Lipids: Introduction & Classification

Lipids are organic substances insoluble in water but soluble in organic solvents like chloroform, ether, and benzene.
Lipids serve various functions, including energy storage, insulation, and structural components of cell membranes.
They also act as precursors for bile acids, steroid hormones, and vitamin D, and help in transporting fat-soluble vitamins.
Lipids are classified into three main categories: simple, complex, and derived.

Simple lipids are esters of fatty acids (FAs) with various alcohols.
Subcategories of simple lipids include:
- Neutral fats (or oils): The alcohol is glycerol.
- Waxes: The alcohol is other than glycerol.
Neutral fats and oils: Examples include triglycerides, made up of glycerol and three fatty acid chains. Different fatty acid compositions make those "mixed triglycerides."
Waxes are esters of FAs with higher molecular weight monohydric alcohols; examples include lanolin, beeswax, and whale sperm oil.
Waxes are not hydrolyzed and lack nutritional value. They are important for protection and waterproofing

Complex lipids are esters of FAs with alcohols containing additional prosthetic groups.
Subcategories of complex lipids include:
- Phospholipids
- Glycolipids
- Lipoproteins

Derived lipids are formed from the hydrolysis (breakdown) of simple and compound lipids.
They maintain the characteristics of lipids.
Subcategories of derived lipids include:
- Fatty acids
- Steroids
- Cholesterol
- Lipid-soluble vitamins
- Hormones
- Ketone bodies

Lipid peroxidation is the oxidative degradation of lipids.
This process involves free radicals "stealing" electrons from lipids in cell membranes, causing cell damage.
The process follows a free radical chain reaction mechanism, often affecting polyunsaturated fatty acids.
The byproducts of peroxidation can be mutagenic and carcinogenic.

A free radical is an atom or molecular fragment with an unpaired electron.
Free radicals are highly reactive, damaging cell membranes, proteins, and DNA.
Free radicals are naturally produced during metabolism but excessive amounts are harmful.

Free radical formation happens when heat or light causes homolytic cleavage of molecules

Free radicals easily react with biological molecules, generating new radicals and initiating chain reactions.
Polyunsaturated fatty acids (PUFAs) in cell membranes are vulnerable to peroxidation by free radicals.
Oxygen-involved free radicals are known as reactive oxygen species (ROS).

Exogenous sources: foods, air pollutants, radiation, cigarette smoke
Endogenous sources: metabolism (mitochondria and peroxisomes), detoxification (cytochrome P450), immune cells

Photoxidation: involves singlet oxygen and sensitizers (porphyrins, myoglobin, riboflavin, bilirubin)
Enzymatic oxidation: cyclooxygenase and lipoxygenase catalyze reactions between oxygen and polyunsaturated fatty acids
Autoxidation: free radical chain reaction

ROS are oxygen-containing free radicals and non-radicals like hydrogen peroxide, hypochlorous acid, singlet oxygen, ozone, and triplet oxygen.

Oxidative stress occurs when there's an imbalance between free radicals and antioxidants in the body, potentially leading to damage to cells and tissues.
Oxidative stress is a natural part of aging.

Structural changes in cell membranes: alteration of membrane fluidity, bound signaling proteins, and permeability, leading to damage and loss of integrity.

Inflammation can lead to a "vicious cycle" of damage with immune cells releasing oxidants to kill bacteria, but those oxidants also damage tissues.

Consuming antioxidant-rich foods (vitamins A, C, E, selenium, phytonutrients, and polyphenols) can neutralize free radicals, minimizing cellular damage.

Antioxidants prevent or delay oxidation by interfering with free-radical chain reactions and converting them into less reactive forms.
Antioxidants can be fat-soluble or water-soluble and examples include:
- Vitamin E, beta-carotene, coenzyme Q10
- Vitamin C, glutathione peroxidase, superoxide dismutase, catalase

Ideal antioxidants are effective at low concentrations, non-toxic to the body, and add no undesired flavor, color, or odor to foods. They must maintain their effectiveness after processing.