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lipids

Wednesday 4 June 2003

Definition: Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others.

Lipids are broadly defined as any fat-soluble (lipophilic), naturally-occurring molecule. Lipids are usually defined as hydrophobic or amphipathic biological molecules that will dissolve in organic solvents such as benzene or chloroform.

The emulsion test is a crude method for determining the presence or absence of lipids in a given sample.

Although the term lipid is sometimes used as a synonym for fats, fats are a subgroup of lipids called triglycerides.

Lipids encompass molecules such as fatty acids and their derivatives (including tri-, di-, and monoglycerides and phospholipids), as well as sterol-containing metabolites such as cholesterol.

Although humans and other mammals use various biosynthetic pathways to both break down and synthesize lipids, some essential lipids cannot be made this way and must be obtained from the diet.

Functions

Lipids are a diverse group of compounds that have many key biological functions, such as acting as structural components of cell membranes, serving as energy storage sources and participating in signaling pathways.

The main biological functions of lipids include energy storage, as structural components of cell membranes, and as important signaling molecules.

Their main structural uses are as part of biological membranes such as the cell membrane, or as a source of energy.

Structure

Lipids may be broadly defined as hydrophobic or amphiphilic small molecules that originate entirely or in part from two distinct types of biochemical subunits or "building blocks": ketoacyl and isoprene groups.

Most lipids consist of a polar or hydrophilic head and one to three nonpolar or hydrophobic fatty acid tails, and therefore they are amphiphilic.

Types

Lipids are the most diverse group of biochemicals. Lipids encompass molecules such as fatty acids and their derivatives (including tri-, di-, and monoglycerides and phospholipids), as well as other sterol-containing metabolites such as cholesterol.

- The fats (or glycerids) are a large group of compounds that contain fatty acids and glycerol; a glycerol molecule attached to three fatty acid esters is a triacylglyceride.

- Several variations on this basic structure exist, including alternate backbones such as sphingosine in the sphingolipids, and hydrophilic groups such as phosphate in phospholipids.

- Steroids such as cholesterol are another major class of lipids that are made in cells.

Using this approach, lipids may be divided into eight categories :

- fatty acyls
- glycerolipids
- glycerophospholipids
- sphingolipids
- saccharolipids
- polyketides (derived from condensation of ketoacyl subunits)
- sterol lipids and prenol lipids (derived from condensation of isoprene subunits).

Fatty acids

Lipids are chiefly fatty acid esters, and are the basic building blocks of biological membranes. Another biological role is energy storage (e.g., triglycerides).

Fatty acids consist of unbranched chains of carbon atoms that are connected by single bonds alone (saturated fatty acids) or by both single and double bonds (unsaturated fatty acids). The chains are usually 14-24 carbon groups long.

Biological membranes

For lipids present in biological membranes, the hydrophilic head is from one of three classes:

- glycolipids, whose heads contain an oligosaccharide with 1-15 saccharide residues.
- phospholipids, whose heads contain a positively charged group that is linked to the tail by a negatively charged phosphate group.
- sterols, whose heads contain a planar steroid ring, for example, cholesterol.

By cellular localization

- intranuclear lipids

By types

- triglycerides (fatty acids)
- steroids
- glyoclipids
- phospholipids
- polyisoprenoids
- ether lipids
-  inositol lipids

Lipid intakes

Humans have a requirement for certain essential fatty acids, such as linoleic acid (an omega-6 fatty acid) and alpha-linolenic acid (an omega-3 fatty acid) in the diet because they cannot be synthesized from simple precursors in the diet.

Both of these fatty acids are 18-carbon polyunsaturated fatty acids differing in the number and position of the double bonds.

Most vegetable oils are rich in linoleic acid (safflower, sunflower, and corn oils).

Alpha-linolenic acid is found in the green leaves of plants, and in selected seeds, nuts and legumes (flax, canola, walnuts and soy).

Fish oils are particularly rich in the longer-chain omega-6 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Most of the lipid found in food is in the form of triacylglycerols, cholesterol and phospholipids.

Most of the saturated fatty acids (as triacylglycerols) in the diet are incorporated into adipose tissue stores, because the absence of double bonds allows a higher energy yield per carbon than is obtained from oxidation of unsaturated fatty acids.

The longer chain fatty acids are incorporated into cell membranes as phospholipids regardless of degree of saturation. Since dietary fatty acids are exchanged with membrane fatty acids, dietary fat composition is reflected in membrane lipid composition.

Thus dietary fatty acids can influence cell function through effects on membrane properties. Dietary fat provides an average energy intake which is approximately twice that of carbohydrate or protein.

A minimum amount of dietary fat is necessary to facilitate absorption of lipid vitamins (vitamin A, vitamin D, vitamin E and vitamin K) and carotenoids.

High fat intake

High fat intake contributes to increased risk of obesity, diabetes and atherosclerosis. Abnormal levels of certain lipids, particularly cholesterol (in hypercholesterolemia) and trans fatty acids, are risk factors for heart disease amongst others.

Atherosclerosis is the primary cause of coronary and cardiovascular diseases and is primary due to the buildup of plaque on the inside walls of arteries.

Plaque is made up of cholesterol-rich low density lipoproteins (LDL), macrophages, smooth muscle cells, platelets, and other substances.

In North America and most other western countries, atherosclerosis is the leading cause of illness and death, almost doubling the number of deaths from cancers.

Despite significant medical advances, coronary artery disease and atherosclerotic stroke are responsible for more deaths than all other causes combined.

A substantial amount of scientific evidence supports the impact of dietary fatty acids on cardiovascular health.

Saturated fatty acids have a profound hypercholesterolemic (increase blood cholesterol levels) effect and tend to increase plasma LDL.

Saturated fatty acids are found predominantly in animal products (butter, cheese and meat) but coconut oil and palm oil are common vegetable sources.

Intake of monounsaturated fats in oils such as olive oil is thought to be preferable to consumption of polyunsaturated fats in oils such as corn oil because the monounsaturated fats apparently do not lower high-density-lipoprotein (HDL) cholesterol levels.

Keeping cholesterol in the normal range not only helps prevent heart attacks and strokes but may also prevent the progression of atherosclerosis.

Statins are a class of drugs that lowers the level of cholesterol in the blood by inhibiting the enzyme HMG-CoA reductase, a key enzyme involved in the biosynthesis of cholesterol in the liver.

See also

- lipid homeostasis
- lipid biosynthesis
- lipid catabolism