Chapter 5 – Lipids Comprehensive Bullet-Point Notes

The Basics

  • Definition: Lipids are compounds that are soluble in organic (non-polar) solvents but not in water—important because the human body is mostly water.
  • Table 5-1 “Usefulness of Fats” (high-level points)
    • Energy source (concentrated kcal)
    • Energy storage (adipose tissue)
    • Padding & insulation of organs
    • Cell membrane structure (phospholipid bilayer)
    • Raw material for compounds such as hormones, bile, vitamin D

Dietary Recommendations for Fat

  • General (Table 5-2)
    • Emphasize unsaturated fats; limit saturated & trans fats.
  • Polyunsaturated Fatty Acids (PUFA)
    • Linoleic acid ((\omega 6)): 5\text{–}10\% of total kcal.
    • Linolenic acid ((\omega 3)): 0.6\text{–}1.2\% of total kcal.
  • Cholesterol
    • DRI: “Minimize” within a healthy diet rather than meet a fixed ceiling.
    • Only about \frac13 of the population shows a marked blood-cholesterol drop when intake is lowered.
    • Nutritious, cholesterol-rich foods—eggs, shellfish, liver—should be limited but not banned.

Classification of Lipids

  • Triglycerides
    • \approx95\% of dietary & body fat.
    • Structure: Glycerol backbone + 3 fatty acids (FA) (Fig 5-3).
    • Glycerol can be synthesized from glucose or amino acids.
  • Fatty-acid chain length
    • Short < 10 C; Medium 12\text{–}14 C; Long > 16 C.
    • Shorter chains → softer, melt more readily.
  • Degree of saturation (Fig 5-5)
    • Saturated (no C=C), solid at room T (butterfat, coconut oil, palm oil).
    • Monounsaturated (1 C=C) – e.g., canola & olive oils.
    • Polyunsaturated (≥2 C=C) – liquid oils from plants & fish.

Processing Effects & Trans Fats

  • Hydrogenation (margarine manufacture, Fig 5-12)
    • Adds H to C=C, yielding “hard” fats that store better, resist rancidity, tolerate high frying temps.
    • Produces trans-fatty acids → behave biologically like saturated FA; raise CVD risk.
  • Regulatory status
    • Labels must list trans fats separately.
    • 2018: Health Canada banned partially hydrogenated oils.
  • Alternative: blending non-hydrogenated oils → “soft margarines” (e.g., Becel).

Other Lipids

  • Phospholipids
    • Structure: Glycerol + 2 FA + phosphate compound.
    • Dual solubility (water & fat) → natural emulsifiers (lecithin in egg yolk).
    • No special therapeutic properties claimed.
  • Sterols
    • Cholesterol in cell membranes & bile; precursor for vitamin D, sex hormones (estrogen, testosterone) & stress hormones (epinephrine).

Digestion, Absorption & Transport

  • Stomach: fat separates from watery chyme; minimal digestion.
  • Small intestine (Fig 5-6)
    • Bile from liver/GB emulsifies fat droplets.
    • Pancreatic lipase splits TG → monoglycerides + FA + glycerol.
  • Absorption (Fig 5-9)
    • Small lipids (short/medium FA & glycerol) enter portal blood directly.
    • Large lipids (long FA & monoglycerides) are packed into chylomicrons → lymph → bloodstream.
  • Lipoproteins
    • Chylomicron – transports dietary lipids.
    • VLDL – exports TG made in liver to cells.
    • LDL – delivers cholesterol to tissues (“bad”).
    • HDL – scavenges cholesterol back to liver for excretion (“good”).

Metabolism & Energy Use

  • At rest, liver & muscle burn a mix ≈ 50\% carbohydrate / 50\% lipid.
  • For FA oxidation, carbohydrate must be available; if not, incomplete breakdown → ketones accumulate in blood & urine.

Blood Cholesterol & CVD Risk

  • High LDL raises heart-disease risk; low HDL also harmful.
  • Key modifiable risk pattern: diet high in saturated & trans fats + low produce & whole-grains.
  • Lifestyle + low-fat, antioxidant-rich diet recommended.

Cholesterol – Nuances

  • Essential precursor; 75\text{–}80\% synthesized de novo.
    • Statin drugs inhibit internal synthesis.
  • Saturated fat intake stimulates endogenous synthesis more than dietary cholesterol itself.
  • ~60\% of people are “non-responders” to dietary cholesterol.

Essential Fatty Acids (EFA)

  • Only FA the body cannot synthesize:
    • Linoleic acid ((\omega 6))
    • Linolenic acid ((\omega 3))
  • Both are PUFA → converted into eicosanoids (local regulators of immunity, blood pressure, clotting, etc.).
  • Severe PUFA deficiency (rare) → skin lesions, growth & reproduction issues, impaired learning/vision, fatty liver, kidney disorders.

Omega-6 Fatty Acids

  • First C=C located 6 C from methyl end.
  • DRI: 5\text{–}10\% of kcal.
  • Food sources: vegetable oils, seeds, nuts, whole-grains (Table 5-5).

Omega-3 Fatty Acids & EPA/DHA

  • First C=C located 3 C from methyl end.
  • DRI: 0.6\text{–}1.2\% of kcal.
  • Food sources: flaxseed/-oil, canola oil, deep-sea fish.
  • n-6 : n-3 ratio should fall; modern diets high in n-6.
  • Recommendation: 2\text{–}3 fatty-fish meals/week for heart health (EPA & DHA). Fried fish loses EPA/DHA.
  • Research findings (Table 5-4)
    • Strongest support: lower CHD risk.
    • Promising: mental health, cognition.
    • Early evidence: some cancers.

Seafood: Risks vs Benefits (Mercury)

  • Most species carry low mercury; effect on existing CVD unclear.
  • High-mercury fish: shark, swordfish, king mackerel, fresh albacore tuna, tilefish.
  • Lower-mercury options: shrimp, canned light tuna, salmon, pollock, catfish.
  • Guidance
    • Eat a variety to dilute any single toxin.
    • Pregnant people & children: choose lower-mercury fish within advised amounts.

Major Dietary Sources of Fat (descending)

  • Dairy (cheese, ice cream, etc.)
  • Processed meat
  • Margarine
  • Salad & cooking oils (mainly unsaturated)
  • Bakery products, fresh meats, butter, fluid milk (sources of saturated fat except salad/oils)

Example Foods – g fat & % kcal (sample)

  • Ice cream (1 c, 357\,kcal): 24\,g, 61\% kcal
  • Ice milk (1 c, 182\,kcal): 6\,g, 30\% kcal
  • Cheddar (50 g, 205\,kcal): 16\,g, 70\% kcal
  • Low-fat mozzarella (50 g, 142\,kcal): 9\,g, 57\% kcal
  • Wieners for hot dog (2, 360\,kcal): 32\,g, 80\% kcal
  • Lean beef (4 oz, 250\,kcal): 11\,g, 40\% kcal
  • Mac & cheese (1 c, 430\,kcal): 22\,g, 46\% kcal
  • Spaghetti + tomato sauce (1 c, 260\,kcal): 9\,g, 31\% kcal

Defensive Dining & Fat Replacers (Tables 5-6, 5-7, 5-8; Fig 5-19)

  • Ingredient swaps: puréed fruit for oil, applesauce for fat in baking, yogurt for sour cream.
  • Cooking methods: bake, broil, grill instead of fry.
  • Add water/air, use non-fat milk, choose lean cuts.
  • Industry fat replacers & artificial fats exist but must also maintain palatability & nutrient density.

Controversy: “Is Butter Back?” – Lipid Guidelines Debate

  • Public media often simplifies or contradicts research.
  • Historic advice: cut total fat → unintended rise in refined carbs & sugars → diabetes/CVD.
  • Seven Countries Study
    • Sat-fat intake closely tracks CHD deaths; total fat less predictive.
    • Finland & Crete both high-fat; Finland had highest CHD deaths, Crete lowest (diet: mostly olive oil, sat-fat < 10\% kcal).
  • Current consensus
    • Replace saturated fats with unsaturated (especially PUFA) & avoid trans fats.
    • Health Canada claim: swapping sat-fat for MUFA/PUFA lowers cholesterol.
  • Ongoing questions
    • Exact biological mechanisms; genetic variability; role of specific FA types.
    • Some sat-FAs may be neutral; excess of certain PUFAs may be harmful.
  • Controversial meta-analysis found no sat-fat–CVD link → criticized for design flaws.
  • Practical takeaway: focus on overall “portfolio” eating patterns
    • Healthy vegetarian, Healthy American/Canadian, Mediterranean diets (Table C5-2).