It is now known that lipids play a much more important role in the body than previously believed. It was previously known that lipids played the role of storage of energy or forming cell membranes alone. Researchers have found that lipids have a much more diverse and widespread biological role in the body in terms of intracellular signalling or local hormonal regulation etc.
Lipids are synthesized in the body using complex biosynthetic pathways. However, there are some lipids that are considered essential and need to be supplemented in diet.
In 1929, for example, George and Mildred Burr demonstrated that linoleic acid was an essential dietary constituent. Bergström, Samuelsson and others in 1964 added to the knowledge of role of lipids in the body by finding that essential fatty acid arachidonate was the biosynthetic precursor of the prostaglandins with their effects on inflammation and other diseases.
In 1979 the first biologically active phospholipid, platelet activating factor was discovered and there was a raised awareness regarding phosphatidylinositol and its metabolites in cellular signally and messaging.
Role of lipids in the body
Lipids have several roles in the body, these include acting as chemical messengers, storage and provision of energy and so forth.
All multicellular organisms use chemical messengers to send information between organelles and to other cells. Since lipids are small molecules insoluble in water, they are excellent candidates for signalling. The signalling molecules further attach to the receptors on the cell surface and bring about a change that leads to an action.
The signalling lipids, in their esterified form can infiltrate membranes and are transported to carry signals to other cells. These may bind to certain proteins as well and are inactive until they reach the site of action and encounter the appropriate receptor.
Storage and provision of energy
Storage lipids are triacylglycerols. These are inert and made up of three fatty acids and a glycerol.
Fatty acids in non esterified form, i.e. as free (unesterified) fatty acids are released from triacylglycerols during fasting to provide a source of energy and to form the structural components for cells.
Dietary fatty acids of short and medium chain size are not esterified but are oxidized rapidly in tissues as a source of ‘fuel”.
Longer chain fatty acids are esterified first to triacylglycerols or structural lipids.
Maintenance of temperature
Layers of subcutaneous fat under the skin also help in insulation and protection from cold. Maintenance of body temperature is mainly done by brown fat as opposed to white fat. Babies have a higher concentration of brown fat.
Membrane lipid layer formation
Linoleic and linolenic acids are essential fatty acids. These form arachidonic, eicosapentaenoic and docosahexaenoic acids. These for membrane lipids.
Membrane lipids are made of polyunsaturated fatty acids. Polyunsaturated fatty acids are important as constituents of the phospholipids, where they appear to confer several important properties to the membranes. One of the most important properties are fluidity and flexibility of the membrane.
Much of the cholesterol is located in cell membranes. It also occurs in blood in free form as plasma lipoproteins. Lipoproteins are complex aggregates of lipids and proteins that make travel of lipids in a watery or aqueous solution possible and enable their transport throughout the body.
The main groups are classified as chylomicrons (CM), very low density lipoproteins (VLDL), low density lipoproteins (LDL) and high density lipoproteins (HDL), based on the relative densities
Cholesterol maintains the fluidity of membranes by interacting with their complex lipid components, specifically the phospholipids such as phosphatidylcholine and sphingomyelin. Cholesterol also is the precursor of bile acids, vitamin D and steroidal hormones.
Prostaglandin formation and role in inflammation
The essential fatty acids, linoleic and linolenic acids are precursors of many different types of eicosanoids, including the hydroxyeicosatetraenes, prostanoids (prostaglandins, thromboxanes and prostacyclins), leukotrienes (and lipoxins) and resolvins etc. these play an important role in pain, fever, inflammation and blood clotting.
The "fat-soluble" vitamins
The "fat-soluble" vitamins (A, D, E and K) are essential nutrients with numerous functions.
Acyl-carnitines transport and metabolize fatty acids in and out of mitochondria.
Polyprenols and their phosphorylated derivatives help on transport of molecules across membranes.
Cardiolipins are a subtype of glycerophospholipids with four acyl chains and three glycerol groups. They activate enzymes involved with oxidative phosphorylation.