Macronutrients Quick Review

Atoms & Molecules

  • Human body uses ≈3\% of the 118 known elements
  • Major atomic composition: O=65\%, C=18\%, H=10\%, N=3\%
  • Molecules = two or more bonded atoms; adding N to C, H, O forms proteins

Carbohydrates (CHO)

  • Basic formula for simple sugars: C6H{12}O_6
  • Categories
    • Monosaccharides: glucose (blood sugar), fructose (fruit sugar), galactose (part of lactose)
    • Disaccharides: sucrose (glu+fru), lactose (glu+gal), maltose (glu+glu)
    • Oligosaccharides: 3\text{–}9 linked units (legumes)
    • Polysaccharides: starch & fiber (plants), glycogen (animals)
  • Starch forms: amylose & amylopectin; main dietary CHO
  • Fiber: non-starch plant polysaccharide; no kcal; lowers obesity, insulin resistance, dyslipidemia, hypertension, etc.
    • Daily fiber (g): children 1\text{–}3 yr 19, men 19\text{–}50 yr 38, women 19\text{–}50 yr 25
  • Glycogen: storage CHO in muscle & liver; max ≈15\,g·kg^{-1} BW
  • Key conversions
    • Glucogenesis: \text{glucose}\rightarrow\text{glycogen}
    • Glycogenolysis: \text{glycogen}\rightarrow\text{glucose}
    • Gluconeogenesis: \text{protein}\rightarrow\text{glucose}
  • Hormonal control: insulin ↓ blood glucose; glucagon ↑ blood glucose via glycogenolysis & gluconeogenesis
  • Intake targets: sedentary 40\text{–}50\% kcal (≈300\,g for 70\,kg), active ≈60\% kcal (400\text{–}600\,g)
  • Four metabolic roles: primary energy, protein-sparing, metabolic primer (prevents ketosis), sole CNS fuel

Lipids

  • H:O ratio \approx18.3{:}1 (CHO 2{:}1) → higher energy density ( 9\,kcal·g^{-1} )
  • Classes
    • Simple: triglycerides (neutral fats), waxes
    • Compound: phospholipids, glycolipids, lipoproteins
    • Derived: fatty acids, steroids, hydrocarbons
  • Fatty acids
    • Saturated: no double bonds (mainly animal)
    • Monounsaturated: one double bond
    • Polyunsaturated: \ge2 double bonds (includes essential \omega\text{-}3, \omega\text{-}6)
  • Lipoproteins
    • Chylomicrons: dietary TG transport
    • VLDL: TG to muscle/adipose
    • LDL: cholesterol to tissues ("bad")
    • HDL: reverse cholesterol transport ("good")
  • Five key functions: energy reserve, organ protection, insulation, vitamin A/D/E/K transport, hunger suppression

Proteins

  • Energy yield 4\,kcal·g^{-1}; body stores 10\text{–}23\,kg mainly in muscle
  • Building blocks: 20 amino acids (AA); 9 essential (must be diet): histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine
  • Protein quality = biologic value (completeness of essential AA)
  • Vegetarian patterns: vegan, lacto-, ovo-lacto-; plant diets low in cholesterol, high fiber & unsaturated fat
  • RDA: adults 0.8\,g·kg^{-1} BW; athletes 1.2\text{–}1.8\,g·kg^{-1} (≤2.0\,g·kg^{-1})
  • Roles
    • Structural: muscle, connective tissue, skin, hair, nails
    • Functional: enzymes (≈2000 types), hormones, hemoglobin/myoglobin, electron carriers (NAD^+, FAD)
    • Acid–base buffering, plasma proteins (thrombin, fibrin)
  • Nitrogen balance
    • Positive: intake > excretion (growth, pregnancy, recovery)
    • Negative: excretion > intake (starvation, low CHO)
  • Metabolic pathways
    • Deamination (liver) → ammonia → urea
    • Transamination (muscle) shuttles N
    • Alanine-glucose cycle supplies liver with substrate to regenerate blood glucose during exercise