Lipids refer to organic biochemicals such as fats, oils, sterols, phospholipids, lipoproteins and waxy elements. Lipids store energy, are structural components of cell membranes and also help in the synthesis of vitamins and hormones. Whenever there is a shortage of carbohydrates, your body breaks down lipids to release calories through a process known as ketosis. According to the U.S. Food and Drug Administration, the recommended daily intake of lipids is 65 grams per day.
With the exception of phospholipids, which partially dissolve in water, all lipids are generally insoluble in water. This is due to the presence of long-chain fatty acids in their structure. The cell membranes of your body are composed of phospholipids. They help create a semi-permeable barrier between the cells and the external environment. If water-soluble compounds were used as components of cell membranes, they wouldn’t be as durable since they would dissolve in water.
High Energy Content
Also known as calorie density, energy density is the total amount of calories per specific weight of food. Lipids have high energy density, which means the amount of energy they release per unit of mass is high. 1 gram of a lipid contains 9 calories per gram, which is about 2.25 times greater than proteins and carbohydrates. The inability of lipids to dissolve in water and hence bond with it means that no excess weight is added to their existing mass without any incremental increase in calorie density.
Digestion and Absorption
Unlike carbohydrates and proteins, lipids barely dissolve in the aqueous medium of the digestive system. The digestion of lipids is done by a water-soluble enzyme called lipase, which can only act on the surface of lipid droplets. Emulsification --a process in which the lipid globules are broken down into tiny emulsion droplets -- makes the digestion of lipids much easier. This is because it significantly increases the surface area that lipase can act on to break down lipids into glycerol and fatty acids. Phospholipids, bile salts and protein known as colipase help emulsify the lipid globules and prevent them from reassociating.
Types of Lipids
While there are several types of lipids, the two major lipids are triglycerides and sterols. Triglycerides contain three molecules of fatty acids and one glycerol molecule in their structure. Since they act as a food store, triglycerides are a major component of storage fat or adipose tissue in your body. A sterol is a type of lipid whose structure is composed of four fused carbon rings and one oxygen molecule. The most common sterol is cholesterol, which is a structural component of cell membranes. It is also converted to bile salts and hormones such as testosterone and estrogen.
- Michigan State University Department of Chemistry: Lipids
- University of Cincinnati Clermont College: Lipids; Fats, Oils, Waxes, etc.
- Essential Cell Biology; Bruce Alberts
- Rose-Hulman Institute of Technology: Introduction to Lipid Metabolism
- Biological Science; Scott Freeman
- Davidson College Department of Biology: How Normal Fats are Digested
- Proteins: Computational Study of Colipase Interaction with Lipid Droplets and Bile Salt Micelles.
- Human Biology; Sylvia Mader, et al.
- University of Wisconsin- Green Bay: Lipoproteins
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