Introduction
We've come a long way from the days when one of the knee-jerk answers to the question "What should I eat?" was "You can't go wrong with carbohydrates." We now know that carbohydrates, the staple of most diets, aren't all good or all bad. Some kinds promote health while others, when eaten often and in large quantities, actually increase the risk for diabetes and coronary heart disease.The wild popularity of the Atkins, South Beach, and other low-carbohydrate diets led many Americans to believe that carbohydrates are "bad," the source of unflattering flab, and a cause of the obesity epidemic. That's a dangerous oversimplification, on a par with "fat is bad." Easily digested carbohydrates from white bread, white rice, pastries, sugared sodas, and other highly processed foods may, indeed, contribute to weight gain and interfere with weight loss. Whole grains, beans, fruits, vegetables, and other sources of intact carbohydrates do just the opposite—they promote good health.
Don't be misled by the blanket pronouncements on the dangers of carbohydrates. They are an important part of a healthy diet. Carbohydrates provide the body with the fuel it needs for physical activity and for proper organ function. The best sources of carbohydrates—fruits, vegetables, beans, and whole grains—deliver essential vitamins and minerals, fiber, and a host of important phytonutrients.
What Are Carbohydrates?
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.
When Sugar Management Goes Awry: Insulin and Diabetes
When you eat a food containing carbohydrates, the digestive system breaks down the digestible ones into sugar, which then enters the blood. As blood sugar levels rise, special cells in the pancreas churn out more and more insulin, a hormone that signals cells to absorb blood sugar for energy or storage. As cells sponge up blood sugar, its levels in the bloodstream begin to fall. That's when other cells in the pancreas start making glucagon, a hormone that tells the liver to start releasing stored sugar. This interplay of insulin and glucagon ensure that cells throughout the body, and especially in the brain, have a steady supply of blood sugar.In some people, this cycle doesn't work properly. People with type 1 diabetes (once called insulin-dependent or juvenile diabetes) don't make enough insulin, so their cells can't absorb sugar. People with type 2 diabetes (once called non-insulin-dependent, or adult-onset diabetes) generally start out with a different problem—their cells don't respond well to insulin's "open up for sugar" signal. This condition, known as insulin resistance, causes blood sugar and insulin levels to stay high long after eating. Over time, the heavy demands made on the insulin-making cells wears them out, and insulin production slows, then stops.
http://www.hsph.harvard.edu/nutritionsource/what-should-you-eat/carbohydrates-full-story/index.html
