lipids

Fundamentals of Nutrition - Chapter 5: Lipids Part II

Lipid Transport

Chylomicrons

• Definition: Chylomicrons are large lipoproteins that serve as transport vehicles for lipids.

• Function: They transport a significant amount of triglycerides (TGs) to peripheral tissues, which include:

  • Adipose Tissue

  • Muscle

  • Liver

• Process: As the triglycerides are delivered to various tissues, chylomicrons decrease in size;

  • The smaller remnants return to the liver where they are repackaged into very low-density lipoproteins (VLDLs) containing triglycerides.

Very Low Density Lipoproteins (VLDL)

• Definition: VLDLs are lipoproteins synthesized in the liver.

• Composition: They are rich in triglycerides, but contain less than chylomicrons.

• Function: VLDLs transport triglycerides to body tissues similar to chylomicrons, namely:

  • Adipose Tissue

  • Muscle

  • Liver

• Outcome: After triglyceride delivery, VLDLs shrink and transition into low-density lipoproteins (LDLs).

Low Density Lipoproteins (LDL)

• Definition: LDLs are formed from VLDLs and have a lower triglyceride content.

• Composition: Primarily composed of cholesterol.

• Function: LDLs transport cholesterol to cells for various crucial functions, including:

  • Hormonal synthesis

  • Bile production

  • Construction of new cell membranes

• Health Implications: Excessive levels of LDLs in the bloodstream can pose health risks which will be elaborated on later.

High Density Lipoproteins (HDL)

• Definition: HDLs are lipoproteins produced in the liver; they are not derived from LDLs.

• Function: They play a role in scavenging cholesterol from cells and transporting it back to the liver for recycling or disposal.

• Health Implications: Having low levels of HDL can diminish protective cardiovascular benefits.

Triglyceride Metabolism

Fed State

• Definition: This state occurs after meals when there's an abundance of triglycerides (TGs).

• Energy Use Options: Triglycerides can either be stored for later use or utilized immediately.

  1. Source of TGs: Originating from chylomicrons and VLDLs.

  2. Breakdown: Lipoprotein Lipase (LPL), secreted from adipose and muscle tissues, breaks down triglycerides, releasing free fatty acids (FFAs) and glycerol.

  3. Tissue Utilization: FFAs are used for energy in:

  • Muscle: Direct energy use.

  • Adipose Tissue: FFAs are re-esterified back into triglycerides for energy storage.

Fasted State

• Definition: This refers to a low energy availability state occurring between meals or overnight.

• Energy Supply: It provides about 60% of energy at rest.

  1. Trigger Factors: Low glucose levels lead to low insulin and high glucagon, activating Hormone Sensitive Lipase.

  2. Breakdown Process: This enzyme breaks down triglycerides into FFAs and glycerol.

  3. Circulation: FFAs and glycerol then enter circulation:

  • Utilized by liver and muscle for energy.

  • Some FFAs can be converted into ketone bodies, but FFAs cannot be converted into glucose.

Main Roles of Body Lipids

• Lipids serve numerous functions, including:

  • Long-term energy storage.

  • Facilitation of fat-soluble vitamin absorption.

  • Organ protection through insulation.

  • Structural components of cells.

  • Precursors for hormone production.

  • Aiding digestion.

Blood Lipids and Health Risks

Blood Lipid Profile

• Definition: Refers to the lipids that circulate within the bloodstream, predominantly triglycerides and total cholesterol (sum of LDL and HDL).

• Health Outcomes: Abnormal blood lipid levels can be associated with:

  • Cardiovascular disease (CVD)

  • Stroke

High HDL Considerations

• Evaluating HDL is crucial as high levels often suggest protection against heart disease. Conversely, combinations of high levels of LDL and triglycerides, combined with low HDL levels, can be detrimental.

• Recommended Blood Lipid Levels:

  • Total Cholesterol: < 200 mg/dL

  • LDL Cholesterol: < 100 mg/dL

  • Triglycerides: < 150 mg/dL

  • HDL Cholesterol: > 60 mg/dL

Dietary Lipids and Health Risks

Effect of Dietary Lipids on Blood Lipid Composition

• Dietary Components:

  • Cholesterol

  • Saturated Fat

  • Trans Fat

• Impacts on lipid levels, particularly HDL, LDL, and triglycerides, increasing CVD risk.

Cholesterol Regulation and Interactions

• The liver plays a critical role in managing cholesterol levels through:

  • Recycling of bile acids and lipoproteins.

  • Regulatory actions influenced by dietary cholesterol.

• Excessive saturated fat intake can disrupt this regulatory balance.

Dietary Lipids and Health Promotion

Effects of Dietary Lipids on Blood Lipids

• Key types of dietary fats:

  • Monounsaturated Fatty Acids (MUFAs)

  • Polyunsaturated Fatty Acids (PUFAs)

• Special Functions of PUFAs:

  • Omega-3: beneficial for brain, eyes, and immune function (anti-inflammatory).

  • Omega-6: impacts skin and immune function (pro-inflammatory).

Dietary Reference Intakes (DRIs) for Fat

Current Recommendations and Intake Trends

• DRIs for Total Fat (Adults):

  • RDA: Not applicable

  • Acceptable Macronutrient Distribution Range (AMDR): 20-35%

• Habitual Intake: Most adults consume approximately 88 g/day, which surpasses the upper limit even without established recommended intake.

• Issues: Focus should not just be on total fat but the quality and types of fats consumed,

• Specific Total Fat Guidance:

  • Lower range:

  • 2000 kcal diet example:

    • 20% AMDR = 400 kcal, which is ~44 g (9 kcal/g),

    • 35% AMDR = 700 kcal, which is ~78 g (9 kcal/g).

Omega-3 Recommendations

• Recommendation highlights:

  • Increased emphasis on omega-3 intake rather than lowering omega-6.

• Current Intake Trends:

  • Males: 1 g/day for Omega-3, 22 g/day for Omega-6 (20:1 ratio).

  • Females: 1 g/day for Omega-3, 18 g/day for Omega-6 (18:1 ratio).

Recommendations based on Dietary Guidelines for Americans (DGAs)

Saturated Fat and Cholesterol

• Definitions:

  • Saturated fats: high in saturated fatty acids, lacking double bonds;

  • Aim to reduce trans fat to as close to 0% as possible;

  • Decrease saturated fat intake to < 10% of total daily calories;

  • Limit dietary cholesterol.

• Rationale: Lowering saturated fats is critical as most high-cholesterol foods are correlated with high saturated fats.

Food Label Comparisons and Healthiness Evaluation

• Practical exercise involving analysis of food labels to determine healthier options based on fat content and quality.

Dietary Fats - Sources and Recommendations

Top Sources of Saturated Fats

• The United States does not meet recommended levels of saturated fat intake, targeting around 12% daily, approximately 240 kcal/day.

Dietary Intake Patterns for Key Food Groups

• Fruit/Veggies: Generally low in fat, with exceptions being avocados and olives.

• Dairy: Preference towards low-fat options without added sugars.

• Protein Foods: Choose a variety of lean meats, avoiding fatty processed foods.

Oils and Cooking Recommendations

• Cooking Oils: Emphasizes the importance of selecting healthy oils (higher in MUFAs & PUFAs) for cooking and dietary intake.

Wrap-up/Reminders

• Important course reminders include upcoming classes, assignments, and assessments.