BIOLOGY Volume 1 - A Guide to General Biology - 2004

8. HETEROTROPHIC NUTRITION

8.7. Human Nutrition

8.7.7. Lipids (Fats and Oils)

Lipids include fats, which are solid at room Temperature, and oils, which are liquid at room temperature. Like CARBOHYDRATES, lipids serve as a major source of energy. Adipose tissue provides a long-term energy reserve in the body. Dietary fats also act as precursors for Fat-soluble Vitamins (A, D, E, K).

The Main characteristics of lipids are outlined below.

1. Dietary lipids primarily consist of triglycerides, which are composed of glycerol and three Fatty acids.

2. A vast number of fatty acids exist in food, but all of them are classified as either saturated or unsaturated.

3. Saturated fatty acids contain no double bonds (they are saturated with hydrogen). Fats are rich in saturated fatty acids. Indeed, the consumption of high amounts of saturated fatty acids is linked to an increased risk of cardiovascular disease (Ch. 15).

4. Unsaturated fatty acids contain one or more double bonds in their Structure. Oils are rich in Unsaturated fatty acids.

5. Unsaturated fatty acids can be either monounsaturated (containing one double bond) or polyunsaturated (containing more than one double bond).

6. Unsaturated fatty acids can be converted into saturated ones through a reaction with hydrogen, a process known as hydrogenation. This is how liquid vegetable oil is hardened to produce margarine.

7. Most naturally occurring fatty acids exist in the cis-form, in which the molecules are completely straight. When unsaturated Fatty acids are partially hydrogenated, some of the remaining double bonds shift into the trans-form, causing the molecules to bend. Within the body, these molecules exhibit properties similar to those of saturated fatty acids.

A balanced diet should include both saturated and unsaturated fatty acids. However, since saturated fats have been linked to cardiovascular disease (Ch. 15), most people are advised to reduce their intake of saturated fatty acids.

Essential Fatty Acids

Essential fatty acids (EFAs) must be obtained from the diet because the body cannot synthesize them on its own. A deficiency in these acids leads to various disorders. Strictly speaking, only two fatty acids are truly essential: linoleic acid and α-linolenic acid. Both are polyunsaturated fatty acids (PUFAs) that occur in the cis-form. Linoleic acid has two double bonds, while linolenic acid has three. These acids perform several vital Functions:

1. EFAs are used to synthesize Phospholipids, which are fundamental components of Cell membranes.

2. These acids are involved in the transport, breakdown, and excretion of Cholesterol. Cholesterol is an essential membrane component and is required for the synthesis of Steroids, including Sex Hormones and vitamin D. However, excess cholesterol can be hazardous to health because it contributes to The Development of atherosclerosis (fatty deposits in the Arteries), which in turn leads to cardiovascular disease. Therefore, Cholesterol METABOLISM must be tightly regulated. Linoleic acid and certain other PUFAs help lower Blood cholesterol levels, whereas saturated fatty acids raise them. The best recommendation in this regard is to minimize Dietary intake of saturated fats wherever possible.

3. Linolenic acid reduces blood clotting associated with atherosclerosis and lowers the risk of recurrent myocardial infarction.

4. EFAs are required for the Synthesis of Other crucial fatty acids, such as Prostaglandins, which possess a wide range of physiological effects. For example, they modulate The activity of certain hormones, promote inflammatory responses, and regulate blood flow to Organs. They are involved in the labor process and are incorporated into oral contraceptives as an antiprogesterone agent to prevent the implantation of a fertilized egg.

5. Linolenic acid is one of the fatty acids essential for the normal development and functioning of the retina and the Brain.

EFA deficiency is rare because the body stores them in fat reserves, and daily dietary intake through fats is usually more than sufficient.