Difference Between Carbohydrate & Lipid Digestion

Carbohydrates and lipids, or fats, are two major sources of energy for humans. Although both are composed of carbon, hydrogen and oxygen, they have very different chemical properties. Carbohydrates are soluble in water and are present in foods in different forms, ranging from the very small sugar to the very large starch molecule. Dietary fats are not soluble in water, and are nearly always triglycerides. Because of these inherent differences between carbohydrates and lipids, the way the digestive system processes them is also very different.

Carbohydrates Overview

Three major types of carbohydrates are in human diets: polysaccharides, disaccharides and monosaccharides. Monosaccharides are single molecules of sugar, and include glucose, fructose and galactose. Disaccharides are simply two monosaccharides joined by a chemical bond. They include maltose, sucrose and lactose. Monosaccharides and disaccharides are often referred to as simple sugars. Polysaccharides are long chains of glucose. In foods, polysaccharides are in the form of starch or fiber, and are collectively called complex carbohydrates.

Digestion of Complex Carbohydrates

Although starch and fiber are both polysaccharides, they have very different physiological effects. Fiber is not digestible by human enzymes, and thus contains no calories. In contrast, starch can be broken down into many molecules of glucose; thus, it is a rich source of calories. Digestion of starch begins in the mouth, where an enzyme in saliva starts the process. However, most carbohydrate digestion occurs in the small intestine. Here, pancreatic enzymes continue breaking apart large starch molecules into the disaccharide maltose.

Digestion of Simple Carbohydrates

Cells in the digestive tract are only able to absorb monosaccharides into the bloodstream. Therefore, additional enzymes in the small intestine break down maltose and other disaccharides into their component monosaccharides. These sugars --- glucose, fructose and galactose --- then enter the blood. However, because glucose is the preferred source of energy for many cells, fructose and galactose must be converted into glucose by the liver. Fructose, and to a lesser extent glucose, exist in their monosaccharide forms in some foods and beverages; in these instances, they require no digestion at all.

Digestion of Lipids

Whereas dietary carbohydrates come in many forms, the vast majority of the in foods is in the form of triglycerides. A triglyceride is a molecule that contains three fatty acids. Unlike carbohydrates, fats are not water soluble. To break up fat into small enough droplets, the digestive system needs bile from the liver. Bile acts like a soap that enables the mixing of water-soluble digestive enzymes and the fat droplets. This mixing enables enzymes from the pancreas to break down triglycerides into fatty acids, which can then enter intestinal cells, where they are repackaged for transport to the blood via the lymphatic system.