Macronutrients Quick Notes

Carbohydrates

  • Definition: Carbohydrates are organic compounds (saccharides—starches and sugars) composed of carbon, hydrogen, and oxygen; H:O ratio usually 2:1 as in H_2O. They range from simple sugars (3 ext{ to } 7 carbon atoms) to very complex polymers.
  • Major energy source: they provide the major source of energy for the body, supplying between 80\%\text{ to }100\% of calories.
  • Classification
    • Monosaccharides (simple sugars): absorbed directly without digestion.
    • Glucose (dextrose): principal form used by the body; abundant in fruits, corn, and corn syrup; formed mainly from starch digestion.
    • Fructose (levulose): sweetest; in honey and many fruits; converted to glucose in the body.
    • Galactose: not free in foods; produced from lactose; can be converted to glucose; galactosemia can occur in infants.
    • Disaccharides (double sugars): composed of 2 monosaccharides; must be hydrolyzed before absorption.
    • Sucrose: from cane/beet sugar; digested to glucose + galactose.
    • Lactose (milk sugar): in milk/milk products (except cheese); digested to glucose + galactose; cow milk ~4.8\%\text{ lactose}; human milk ~7\%.
    • Maltose: from starch hydrolysis; digested to glucose.
    • Polysaccharides (complex sugars): many monosaccharides; slower digestion.
    • Starch: major polysaccharide; sources include cereals, potatoes; digested to glucose; slower digestion.
    • Dextrins: intermediate products from starch breakdown.
    • Cellulose: plant framework; in unrefined grains/vegetables/fruits; non-digestible by humans; adds bulk and stimulates peristalsis; soluble vs insoluble varieties.
      • Soluble cellulose (fibers like in fruits/legumes, barley, oats): delays transit and glucose absorption; may lower blood cholesterol.
      • Insoluble cellulose (wheat bran, corn bran, whole grains): accelerates transit and increases fecal bulk.
    • Pectins: non-digestible, gel-forming polysaccharides; used in jellies; can absorb toxins and bacteria.
    • Glycogen (animal starch): stored in liver and muscle; liver glycogen release is hormonally regulated by glucagon.
  • Digestion
    • Mouth: amylase (salivary) initiates starch digestion.
    • Stomach: no significant carbohydrate digestion.
    • Small intestine: pancreatin/amylopsin digests starch to dextrins/maltose; disaccharidases (sucrase, lactase, maltase) hydrolyze disaccharides to monosaccharides (glucose, galactose, fructose).
  • Functions
    • Primary energy source; must be supplied regularly.
    • Protein-sparing: prevents use of protein for energy.
    • Essential for fat metabolism; insufficient carbs lead to ketone production.
    • Fiber (cellulose) aids elimination and GI health; laxative/slowing/accelerating transit depending on type.
    • Glucose is crucial for brain and nerve tissue; constant supply needed.
  • Carbohydrate stores and energy
    • Body carbohydrate stores (example in 70 kg man):
    • Liver glycogen: 110\text{ g}
    • Muscle glycogen: 245\text{ g}
    • Blood glucose: 20\text{ g}
    • Total: 375\text{ g} ≈ 1460\text{ kcal}, enough for ~13\text{ hours} of moderate activity.
  • Sources
    • Whole grains (iron, thiamine, niacin, etc.), potatoes, milk (lactose source), sugars/sweets; note "empty calories" when foods provide carbs with little other nutrients.
  • Health effects of starch and fibers
    • Weight control: fiber-rich complex carbs tend to be low in fat/sugars and aid weight loss.
    • Heart disease: high-carb diets rich in whole grains can protect against heart disease/stroke.
    • Cancer: high-carb diets may help prevent various cancers.
    • Diabetes: high-carb, low-fat diets help with weight control.
    • GI health: dietary fiber supports large intestine health.

Fats

  • Definition: Fats (lipids) are organic compounds rich in carbon/hydrogen/oxygen that provide energy and structural functions. Triglycerides (fats/oils) are the main fats and are glycerol esters of three fatty acids.
  • Energy role: fats are a concentrated energy source, providing more than twice the energy of carbohydrates and account for about 34\% of the body's energy reserves.
  • Classification
    • Simple lipids (neutral fats): triglycerides; waxes (esters of fatty acids with high-molecular-weight alcohols).
    • Compound lipids: phospholipids, glycolipids, lipoproteins.
    • Derived lipids: from fat digestion or breakdown; include fatty acids, glycerol, cholesterol.
  • Phospholipids
    • Lecithins: emulsifiers; abundant in liver/egg yolk; used in foods.
    • Cephalins: involved in blood clotting.
    • Sphingomyelins: in brain/nerve tissue; insulate nerve fibers.
  • Glycolipids
    • Cerebrosides: nerve membranes; contain galactose.
    • Gangliosides: contain glucose/galactose/amino sugars.
  • Lipoproteins
    • Lipids bound to proteins; transport cholesterol, fats; found in membranes and organelles; insoluble in water.
  • Derived lipids
    • Fatty acids: basic fat building blocks; can be saturated or unsaturated.
    • Saturated: no hydrogen added; examples ext{palmitic}, ext{stearic}; common in animal fats.
    • Monounsaturated (MUFA): 1 double bond; example ext{oleic acid}; found in olive/peanut oils; can improve cholesterol profile.
    • Polyunsaturated (PUFA): 2+ double bonds; examples ext{linoleic acid} (Omega-6) and ext{linolenic acid} (Omega-3); cannot be synthesized; essential in diet.
  • Omega-6 and Omega-3 ratio
    • Plant sources provide Omega-6; more Omega-3 recommended from plant/marine sources (e.g., flaxseed, canola, walnuts, soy, fish oils).
  • Digestion
    • Mouth: no significant fat digestion.
    • Stomach: minimal enzymatic activity on fats.
    • Small intestine: bile emulsifies fats; pancreatic lipase hydrolyzes triglycerides to fatty acids and glycerol; products form micelles for absorption.
  • Functions
    • Energy source; energy storage.
    • Protein-sparing (fat provides energy so protein can be used for tissue repair).
    • Thermal insulation and protection of organs; cellulite and fat layers cushion organs.
    • Absorption of fat-soluble vitamins (A, D, E, K).
    • Satiety and palatability.
  • Food sources
    • Visible fats: lard, butter, margarine, shortenings, oils.
    • Invisible fats: present in dairy, eggs, nuts, meat.
    • Margarine/shortenings are often hydrogenated; may contain trans fats; some variants are trans-fat-free.
  • Trans fats
    • Artificial fats formed by partial hydrogenation; raise LDL and lower HDL; increase triglycerides; associated with higher heart disease risk.
    • Naturally occurring trans fats exist in some meat/dairy but usually smaller contribution.
    • Public health measures: labeling/regulation; some products now trans-fat-free.
  • Reducing fat intake
    • Eat moderate portions; favor natural foods with healthy fats; avoid hydrogenated/trans fats; include fruits, vegetables, whole grains, and lean proteins.
  • Cholesterol in foods
    • Present in animal products; plant foods have no cholesterol.
    • Risk-based limits: with risk factors, limit to 200\text{ mg/day}; without risk factors, limit to 300\text{ mg/day}.
    • Tables summarize cholesterol content in dairy, fats, and meats.
  • Essential fatty acids
    • Linoleic acid (Omega-6): vegetable oils; can be converted to arachidonic acid.
    • Linolenic acid (Omega-3): flaxseed/canola/walnuts/soy; can be converted to EPA/DHA; important for growth, heart health, and more.

Proteins

  • Definition and nature
    • Proteins consist of amino acids; contain nitrogen in addition to C, H, O; built from amino acids joined by peptide bonds; digestion yields amino acids.
  • Classification
    • Simple proteins: yield only amino acids on hydrolysis (e.g., albumins, globulins, glutelins, prolamins, albuminoids, histones/protamines).
    • Compound (conjugated) proteins: proteins linked with non-protein prosthetic groups (e.g., nucleoproteins, mucoproteins/glycoproteins, lipoproteins, phosphoproteins, chromoproteins, metalloproteins).
    • Derived proteins: products formed during hydrolysis.
  • Essential vs non-essential amino acids
    • Essential (must be supplied in the diet): ext{threonine, lysine, leucine, methionine, isoleucine, phenylalanine, valine, tryptophan}.
    • Non-essential: can be synthesized by the body (e.g., glycine, cystine, alanine, tyrosine, aspartic acid, serine, glutamic acid, arginine, proline, histidine, hydroxyproline).
  • Complete vs incomplete proteins
    • Complete: contain all essential amino acids in sufficient quantities (mainly animal-derived: meat, milk, cheese, eggs).
    • Incomplete: deficient in one or more essential amino acids (plant sources: grains, legumes, seeds, nuts).
  • Digestion
    • Mouth: no enzyme; mechanical chewing only.
    • Stomach: pepsin (activated from pepsinogen by HCl) converts proteins to proteoses/peptones; rennin in infants coagulates casein.
    • Small intestine (pancreatic/intestinal enzymes): trypsin, chymotrypsin, carboxypeptidase; aminopeptidases and dipeptidases in the intestinal wall convert polypeptides to peptides and amino acids.
  • Functions
    • Tissue repair and growth (anabolism) and energy source when needed.
    • Build new tissues; some hormones/secretions are protein or amino acid–based.
    • Maintain osmotic balance among body fluids.
  • Sources
    • Complete protein foods: meat, fish, poultry, eggs, milk, cheese.
    • Legumes and nuts; breads and cereals.
  • Protein requirements
    • Quality matters (biological value, BV; net protein utilization, NPU; protein efficiency ratio, PER).
    • General daily recommendation (Food and Nutrition Board):
    • Adult: 0\text{-}9 \text{ g/kg BW}
    • Children: needs vary with age/growth
    • Pregnancy: additional 30\text{ g} above non-pregnant needs
    • Lactation: additional 20\text{ g} above non-pregnant needs
  • Health effects of protein
    • Higher animal-protein intake linked with higher saturated fat intake and potential heart disease risk.
    • Some associations between high animal-protein intake and certain cancers.
    • Excess protein can affect bone/calcium balance and may influence kidney workload; high-protein diets require adequate fluid intake.
  • PEM (Protein-Energy Malnutrition)
    • Acute PEM: thin for height in children.
    • Chronic PEM: short for age.
    • Marasmus: severe deprivation of both protein and energy.
    • Kwashiorkor: rapid protein deficiency often with illness (e.g., measles).
  • Notes on protein sources and usage
    • Emphasize a mix of complete proteins and complementary plant proteins to ensure all essential amino acids are present.