Fats

Functions of Fats, What is a Fat/Lipid

Functions of Fat

There are many different types of fats in our diets. Fats are a major source of energy. They are the only form of energy that the body can store for a prolonged period. These stored lipids also provide insulation, help to control body temperature, and form some protection for the body's organs. Other functions of lipids include maintenance of cell membrane structure and functional integrity. Lipids are also the precursors to hormones and hormone-like compounds which have many physiological roles within the body.

We also know that different lipids can positively and negatively affect our blood cholesterol levels.

What is a Fat/Lipid?

Lipids are a group of compounds that dissolve in organic solvents, but are insoluble in water. Common lipids are oils which are liquid at room temperature and fats which are solid at room temperature.

It is important to remember that not all fats are created equal. There are many different types of fats in our diets. Triglycerides make up the bulk of lipids in our diet, with phospholipids and sterols making up most of the remainder. Other lipids include fat-soluble vitamins such as vitamin A, E and D, and carotenoids.

In our diet, 95% of the fat we consume is in the form of triglycerides (sometimes seen as triacylglycerol). A triglyceride is a molecule that is formed from a molecule of glycerol with 3 fatty acids attached.

Fatty acids consist of a chain of carbon atoms with hydrogens attached. They have a methyl group at one end and a carboxyl acid group at the other. The methyl end carbon is known as the n minus (n-) or omega (w ) carbon atom. You may have heard of fats that are called omega-3 fatty acids. These are fatty acids which have a double bond 3 carbons away from the methyl carbon atom.

There are 3 main classes of fatty acids. These are saturated, monounsaturated, and polyunsaturated fatty acids.

Saturated fatty acids are those in which carbon-carbon bonds are fully saturated with hydrogen atoms (i.e. 4 hydrogens per carbon-carbon bond).

When 2 hydrogen atoms are absent, the carbons form a double bond with each other. When one double bond is present the fatty acid is known as a monounsaturated fatty acid

When 2 or more double bonds result, the fatty acid is known as a polyunsaturated fatty acid.

Fatty acids can be described by their common name, their chemical name, their chemical structure or by a shorthand notation.

The shorthand notation firstly describes the number of carbon atoms, then the number of double bonds. The third descriptor for the mono- and polyunsaturated fatty acids indicates the position of the first double bond relative to and including the methyl carbon. The methyl carbon is the carbon at the end of the fatty acid which is surrounded by 3 hydrogen atoms. The other end of the fatty acid is known as the carboxyl end.

Examples

Common name: stearic acid

Chemical name: octadecanoic acid

Fatty acid notation: C18:0

Monounsaturated fat

Common name: Oleic acid

Chemical name:  "⁹octadecanoic acid

Fatty acid notation: C18:1n-9

Polyunsaturated fat

Common name: Linoleic

Chemical name: "^{9, 12} -octadecadienoic acid

Fatty acid notation: C18:2n-6

The positions of the double bonds on the monounsaturated and polyunsaturated fatty acids have a profound effect on human health. The position of the first double bond relative to the methyl end indicates the "family" to which the unsaturated fatty acid belongs.

Polyunsaturated fats where the first double bond is 3 carbons away from the methyl end are known as n-3 or w 3 (omega 3) fatty acids. Fatty acids where the first double bond is next to the sixth carbon atom are named n-6 or w 6, and fatty acids where the first double bond is next to the ninth carbon are n-9 or w 9.

You may sometimes hear fatty acids referred to as short (containing less than 8 carbons), medium (8-12 carbons) or long (14 or more carbons) chain fatty acids.

A saturated fat is solid at room temperature. Inserting a double-bond in a saturated fatty acid reduces its melting point. Therefore, fats which contain predominantly saturated fats (e.g. butter) are usually solid at room temperature, whereas oils which contain a predominance of polyunsaturated fat (e.g. soybean oil) are liquid at room temperature.

During food processing the properties of fat can be changed. An example of this is when oils which are usually liquid at room temperature undergo a process called hydrogenation which make these fats solid at room temperature. This process is used by the food industry for such foods as margarine, biscuits, cakes, chocolates, pies and sauces. The process of hydrogenation adds a hydrogen atom to the double bonds in monounsaturated or polyunsaturated acids in the oil which consequently increases the melting point of the fat. This produces a fat with a consistency that is appropriate to the texture of the desired food. Hydrogenation also changes the configuration of some of the remaining double bonds from what we call a cis-configuration to a trans-configuration. Trans-fatty acids behave biologically like saturated fats within the body. For this reason many margarine manufacturers have reduced that amount of trans-fatty acids in their products.

Phospholipids comprise only a small proportion of dietary lipid intake.

There are 4 major phospholipids which comprise of a diglyceride (as opposed to a triglyceride which has 3 fatty acids attached to a glycerol backbone, a diglyceride has 2 fatty acids) in which the third position of the glycerol molecule is occupied by a phosphoric acid residue which has 1 of 4 bases attached to it (choline, inositol, serine, or ethanolamine). These 4 phospholipids, along with sphingomyelin comprise more than 95% of phospholipids found in the body as well as in foods. The most abundant phospholipid found in nature is phosphatidylcholine (aka lecithin).

Phospholipids are found in virtually all animal and vegetable foods. Rich sources include eggs, liver, peanuts, soybeans, and wheatgerm.

Since phospholipids have both polar and non-polar characteristics they make excellent emulsifying agents. In the body phospholipids are important for the structure of the cell wall. Phospholipids are also good sources of essential fatty acids.

Sterols are similar to other lipids, in that they are made up of carbon, hydrogen and oxygen. What makes them different from other lipids is the way in which they are structurally arranged. Their carbon, oxygen and hydrogen atoms are arranged in a series of 4 rings with a range of side chains.

Cholesterol is the major sterol found in animal tissue and is only present in animal foods. Sources of cholesterol include, eggs, meat, dairy products, shellfish, fish and poultry.

Cholesterol has an important structural role in cell membranes and functions as the precursor of bile acids, steroid hormones, and vitamin B.

People often confuse dietary cholesterol and blood cholesterol. The amount of cholesterol in your blood at any one time is not a good reflection of the cholesterol in your diet. The habitual consumption of a diet high in saturated fat is more likely to have a profound affect on your blood cholesterol level than is the amount of cholesterol in your diet per se.

Phytosterols are the sterols found in plants. The major plant sterols are b -sitosterol, campesterol and stigmasterol.

There has been recent interest in the use of plant sterols in the diets of individuals with high blood cholesterol levels. Research suggests that plant sterols reduce the absorption of cholesterol by the body and subsequently reduces blood cholesterol levels.