or inadequate antioxidants increase risk of chronic diseases.

Preserving Vitamin Content

• Minimize cooking time, use less water, avoid high temperatures, and store foods properly.

General Functions and Sources

• Water-soluble vitamins: Coenzymes in metabolism, support immune function.

• B vitamins: Found in whole grains, meats, dairy, legumes, and leafy greens.

Selected Vitamins

• Vitamin A: Vision, immune function; deficiency causes night blindness; toxicity causes liver damage; sources: retinol (animal), beta-carotene (plant).

• Vitamin D: Bone health; sources: sunlight, fortified foods; deficiency in adults causes osteomalacia.

• Vitamin E: Antioxidant; sources: nuts, seeds, oils; destroyed by heat and oxygen.

• Vitamin K: Blood clotting; sources: leafy greens (K1), gut bacteria (K2).

• Thiamin (B1): Energy metabolism; sources: whole grains, pork; deficiency causes beriberi.

• Riboflavin (B2): Energy metabolism; sources: dairy, grains; destroyed by light.

• Folate/Folic Acid: DNA synthesis; sources: leafy greens, fortified grains; deficiency causes neural tube defects; supplementation recommended in pregnancy.

• Niacin (B3): Energy metabolism; sources: meats, grains; deficiency causes pellagra.

• Vitamin B6: Amino acid metabolism; deficiency causes anemia; toxicity can cause nerve damage.

• Vitamin B12: Nerve function; sources: animal foods; requires intrinsic factor for absorption.

• Vitamin C: Collagen synthesis, antioxidant; sources: citrus fruits; deficiency causes scurvy; enhances iron absorption.

• Pantothenic acid and biotin: Deficiency/toxicity rare.

Water and Minerals

Intracellular vs Extracellular Minerals

• Intracellular: Potassium, magnesium, phosphate.

• Extracellular: Sodium, chloride, calcium.

Fluid Needs

• Determined by age, activity, environment, health status.

Tap vs Bottled Water

• Both are generally safe; tap water is regulated by the EPA, bottled by the FDA.

Trace vs Major Minerals

• Major: Needed in amounts >100 mg/day (e.g., calcium, phosphorus, potassium).

• Trace: Needed in smaller amounts (e.g., iron, zinc, iodine).

Selected Minerals

• Sodium: Fluid balance, nerve function; excess intake common; sources: processed foods.

• Chloride: Major extracellular anion; maintains fluid balance.

• Potassium: Fluid balance, muscle function; sources: fruits, vegetables; excess can cause heart issues.

• Magnesium: Enzyme function; sources: nuts, whole grains; deficiency rare but possible.

• Iodine: Thyroid hormone synthesis; sources: iodized salt, seafood; deficiency causes goiter.

• Fluoride: Tooth enamel strength; sources: water, dental products; deficiency increases cavity risk.

• Calcium: Bone health; sources: dairy, leafy greens; absorption interfered by oxalates/phytates; deficiency causes osteoporosis.

• Phosphorus: Bone and cell membrane structure; sources: meat, dairy.

• Zinc: Immune function; sources: meat, seafood; deficiency impairs immunity.

• Iron: Oxygen transport; sources: heme (animal), non-heme (plant); deficiency causes anemia; toxicity can damage organs.

• Vitamin C enhances non-heme iron absorption.

Weight Management and Energy Balance

Obesity and Chronic Disease

• Obesity increases risk for heart disease, type 2 diabetes, hypertension, and certain cancers.

Waist Circumference Guidelines

• Women: >35 inches indicates increased risk.

• Men: >40 inches indicates increased risk.

Body Mass Index (BMI)

• Calculated as:

• Limitations: Does not distinguish between muscle and fat, may misclassify athletes or elderly.

Mortality Risk and BMI

• Both underweight and obese categories are associated with increased mortality risk.

Body Fat Distribution

• Subcutaneous fat: Under the skin; less health risk.

• Visceral fat: Around organs; higher risk for metabolic diseases.

• Apple shape: More visceral fat; higher risk.

• Pear shape: More subcutaneous fat; lower risk.

Body Fat Percentage

• Women naturally have higher body fat % than men due to reproductive needs.

Metabolically Healthy

• Having normal metabolic markers (e.g., blood pressure, glucose) despite higher body weight.

Energy Expenditure

• Basal metabolic rate (BMR): Energy for basic functions.

• Thermic effect of food (TEF): Energy for digestion.

• Physical activity: Energy for movement.

Factors Affecting Metabolic Rate

• Controllable: Physical activity, muscle mass, diet.

• Non-controllable: Age, genetics, sex, hormones.

Measuring Body Fat

Appetite vs Hunger

• Appetite: Psychological desire to eat.

• Hunger: Physiological need for food.

Hormonal Regulation

• Ghrelin: Stimulates hunger.

• Leptin, peptide YY: Promote fullness (satiety).

Gene-Environment Interaction

• Genetics and environmental factors (e.g., food availability, lifestyle) both influence obesity risk.

Calorie Deficit for Weight Loss

• To lose 1 pound of body fat, a deficit of approximately 3,500 kcal is needed.