Wednesday, June 05, 2013
Carbohydrates are found in a wide array of foods—bread, beans, milk, popcorn, potatoes, cookies, spaghetti, soft drinks, corn, and cherry pie. They also come in a variety of forms. The most common and abundant forms are sugars, fibers, and starches.
The basic building block of every carbohydrate is a sugar molecule, a simple union of carbon, hydrogen, and oxygen. Starches and fibers are essentially chains of sugar molecules. Some contain hundreds of sugars. Some chains are straight, others branch wildly.
Carbohydrates were once grouped into two main categories. Simple carbohydrates included sugars such as fruit sugar (fructose), corn or grape sugar (dextrose or glucose), and table sugar (sucrose). Complex carbohydrates included everything made of three or more linked sugars. Complex carbohydrates were thought to be the healthiest to eat, while simple carbohydrates weren’t so great. It turns out that the picture is more complicated than that.
The digestive system handles all carbohydrates in much the same way—it breaks them down (or tries to break them down) into single sugar molecules, since only these are small enough to cross into the bloodstream. It also converts most digestible carbohydrates into glucose (also known as blood sugar), because cells are designed to use this as a universal energy source.
Fiber is an exception. It is put together in such a way that it can’t be broken down into sugar molecules, and so it passes through the body undigested. Fiber comes in two varieties: soluble fiber dissolves in water, while insoluble fiber does not. Although neither type nourishes the body, they promote health in many ways. Soluble fiber binds to fatty substances in the intestines and carries them out as a waste, thus lowering low-density lipoprotein (LDL, or bad cholesterol). It also helps regulate the body’s use of sugars, helping to keep hunger and blood sugar in check. Insoluble fiber helps push food through the intestinal tract, promoting regularity and helping prevent constipation. Adults need at least 20 to 30 grams of fiber per day for good health. But most Americans get only about 15 grams a day.
The Nutrition Source
Harvard School of Public Health