Carbohydrates are your body's primary source of glucose, and glucose is your cellular fuel. The body begins to convert carbohydrates almost entirely into glucose shortly after carb-containing foods are eaten, and insulin helps to “unlock” the cells to move the glucose from the bloodstream into the cells for energy. If you have insufficient insulin production or your body is resistant to insulin, consuming too many carbohydrates can cause blood glucose to rise unduly.

All foods that contain starches and/or sugars — including fruits, vegetables, milk, yogurt, breads, grains, beans, and pasta — contain carbs. Virtually the only whole foods that are carbohydrate-free are protein-rich meats, poultry, and finfish (when prepared without additional ingredients such as breading and marinades), and fats such as cooking oils and shortening.

To avoid all carb-containing foods is both impossible and unadvisable — your body needs the important micronutrients and phytochemicals contained in these foods. But you do need to learn the basics of assessing the quantity and quality of carbohydrates in your food, how your body reacts to them, and how to make smart carb choices based on this information.

Carb Science

Carbohydrates are categorized by their chemical structure. A monosaccharide, or simple sugar, is composed of a single saccharide (sugar) chain. Glucose, fructose (fruit sugar), and galactose are all simple sugars. Disaccharides are two simple sugars joined together, and include lactose (milk sugar), maltose (malt sugar), and sucrose (table sugar). Polysaccharides, or complex carbohydrates, are ten or more simple sugar chains joined. Glycogen, starches, and fiber are polysaccharides. (A fourth type of carbohydrate, oligosaccharides, is composed of three to ten sugar chains, but most of these are usually formed from the breakdown of polysaccharides.)

All carbohydrates are hydrolyzed, or broken down, into monosaccharides before they can be processed by the body. Amylase, a type of enzyme found in the saliva and secreted by the pancreas, helps to break down carbs into glucose during their journey through the small intestine. Once hydrolysis occurs, the resulting glucose is absorbed into the bloodstream. Any excess is converted to glycogen and stored in the liver, along with fructose and galactose.

The Glycemic Index

Does it matter what kind of carbs you consume? At one time, nutritionists believed that people with diabetes should avoid simple sugars (monosaccharides and disaccharides) and eat foods containing complex carbohydrates instead, with the mistaken belief that simple sugars would raise glucose levels faster and more dramatically across the board. But it's now known that gram for gram, complex carbohydrates found in breads, cereals, potatoes, vegetables, and other foods raise the blood sugar approximately the same amount as do simple sugars like honey, fructose, or table sugar.

However, there may be a difference in how rapidly certain foods raise blood sugar levels. The glycemic index, or GI, is a measure of how quickly the carbs in certain foods are transformed into blood glucose. Foods with a low GI (e.g., beans, multigrain bread) raise glucose levels at a slower and steadier rate than a high-GI food (e.g., rice, potatoes).

Carbs and Blood Glucose Control

Carbohydrates and the glucose they generate are an energy source — the dietary fuel of the human body. Insulin produced by the pancreas enables our cells to burn this carb-generated glucose. This is why determining the amount of carbohydrates in a meal is so important for blood sugar control.

People with type 1 diabetes, and some with type 2 diabetes, have to inject enough insulin to accommodate, or “cover,” all the carbohydrates they eat. And many people with type 2 diabetes have the added task of having to adjust their diets to lose excess weight, as well. Concepts like carbohydrate counting and dietary exchanges, covered in the next chapter, enable people with diabetes to manage their blood sugar levels effectively.

It's also important to understand that carbohydrates don't work in isolation. Other nutrient components of the food you eat also can affect carb absorption. High-fat and high-protein foods can delay carb absorption. And although fiber is considered a carbohydrate, high-fiber foods also slow the absorption of glucose since they slow the passage of food through the digestive tract.