As a dietitian, I'm often asked to explain the difference between simple and complex carbohydrates. While most people have heard that complex carbohydrates are better for you than simple carbohydrates, what's often unclear is how the two differ, and why the differences between the two affect your health.

There are three types of carbohydrates: starch, sugar and fiber. Starches and sugars provide your body with its main source of energy. They're all comprised of carbon, oxygen and hydrogen, which are organized into single units. Sugars contain just one or two of these units and are "simple," while starches and fibers have many units of sugar, making them "complex".

Simple Carbohydrates

Sugars can occur naturally or artificially in foods, but your body can't distinguish between the sources. When people talk about "sugar," they're likely referring to table sugar, which is sucrose (a two-unit sugar). Table sugar belongs to a larger group of sugars, though, known as simple carbohydrates.

Simple carbohydrates include monosaccharides (one-unit sugars) and disaccharides (two-unit sugars). Monosaccharides include glucose, fructose and galactose. These can be absorbed directly and don't require further breakdown from enzymes, unlike disaccharides and polysaccharides.

Disaccharides are formed chemically when two monosaccharides combine to create once of the following:

Lactose: glucose and galactose
Sucrose: glucose and fructose
Maltose: glucose and glucose

Again, sucrose is just table sugar, and it occurs naturally in several fruits, grains and vegetables. Lactose is also a natural sugar, and it can be found in milk and other dairy products. Maltose forms naturally when starches break down from complex carbohydrates into simple sugars.

Complex Carbohydrates

Both fiber and starch are polysaccharides, meaning they are made up of many units of sugar and resemble a long chain. Plant foods, including grains, potatoes and legumes, contain starches.

All carbohydrates, except for fiber, are broken down by your body into monosaccharides as your body digests them. Your body breaks them down into simple sugars so they can be absorbed in your bloodstream and then transported to your cells and converted to energy.

What about fiber, you ask? Fiber isn't completely broken down in your digestive tract, and some of it remains whole in your body because you lack the enzymes to break it down. This actually confers numerous health benefits.

What's the Difference?

Regarding your health, the real difference is where the sugar comes from. While your body can't distinguish the difference between the source of sugar once it's broken down and absorbed, the food from which the sugar originated has a huge impact on your overall health. This is due to the nature other nutrients that may be in the food you consumed.

Since complex carbohydrates come from plant-based foods, we know that those foods also contain a plethora of beneficial nutrients in addition to their carbs, including vitamins, minerals and antioxidants.

The complex carbs are broken down into simple sugars. However, some simple sugars that are added to foods don't give you any beneficial nutrients. For example, fructose can be found in candies, soda, and other sweets lacking in health-promoting nutrients, but fructose is also present in fruit.

Even though both foods contain fructose, fruit is obviously a healthier choice because it's not solely made up of simple carbohydrates -- it also contains fiber, vitamins and antioxidants. The fiber in fruit helps slow the digestion of carbs, which is why your blood sugar doesn't spike as much after eating fiber-filled fruit like it does when you gulp down a soda or candy bar.

Vegetables and grains also contain some simple sugars in addition to their starches, mostly in the form of sucrose, but they give you a hefty dose of vitamins, minerals and antioxidants too, none of which you'll find in candies, soda and other sucrose-packed sweets.

The Bottom Line

When given the option, you should choose complex carbohydrates, such as those found in vegetables, whole grains and legumes, more often than simple carbohydrates. Not only will complex carbohydrates provide a more steady supply of energy and cause a less dramatic increase in your blood glucose levels, the foods in which complex carbs are found also provide a plethora of beneficial nutrients.