Carbohydrates are a readily available source of energy for the body. (click on any picture below for larger image)
As monosaccharides, they provide simple sugars such as glucose (blood sugar), fructose (fruit sugar), galactose (milk sugar), and ribose (the backbone of RNA). As disaccharides, the more complex carbohydrates make up sucrose (table sugar = glucose + fructose) and lactose (milk sugar = glucose + galactose).
Starches (amylose + amylopectin) are another form of carbohydrates. These are the storage form of sugar in plants. Analagously, animals store their sugar as glycogen. Starches and glycogen are long strands of sugars (polysacharides) that branch off at various points.
Cellulose makes up the support network of plant walls, while chitin is contained in the body armors of many bugs, crabs, lobsters, shrimp, etc. While we eat a lot of cellulose via fruits, vegetables, whole grains, legumes, and nuts, we don’t actually absorb it. Instead, it is passed through to the large intestines (as dietary fiber). There, it promotes regular bowel movements, thus decreasing transit time associated with digestive distress and colon cancer. It also binds to bile acids, eliminating them from the body via the stool and thus lowering serum cholesterol.
As the body breaks down starches contained in breads, root vegetables, legumes, and so forth, the glucose molecules are released, giving us the taste sensation of sweetness.
The glycemic index measures the speed at which starches are broken down, making the glucose units available to all the cells in the body. Hence, more complex and unrefined foods will have a lower glycemic index, because the body has to work harder at breaking them down. This is actually desirable for optimal blood sugar regulation. (click on any picture for larger image)
When we eat highly processed foods or indulge our sweet tooth with candy, cookies, muffins, cake, pie, etc., the blood gets flooded with glucose units. Insulin rushes in to usher them into the cells, and the liver tries to mop up the excess glucose by converting it to glycogen to be stored in the liver, or to triglycerides to be stored in the cells of the fatty tissue (adipocytes). Regular consumption of such foods is a recipe for disaster, leading one down the road of diabetes with its many complications such as hypertension, heart disease, stroke, peripheral neuropathy, retinopathy, gastroparesis, and kidney disease/failure, among others.
While the glycemic index helps us to pay attention to blood sugar, it says little about the nutritional aspect of the foods that we eat. Foods not only contain carbohydrates but also proteins, fats, vitamins, minerals, trace elements, and other important biochemicals. Food eaten in its natural state will give us the greatest amount of nourishment for our bodies, as they provide us with the biochemicals that are necessary for the daily metabolic processes of life.
Carbohydrates deliver glucose molecules to the body for energy production. Hence, they are an important part of our foods. Whole, unprocessed or little processed fruits, vegetables, grains, legumes, nuts, and dairy are valuable sources. Cake, soda, cookies, candy, or highly processed foods are poor sources because they bring to the table nothing in the form of proteins, healthy fats, vitamins or minerals. Hence, they are referred to as empty calories.
The source of carbohydrates matters! Be wise in your choice of carbohydrates and be well.