Carbohydrates can be classified into two general categories based on their chemical structure: simple carbohydrates (or sugars) and complex carbohydrates (or starches).

Much of the sugar consume comes from what is added to processed foods (such as cookies, cakes, muffins, ice cream, candies, breakfast cereals, sports bars, and especially sodas). The most common form of added sugar is plain white table sugar (or sucrose). Other sugars that are added to foods include molasses, honey, high fructose corn syrup, and fruit juice concentrate. Read labels to identify these sources.

Sugar is also naturally occurring in some foods. For example, fruit has fruit sugar (called fructose), and milk has milk sugar (called lactose). Just because these foods are high in sugar doesn't make them "bad" foods. On the contrary, these foods are loaded with important vitamins and minerals, and they are relatively low in calories. It's only when sugar is concentrated or extracted from natural sources and then added to nutrient-poor processed foods that sugar can become a problem.

Complex carbohydrates are in foods such as bread, cereal, rice, pasta, tortillas, crackers, pretzels, beans, and starchy vegetables (like potatoes, peas, corn, and yams). Because whole pieces of fruit have fiber (unlike fruit juices or fruit juice concentrate sweeteners), they too can be considered complex carbohydrates. Much of the complex carbohydrate Americans consume comes from refined and processed products (such as white breads, white rice, white pasta, and white instant potatoes or French fries).

The refinement of these foods takes away many of their nutrients and fiber and leaves just a white starchy final product. These foods are “enriched,” but only with five nutrients (four B vitamins and iron). All the other nutrients are forever lost. Foods in their whole form (such as whole wheat bread, brown rice, oats, and whole grain cereals) retain all of their nutrients and fiber.

These are often confused with refined and unrefined carbohydrates, but the terms simple and complex refer to how complicated the chemical structure of a carbohydrate is rather than to whether it's wholegrain or not. Complex carbohydrates are the most common and there are three kinds:

Glycogen : This is your body's major fuel source and is sometimes referred to as blood sugar. It's formed from glucose, which is found in almost all foods, and is converted into energy.

Starch : This is only found in plants and, contrary to popular belief, isn't fattening (it's the rich sauces, fats and oils often added to pasta, potatoes, rice, noodles and bread that are the culprits!).

Fibre (non-starch polysaccharide) : This is abundant in unrefined carbohydrates, fruit and vegetables, and is important because it helps your body to process waste efficiently and helps you to feel fuller for longer.

All carbohydrates (whether simple or complex) are ultimately broken down in the body to the same thing: sugar (i.e. blood sugar or blood glucose). And all carbohydrates ultimately fuel your brain and body the same way. So does it matter whether carbohydrate fuel comes from a diet of mostly added sugars in gummy bears, Oreo cookies, and soda vs. a diet of mostly whole grains, beans, vegetables, and fruits? Absolutely! Here’s why…

1.Simple sugars (when added to foods) are a more concentrated form of carbohydrate and calories. Naturally occurring sugars in whole pieces of fruit and complex carbohydrates in grains and vegetables are diluted with fiber and water, which makes them more bulky (so they fill you up) without extra calories.
Example:
1Tbs. sugar provides 12 g carbohydrate and 48 calories. vice versa,
1c. berries, which also provides 12 g carbohydrate and 48 calories.
Similarly, naturally occurring sugar in nonfat milk is diluted with water, protein, and many important vitamins and minerals.

2. Simple “added sugars” have few (if any) vitamins or minerals. They’re just “empty calories.”
Example:
1Tbs. sugar provides only 12 g carbohydrates and 48 calories.vice versa,
1c. berries provides 12 g carbohydrates, 48 calories PLUS 85 mg vitamin C, 26 mg folate, 247 mg potassium, and 3 g fiber.
Calories from added sugars are hiding in many foods. Take a look at the calorie difference between foods that have a lot of hiding added sugar vs. no hiding added sugar:

3. Simple added sugars (as well as many refined, low fiber starchy foods) are digested and absorbed more quickly, resulting in a rapid increase in blood sugar levels. This gives you a rapid, immediate burst of energy. But, this energy is short lasting. High blood sugar levels trigger a rapid surge of the hormone insulin. Insulin causes a rapid drop in blood sugar, leaving you feeling tired, hungry, and craving more sugar shortly after you eat.