The biochemical word "lipid" is just another way of saying fat. When you consume dietary fats, you're taking in large molecules that your intestine can't absorb directly. You therefore digest these fats into smaller pieces, and then absorb them into the bloodstream. From there, the lipids go through many stages of metabolism as your cells burn them for energy.
Lipids
There are two major sources of lipids that your cells can use as energy. You can obtain fats directly from foods--oils and animal lard are both sources of fat--or your cells can get fats for energy from stored body fat. Whether your cells are metabolizing fats you've consumed or fats you've released from body fat stores, the physiological process of metabolism is the same, where metabolism refers to chemical reactions involved in breaking down--or making--substances in the body.
Lipid Breakdown
To metabolize fats for energy, your cells engage in a process called beta-oxidation of fatty acids, explain Drs. Reginald Garrett and Charles Grisham in their book "Biochemistry." This involves chopping a long fatty acid molecule--a lipid--into many smaller pieces. The smaller molecules then enter a metabolic reaction series called the Kreb's Cycle. Through the reactions of the Kreb's Cycle, fat pieces are burned for energy and reduced to carbon dioxide and water.
Lipid Synthesis
Just as lipids break down through a series of reactions that occur in stages, the body can also build lipids through a series of metabolic stages. This is called lipid synthesis, and your cells engage in it any time you consume more energy than your cells require for immediate purposes. If you overconsume protein or carbohydrate, your cells convert the excess into molecules of a compound called acetyl-CoA. The cells then use this to build fatty acids, explain Drs. Mary Campbell and Shawn Farrell in their book "Biochemistry."
Affecting Lipid Metabolism
Different actions on your part can affect your lipid metabolism. If you routinely provide your body with more calories than you require to maintain cellular function, you'll favor lipid synthesis, which leads to weight gain. If you routinely consume fewer calories than you need to maintain function, you'll favor release of lipids from fat cells into the bloodstream, and will also favor the breakdown of those lipids by cells for energy.
References
- "Biochemistry"; Reginald Garrett, Ph.D. and Charles Grisham, Ph.D.; 2007
- "Biochemistry"; Mary Campbell, Ph.D. and Shawn Farrell, Ph.D.; 2005
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