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.
Some high-fat foods
- Cakes, cookies and biscuits
- Cheese (especially cream cheese and processed cheese)
- Creamy desserts
- Creamy dips (e.g. taramasalata)
- Creamy sauces (e.g. mayonnaise, Hollandaise)
- Crispy snacks
- Dairy cream
- Deep-fried foods
- Fatty meats (e.g. burgers, streaky bacon, salami, sausages)
- French fries (especially if thinly cut)
- Fried bread
- Fritters and batter
- Ice cream
- Pastries, croissants
- Pork pies
The fat content of food plays a large part in its palatability. For instance the taste of meat comes from the flavour of its fat. Fat makes ice cream creamy and pastry crumbly or flaky. Without fat, cakes become rubbery and milk watery. Many people eat a high-fat diet (the average in the western diet is around 40 per cent of the total calorie intake) without realizing this, owing to large quantities of fats being hidden in processed foods such as biscuits and burgers. The law in most countries compounds the problem; for instance in the UK meat may legally be described as ‘lean’ even if it is one third fat.
Dietary fats and oils are composed of units called triglycerides. A triglyceride consists of three fatty acids attached to a ‘backbone’ of glycerol.
Fatty acids are made from chains of carbon, hydrogen and oxygen. They 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 (see diagram on previous page).
Linoleic acid, which is an essential fatty acid, belongs to the family of ‘omega 6’ fatty acids because the first double bond appears after the sixth carbon atom. The other essential fatty acid is known as alpha linolenic acid. It belongs to the family of ‘omega 3’ fatty acids because the first double bond appears after the third carbon atom. Fish oil also belongs to the omega 3 family of fatty acids. Most vegetable oils mainly comprise omega 6 fatty acids.
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
- Leafy vegetables
Saturated fats are not essential and 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, are also non-essential. They have one double bond (one carbon atom is missing 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 which the carbon atoms can attach to. This is how oils can be turned into margarine.
Digestion and metabolism
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. In short, they are added to your body fat. This is the ultimate fate of saturated fats.
EFAs on the other hand have a more vitamin-like purpose. EFAs are broken down by the enzymes shown in the diagram below and turned into prostaglandins (sometimes known as eicosanoids). These are hormone-like substances which when out of balance can cause disorders such as high blood pressure, arthritis, menstrual pain, allergies, asthma, eczema, migraine and fertility problems. Series 1 and 3 prostaglandins are beneficial, but series 2 prostaglandins can encourage inflammation in the body.
The enzyme delta-6-desaturase (D-6-D) in the diagram is vital to metabolize EFAs. EFA deficiency symptoms (which are really prostaglandin deficiency symptoms) can develop if this enzyme is not present in sufficient amounts. Factors which can reduce its efficiency include:
- A high intake or blood level of cholesterol
- A high intake of saturated fats and trans fats
- High adrenaline levels
- A high alcohol consumption
- 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 prostaglandins include:
- Dry eyes
- Eczema, psoriasis or dry skin
- Inflammatory disorders of all types
- Premenstrual syndrome (especially breast pain)
- Tendency to clot formation in blood
- Split fingernails
- Ear problems
- Hyperactivity in children
- Extreme thirst and general dryness