"Lipid" is a biochemical word meaning "fat." In the course of your normal cellular function, your body breaks down fats regularly. This occurs both in the digestive tract and at the cellular level. Chemicals called enzymes help your digestive tract and cells break down lipids, but you can't use enzymes to force lipid breakdown or to lose body fat.
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Your body and cells need to be able to break down lipids for several reasons. First, when you consume lipid-containing food, you have to break the lipids into smaller pieces that your digestive tract can absorb into the bloodstream, explains Dr. Lauralee Sherwood in her book "Human Physiology." Additionally, your fat cells have to break down stored lipids in order to release them into the blood for cellular energy use. Finally, cells break down lipids for energy.
Your cells and organ systems use enzymes to help them accomplish chemical reactions, explain Drs. Reginald Garrett and Charles Grisham in their book "Biochemistry." An enzyme is a protein that causes chemical reactions to take place faster than they otherwise would. You produce many different enzymes throughout the body -- some function in the digestive tract and other organ systems, while some function in the cells. All enzymes are reaction-specific -- enzymes that break down lipids don't take part in other kinds of reactions.
Enzymes called pancreatic lipases break lipids in your food down into smaller pieces; you absorb these through your small intestine into your bloodstream, and cells either burn them or store them as body fat. A different type of lipase then breaks down stored body fats -- also lipids -- when your cells require more energy. Many different enzymes are involved in breaking down lipids for energy at the cellular level -- these burn lipids, producing carbon dioxide and water.
You may have heard that taking certain enzymes can help you break down stored lipids and lose body fat. This is not true, unfortunately. First, stored body fat isn't an indication that you're deficient in lipid breakdown enzymes -- it's simply an indication that you are taking in more energy than you need. Supplementing with enzymes won't help. Second, there's no scientific evidence to suggest that enzymes you take as supplements can make it out of the digestive tract and into the cells -- nor can enzymes you take as supplements help you digest additional fat in your food.