Definition: Lipids are broadly defined as any fat-soluble (lipophilic), naturally-occurring molecule. They include animal or vegetal fats, oils, waxes, cholesterol, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others.
The main biological functions of lipids include energy storage, acting as structural components of cell membranes, and participating as important signaling molecules.
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 emulsion test is a crude method for determining the presence or absence of lipids in a given sample.
Most lipids consist of a polar or hydrophilic head and one to three nonpolar or hydrophobic fatty acid tails, and therefore they are amphiphilic.
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.
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.
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