Types and Functions of Lipids

Phospholipids: Builders and Emulsifiers

While triglycerides are built for storage, phospholipids are built for function and structure. Like triglycerides, they are built on a glycerol backbone, but they are distinguished by the addition of phosphorus compounds.

Key Functions

Phospholipids serve two vital roles in the body:

• Cell Membrane Component: They form the fundamental bilayer that makes up the structure of cell membranes.
• Emulsifiers: An emulsifier is a substance that helps mix fats with water. Because lipids and water are incompatible, emulsifiers are necessary to allow them to blend smoothly.

Common examples of phospholipids include lecithin (often found in egg yolks and soybeans) and bile acids. In the digestive system, bile acids act as emulsifiers to break large fat globules into smaller ones, which increases the surface area for digestive enzymes like lipase to work more effectively.

Exam Tip: Remember that while phospholipids are found in many foods like soybeans and wheat germ, they are not essential dietary components because the human body can synthesize them as needed.

Sterols: The Multi-Ringed Waxy Substances

Unlike the previous two types, sterols do not have a glycerol backbone and do not consist of fatty acid chains. Instead, they possess a unique multi-ringed structure and are characterized as waxy substances.

Cholesterol

The most well-known sterol is cholesterol. It is produced by the liver in both animals and humans. Cholesterol is essential for several bodily functions:

• It is a structural component of cell membranes.
• It serves as a precursor for the synthesis of important hormones, such as estrogen, testosterone, and vitamin D.
• It is a building block for bile acids.

Because the body can produce all the cholesterol it requires, it is not considered an essential dietary component. Dietary cholesterol is found only in animal products, such as meat, eggs, and whole milk.

Phytosterols

Phytosterols are plant-derived sterols found in sources like nuts, seeds, grains, and vegetable oils. They offer a unique health benefit: they can help lower blood cholesterol levels by competing with cholesterol for absorption sites in the small intestine.

Hydrogenation and Trans Fatty Acids

The structure of lipids can be chemically altered through a process called hydrogenation. This is a common industrial process used to turn liquid oils into solid fats to improve texture and increase shelf life.

The Process and the Risk

During hydrogenation, hydrogen is added to the carbon-to-carbon double bonds of unsaturated fatty acids. While this makes the fat more stable, "partial hydrogenation" can create trans fatty acids.

Trans fatty acids are significantly harmful to cardiovascular health because they disrupt the balance of lipoproteins in the blood:

• They increase LDL (low-density lipoprotein), often called "bad" cholesterol.
• They decrease HDL (high-density lipoprotein), often called "good" cholesterol.

This combination increases the overall risk of cardiovascular diseases, such as heart attacks and strokes.

Health Summary: To protect your heart, limit the intake of hydrogenated fats, deep-fried foods, and high-fat baked goods.