Lipids Flashcards

Lecture Objectives

• Outline the four major types of fatty acids with detailed chemical structures and properties.

• Discuss the types of essential fatty acids, their synthesis in the body, and specific health benefits, including mechanisms of action.

• Explain the benefits of including plant sterols in the diet, focusing on their mechanism of action in cholesterol reduction and recommended intake levels.

• Discuss the role of lipoproteins (VLDL, LDL, HDL) in regulating blood lipids, including their composition, function, and metabolism.

• Discuss the impact that trans and saturated fats have on our health, detailing the specific health risks and providing recommendations for dietary intake.

Functions of Lipids

• Palatability of food: Lipids enhance the sensory qualities of food by providing unique flavors and textures.

• Cell membranes, nerve fibres (myelin), and gene activation: Lipids are integral structural components of cell membranes, maintain nerve fiber insulation, and participate in gene activation processes.

• Energy: Lipids are a concentrated source of energy, offering unlimited storage capacity in adipose tissue.

• Insulation and protective padding: Lipids provide thermal insulation and protect vital organs through cushioning.

• Supply of essential fatty acids: Lipids deliver essential fatty acids that the body cannot synthesize, crucial for various physiological functions.

• Hormone production: Lipids are precursors to steroid hormones, which regulate numerous bodily functions.

• Absorption and supply of fat-soluble vitamins: Lipids facilitate the absorption and transport of fat-soluble vitamins (A, D, E, K).

Fatty Acids

• Fatty acids are the fundamental building blocks of lipids, composed of hydrocarbon chains with a carboxyl group at one end.

• They exhibit a range of chain lengths from 4 to 24 carbons, influencing their physical and metabolic properties.

• Most dietary fats consist of long-chain fatty acids known as triglycerides, which are composed of a glycerol molecule esterified with three fatty acids.

• Triglycerides serve as the body's primary storage form of fat, efficiently storing energy in adipose tissue.

Types of Fatty Acids

• Saturated Fatty Acids:

  • Predominantly found in animal fats, characterized by single bonds between carbon atoms.

  • Solid at room temperature (e.g., lard, butter, dripping) due to their straight-chain structure allowing tight packing.

• Monounsaturated Fatty Acids:

  • Abundant in nuts, seeds, avocado, canola oil, and olive oil, featuring one double bond in their hydrocarbon chain.

• Polyunsaturated Fatty Acids:

  • Present in plants, fish, grains, and cereals, distinguished by multiple double bonds in their hydrocarbon chains.

  • Liquid at room temperature due to the kinks introduced by double bonds that prevent tight packing.

  • Includes essential fatty acids necessary for human health.

• Trans Fatty Acids:

  • Artificially produced during the hydrogenation of liquid fats (e.g., margarine), resulting in a trans configuration around the double bond.

Essential Fatty Acids

• The human body is capable of synthesizing most fatty acids, except for two essential subtypes:

  • Linoleic acid and arachidonic acid (omega-6 fatty acids), which are precursors for eicosanoids involved in inflammation and immune response.

  • Linolenic acid, EPA (eicosapentaenoic acid), and DHA (docosahexaenoic acid) (omega-3 fatty acids), known for their anti-inflammatory and cardioprotective effects.

• Consumption of omega-3 fatty acids is associated with reduced risk of heart disease, hypertension, arthritis, osteoporosis, and type II diabetes.

• The dietary ratio of omega-6 to omega-3 fatty acids is a crucial determinant of health benefits, with an ideal ratio of 2:1 promoting optimal physiological function.

• Western countries typically exhibit imbalanced omega fatty acid consumption, with ratios often ranging from 10-15:1.

Omega-3 Fatty Acids

• Linolenic Acid:

  • Function: Reduces triglycerides, lowers LDL-cholesterol, and produces prostaglandins that reduce inflammation, dilate blood vessels, and inhibit blood clotting.

  • Food Sources: Fats and oils (walnut, soybean, canola, linseed), vegetables (soybeans), nuts and seeds (flaxseed).

• EPA and DHA:

  • Function: Reduce triglycerides, decrease LDL-cholesterol, and produce prostaglandins that reduce inflammation, dilate blood vessels, and inhibit blood clotting. Also crucial for brain development and cognitive function.

  • Food Sources: Can be derived from Linolenic acid; fish from cold, deep oceans (salmon, mackerel, tuna, herring, sablefish, flounder, sardines), shellfish (shrimp, oysters).

Omega-6 Fatty Acids

• Linoleic Acid:

  • Function: Essential for maintaining cell membrane integrity and supporting immune responses through eicosanoid production.

  • Food Sources: Leafy vegetables, seeds, nuts, grains and legumes, vegetable oils (cottonseed, safflower, corn, sesame, sunflower), evening primrose oil, spirulina and lecithin.

• Arachidonic Acid:

  • Function: Vital for cell membrane structure and immune response. However, excessive intake can lead to the production of prostaglandins that strongly increase inflammation, constrict blood vessels, and promote blood clotting.

  • Food Sources: Can be derived from Linoleic acid; meats.

Phospholipids

• Functions:

  • Essential components of cell membranes, forming a lipid bilayer that regulates membrane permeability and fluidity.

  • Facilitate lipid transport as part of lipoproteins, encapsulating hydrophobic lipids for transport in the bloodstream.

  • Act as emulsifiers, stabilizing mixtures of lipids and water, crucial for digestion and absorption.

• Food sources: Egg yolks, liver, soybeans, peanuts.

Sterols: Cholesterol

• Functions:

  • Integral component of cell membranes, modulating membrane fluidity and stability.

  • Precursor to other substances, including sterol hormones (e.g., cortisol, estrogen, testosterone), vitamin D, and bile acids, which are essential for various physiological processes.

  • Synthesized in the liver, contributing to endogenous cholesterol levels.

• Food Sources: Exclusively found in animal foods.

Plant Sterols

• Naturally occurring compounds in plants and oils, playing a key role in forming cell membrane structure, analogous to cholesterol in animals.

• Plant sterols effectively lower blood cholesterol levels by competitively inhibiting the absorption of dietary cholesterol in the small intestine.

• To achieve a significant cholesterol-lowering effect, a daily intake of at least 2 grams is recommended (e.g., found in 2 Weet-Bix ® cholesterol-lowering biscuits).

• Most individuals typically consume between 150-360mg/day, contingent on their dietary habits.

Lipoproteins

• Very-low-density lipoproteins (VLDL): Primarily deliver triglycerides from the liver to cells throughout the body.

• Low-density lipoproteins (LDL): Transport cholesterol from the liver to cells, contributing to cholesterol deposition in arterial walls.

• High-density lipoproteins (HDL): Scavenge cholesterol from cells and arterial walls, facilitating its transport back to the liver for excretion or recycling.

Guidelines for Daily Fat Intake

• Low-fat diets: 30 – 40g

• Most women and children: 30 – 50g

• Most men: 40 – 60g

• Teenagers and active children: 70g

• Larger and very active athletes / workers: 80 – 100g

Fat Content in Food

• 2 chocolate biscuits: 6g

• 1 small chocolate bar (40g): 12g

• 1 Danish pastry: 14g

• 1 croissant: 23g

• 1 packet of potato chips (50g): 18g

• 1 regular muesli bar: 6g

• 1 meat pie: 24g

• 1 sausage roll: 23g

• 2 slices of cheddar cheese (40g): 14g

• 1 chicken-filled bread roll: 10g

• 1 McDonald’s hamburger and regular fries: 28g

• 1 slice of pizza: 10g

• 1 piece of battered fish and 1 scoop of chips: 63g

• 2 pieces of KFC chicken thighs, regular fries: 80g

Lipids and Health

• Diets high in total fat are more likely to promote weight gain and lead to obesity due to the high caloric density of fats.

• High amounts of trans fat intake raise LDL cholesterol, lipoprotein (a), and triglycerides, thereby increasing the risk of atherosclerosis and coronary heart disease (CHD).

• A diet high in trans fatty acids is also associated with an elevated risk of insulin resistance and diabetes in humans.

• The prevailing health belief that low-fat high-carbohydrate diets reduce CHD is not consistently supported by robust scientific evidence.

• Findings from a 14-year follow-up from the Nurses’ Health Study indicate that replacing saturated and trans fat with unhydrogenated unsaturated fats is more effective in preventing CHD than merely reducing overall fat intake.

• Hu, FB et al. (2001) Types of dietary fat and risk of coronary heart disease: A critical review. J Am Coll Nutr. 20(1): 5-19.

Dietary Recommendations for Reducing CHD Risk

A diet that:

1. Is abundant in whole grains and other minimally processed carbohydrates;

2. Includes moderate amounts of fats (\sim 30-40\% of total energy), with an emphasis on unsaturated fats and omega-3 polyunsaturated fats from seafood and plant sources;

3. Limits refined grains and carbohydrates;

4. Excludes packaged foods, baked goods, and fast foods containing trans fatty acids

…will likely reduce the risk of CHD.

Mozaffarian, D.(2005) Effects of dietary fats versus carbohydrates on coronary heart disease: a review of the evidence. Curr Atheroscler Rep. 7(6): 435-45.

Trans Fats in Our Diet

Foods that contain trans fats include:

• Margarine

• Instant noodles and soups

• Fast food (mainly fries)

• Baked goods (muffins, pizza, pies, crackers)

• Pre-mixed cake mixes, pancake mixes, and chocolate drink mixes

• Biscuits and cakes

• Donuts

• Snacks (potato and corn chips and packaged or microwave popcorn)

Summary

• Fatty acids are categorized as saturated, monounsaturated, polyunsaturated, or trans fatty acids, each with distinct chemical structures and health implications.

• The essential omega-3 fatty acids provide significant health benefits and should be consumed in a balanced ratio with omega-6 fatty acids on a daily basis to promote overall well-being.

• Consumption of plant sterols can effectively reduce blood cholesterol levels, especially when combined with a reduction in saturated fat intake.

• Lowering LDL-cholesterol and increasing HDL-cholesterol are crucial for maintaining cardiovascular health and overall well-being.

• Limiting the intake of trans fats has the most substantial impact on reducing the risk of CHD.