Lesson 6.4 Lipid Transport, Storage, and Utilization

Introduction

  • Dietary lipids must be digested in the gastrointestinal tract and absorbed from the small intestine.
  • They are transported around the body to be either utilized by cells or stored for later use.
  • Since lipids are not water-soluble, they require mechanisms for transport through the body’s aqueous environment.

1. Lipoproteins Transport Lipids

  • Lipoproteins: Transport vehicles for moving water-insoluble lipids around the body.
  • They consist of four basic components:
    • Cholesterol
    • Triglycerides
    • Phospholipids
    • Proteins
  • Lipid Core: Interior of a lipoprotein carrying triglycerides and cholesterol.
  • Surface Coat: Exterior of lipoproteins composed of components that are at least partially water-soluble:
    • Proteins called apolipoproteins
    • Phospholipids
    • Cholesterol
  • Phospholipids are oriented with their water-soluble heads facing the exterior and fat-soluble tails inwards.
  • Apolipoproteins are amphipathic (soluble in both fat and water), aiding in lipid transport in the blood.

2. Types and Functions of Lipoproteins

  • All lipoproteins have the same structure but vary in component amounts, size, and function.

  • Four main types of lipoproteins:

    • Very Low-Density Lipoprotein (VLDL)
    • Intermediate-Density Lipoprotein (IDL)
    • Low-Density Lipoprotein (LDL)
    • High-Density Lipoprotein (HDL)

  • Naming convention derived from their density; protein is denser than lipids.

  • Comparison of lipoproteins based on density:

    • HDL has the highest protein content, thus is the most dense.
    • LDL contains less protein and more triglycerides compared to HDL.
    • VLDL and IDL are also carriers of cholesterol but not included in routine blood lipid panels due to measurement difficulties.

  • Emphasis on LDL and HDL:

    • Hyperlipidemia of LDL is often referred to as bad cholesterol.
    • HDL is often called good cholesterol, but remember both are carriers of lipids, not cholesterol.

3. Functions of Lipoproteins

  • Chylomicron Function:
    • Formed in the small intestine, absorbed into lymph vessels, delivered into the bloodstream.
    • Delivers triglycerides derived from digested food to cells for energy or storage as adipose tissue.
  • Triglyceride transfer:
    • Mediated by lipoprotein lipase that breaks down triglycerides into fatty acids and glycerol.
    • Fatty acids either oxidized for ATP generation or reassembled and stored as triglycerides.

4. Lipoprotein Transformation and Pathway

  • Lipid Transport from the Liver:
    • VLDL's primary job is triglyceride delivery to body’s cells.
    • As triglycerides diminish, VLDL transitions to IDL and then LDL, which primarily carries cholesterol.
    • Excess LDL can increase cardiovascular disease risk.
    • HDL's role is to return excess cholesterol from body cells to the liver for disposal.

5. Dietary Fat and Storage

  • Caloric Intake:
    • Fat storage is proportional to caloric intake versus expenditure.
    • Excess caloric consumption from any macronutrient will increase fat storage.
  • Caloric Density:
    • Fat has a caloric density of 9 kcal/g, whereas carbohydrates and proteins both have 4 kcal/g.
    • Attention to portion sizes on a high-fat diet is advised, due to potential health risks associated with fat types consumed.

6. Understanding a Blood Lipid Panel

  • Differentiation between Cholesterols:
    • Dietary cholesterol refers to cholesterol from animal foods.
    • Blood cholesterol refers to total cholesterol levels in the bloodstream, primarily through lipoproteins.
  • A blood lipid panel measures values of LDL, HDL, and triglycerides.
  • Comprehensive risk assessment for cardiovascular disease includes evaluating these values alongside other factors such as family history and lifestyle choices.

7. Misconceptions about Cholesterols

  • Terms like bad and good cholesterol can simplify the discussion too much.
  • Cholesterol is a uniform substance regardless of its source or transport mechanism.
  • Atherosclerosis Risk:
    • High levels of LDL may lead to the formation of arterial plaques, impeding blood flow and contributing to diseases such as:
    • Coronary artery disease
    • Carotid artery disease
    • Peripheral artery disease
    • Chronic kidney disease
  • Blocked arteries can result in serious health events such as heart attacks or strokes.

8. LDL Particle Size and Cardiovascular Risk

  • Physicians may conduct additional tests to assess LDL particle size and quantity.
  • Smaller, denser LDL particles are more atherogenic compared to larger, buoyant ones.
  • Individuals with smaller LDL particles could have a higher risk for heart disease despite identical total LDL readings.
  • Assessing LDL particle size is not routine for all patients but may be significant in those with diabetes or insulin sensitivity issues.

9. Updated Understanding of HDL

  • Initially considered the protector against cardiovascular disease, recent studies reveal that high HDL levels do not guarantee cardiovascular protection.
  • Raising HDL levels pharmacologically has not reduced heart disease incidents in clinical trials.
  • Evidence suggests that increasing HDL levels may not directly correlate with decreased cardiovascular risks, prompting a shift in focus primarily to lowering LDL cholesterol instead.