Everything About Fat

Fats and oils are the most energy dense component of foods, supplying 9 kcal per gram of the pure substance. Because of this high energy density fat supplies 40% or more of the energy in the diet in many developed countries,although in some developing countries, mainly reliant on plant sources for their energy, the proportion may be as low as 10%. Chemically, fats and oils are lipids,defined as substances that are insoluble in water, but soluble in organic solvents such as alcohol. Fats are usually solid at room temperature, e.g. butter or lard, while oils are liquid but very similar chemically. As well as triglycerides, which make up the main part of dietary fat, lipid materials include such things as sterols and phospholipids. Lipids are very important in the body as well as providing an energy store (triglycerides), they are important in maintaining the structure of cell membranes, and sterols such as cholesterol also provide the basis for a wide range of hormones, including the reproductive hormones....

Phospholipids are important because they are miscible in both lipids and water and act to stabilize emulsions. For example, in the body they help to maintain cell membrane structure, and in foods lecithins,found naturally in egg yolk, peanuts, and soya, are used to stabilize foods such as chocolate and mayonnaise.

Although fats and oils have generally similar structures, there are differences that are important to health.

In the diet the most important fats and oils are triglycerides, whose chemical structure consists of three fatty acid molecules attached to a molecule of glycerol. Fatty acids are composed of chains of carbon atoms of varying length, with hydrogen atoms attached at the bonding sites. Depending on how many hidrogen atoms are attached at each carbon bond, the fatty acids may be termed saturated or unsaturated. If sites are saturated there is a single bond between the carbon atoms, and where sites are unsaturated a double bond occurs. Fatty acids are therefore classified according to the length of the carbon chain and the number of double bonds.

* Short chain fatty acids have 4–6 carbon atoms, medium chain 8–12, long chain 14–18, and            very long chain 20 or more.

* Saturated fatty acids have no double bond, monounsaturated fatty acids have one, and           

   polyunsaturated fatty acids have several double bonds.

The longer the chain length of the constituent fatty acids and the more saturated these are, the harder the fat.Thus lard contains a high proportion of saturated fatty acids, corn oil contains a high proportion of long chain polyunsaturated fatty acids, and fish oils contain very long-chain polyunsaturated fatty acids. Unsaturated fatty acids may exist in two distinct forms,cis or trans. 

The trans form is less common in natural fats but is produced during processing, e.g. in the manufacture of margarines. High intakes of trans polyunsaturated fatty acids, it has been suggested, act in a similar way to saturated fatty acids in their effects on blood cholesterol levels.

A further complication in the chemistry of unsaturated fatty acids is that they exist in three different ‘families’, according to the position of the first double bond in the molecule. Thus fatty acids may belong to the n-3, n-6, or n-9 family. Humans can insert a double bond at position 9 but not at positions 3 or 6. The diet must therefore include some n-3 and n-6 fatty acids, and these are termed the essential fatty acids and sometimes also called omega-3 and omega-6

fatty acids. Recommendations from organizations such as theFood and Agriculture Organization and the World Health Organization are that linoleic acid (n-6) should supply 4–10% and linolenic acid (n-3) 0.5–4% of dietary energy, with a ratio of 0.1:0.4 for n-3:n-6.

Fats and oils in foods contain mixtures of fatty acids

Plant foods contain fats with mainly polyunsaturated fatty acids (40–60%)and monounsaturated fatty acids (30–40%), with up to about 20% of saturated fats. In animal fats the greatest proportion are saturated fatty acids (40–60%), with some monounsaturated fatty acids(30–50%) and less polyunsaturated fatty acids (approximately 10%). There are exceptions to all rules, and some plant fats such as palm oil and coconut oil contain large amounts of saturated fats, while poultry and game tend to have higher proportions of polyunsaturated fatty acids.