It's hard to believe that 10 years ago, dieters were advised to eat a low-fat diet high in pasta, bread and other carbohydrates. In today's most popular diets—the Atkins diet, The South Beach Diet and The Zone—carbohydrates are often banished from the menu, or only permitted in very limited amounts.

Many Americans are left wondering if the success of these diets means that all carbohydrates are bad for your health. And some might be surprised to know that carbs are found in fruits and vegetables as well as bread and pasta. Below, Martha McKittrick, RD, a certified diabetes educator at New York-Presbyterian Hospital in New York City, explains exactly what carbohydrates are and what role they play in nutrition and weight loss.

What is a carbohydrate?

A carbohydrate is the body's main source of energy. There are two major carbohydrates: sugars or simple carbohydrates and starches or complex carbohydrates. Fiber, another carbohydrate, is not digested or absorbed by the body. However, it does aid in digestion and offers protection against some diseases.

There are three major kinds of sugars, which are called monosaccharides. They're called glucose, fructose and galactose. And examples are brown sugar, honey, maple syrup, white table sugar and high fructose corn syrup, which is in many juices.

A complex carbohydrate is basically sugar molecules linked together in a chain. Examples of complex carbs are rice, bread, pasta, cereal and crackers. But almost all foods contain a mixture of sugars and starches, unless we're talking just plain brown sugar or maple sugar; those kinds of things are just pure sugar.

How are simple and complex carbohydrates broken down differently?

During digestion, complex carbs are broken down into sugars. These sugar molecules move into the bloodstream and then into the body's cells where they are converted into energy. Simple sugars do not need to be broken down, so they get absorbed into the bloodstream more quickly than the complex carbohydrates. An exception is fructose, a sugar that's found in a lot of fruits and juice. Fructose gets broken down very slowly, even more slowly than many complex carbs.

How does one measure how quickly a food causes the blood sugar to rise?

The glycemic index is a measure of how rapidly a particular food will cause your blood sugar to increase. One would think if you had something sweet like sugar, it would cause your sugar to rise more rapidly than something that's not sweet, like a potato. But this is not always true. For example, potatoes have what's called high glycemic index; that means they cause your blood sugar to rise very quickly.

One problem with the glycemic index is that it assumes that you eat that food by itself. Most people do not sit down and eat a huge baked potato for dinner. They might put some butter on it; they might have a piece of chicken with it. So any time you're adding other foods containing protein and/or fat to it, you're changing the glycemic index. Also the more you process a food, the higher the glycemic index. For example, if you make mashed potatoes, it can affect the glycemic index of the potatoes. So a lot of experts don't really believe the glycemic index is that helpful because most people eat meals that contain more than one food.

Basically, the glycemic index can be a useful tool, but I think more research needs to be done on it.

Why don't you want your blood sugar to shoot up?

Foods with a high glycemic index can cause blood sugar to rise rapidly. This can cause large amounts of insulin—which is a hormone that regulates blood sugar—to be secreted, which in turn can cause a rapid drop in blood sugar. This rapid drop can lead to weakness, headaches and fatigue.

In addition, high levels of insulin can contribute towards increasing risk of heart disease in some people by raising blood pressure, elevating triglycerides and lowering HDL cholesterol, which is the "good cholesterol." The combination of these factors is called metabolic syndrome or Syndrome X.

High levels of insulin can also contribute to fat storage.

Do you think people should choose complex carbs over simple ones?

In general, yes. However, I like to use the words "processed" and "unprocessed." Just because a food contains complex carbs, it does not mean it is healthy. For example, white bread contains complex carbs, but it does not contain many nutrients. I would much rather see you eat whole grain bread.

The more processed something is, the more quickly it will raise blood sugar, which in turn can cause you to secrete more insulin. In addition, processed foods tend to contain fewer vitamins, minerals and disease-fighting phytochemicals as well as fiber.

How can someone know what foods are processed and unprocessed?

Foods that are refined or processed, I call "white" foods. White rice, white bread, white pasta, or sugary foods like candy. Examples of whole-grain or unprocessed foods are oatmeal and other whole-grain cereals, brown rice, couscous, bulgur, whole-grain bran and fruit.

Do you think processed foods are contributing to rising rates of obesity?

I definitely think we're eating more processed foods than we were years ago. But I think you also have to look at portion sizes. I think our portion sizes of all foods are huge. If you had a 100 calories of a processed, sugary food, it's not a big deal. But look at the giant muffins or cinnamon buns in malls, they're 8 ounces and 800 calories.

Why do people lose weight on low-carb diets?

First of all, when you cut carbs out of your diet, you tend to cut a lot of calories. Whether the carbs are from ice cream, cake, cookies, cereal, milk, yogurt or fruit, if you cut all that stuff out of your diet, you're going to have a huge decrease in your calories.

The second thing is: Carbohydrate holds in water. When we eat carbohydrate, a lot of goes to our glycogen stores in the liver and the muscles where carbohydrates are stored for energy. And in order to hold in one little molecule of glycogen, you have to hold in three molecules of water. So basically our cells are stuffed full of water, holding in this one little molecule of sugar stored as glycogen.

As soon as you go on a low-carb diet, your body takes all those stored sugar molecules and uses it for energy and you will urinate out pounds of water. So carbohydrate storage holds in water. When you go on a low-carb diet, you use up your carb stores, you release water, and you can lose four pounds easy in five days.

Some studies show that some people are able to lose weight more easily on low-carb diets than on low-fat diets even if the calories are the same in both diets.

What are some of the nutritional problems with a low-carb diet?

It depends really on how low you're talking. I think there needs to be more long-term studies to address if there is any danger from being on a real low-carb diet for many years. If you're eating low amounts of fruit, whole grains and vegetables, will that increase your risk of cancer 10 years from now? We just don't know.

Some people find, when they go on a low-carb diet, they have poor energy levels, especially if they're athletic and they work out a lot. So it doesn't tend to be a great choice if somebody is really active.

But moderately low-carb diets work well for a lot of people. A lot of people find that they're not as hungry when they cut their carbs down. Maybe a diet that is about 40 percent carbs, like the Zone Diet.

But if somebody did want to go on a lower-carb diet, they should just make sure that the carbs they're eating are healthy: the fruits, the vegetables and a moderate amount of whole grains.

What is the biggest misconception about carbohydrates?

I think people believe carbohydrates are bad. It's like we're just brainwashed now. I see people who will eat a 16-ounce steak for dinner and four ounces of cheese and six ounces of nuts for snacks but avoid eating a baked potato because they think the potato will make them fat. I think what people just need to realize is that you can fit carbs into your diet in moderate amounts.