Also known as lipids, fats are components of the diet and the human or animal body which are insoluble in water but soluble in organic solvents. Fats may be solid or liquid, in which case they are known as oils. Butter, lard, meat fat, oils and margarine are the foods with the highest fat content.

Dietary fats and oils are composed of units called triglycerides which in turn consist of carbon, hydrogen and oxygen formed into chains of fatty acids attached to a 'backbone' of glycerol. Fatty acids are classed as 'essential' or 'non-essential', depending on whether the body is capable of synthesizing them or not. Those which it cannot make and must obtain from the diet are known as essential fatty acids (EFAs). A deficiency of EFAs can have a widespread impact on health.

Fatty acids are classified according to four characteristics:

• Whether or not they are essential
• The length of the chain
• Whether they are saturated or unsaturated
• The position of the first double bond (a double bond occurs in an unsaturated fatty acid molecule at points where there is no hydrogen atom for a carbon atom to bond with).

Linoleic acid, which is an essential fatty acid, is described as an 'omega 6' fatty acid because the first double bond appears after the 6th carbon atom. The other essential fatty acid is known as alpha linolenic acid. It is described as an 'omega 3' fatty acid because the first double bond appears after the third carbon atom.

Good food sources of linoleic acid

• Corn oil
• Fresh nuts and seeds
• Groundnut oil
• Safflower oil
• Sunflower seed oil

Good food sources of alpha linolenic acid

• Linseed (flax seed) oil
• Soybean oil
• Vegetable leaves
• Walnuts

Saturated fats have no double bonds - all the carbon atoms in their fatty acid molecules are attached to a hydrogen atom. Monounsaturated fatty acids, such as oleic acid, found in olive oil, have one double bond - one carbon atom is not attached to a hydrogen atom. Polyunsaturated fatty acids have two or more double bonds. The more unsaturated a fat, the more it tends to be liquid at room temperature. However, unsaturated fats can be turned into saturated fats by artificially adding hydrogen atoms to which the carbon atoms can attach. This is how oils can be turned into margarine.

Functions

After a meal, emulsified fat droplets are absorbed from the gut into the lymphatic system and then drain into the bloodstream at the thoracic duct in the neck. Within hours, the triglycerides are broken down into fatty acids and glycerol and then removed from the blood and into the adipose (fat) cells where they are reconstituted into triglycerides and stored for use as a future source of energy. This is the ultimate function of dietary saturated fat.

The essential fatty acids, which are polyunsaturated, have many other functions:

• Components of plasma and mitochondrial lipoproteins
• Controlling cholesterol levels
• Fat transport and metabolism
• Precursors of prostaglandins and leukotrienes.

Prostaglandins are hormone-like substances which control many essential functions in the body in relation to disorders such as high blood pressure, arthritis, menstrual pain, allergies, asthma, eczema, migraine and fertility.

As the fatty acids are metabolized, their chains become longer and more unsaturated. Each step in metabolism depends on an enzyme, the name of which is shown in the figure, and always ends in '-ase'. If these enzymes are in short supply, the production of prostaglandins may be impaired. Factors which we know interfere with the optimum function of delta-6-desaturase (D-6-D) are:

• A high intake or blood level of cholesterol
• A high intake of saturated fats and 'trans' fats (see below)
• High adrenaline levels
• A high alcohol consumption
• Diabetes
• Atopy (an inherited susceptibility to allergic diseases)
• Deficiencies of magnesium, vitamin B6, biotin or zinc.

Some health problems which make us suspect a deficiency of EFAs, or a lack of efficiency in converting them to prostaglandins are:

• Dry eye syndrome
• Eczema, psoriasis or dry skin
• Inflammatory disorders of all types
• Premenstrual syndrome (especially breast pain)
• Tendency to clot formation in blood.

Supplements prescribed to correct these deficiencies may be more effective if they bypass a defective D-6-D enzyme. This is why GLA and EPA supplements are sometimes given instead of their precursors, and studies have shown them to be often effective in the clearance of the above conditions. GLA is the active ingredient in evening primrose oil, borage oil or blackcurrant seed oil, and EPA is obtained from fish oils. Other possible signs of essential fatty acid deficiency are split fingernails, a history of ear problems or hyperactivity in children, and extreme thirst (where no other cause can be found).

Essential fatty acids have been found helpful in the treatment of schizophrenia.

Cis and Trans

Essential fatty acids have an active form (known as 'cis') and an inactive form (known as 'trans') depending on the arrangement of their atoms. Only the cis form can be turned into prostaglandins.

Plant oils contain large amounts of cis fatty acids. But commercial processing of oils to improve stability and odour, and the partial hydrogenation of oils to form magarines or 'vegetable fat', converts a large proportion of cis fatty acids into the inactive trans form. So it is possible to develop an essential fatty acid deficiency even if we appear to eat adequate amounts of oils.

Though not thought to be actively toxic, the primary adverse effect of trans fatty acids is to inhibit the metabolism of EFAs by competing with them for the important liver enzyme delta-6-desaturase. This increases EFA requirements, affects EFA utilization, and aggravates the symptoms of EFA deficiency. Trans fatty acids are also incorporated differently into cell membranes, triglycerides and phospholipids within the body, and there is evidence that a high consumption of partially hydrogenated vegetable fats (which are high in trans fatty acids), is associated with a greater risk of heart disease.