How the Absorption of Fat Differs From That of Carbohydrates

Fats require special processing before absorption from your digestive system.

Fats require special processing before absorption from your digestive system.

The fats and carbohydrates in your diet provide most of the energy required to fuel the essential processes that keep your body running. Along with dietary proteins, fats and carbohydrates also provide building blocks to maintain the structure of all of your tissues. The products of carbohydrate digestion are simple sugars that dissolve in water and are easily absorbed through the walls of your small intestine into your blood. In contrast, the products of fat digestion are oily substances that require special processing and packaging before they can be transported in the water environment of your blood and distributed to your tissues.

Digestion

Starches and fats must be broken down into smaller units in a process called digestion before they can be absorbed into your blood. Starches are digested mainly into a sugar called maltose, which is two glucose molecules linked together, by the amylase enzymes present in saliva and pancreatic juices secreted into your small intestine. Pancreatic juices also contain lipase enzymes that break down dietary fats, mainly triglycerides, into fatty acids and monoglycerides. Digestion of dietary fats requires emulsification of the fat globules into structures called micelles by bile acids and phospholipids present in bile secretions from your liver.

Carbohydrate Absorption

An enzyme called maltase, residing at the surface of the cells lining your small intestine, breaks down maltose into two molecules of glucose. Similarly, dietary sucrose, or table sugar, is broken down into one molecule of glucose plus one molecule of fructose by an enzyme called sucrase. Lactose, or milk sugar, is broken down into one molecule of galactose plus one molecule of glucose by an enzyme called lactase. Glucose, fructose and galactose are all taken into your intestinal cells through transporter channels and directly passed into your blood.

Fat Absorption

Micelles shuttle emulsified and digested fats to your intestinal wall. Cholesterol requires a specific transport protein for passage into the cells of your small intestine. In contrast, fatty acids and monoglycerides can freely diffuse into your intestinal cells. Once inside, triglycerides are reformed from the digested fatty acids and monoglycerides. The triglycerides are then packaged with cholesterol, phospholipids and proteins into very large lipoproteins called chylomicrons. Chylomicrons pass through your intestinal cells and are absorbed into lymph vessels called lacteals. Chylomicrons carrying the fats from your last meal make their way into your blood where lymph empties into your blood vessels under your left shoulder.

Dietary Fiber

Dietary fiber is a type of carbohydrate found in whole foods that is indigestible in your small intestine and helps to slow the rate of fat and carbohydrate absorption. If you're trying to lose weight, fiber helps to discourage you from overeating by giving you a feeling of fullness. Fiber can also increase the amount of dietary fat that passes through your intestines unabsorbed and is excreted with your stool. Some forms of indigestible fiber are broken down and fermented by bacteria that inhabit your large intestine into short-chain fatty acids, which are absorbed into your blood and can be used in your liver for energy.

 

About the Author

Michael Peluso is a semi-retired scientist in the field of nutritional biochemistry. He received his M.S. in nutrition from the University of California, Davis and Ph.D. in nutrition from the University of Missouri. Peluso's work has appeared in scholarly publications such as the "Journal of Nutrition," "Lipids" and "Experimental Biology and Medicine."

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