Fats, or lipids, are a major source of energy from your diet. According to the U.S Department of Health and Human Services, few Americans obtain less than 20 percent of their total calories from fat. Compared to carbohydrates and proteins -- the other two principal sources of dietary energy -- fats are harder to digest, but your body possesses the means to extract the maximal number of calories from each gram of fat you eat.
Composition
"The Merck Manual of Diagnosis and Therapy" reports that more than 95 percent of dietary fats are triglycerides, with the remainder composed of phospholipids, cholesterol and free fatty acids. The triglyceride molecule consists of a glycerol backbone to which three fatty acids are attached. Triglycerides and large phospholipids cannot be directly absorbed through your intestinal wall, so they must be broken down into simpler molecules. In addition, fats don't mix well with water, so they must be emulsified in the aqueous environment of your gastrointestinal tract to enable their digestion.
Mouth and Stomach
Chewing -- the initial step in fat digestion -- mechanically separates your food and helps expose dietary lipids to salivary lipase, a fat-digesting enzyme produced by your salivary glands. However, salivary lipase plays a relatively minor role in breaking down fats. Once your food is swallowed, another enzyme called gastric lipase begins to earnestly separate fatty acids from triglycerides and breaks phospholipids apart. Vigorous peristalsis -- coordinated muscular activity in your stomach wall -- mixes your food with stomach acid and improves the efficiency of gastric lipase.
Intestine
Following liquefaction in your stomach, food passes into your small intestine, where yet another digestive enzyme -- pancreatic lipase -- continues the process of digesting triglycerides into free fatty acids, glycerol and monoglycerides, which are molecules of glycerol with a single fatty acid attached. Bile released from your gallbladder emulsifies the fats in your intestine and helps expose them to water-soluble lipases. Eventually, free fatty acids, cholesterol, glycerol and monoglycerides are absorbed by enterocytes, the cells that line your intestine.
Reassembly and Packaging
Once inside the enterocytes, free fatty acids and monoglycerides are reassembled into triglycerides and packaged with cholesterol and proteins into large lipid-protein complexes called chylomicrons, which are subsequently transferred into the lymphatic channels surrounding your intestine. Lymphatic vessels slowly carry chylomicron-rich lymph fluid to a channel in your chest called the thoracic duct, which finally empties into veins leading back to your heart. In this way, chylomicrons derived from the fat in your diet find their way into your bloodstream, where they are transported to various organs and tissues for processing.
References
- U.S. Department of Health and Human Services: Dietary Guidelines for Americans 2005
- "Staying Healthy With Nutrition: Lipids -- Digestion and Absorption"; Elson M. Haas, M.D.; 2006
- "The Merck Manual of Diagnosis and Therapy, 18th Edition: Lipid Disorders"; Mark H. Beers, M.D.; 2006
