Our bodies break carbohydrates down into sugar and then burn the sugar as fuel for our brain and body. Some people are afraid of carbohydrates because they think sugar automatically means weight gain, mood swings, and all things bad. But the fact is that your brain and body must have a steady supply glucose, the form of sugar that the body breaks carbohydrates down to.

Complex Carbohydrates
Complex carbohydrates are the “good carbs”. Examples of complex carbs are whole grains, starchy vegetables and beans. They have complicated molecules that breakdown slowly, delivering a steady supply of sugar to the bloodstream. When sugar is delivered to the cells gradually, they can burn it for energy and our energy levels and moods stay stable. The glycemic index is the measure how fast and how much a food increases blood sugar levels.

Simple Carbohydrates vs Bad Carbohydrates
Simple carbohydrates are made of simple molecules that are easy for your body to breakdown and deliver sugar to the bloodstream quickly.

Bad carbs are in the simple carbohydrate category. White sugar and white flour based foods are examples of simple carbohydrates. They breakdown quickly causing a fast spike in blood sugar. That’s where you get a rush of energy, then a big drop — and your mood goes right along with it. Plus, when too much sugar floods the system all at once, your body can’t use it all for energy and converts it to…guess what? Fat. Yes, your body ultimately converts excess sugar into glycogen and then into fat for storage. And worse, constantly overwhelming the cells with high levels of sugar is associated with blood sugar related diseases like hypoglycemia, diabetes, and insulin resistance.

Not all simple carbohydrates are “bad” bad carbs. Some, like fruit, are simple, nutritious carbs. Fresh fruit gives us enzymes, vitamins, minerals and fiber. Milk products also offer important nutritional benefits like protein, vitamins, and fats. The point is to recognize that these simple carbohydrates, that are not so “bad”, are still fast sugars that do not sustain our energy in the same way that complex carbohydrates do.

Good Carbs Gone Bad
One of the confusing things about carbohydrates is difference between whole and refined foods. Whole grains are a good example. When a grain is refined, the nutrients and fiber have been taken from it making it quick to digest and less nutritious. Let’s look at the making of white bread flour: A whole wheat kernel with all its parts includes the bran, the endosperm, and the germ.

The bran gives us fiber which has many digestive benefits including slowing the breakdown of the food and the absorption of fat. And the germ is the part that has the most nutrients. But when whole wheat is refined to make white flour the bran and germ are removed, leaving just the less nutritious endosperm.

The refining process is applied to other nutritious whole grains like corn and rice as well. Many of us only experience these formerly nutritious whole grains when are turned into refined flour and used to make processed foods like commercial breakfast cereals, breads, tortillas, white rice and junk food. These denatured carbs are not likely to help you sustain your energy.
So the truly bad carbohydrates are the ones that offer little in the way of nutrition and pump a lot of simple, energy-destabilizing sugars into the body. Examples of bad, bad carb foods are those made with refined sugar and refined flour like candy, cake, muffins and white bread — no nutrients and no steady energy.

Get Good Carb Energy
My favorite image for carbohydrate digestion is to think of a campfire. Complex carbs are like good logs. They catch easily and burn steadily for a long time. The bad, simple carbs are like dry tender. When we put them into our digestive fire they flare up quickly and then burn out leaving us with little to run on.

A balanced diet includes carbohydrates with an emphasis on the complex, along with healthy fats and proteins. Those of us who want to be healthy and get in shape and stay there, need to take advantage of all three food groups.

https://www.verywell.com/good-carbs-vs-bad-carbs-2704423
Sources:
”Inside the [food] Pyramid”, MyPyramid.gov 
Whole grains: Hearty options for a healthy diet, Mayo clinic
Staying Healthy with Nutrition, Haas, Elson
The New Optimum Nutrition Bible, Holford, Patrick