In terms of nutrition "good" carbohydrates are complex carbohydrates such as those found in beans, whole grains and many green vegetables. "Bad" carbohydrates are simple carbs, such as those in cane sugar, white flour and starchy vegetables such as potatoes.
When the body digests a carbohydrate, the speed with which it breaks down the molecule depends on how complex it is. Simple sugars are absorbed quickly into the bloodstream, while complex carbohydrates take time and enzymatic action to break down. This means simple sugars lead to rapid blood sugar elevation, while complex carbohydrates raise blood sugar more slowly and gradually.
When someone eats a lot of "bad" carbs, simple sugars flood the bloodstream. The pancreas responds by secreting a large amount of insulin to handle this excess sugar, quickly using it up. This can make the consumer hungry, or even dizzy and disoriented if blood glucose concentration drops to dangerously low levels. A diet containing excessive levels of simple carbs can even lead to insulin resistance and type 2 diabetes over time.
The effect of simple carbohydrates can be moderated by the presence of fiber. For instance, a glass of orange juice is high in simple sugars and results in a high blood sugar spike. Eating an orange, on the other hand, provides fiber in the fruit's pulp that takes longer to digest, slowing down the absorption of sugar into the bloodstream.